3.2.1 Pure substances and mixtures

In chemistry, pure means one substance and nothing else

1. In everyday use, pure usually means clean, natural or with nothing added.
2. Pure orange juice has nothing added to it, yet it still contains water, sugars and acids.
3. Chemistry uses the word far more strictly, allowing only one element or one compound.
4. Every sample of a pure substance therefore has the same fixed composition.
5. The substances in a mixture are not chemically bonded to each other, so each keeps its own properties.
6. The proportions in a mixture can vary from one sample to the next.
7. A mixture can be separated by physical methods, with no chemical reaction involved.

A pure substance melts at one sharp temperature

1. A pure solid melts at a single temperature, giving the same start and finish reading.
2. That single value is described as a sharp melting point.
3. A mixture melts over a range, beginning at one temperature and finishing at a higher one.
4. The range appears because the substances in the mixture melt at different temperatures.
5. Recording both the start and the finish temperature is what makes the comparison possible.
6. A measured melting point well away from the accepted value is itself evidence of impurity.

Boiling point data tells the same story

1. A pure liquid boils at a single temperature, which holds steady while it boils.
2. A mixture boils over a range, and its temperature keeps climbing as boiling goes on.
3. Pure water boils at $100{}^{\circ }C$ at normal pressure.
4. Salt water boils above $100{}^{\circ }C$, and the temperature rises further as water leaves.
5. Melting and boiling data are read the same way: one fixed temperature points to a pure substance and a range points to a mixture.

Reading purity from a set of data

1. Read the temperature at which melting or boiling begins.
2. Read the temperature at which it finishes.
3. Two equal values point to a pure substance.
4. A finish higher than the start points to a mixture.
5. Quote both figures in the conclusion, because those numbers are the evidence.

3.2.2 Techniques for separating mixtures

Separation uses a physical difference between the components

1. The substances in a mixture are not chemically bonded, so each keeps its own physical properties.
2. A separation exploits one of those differences: particle size, solubility or boiling point.
3. A solution forms when a solute dissolves in a solvent, as salt does in water.
4. No new substance is made, so every separation here is a physical process.
5. Choosing a method starts with two questions: which property differs, and which component you want to keep.

Distillation separates by boiling point

1. Simple distillation recovers the solvent from a solution, such as water from salt water.
2. The mixture is heated, the component with the lower boiling point evaporates first, and its vapour passes into a condenser.
3. The condenser cools that vapour back to a liquid, which runs into a receiving flask as the distillate.
4. The dissolved solid stays behind in the distillation flask, because it does not evaporate.
5. Simple distillation also separates two liquids whose boiling points are far apart.
6. When the liquids are miscible and their boiling points sit close together, fractional distillation takes over.
7. A fractionating column sits between the flask and the condenser, and vapours condense and evaporate repeatedly as they rise through it.
8. Each of those steps enriches the vapour in the lower-boiling liquid, which makes the separation far sharper.

Filtration and crystallisation separate by particle size and solubility

1. Filtration separates an insoluble solid from a liquid, because those solid particles are too large to pass through the filter paper.
2. The liquid that passes through is the filtrate, and the solid held on the paper is the residue.
3. Dissolved particles are small enough to pass straight through, so filtration cannot remove a solute.
4. Crystallisation recovers a soluble solid from its solution.
5. The solution is heated gently so that some solvent evaporates and the solution becomes more concentrated.
6. Heating stops well before the solution is dry, and the concentrated solution is left to cool.
7. Crystals form as it cools, because less solute can stay dissolved at the lower temperature.
8. The crystals are then filtered off and dried between filter papers.

Chromatography separates by how far each component travels

1. Paper chromatography separates substances that dissolve in the same solvent, such as the dyes in an ink.
2. A pencil line is drawn near the bottom of the paper, because pencil does not dissolve and will not run.
3. A small spot of the mixture goes on that line, and the paper stands in solvent kept below it.
4. The solvent rises through the paper and carries the dissolved substances up with it.
5. Each substance travels its own distance, because they differ in solubility and in how strongly the paper holds them.
6. The separated spots left behind make a chromatogram.

Choosing the right technique

1. An insoluble solid in a liquid calls for filtration.
2. A soluble solid you want to keep calls for crystallisation.
3. A solvent you want to recover from a solution calls for simple distillation.
4. Miscible liquids with close boiling points call for fractional distillation.
5. Soluble substances to be separated or identified call for paper chromatography.
6. Sand, salt and water needs two stages: filter off the sand, then crystallise the salt from the filtrate.
7. The same mixture is handled differently depending on which component the question asks for.

3.2.3 Paper chromatography and Rf values

The solvent moves and the paper stays still

1. In paper chromatography the paper is the stationary phase and the solvent is the mobile phase.
2. A pencil baseline is drawn near the bottom, because pencil will not dissolve and run.
3. A small spot of the mixture is placed on that baseline.
4. The paper stands in solvent kept below the baseline, so the spot is not washed off into the solvent.
5. The solvent rises through the paper and carries the dissolved substances with it.
6. A substance that is more soluble in the solvent, and held less strongly by the paper, travels further.
7. The furthest point the solvent reaches is the solvent front, marked before the paper dries.

Reading a chromatogram

1. Each separated substance shows up as its own spot.
2. A sample giving one spot is consistent with a pure substance.
3. A sample giving more than one spot has to be a mixture.
4. One spot is not proof of purity, because two substances can happen to travel the same distance.
5. Running known substances alongside the unknown on the same paper allows the positions to be compared.
6. A spot level with a known suggests that the unknown contains that substance.
7. Every comparison needs the same paper, the same solvent and the same conditions.

$R_f$ values put a number on how far a substance travels

1. Measure from the baseline to the centre of the spot.
2. Measure from the baseline to the solvent front.
3. Divide the first distance by the second: $$R_f=\frac{distance\; travelled\; by\; the\; substance}{distance\; travelled\; by\; the\; solvent\; front}$$
4. Both distances are in the same unit, so $R_f$ carries no unit.
5. $R_f$ always lies between $0$ and $1$, because nothing travels further than the solvent itself.
6. A spot $3.0cm$ from the baseline with the solvent front at $6.0cm$ gives $R_f=3.0÷6.0=0.50$.

Drawing a conclusion from a chromatogram

1. Count the spots to decide between a pure substance and a mixture.
2. Compare the spot positions with the knowns run alongside.
3. Calculate $R_f$ for each spot where the question asks for it.
4. Match each value against the $R_f$ values of the known substances.
5. Quote the evidence, which is the number of spots and the values that match.

3.2.4 Making water potable

Potable water is safe to drink, not chemically pure

1. Water is described as potable when it is safe to drink, which is not the same as being chemically pure.
2. Potable water still contains dissolved substances, and at the levels present they do no harm.
3. Fresh water from rivers, lakes and the ground carries insoluble particles, dissolved substances and harmful microorganisms.
4. Treatment has to deal with the particles and the microorganisms before the water is safe.
5. Sea water needs different treatment, because its problem is the dissolved salt rather than the particles.

Fresh water is treated in three stages

1. In sedimentation the water is left to stand so that suspended insoluble particles sink under gravity.
2. The settled sludge is drawn off, leaving far fewer solids for the next stage to handle.
3. Filtration then passes the water through beds of sand and gravel, which trap the insoluble particles that remain.
4. Dissolved substances pass straight through the filter beds along with the water.
5. In chlorination, chlorine is added to kill the harmful microorganisms that filtration leaves behind.
6. The three stages run in that order because each one prepares the water for the next.

Sea water is made potable by distillation

1. Sea water holds so much dissolved salt that filtration and chlorination cannot make it drinkable.
2. In distillation the sea water is heated until the water turns to vapour.
3. The dissolved salts do not vaporise, so they stay behind in the original container.
4. The vapour is cooled in a condenser and collected as liquid water carrying no dissolved salt.
5. Removing the salt from sea water this way is called desalination.
6. Boiling that much water takes a great deal of energy, so desalination is expensive and is used mainly where fresh water is scarce.

Water for chemical analysis must contain no dissolved salts

1. Chemical analysis tests a sample to find out which substances it contains.
2. The water used in analysis must have no dissolved salts in it at all.
3. Dissolved salts separate into ions, and those ions can react with the chemicals used in the test.
4. A test would then respond to something that came from the water rather than from the sample.
5. Tap water contains chloride ions, so it would give a white precipitate with acidified silver nitrate even when the sample contained none.
6. Distilled water is used instead, because distillation leaves every dissolved salt behind.