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.
- 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 25 cm325\,\text{cm}^325cm3 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 000 to 110 ∘C110\ ^{\circ}\text{C}110 ∘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 100 ∘C100\ ^{\circ}\text{C}100 ∘C.
- Stop heating before the flask boils dry.
- Test the distillate by showing that it boils at 100 ∘C100\ ^{\circ}\text{C}100 ∘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.
- 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 78 ∘C78\ ^{\circ}\text{C}78 ∘C, from water, which boils at 100 ∘C100\ ^{\circ}\text{C}100 ∘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?
