2.1.1 Pure substances and melting point data

A pure substance contains one element or compound

Definition

Pure substance

A single element or a single compound, with nothing else mixed in.

Definition

Mixture

A combination of two or more substances that are not chemically bonded together.

Every particle in a pure element, such as a copper wire, is an atom of that one element.
Every particle in a pure compound, such as distilled water, is the same $H2O\text{H}_2\text{O}$ $H_{2} O$ molecule.
1. A pure compound still contains more than one element, because its atoms are bonded together in a fixed ratio.
Any sample with two or more substances mixed together is not pure in the chemical sense.
1. Sea water is not pure, because it contains water with dissolved salts such as sodium chloride.

Common Mistake

Do not call a compound impure because it contains more than one element, since its elements are bonded into a single substance.

Everyday 'pure' is looser than the scientific meaning

Definition

Purity

The extent to which a sample contains only one substance.

In everyday language, pure usually means natural or with nothing added.
1. A carton labelled pure orange juice has nothing added, yet the juice is a mixture of water, sugars, acids and many other substances.
2. Bottled spring water is sold as pure, but it contains dissolved calcium and magnesium compounds.
In chemistry, a sample is pure only if it contains a single substance.
Medicines and food additives are made to high purity, because other substances could change how they work or cause harm.

Key Idea

A chemically pure sample contains one substance only, however natural or unprocessed it is.

A pure substance melts at a sharp, fixed temperature

Definition

Melting point

The temperature at which a solid changes to a liquid at a stated pressure.

Every pure substance has its own fixed melting point at a given pressure.
1. Pure ice melts at $∘C0\ ^{\circ}\text{C}$ at normal atmospheric pressure.
A pure substance melts sharply, changing from solid to liquid over a very narrow temperature range.
1. All its particles are the same, so the attractions holding the solid together all need the same energy to overcome.
Comparing a measured melting point with a data book value can identify a substance.

Example

Pure benzoic acid melts at $∘C122\ ^{\circ}\text{C}$ , so a white solid that melts sharply at $∘C122\ ^{\circ}\text{C}$ is very likely to be pure benzoic acid.

Impurities lower the melting point and widen its range

Definition

Impure substance

A sample that contains more than one substance.

Impurities usually lower the melting point of a substance.
An impure solid also melts over a range of temperatures rather than at one sharp value.
1. The impurity particles disrupt the regular arrangement of the main substance, so some parts of the solid break down at lower temperatures than others.
The more impurity a sample contains, the lower and wider its melting range usually becomes.
Salt is spread on icy roads for the same reason, because salty water freezes below $∘C0\ ^{\circ}\text{C}$ .

Example

A sample of aspirin melts between $∘C128\ ^{\circ}\text{C}$ and $∘C133\ ^{\circ}\text{C}$ , while pure aspirin melts at $∘C136\ ^{\circ}\text{C}$ .
The sample melts below the pure value and over a range, so it is impure.

Self review

What does 'pure' mean in chemistry?
Why is a carton of pure orange juice not pure in the chemical sense?
How does the melting of a pure substance differ from the melting of an impure one?
A sample melts between $∘C76\ ^{\circ}\text{C}$ and $∘C80\ ^{\circ}\text{C}$ , and the pure substance melts at $∘C82\ ^{\circ}\text{C}$ ; is the sample pure?
Why does an impurity make a solid melt over a range of temperatures?

2.1.2 Relative formula mass and empirical formula

Adding relative atomic masses gives relative formula mass

Definition

Relative atomic mass

The average mass of an atom of an element compared with one-twelfth of the mass of a carbon-12 atom.

Definition

Relative formula mass

The mass of one formula unit of a substance compared with one-twelfth of the mass of a carbon-12 atom, given the symbol Mr.

Each element's relative atomic mass, $A_r$ , is given on the Periodic Table.
To find $M_r$ , multiply each $A_r$ by the number of atoms of that element in the formula and add the results.
A small number after a symbol applies only to that atom, so $H2O\text{H}_2\text{O}$ contains two hydrogen atoms and one oxygen atom.
A number after a bracket multiplies everything inside it, so $Ca(OH)2\text{Ca(OH)}_2$ contains one calcium, two oxygen and two hydrogen atoms.
Relative formula masses have no units, because they compare masses with the carbon-12 standard.

Example

Calculate the relative formula mass of calcium hydroxide, $Ca(OH)2\text{Ca(OH)}_2$ ( $A_r$ : Ca = 40.1, O = 16.0, H = 1.0).
Count the atoms: $1$ Ca, $2$ O and $2$ H.
Multiply and add: $Mr=40.1+(2×16.0)+(2×1.0)=74.1M_r = 40.1 + (2 \times 16.0) + (2 \times 1.0) = 74.1$ .

Relative formula masses balance on both sides of equations

A large number in front of a formula multiplies the whole formula, so $2H2O2\text{H}_2\text{O}$ means two water molecules.
To find the mass of a species in an equation, multiply its $M_r$ by this balancing number.
In $2Mg+O2→2MgO2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}$ $2 Mg + O_{2} \to 2 MgO$ , the reactants total $(2×24.3)+32.0=80.6(2 \times 24.3) + 32.0 = 80.6$ $(2 \times 24.3) + 32.0 = 80.6$ .
1. The product also totals $\times (24.3 + 16.0) = 80.6$ , because no atoms are gained or lost.
If the two totals differ, the equation has not been balanced correctly.

Common Mistake

Do not count oxygen once for $O2\text{O}_2$ , because an oxygen molecule contains two atoms, giving $M_r = 32.0$ .
Do not round $A_r$ values such as Mg = 24.3 to whole numbers unless the values you are given are whole numbers.

The empirical formula is the simplest whole-number ratio

Definition

Empirical formula

The simplest whole-number ratio of atoms of each element in a compound.

Definition

Molecular formula

The actual number of atoms of each element in one molecule of a compound.

To find an empirical formula from a molecular formula, divide the numbers of atoms by the largest number that goes into all of them.
1. Ethane, $C2H6\text{C}_2\text{H}_6$ , divides by $2$ to give the empirical formula $CH3\text{CH}_3$ .
2. Glucose, $C6H12O6\text{C}_6\text{H}_{12}\text{O}_6$ , divides by $6$ to give $CH2O\text{CH}_2\text{O}$ .
When the numbers share no common factor, as in $H2O\text{H}_2\text{O}$ or $CO2\text{CO}_2$ , the molecular formula is already the empirical formula.
Ethene, $C2H4\text{C}_2\text{H}_4$ , and propene, $C3H6\text{C}_3\text{H}_6$ , share the empirical formula $CH2\text{CH}_2$ , so an empirical formula alone may not identify a compound.

Note

Ionic compounds such as sodium chloride are giant lattices with no separate molecules, so their formula, $NaCl\text{NaCl}$ , is always an empirical formula.

Reading empirical formulae from models and diagrams

In a ball and stick model or displayed formula, count the atoms of each element first.
Then write the counts as a ratio and simplify it to the smallest whole numbers.
For a giant structure diagram, such as part of a sodium chloride lattice, work out the ratio in which the ions repeat.
Going the other way, the empirical formula $CH2\text{CH}_2$ $CH_{2}$ tells you that a matching model must contain twice as many hydrogen atoms as carbon atoms.
1. A model of ethene with $2$ carbon and $4$ hydrogen atoms fits this ratio, while one with $2$ carbon and $6$ hydrogen atoms does not.

Example

A diagram of a molecule shows $4$ carbon atoms, $8$ hydrogen atoms and $2$ oxygen atoms; find its empirical formula.
Write the ratio of atoms: C : H : O = $4 : 8 : 2$ .
Divide each number by $2$ , the largest number that goes into all three: $2 : 4 : 1$ .
Write the empirical formula: $C2H4O\text{C}_2\text{H}_4\text{O}$ .

Self review

How do you calculate the relative formula mass of a compound?
What is the relative formula mass of $MgCl2\text{MgCl}_2$ ( $A_r$ : Mg = 24.3, Cl = 35.5)?
What does the number $3$ in $3CO23\text{CO}_2$ tell you?
What is the empirical formula of $C4H10\text{C}_4\text{H}_{10}$ ?
Why is the formula of sodium chloride always an empirical formula?

2.1.3 Formulations as useful mixtures

A formulation is a mixture with a purpose

Definition

Formulation

A mixture designed for a particular use.

Definition

Component

One of the substances present in a mixture.

Each component has a specific purpose, such as giving colour, protecting a surface or making the product easier to use.
The components are mixed in measured quantities, because changing the proportions changes the properties.
The components are mixed rather than reacted, so each keeps its own properties within the product.
1. A formulation is therefore a mixture, and it is not pure in the chemical sense.
Manufacturers follow a precise recipe, so every batch has the same properties.

Key Idea

A formulation is useful because both its components and their amounts are chosen deliberately for one purpose.

Paints, medicines, cleaning products and fertilisers are everyday formulations

Paint contains a pigment for colour, a binder that holds the pigment on the surface and a solvent that keeps the paint runny until it dries.
A medicine tablet contains the active drug with fillers that bulk it out, binders that hold it together and a coating that makes it easier to swallow.
Washing-up liquid combines detergents that lift grease with water, fragrance and colouring.
Fertilisers are blends of nitrogen, phosphorus and potassium compounds in proportions suited to particular crops.
Fuels, cosmetics, sun creams and many foods are also formulations.

Practical

Making Prussian blue paint

Aim: make a pure, dry sample of the pigment Prussian blue and find out which binder turns it into the best paint.
Apparatus: two $cm3100\,\text{cm}^3$ conical flasks, a $cm325\,\text{cm}^3$ measuring cylinder, a glass rod, a filter funnel and filter paper, a wash bottle of distilled water, a dropping pipette, propanone, a watch glass, a pestle and mortar, a spatula, a digital balance, iron(II) sulfate, potassium hexacyanoferrate, binders such as linseed oil, egg yolk and dilute PVA glue, and white paper.

Part 1: making the pigment

Dissolve a spatula of iron(II) sulfate in about $cm320\,\text{cm}^3$ of distilled water in one conical flask.
Dissolve a spatula of potassium hexacyanoferrate in about $cm320\,\text{cm}^3$ of distilled water in the second flask.
Pour one solution slowly into the other while stirring, so that a dark blue precipitate forms.
Filter the mixture, then wash the solid on the filter paper with distilled water to remove the soluble by-products.
Rinse the solid with a few drops of propanone, which mixes with the water left on it and evaporates quickly.
Scrape the solid onto a watch glass and leave it to dry.

Part 2: testing the binders

Grind the dry pigment to a fine powder with a pestle and mortar.
Weigh equal masses of the pigment and mix each one with the same volume of a different binder.
Paint each mixture onto the same kind of paper, using the same number of brush strokes.
Compare how easily each paint spreads, how even the colour is and how long it takes to dry.

Accuracy and safety

Use the same mass of pigment and the same volume of binder each time, so that the paints are compared fairly.
Keep acids well away from potassium hexacyanoferrate, because they react to release a very toxic gas.
Keep propanone away from flames, because it is highly flammable.
Wear eye protection throughout.

Changing one component changes the whole formulation

Adding more pigment gives a paint a stronger colour, but too much can make it crack as it dries.
Adding more solvent makes a paint thinner and easier to spread, but it covers the surface less well.
In a medicine, the dose of active ingredient must be exact, because too little does not work and too much can be harmful.
Formulations are tested so that the product is safe, effective and the same from batch to batch.

Common Mistake

Do not describe a formulation as a compound, because its components are mixed and are not chemically bonded together.

A formulation melts over a range of temperatures

Like any mixture, a formulation has no single melting point.
A heating curve for a formulation shows the solid melting over a range, with no flat section at one temperature.
1. A pure substance gives a flat section at its melting point, because the temperature stays constant while it melts.
This lets a chemist tell a formulation apart from a pure substance using melting data.
Chocolate is a formulation of cocoa solids, cocoa butter, sugar and milk, which is why it softens gradually as it warms.

Example

Sample A stays at $∘C80\ ^{\circ}\text{C}$ while it melts, but sample B melts between $∘C55\ ^{\circ}\text{C}$ and $∘C62\ ^{\circ}\text{C}$ , so sample B is the formulation.

Self review

What is a formulation?
Why are the components of a formulation measured carefully?
Name three components of paint and give the job of each.
Why is a formulation not pure in the chemical sense?
How would a heating curve show that a substance is a formulation?

2.1.4 Techniques for separating mixtures

Filtration separates an insoluble solid from a liquid

Definition

Residue

The insoluble solid left behind in the filter paper after filtration.

Definition

Filtrate

The liquid that passes through the filter paper during filtration.

The mixture is poured through filter paper folded into a funnel.
1. The paper has tiny holes that let the liquid and any dissolved particles through but stop the larger solid particles.
Filtration separates sand from water, or removes an excess solid after a reaction.
It cannot separate a dissolved solid from its solvent, because dissolved particles pass through the paper.
Rinsing the solid with a little distilled water washes off traces of the solution.

Example

A paper coffee filter works the same way, with the flavoured solution passing through and the grounds staying behind.

Crystallisation recovers a soluble solid from its solution

Definition

Crystallisation

A separation technique used to obtain solid crystals of a dissolved substance from a solution.

Definition

Saturated solution

A solution in which no more solute can dissolve at that temperature.

Heat the solution in an evaporating basin to drive off some of the water.
Stop heating once crystals form on a glass rod dipped into the solution and taken out.
Leave the solution to cool slowly, because most solids are less soluble at lower temperatures, so crystals form.
Filter off the crystals and dry them between filter papers or in a warm oven.
Crystallisation gives larger, purer crystals than boiling off all the water, which can also break down some solids.

Practical

Aim: obtain dry crystals of hydrated copper(II) sulfate from its solution.
Apparatus: copper(II) sulfate solution, an evaporating basin, a beaker of water to use as a water bath, a Bunsen burner, a tripod and gauze, a heatproof mat, a glass rod, tongs, a filter funnel and filter paper.

Method

Pour about $cm325\,\text{cm}^3$ of copper(II) sulfate solution into an evaporating basin.
Stand the basin on a beaker of water on a tripod and gauze, and heat the water, which warms the solution gently and stops it boiling and spitting.
Dip a glass rod into the solution from time to time and lift it out to cool.
Stop heating when small crystals form on the cooled rod, which shows the solution is nearly saturated.
Leave the basin in a cool place for a day or more, so that larger crystals grow as the solution cools and more water evaporates.
Pour off the remaining solution and pat the crystals dry between filter papers, or leave them somewhere warm to dry.

Accuracy and safety

Do not heat the crystals strongly, because they lose their water of crystallisation and turn into white anhydrous copper(II) sulfate.
Cooling the solution slowly gives larger, better shaped crystals than cooling it quickly.
Wear eye protection, because hot solution can spit.
Wash your hands afterwards, because copper(II) sulfate is harmful if swallowed.

Simple distillation collects the solvent from a solution

Definition

Distillate

The liquid collected after vapour has been condensed during distillation.

Heating the solution boils the solvent, while the dissolved solid stays behind in the flask.
The vapour passes into a condenser, where cold water flowing around the outside cools it back into a liquid.
The condensed liquid drips into a collecting beaker.
The thermometer bulb sits level with the side arm, so it reads the temperature of the vapour leaving the flask.
Simple distillation produces pure water from sea water, because the dissolved salts stay behind when the water boils.
Heating chopped orange peel in water distils out an oil called limonene, which forms a separate layer on top of the water in the distillate.

Practical

Aim: collect pure water from salty water by simple distillation.
Apparatus: a round-bottomed flask, anti-bumping granules, a thermometer from $0$ to $∘C110\ ^{\circ}\text{C}$ , a still head with a side arm, a Liebig condenser with rubber tubing, a collecting beaker, a Bunsen burner or heating mantle, clamps and stands, salty water and a watch glass.

Part 1: setting up the apparatus

Half-fill the round-bottomed flask with salty water and add a few anti-bumping granules, so that it boils smoothly.
Clamp the flask firmly and fit the still head and thermometer, with the thermometer bulb level with the side arm.
Connect the side arm to the condenser, clamping the condenser so that it slopes down into the collecting beaker.
Connect the rubber tubing so that cold water runs into the bottom of the condenser and out at the top, which keeps the outer jacket full of cold water.

Part 2: distilling and testing the water

Heat the flask gently until the salty water boils.
Collect the liquid that drips from the condenser while the thermometer reads about $∘C100\ ^{\circ}\text{C}$ .
Stop heating before the flask boils dry.
Test the distillate by showing that it boils at $∘C100\ ^{\circ}\text{C}$ , or that a few drops evaporated on a watch glass leave no solid.

Accuracy and safety

The thermometer bulb must be level with the side arm, so that it measures the temperature of the vapour that is distilling over.
Never heat a closed system, so check that the end of the condenser is open to the air.
Stop before the flask boils dry, because the hot salt left behind can crack the glass.
Wear eye protection and let the apparatus cool before taking it apart.

Fractional distillation separates liquids with different boiling points

Definition

Fractionating column

A vertical tube in which vapour repeatedly condenses and evaporates, improving the separation of liquids with different boiling points.

The mixture of liquids is heated in a flask fitted with a fractionating column, often packed with glass beads.
The column is hottest at the bottom and coolest at the top.
The liquid with the lowest boiling point reaches the top of the column first and passes into the condenser.
1. Vapour of the higher-boiling liquid condenses on the beads and drips back into the flask.
When the thermometer reading rises, the next liquid has started to distil, so the collecting beaker is changed.
This separates ethanol, which boils at $∘C78\ ^{\circ}\text{C}$ , from water, which boils at $∘C100\ ^{\circ}\text{C}$ .

Common Mistake

Do not heat flammable liquids such as ethanol over a Bunsen flame; use an electric heater or a water bath instead.

Choosing a separation technique from substance properties

Definition

Solubility

The amount of a substance that can dissolve in a particular solvent at a given temperature.

An insoluble solid in a liquid is separated by filtration.
A soluble solid is recovered from its solution by crystallisation.
The solvent from a solution is collected by simple distillation.
Liquids that mix together but have different boiling points are separated by fractional distillation.
Some mixtures need two steps, such as rock salt, which is dissolved and filtered to remove sand, then crystallised to recover the salt.

Exam technique

When a question gives data about a mixture, first pick out whether each substance dissolves and how far apart the boiling points are.
Then name the technique and justify it with that property, for example that sand is insoluble so it stays in the filter paper.

Self review

Which technique separates sand from water, and why does it work?
Why is a solution left to cool during crystallisation?
Where should the thermometer bulb sit during distillation?
How does a fractionating column separate ethanol from water?
Which two techniques would you use to get pure salt from rock salt?

2.1.5 Chromatography and Rf values

Chromatography separates using stationary and mobile phases

Definition

Stationary phase

The part of a chromatography system that stays in place while the solvent moves through it.

Definition

Mobile phase

The part of a chromatography system that moves, carrying the substances being separated.

Paper chromatography uses paper as the stationary phase and a solvent as the mobile phase.
Thin layer chromatography uses a thin layer of silica gel on a glass, metal or plastic plate as its stationary phase.
The substances in a sample start on the stationary phase and begin to move once the solvent reaches them.
Each substance is distributed between the two phases, spending part of its time dissolved in the solvent and part held on the stationary phase.
1. A substance that spends more time in the mobile phase travels further, so the substances separate.

Key Idea

Separation depends on how each substance is shared between the two phases, so two substances that are shared in the same way cannot be separated by that solvent.

Paper and thin layer chromatography follow the same method

Draw a start line in pencil about $1 cm1\,\text{cm}$ $1 cm$ from the bottom of the paper or plate.
1. Pencil is used because it does not dissolve in the solvent, whereas ink would run and add its own spots.
Put a small, concentrated spot of each sample on the line.
Stand the paper in a beaker of solvent with the solvent level below the start line, then cover the beaker.
1. If the solvent were above the line, the spots would dissolve into the solvent in the beaker instead of moving up the paper.
Remove the paper when the solvent is near the top, mark how far the solvent reached and leave the paper to dry.

Practical

Separating the dyes in food colourings

Aim: find out which dyes are present in several food colourings.
Apparatus: chromatography paper, a pencil and ruler, capillary tubes, food colourings, samples of known dyes, a beaker with a lid or a cover, water as the solvent, a glass rod or splint, sticky tape and a hairdryer or warm place for drying.

Method

Draw a pencil start line about $cm2\,\text{cm}$ from the bottom of the chromatography paper.
Use a capillary tube to put a small spot of each food colouring and each known dye on the line, at least $cm1\,\text{cm}$ apart.
Build up each spot by adding a drop, letting it dry and adding another, so the spot stays small but holds enough dye to see.
Pour water into the beaker to a depth of about $cm1\,\text{cm}$ , so that the solvent will be below the start line.
Tape the top of the paper to a glass rod resting across the beaker, so that the bottom edge dips into the water, and cover the beaker.
Remove the paper when the solvent is about $cm1\,\text{cm}$ from the top, and draw a pencil line at the solvent front straight away.
Let the paper dry, then measure from the start line to the centre of each spot and to the solvent front.
Calculate each Rf value using $solvent\text{R}_\text{f} = \dfrac{\text{distance moved by spot}}{\text{distance moved by solvent}}$ .

Accuracy and safety

Draw the start line in pencil, because ink would dissolve in the solvent and run up the paper with the dyes.
Keep the solvent below the start line, or the spots will dissolve into the solvent in the beaker.
Cover the beaker, so that the air inside stays full of solvent vapour and the solvent does not evaporate from the paper.
A colouring that matches a known dye in colour and Rf value contains that dye, and one that gives more than one spot is a mixture.
Thin layer chromatography follows the same steps on a plate coated with silica, and it runs faster and gives sharper spots.

A chromatogram shows how many substances a mixture contains

Definition

Chromatogram

The pattern of separated spots produced after a mixture has been analysed by chromatography.

Definition

Solvent front

The furthest point reached by the solvent as it moves through the stationary phase.

Each separate spot is a different substance.
A mixture of three dyes gives three spots, while a pure dye gives only one.
A spot that stays on the start line is insoluble in that solvent.
If two samples produce spots at the same heights in the same solvent, they are likely to be the same substance.
1. Repeating the run in a second solvent makes the match more certain, because two different substances rarely match in both.

Example

Forensic scientists compare the ink on a forged document with inks from suspects' pens, and a match in several solvents points to the pen that was used.

The Rf value compares substance and solvent distances

Definition

Rf value

A value that compares how far a substance travels in chromatography with how far the solvent travels.

Measure from the start line to the centre of the spot, and from the start line to the solvent front.
Calculate the Rf value by dividing the two distances: $solventR_f = \frac{\text{distance moved by substance}}{\text{distance moved by solvent}}$
An Rf value is always between 0 and 1, because a substance cannot travel further than the solvent.
A substance has the same Rf value in the same solvent and stationary phase, so comparing Rf values with known values identifies it.
Rf values have no units, because they are a ratio of two lengths.

Example

A dye moves $cm3.6\,\text{cm}$ from the start line while the solvent front moves $cm8.0\,\text{cm}$ ; calculate its Rf value.
Write the ratio: $cmR_f = \dfrac{3.6\,\text{cm}}{8.0\,\text{cm}}$ .
Divide: $R_f = 0.45$ .

Chromatography can show whether a substance is pure

A pure substance gives a single spot in every solvent tried.
An impure substance gives two or more spots, showing the extra substances present.
Running the sample alongside a pure reference sample shows whether its main spot matches.
A single spot in one solvent is not conclusive, because two substances might move the same distance in that solvent.
1. Testing in different solvents gives stronger evidence that the sample is pure.

Exam technique

When a question asks you to identify substances from a chromatogram, compare the height or Rf value of each spot with the reference spots and name each match.

Self review

What are the stationary and mobile phases in paper chromatography?
Why is the start line drawn in pencil?
What does a chromatogram with four spots tell you about a sample?
A spot moves $cm2.4\,\text{cm}$ and the solvent moves $cm6.0\,\text{cm}$ ; what is its Rf value?
How can chromatography show that a substance is pure?

2.1.1 Pure substances and melting point data

A pure substance contains one element or compound

Definition

Pure substance

A single element or a single compound, with nothing else mixed in.

Definition

Mixture

A combination of two or more substances that are not chemically bonded together.

Every particle in a pure element, such as a copper wire, is an atom of that one element.
Every particle in a pure compound, such as distilled water, is the same $H2O\text{H}_2\text{O}$ $H_{2} O$ molecule.
1. A pure compound still contains more than one element, because its atoms are bonded together in a fixed ratio.
Any sample with two or more substances mixed together is not pure in the chemical sense.
1. Sea water is not pure, because it contains water with dissolved salts such as sodium chloride.

Common Mistake

Do not call a compound impure because it contains more than one element, since its elements are bonded into a single substance.

Everyday 'pure' is looser than the scientific meaning

Definition

Purity

The extent to which a sample contains only one substance.

In everyday language, pure usually means natural or with nothing added.
1. A carton labelled pure orange juice has nothing added, yet the juice is a mixture of water, sugars, acids and many other substances.
2. Bottled spring water is sold as pure, but it contains dissolved calcium and magnesium compounds.
In chemistry, a sample is pure only if it contains a single substance.
Medicines and food additives are made to high purity, because other substances could change how they work or cause harm.

Key Idea

A chemically pure sample contains one substance only, however natural or unprocessed it is.

A pure substance melts at a sharp, fixed temperature

Definition

Melting point

The temperature at which a solid changes to a liquid at a stated pressure.

Every pure substance has its own fixed melting point at a given pressure.
1. Pure ice melts at $∘C0\ ^{\circ}\text{C}$ at normal atmospheric pressure.
A pure substance melts sharply, changing from solid to liquid over a very narrow temperature range.
1. All its particles are the same, so the attractions holding the solid together all need the same energy to overcome.
Comparing a measured melting point with a data book value can identify a substance.

Example

Pure benzoic acid melts at $∘C122\ ^{\circ}\text{C}$ , so a white solid that melts sharply at $∘C122\ ^{\circ}\text{C}$ is very likely to be pure benzoic acid.

Impurities lower the melting point and widen its range

Definition

Impure substance

A sample that contains more than one substance.

Impurities usually lower the melting point of a substance.
An impure solid also melts over a range of temperatures rather than at one sharp value.
1. The impurity particles disrupt the regular arrangement of the main substance, so some parts of the solid break down at lower temperatures than others.
The more impurity a sample contains, the lower and wider its melting range usually becomes.
Salt is spread on icy roads for the same reason, because salty water freezes below $∘C0\ ^{\circ}\text{C}$ .

Example

A sample of aspirin melts between $∘C128\ ^{\circ}\text{C}$ and $∘C133\ ^{\circ}\text{C}$ , while pure aspirin melts at $∘C136\ ^{\circ}\text{C}$ .
The sample melts below the pure value and over a range, so it is impure.

Self review

What does 'pure' mean in chemistry?
Why is a carton of pure orange juice not pure in the chemical sense?
How does the melting of a pure substance differ from the melting of an impure one?
A sample melts between $∘C76\ ^{\circ}\text{C}$ and $∘C80\ ^{\circ}\text{C}$ , and the pure substance melts at $∘C82\ ^{\circ}\text{C}$ ; is the sample pure?
Why does an impurity make a solid melt over a range of temperatures?

2.1.2 Relative formula mass and empirical formula

Adding relative atomic masses gives relative formula mass

Definition

Relative atomic mass

The average mass of an atom of an element compared with one-twelfth of the mass of a carbon-12 atom.

Definition

Relative formula mass

The mass of one formula unit of a substance compared with one-twelfth of the mass of a carbon-12 atom, given the symbol Mr.

Each element's relative atomic mass, $A_r$ , is given on the Periodic Table.
To find $M_r$ , multiply each $A_r$ by the number of atoms of that element in the formula and add the results.
A small number after a symbol applies only to that atom, so $H2O\text{H}_2\text{O}$ contains two hydrogen atoms and one oxygen atom.
A number after a bracket multiplies everything inside it, so $Ca(OH)2\text{Ca(OH)}_2$ contains one calcium, two oxygen and two hydrogen atoms.
Relative formula masses have no units, because they compare masses with the carbon-12 standard.

Example

Calculate the relative formula mass of calcium hydroxide, $Ca(OH)2\text{Ca(OH)}_2$ ( $A_r$ : Ca = 40.1, O = 16.0, H = 1.0).
Count the atoms: $1$ Ca, $2$ O and $2$ H.
Multiply and add: $Mr=40.1+(2×16.0)+(2×1.0)=74.1M_r = 40.1 + (2 \times 16.0) + (2 \times 1.0) = 74.1$ .

Relative formula masses balance on both sides of equations

A large number in front of a formula multiplies the whole formula, so $2H2O2\text{H}_2\text{O}$ means two water molecules.
To find the mass of a species in an equation, multiply its $M_r$ by this balancing number.
In $2Mg+O2→2MgO2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}$ $2 Mg + O_{2} \to 2 MgO$ , the reactants total $(2×24.3)+32.0=80.6(2 \times 24.3) + 32.0 = 80.6$ $(2 \times 24.3) + 32.0 = 80.6$ .
1. The product also totals $\times (24.3 + 16.0) = 80.6$ , because no atoms are gained or lost.
If the two totals differ, the equation has not been balanced correctly.

Common Mistake

Do not count oxygen once for $O2\text{O}_2$ , because an oxygen molecule contains two atoms, giving $M_r = 32.0$ .
Do not round $A_r$ values such as Mg = 24.3 to whole numbers unless the values you are given are whole numbers.

The empirical formula is the simplest whole-number ratio

Definition

Empirical formula

The simplest whole-number ratio of atoms of each element in a compound.

Definition

Molecular formula

The actual number of atoms of each element in one molecule of a compound.

To find an empirical formula from a molecular formula, divide the numbers of atoms by the largest number that goes into all of them.
1. Ethane, $C2H6\text{C}_2\text{H}_6$ , divides by $2$ to give the empirical formula $CH3\text{CH}_3$ .
2. Glucose, $C6H12O6\text{C}_6\text{H}_{12}\text{O}_6$ , divides by $6$ to give $CH2O\text{CH}_2\text{O}$ .
When the numbers share no common factor, as in $H2O\text{H}_2\text{O}$ or $CO2\text{CO}_2$ , the molecular formula is already the empirical formula.
Ethene, $C2H4\text{C}_2\text{H}_4$ , and propene, $C3H6\text{C}_3\text{H}_6$ , share the empirical formula $CH2\text{CH}_2$ , so an empirical formula alone may not identify a compound.

Note

Ionic compounds such as sodium chloride are giant lattices with no separate molecules, so their formula, $NaCl\text{NaCl}$ , is always an empirical formula.

Reading empirical formulae from models and diagrams

In a ball and stick model or displayed formula, count the atoms of each element first.
Then write the counts as a ratio and simplify it to the smallest whole numbers.
For a giant structure diagram, such as part of a sodium chloride lattice, work out the ratio in which the ions repeat.
Going the other way, the empirical formula $CH2\text{CH}_2$ $CH_{2}$ tells you that a matching model must contain twice as many hydrogen atoms as carbon atoms.
1. A model of ethene with $2$ carbon and $4$ hydrogen atoms fits this ratio, while one with $2$ carbon and $6$ hydrogen atoms does not.

Example

A diagram of a molecule shows $4$ carbon atoms, $8$ hydrogen atoms and $2$ oxygen atoms; find its empirical formula.
Write the ratio of atoms: C : H : O = $4 : 8 : 2$ .
Divide each number by $2$ , the largest number that goes into all three: $2 : 4 : 1$ .
Write the empirical formula: $C2H4O\text{C}_2\text{H}_4\text{O}$ .

Self review

How do you calculate the relative formula mass of a compound?
What is the relative formula mass of $MgCl2\text{MgCl}_2$ ( $A_r$ : Mg = 24.3, Cl = 35.5)?
What does the number $3$ in $3CO23\text{CO}_2$ tell you?
What is the empirical formula of $C4H10\text{C}_4\text{H}_{10}$ ?
Why is the formula of sodium chloride always an empirical formula?

2.1.3 Formulations as useful mixtures

A formulation is a mixture with a purpose

Definition

Formulation

A mixture designed for a particular use.

Definition

Component

One of the substances present in a mixture.

Each component has a specific purpose, such as giving colour, protecting a surface or making the product easier to use.
The components are mixed in measured quantities, because changing the proportions changes the properties.
The components are mixed rather than reacted, so each keeps its own properties within the product.
1. A formulation is therefore a mixture, and it is not pure in the chemical sense.
Manufacturers follow a precise recipe, so every batch has the same properties.

Key Idea

A formulation is useful because both its components and their amounts are chosen deliberately for one purpose.

Paints, medicines, cleaning products and fertilisers are everyday formulations

Paint contains a pigment for colour, a binder that holds the pigment on the surface and a solvent that keeps the paint runny until it dries.
A medicine tablet contains the active drug with fillers that bulk it out, binders that hold it together and a coating that makes it easier to swallow.
Washing-up liquid combines detergents that lift grease with water, fragrance and colouring.
Fertilisers are blends of nitrogen, phosphorus and potassium compounds in proportions suited to particular crops.
Fuels, cosmetics, sun creams and many foods are also formulations.

Practical

Making Prussian blue paint

Aim: make a pure, dry sample of the pigment Prussian blue and find out which binder turns it into the best paint.
Apparatus: two $cm3100\,\text{cm}^3$ conical flasks, a $cm325\,\text{cm}^3$ measuring cylinder, a glass rod, a filter funnel and filter paper, a wash bottle of distilled water, a dropping pipette, propanone, a watch glass, a pestle and mortar, a spatula, a digital balance, iron(II) sulfate, potassium hexacyanoferrate, binders such as linseed oil, egg yolk and dilute PVA glue, and white paper.

Part 1: making the pigment

Dissolve a spatula of iron(II) sulfate in about $cm320\,\text{cm}^3$ of distilled water in one conical flask.
Dissolve a spatula of potassium hexacyanoferrate in about $cm320\,\text{cm}^3$ of distilled water in the second flask.
Pour one solution slowly into the other while stirring, so that a dark blue precipitate forms.
Filter the mixture, then wash the solid on the filter paper with distilled water to remove the soluble by-products.
Rinse the solid with a few drops of propanone, which mixes with the water left on it and evaporates quickly.
Scrape the solid onto a watch glass and leave it to dry.

Part 2: testing the binders

Grind the dry pigment to a fine powder with a pestle and mortar.
Weigh equal masses of the pigment and mix each one with the same volume of a different binder.
Paint each mixture onto the same kind of paper, using the same number of brush strokes.
Compare how easily each paint spreads, how even the colour is and how long it takes to dry.

Accuracy and safety

Use the same mass of pigment and the same volume of binder each time, so that the paints are compared fairly.
Keep acids well away from potassium hexacyanoferrate, because they react to release a very toxic gas.
Keep propanone away from flames, because it is highly flammable.
Wear eye protection throughout.

Changing one component changes the whole formulation

Adding more pigment gives a paint a stronger colour, but too much can make it crack as it dries.
Adding more solvent makes a paint thinner and easier to spread, but it covers the surface less well.
In a medicine, the dose of active ingredient must be exact, because too little does not work and too much can be harmful.
Formulations are tested so that the product is safe, effective and the same from batch to batch.

Common Mistake

Do not describe a formulation as a compound, because its components are mixed and are not chemically bonded together.

A formulation melts over a range of temperatures

Like any mixture, a formulation has no single melting point.
A heating curve for a formulation shows the solid melting over a range, with no flat section at one temperature.
1. A pure substance gives a flat section at its melting point, because the temperature stays constant while it melts.
This lets a chemist tell a formulation apart from a pure substance using melting data.
Chocolate is a formulation of cocoa solids, cocoa butter, sugar and milk, which is why it softens gradually as it warms.

Example

Sample A stays at $∘C80\ ^{\circ}\text{C}$ while it melts, but sample B melts between $∘C55\ ^{\circ}\text{C}$ and $∘C62\ ^{\circ}\text{C}$ , so sample B is the formulation.

Self review

What is a formulation?
Why are the components of a formulation measured carefully?
Name three components of paint and give the job of each.
Why is a formulation not pure in the chemical sense?
How would a heating curve show that a substance is a formulation?

2.1.4 Techniques for separating mixtures

Filtration separates an insoluble solid from a liquid

Definition

Residue

The insoluble solid left behind in the filter paper after filtration.

Definition

Filtrate

The liquid that passes through the filter paper during filtration.

The mixture is poured through filter paper folded into a funnel.
1. The paper has tiny holes that let the liquid and any dissolved particles through but stop the larger solid particles.
Filtration separates sand from water, or removes an excess solid after a reaction.
It cannot separate a dissolved solid from its solvent, because dissolved particles pass through the paper.
Rinsing the solid with a little distilled water washes off traces of the solution.

Example

A paper coffee filter works the same way, with the flavoured solution passing through and the grounds staying behind.

Crystallisation recovers a soluble solid from its solution

Definition

Crystallisation

A separation technique used to obtain solid crystals of a dissolved substance from a solution.

Definition

Saturated solution

A solution in which no more solute can dissolve at that temperature.

Heat the solution in an evaporating basin to drive off some of the water.
Stop heating once crystals form on a glass rod dipped into the solution and taken out.
Leave the solution to cool slowly, because most solids are less soluble at lower temperatures, so crystals form.
Filter off the crystals and dry them between filter papers or in a warm oven.
Crystallisation gives larger, purer crystals than boiling off all the water, which can also break down some solids.

Practical

Aim: obtain dry crystals of hydrated copper(II) sulfate from its solution.
Apparatus: copper(II) sulfate solution, an evaporating basin, a beaker of water to use as a water bath, a Bunsen burner, a tripod and gauze, a heatproof mat, a glass rod, tongs, a filter funnel and filter paper.

Method

Pour about $cm325\,\text{cm}^3$ of copper(II) sulfate solution into an evaporating basin.
Stand the basin on a beaker of water on a tripod and gauze, and heat the water, which warms the solution gently and stops it boiling and spitting.
Dip a glass rod into the solution from time to time and lift it out to cool.
Stop heating when small crystals form on the cooled rod, which shows the solution is nearly saturated.
Leave the basin in a cool place for a day or more, so that larger crystals grow as the solution cools and more water evaporates.
Pour off the remaining solution and pat the crystals dry between filter papers, or leave them somewhere warm to dry.

Accuracy and safety

Do not heat the crystals strongly, because they lose their water of crystallisation and turn into white anhydrous copper(II) sulfate.
Cooling the solution slowly gives larger, better shaped crystals than cooling it quickly.
Wear eye protection, because hot solution can spit.
Wash your hands afterwards, because copper(II) sulfate is harmful if swallowed.

Simple distillation collects the solvent from a solution

Definition

Distillate

The liquid collected after vapour has been condensed during distillation.

Heating the solution boils the solvent, while the dissolved solid stays behind in the flask.
The vapour passes into a condenser, where cold water flowing around the outside cools it back into a liquid.
The condensed liquid drips into a collecting beaker.
The thermometer bulb sits level with the side arm, so it reads the temperature of the vapour leaving the flask.
Simple distillation produces pure water from sea water, because the dissolved salts stay behind when the water boils.
Heating chopped orange peel in water distils out an oil called limonene, which forms a separate layer on top of the water in the distillate.

Practical

Aim: collect pure water from salty water by simple distillation.
Apparatus: a round-bottomed flask, anti-bumping granules, a thermometer from $0$ to $∘C110\ ^{\circ}\text{C}$ , a still head with a side arm, a Liebig condenser with rubber tubing, a collecting beaker, a Bunsen burner or heating mantle, clamps and stands, salty water and a watch glass.

Part 1: setting up the apparatus

Half-fill the round-bottomed flask with salty water and add a few anti-bumping granules, so that it boils smoothly.
Clamp the flask firmly and fit the still head and thermometer, with the thermometer bulb level with the side arm.
Connect the side arm to the condenser, clamping the condenser so that it slopes down into the collecting beaker.
Connect the rubber tubing so that cold water runs into the bottom of the condenser and out at the top, which keeps the outer jacket full of cold water.

Part 2: distilling and testing the water

Heat the flask gently until the salty water boils.
Collect the liquid that drips from the condenser while the thermometer reads about $∘C100\ ^{\circ}\text{C}$ .
Stop heating before the flask boils dry.
Test the distillate by showing that it boils at $∘C100\ ^{\circ}\text{C}$ , or that a few drops evaporated on a watch glass leave no solid.

Accuracy and safety

The thermometer bulb must be level with the side arm, so that it measures the temperature of the vapour that is distilling over.
Never heat a closed system, so check that the end of the condenser is open to the air.
Stop before the flask boils dry, because the hot salt left behind can crack the glass.
Wear eye protection and let the apparatus cool before taking it apart.

Fractional distillation separates liquids with different boiling points

Definition

Fractionating column

A vertical tube in which vapour repeatedly condenses and evaporates, improving the separation of liquids with different boiling points.

The mixture of liquids is heated in a flask fitted with a fractionating column, often packed with glass beads.
The column is hottest at the bottom and coolest at the top.
The liquid with the lowest boiling point reaches the top of the column first and passes into the condenser.
1. Vapour of the higher-boiling liquid condenses on the beads and drips back into the flask.
When the thermometer reading rises, the next liquid has started to distil, so the collecting beaker is changed.
This separates ethanol, which boils at $∘C78\ ^{\circ}\text{C}$ , from water, which boils at $∘C100\ ^{\circ}\text{C}$ .

Common Mistake

Do not heat flammable liquids such as ethanol over a Bunsen flame; use an electric heater or a water bath instead.

Choosing a separation technique from substance properties

Definition

Solubility

The amount of a substance that can dissolve in a particular solvent at a given temperature.

An insoluble solid in a liquid is separated by filtration.
A soluble solid is recovered from its solution by crystallisation.
The solvent from a solution is collected by simple distillation.
Liquids that mix together but have different boiling points are separated by fractional distillation.
Some mixtures need two steps, such as rock salt, which is dissolved and filtered to remove sand, then crystallised to recover the salt.

Exam technique

When a question gives data about a mixture, first pick out whether each substance dissolves and how far apart the boiling points are.
Then name the technique and justify it with that property, for example that sand is insoluble so it stays in the filter paper.

Self review

Which technique separates sand from water, and why does it work?
Why is a solution left to cool during crystallisation?
Where should the thermometer bulb sit during distillation?
How does a fractionating column separate ethanol from water?
Which two techniques would you use to get pure salt from rock salt?

2.1.5 Chromatography and Rf values

Chromatography separates using stationary and mobile phases

Definition

Stationary phase

The part of a chromatography system that stays in place while the solvent moves through it.

Definition

Mobile phase

The part of a chromatography system that moves, carrying the substances being separated.

Paper chromatography uses paper as the stationary phase and a solvent as the mobile phase.
Thin layer chromatography uses a thin layer of silica gel on a glass, metal or plastic plate as its stationary phase.
The substances in a sample start on the stationary phase and begin to move once the solvent reaches them.
Each substance is distributed between the two phases, spending part of its time dissolved in the solvent and part held on the stationary phase.
1. A substance that spends more time in the mobile phase travels further, so the substances separate.

Key Idea

Separation depends on how each substance is shared between the two phases, so two substances that are shared in the same way cannot be separated by that solvent.

Paper and thin layer chromatography follow the same method

Draw a start line in pencil about $1 cm1\,\text{cm}$ $1 cm$ from the bottom of the paper or plate.
1. Pencil is used because it does not dissolve in the solvent, whereas ink would run and add its own spots.
Put a small, concentrated spot of each sample on the line.
Stand the paper in a beaker of solvent with the solvent level below the start line, then cover the beaker.
1. If the solvent were above the line, the spots would dissolve into the solvent in the beaker instead of moving up the paper.
Remove the paper when the solvent is near the top, mark how far the solvent reached and leave the paper to dry.

Practical

Separating the dyes in food colourings

Aim: find out which dyes are present in several food colourings.
Apparatus: chromatography paper, a pencil and ruler, capillary tubes, food colourings, samples of known dyes, a beaker with a lid or a cover, water as the solvent, a glass rod or splint, sticky tape and a hairdryer or warm place for drying.

Method

Draw a pencil start line about $cm2\,\text{cm}$ from the bottom of the chromatography paper.
Use a capillary tube to put a small spot of each food colouring and each known dye on the line, at least $cm1\,\text{cm}$ apart.
Build up each spot by adding a drop, letting it dry and adding another, so the spot stays small but holds enough dye to see.
Pour water into the beaker to a depth of about $cm1\,\text{cm}$ , so that the solvent will be below the start line.
Tape the top of the paper to a glass rod resting across the beaker, so that the bottom edge dips into the water, and cover the beaker.
Remove the paper when the solvent is about $cm1\,\text{cm}$ from the top, and draw a pencil line at the solvent front straight away.
Let the paper dry, then measure from the start line to the centre of each spot and to the solvent front.
Calculate each Rf value using $solvent\text{R}_\text{f} = \dfrac{\text{distance moved by spot}}{\text{distance moved by solvent}}$ .

Accuracy and safety

Draw the start line in pencil, because ink would dissolve in the solvent and run up the paper with the dyes.
Keep the solvent below the start line, or the spots will dissolve into the solvent in the beaker.
Cover the beaker, so that the air inside stays full of solvent vapour and the solvent does not evaporate from the paper.
A colouring that matches a known dye in colour and Rf value contains that dye, and one that gives more than one spot is a mixture.
Thin layer chromatography follows the same steps on a plate coated with silica, and it runs faster and gives sharper spots.

A chromatogram shows how many substances a mixture contains

Definition

Chromatogram

The pattern of separated spots produced after a mixture has been analysed by chromatography.

Definition

Solvent front

The furthest point reached by the solvent as it moves through the stationary phase.

Each separate spot is a different substance.
A mixture of three dyes gives three spots, while a pure dye gives only one.
A spot that stays on the start line is insoluble in that solvent.
If two samples produce spots at the same heights in the same solvent, they are likely to be the same substance.
1. Repeating the run in a second solvent makes the match more certain, because two different substances rarely match in both.

Example

Forensic scientists compare the ink on a forged document with inks from suspects' pens, and a match in several solvents points to the pen that was used.

The Rf value compares substance and solvent distances

Definition

Rf value

A value that compares how far a substance travels in chromatography with how far the solvent travels.

Measure from the start line to the centre of the spot, and from the start line to the solvent front.
Calculate the Rf value by dividing the two distances: $solventR_f = \frac{\text{distance moved by substance}}{\text{distance moved by solvent}}$
An Rf value is always between 0 and 1, because a substance cannot travel further than the solvent.
A substance has the same Rf value in the same solvent and stationary phase, so comparing Rf values with known values identifies it.
Rf values have no units, because they are a ratio of two lengths.

Example

A dye moves $cm3.6\,\text{cm}$ from the start line while the solvent front moves $cm8.0\,\text{cm}$ ; calculate its Rf value.
Write the ratio: $cmR_f = \dfrac{3.6\,\text{cm}}{8.0\,\text{cm}}$ .
Divide: $R_f = 0.45$ .

Chromatography can show whether a substance is pure

A pure substance gives a single spot in every solvent tried.
An impure substance gives two or more spots, showing the extra substances present.
Running the sample alongside a pure reference sample shows whether its main spot matches.
A single spot in one solvent is not conclusive, because two substances might move the same distance in that solvent.
1. Testing in different solvents gives stronger evidence that the sample is pure.

Exam technique

When a question asks you to identify substances from a chromatogram, compare the height or Rf value of each spot with the reference spots and name each match.

Self review

What are the stationary and mobile phases in paper chromatography?
Why is the start line drawn in pencil?
What does a chromatogram with four spots tell you about a sample?
A spot moves $cm2.4\,\text{cm}$ and the solvent moves $cm6.0\,\text{cm}$ ; what is its Rf value?
How can chromatography show that a substance is pure?

2.1 Purity and separating mixtures

2.1.1 Pure substances and melting point data

A pure substance contains one element or compound

Everyday 'pure' is looser than the scientific meaning

A pure substance melts at a sharp, fixed temperature

Impurities lower the melting point and widen its range

2.1.2 Relative formula mass and empirical formula

Adding relative atomic masses gives relative formula mass

Relative formula masses balance on both sides of equations

The empirical formula is the simplest whole-number ratio

Reading empirical formulae from models and diagrams

2.1.3 Formulations as useful mixtures

A formulation is a mixture with a purpose

Paints, medicines, cleaning products and fertilisers are everyday formulations

Making Prussian blue paint

Part 1: making the pigment

Part 2: testing the binders

Accuracy and safety

Changing one component changes the whole formulation

A formulation melts over a range of temperatures

2.1.4 Techniques for separating mixtures

Filtration separates an insoluble solid from a liquid

Crystallisation recovers a soluble solid from its solution

Method

Accuracy and safety

Simple distillation collects the solvent from a solution

Part 1: setting up the apparatus

Part 2: distilling and testing the water

Accuracy and safety

Fractional distillation separates liquids with different boiling points

Choosing a separation technique from substance properties

2.1.5 Chromatography and Rf values

Chromatography separates using stationary and mobile phases

Paper and thin layer chromatography follow the same method

Separating the dyes in food colourings

Method

Accuracy and safety

A chromatogram shows how many substances a mixture contains

The Rf value compares substance and solvent distances

Chromatography can show whether a substance is pure

Review 2.1 Purity and separating mixtures by teaching Genie

Recap your knowledge with an interactive lesson

Put it into practice with exam-style questions

Remember key concepts with flashcards

2.1 Purity and separating mixtures Revision Guide

2.1 Purity and separating mixtures

2.1.1 Pure substances and melting point data

A pure substance contains one element or compound

Everyday 'pure' is looser than the scientific meaning

A pure substance melts at a sharp, fixed temperature

Impurities lower the melting point and widen its range

2.1.2 Relative formula mass and empirical formula

Adding relative atomic masses gives relative formula mass

Relative formula masses balance on both sides of equations

The empirical formula is the simplest whole-number ratio

Reading empirical formulae from models and diagrams

2.1.3 Formulations as useful mixtures

A formulation is a mixture with a purpose

Paints, medicines, cleaning products and fertilisers are everyday formulations

Making Prussian blue paint

Part 1: making the pigment

Part 2: testing the binders

Accuracy and safety

Changing one component changes the whole formulation

A formulation melts over a range of temperatures

2.1.4 Techniques for separating mixtures

Filtration separates an insoluble solid from a liquid

Crystallisation recovers a soluble solid from its solution

Method

Accuracy and safety

Simple distillation collects the solvent from a solution

Part 1: setting up the apparatus

Part 2: distilling and testing the water

Accuracy and safety

Fractional distillation separates liquids with different boiling points

Choosing a separation technique from substance properties

2.1.5 Chromatography and Rf values

Chromatography separates using stationary and mobile phases

Paper and thin layer chromatography follow the same method

Separating the dyes in food colourings