Pure Substances and Mixtures

Lessons

Senior 1 · Chemistry

Pure Substances and Mixtures

5 lessons

Lesson 3 of 5

Separating Mixtures: Filtration, Evaporation and Crystallisation

25 min

Salt from a salty mud

Imagine a bag of cooking salt that has been dropped in the sand. The salt is precious, but you cannot pick out millions of tiny grains by hand. You can separate them, though, by using the fact that salt dissolves in water but sand does not. Every separation method in chemistry works the same way: it uses a property that one part of the mixture has and the other does not.

Experiment: separating salt from sand

Put the mixture in a beaker, add water and stir until all the salt has dissolved.
Filter the mixture (Figure 3.1). Fold a filter paper into a cone (Figure 3.2), put it in a funnel and pour the mixture down a glass rod into the paper. The sand stays behind; the salt solution runs through.
Pour the salt solution into an evaporating basin and heat it gently until most of the water has evaporated (Figure 3.3 A). Turn the heat off as the first crystals appear and let the last of the water evaporate on its own.
Dry the sand on a filter paper. Weigh the dry sand and the salt.

Figure 3.1: Filtration. Pouring down a glass rod stops the liquid splashing past the paper.

Figure 3.2: Folding a filter paper.

Definition

Filtration: separating an insoluble solid from a liquid by passing the mixture through a filter paper. The solid left in the paper is the residue; the liquid that passes through is the filtrate.

Definition

Evaporation: heating a solution so that the solvent turns to vapour and leaves the dissolved solid behind. Example: evaporating salt water leaves salt.

Warning

Safety. Wear goggles: a nearly dry evaporating basin spits hot salt. Use a small flame and stop heating before the basin is dry. The basin stays hot for a long time: move it only with tongs.

Worked example

Problem. Mugisha separates 20.0 g of a sand and salt mixture. He recovers 12.4 g of dry sand and 7.2 g of salt. (a) What percentage of the mixture was salt, according to his results? (b) The masses add up to less than 20.0 g. Suggest two reasons.

Solution.

$\text{percentage of salt} = \frac{7.2\,\mathrm{g}}{20.0\,\mathrm{g}} \times 100\% = 36\%$
$12.4\,\mathrm{g} + 7.2\,\mathrm{g} = 19.6\,\mathrm{g}$ , so 0.4 g was lost. Some salt solution stayed soaked in the filter paper and the sand, and some salt may have spat out of the basin while it was heated.

Crystallisation

Some solids break down or burn if they are heated to dryness: sugar, for example, turns to caramel and then to carbon. To obtain them, chemists use crystallisation. The solution is heated only until it is saturated (a drop placed on a cold glass rod forms tiny crystals), then left to cool slowly. As it cools, less solid can stay dissolved, so crystals grow in the solution (Figure 3.3 B). They are filtered off and dried. At Kabuye Sugar Works, near Kigali, sugar-cane juice is boiled down in this way until sugar crystals form.

Definition

Saturated solution: a solution that holds as much solute as can dissolve at that temperature. Most solids dissolve better in hot water, so a hot saturated solution forms crystals as it cools.

Definition

Crystallisation: obtaining crystals of a dissolved solid by partly evaporating its solution until it is saturated, then letting it cool so that crystals form. It gives purer, well-shaped crystals than evaporating to dryness.

Figure 3.3: Evaporation to dryness and crystallisation.

More ways to separate mixtures

Two immiscible liquids, such as cooking oil and water, are separated with a separating funnel (Figure 3.4): the denser liquid forms the lower layer and is run off through the tap.

Figure 3.4: Separating two immiscible liquids.

Table 3.1: Choosing a separation method

Mixture	Method	Property used
Sand and water	filtration, or decanting (letting the sand settle and pouring off the water)	sand is insoluble and denser than water
Salt and water	evaporation	water turns to vapour; salt does not
Copper(II) sulfate crystals from solution	crystallisation	less solid dissolves in cold water than in hot
Oil and water	separating funnel	the liquids are immiscible and have different densities
Iron filings and sand	magnet	iron is magnetic; sand is not
Beans and small stones or chaff	hand picking, sieving or winnowing	different sizes, or chaff is lighter and blown away

Worked example

Problem. Choose a method to obtain (a) dry sand from a mixture of sand and water, (b) sugar crystals from sugar solution, (c) iron filings from a mixture with sulfur, (d) water from a mixture of water and cooking oil.

Solution. (a) filtration, then dry the residue; (b) crystallisation, because sugar chars if heated to dryness; (c) a magnet, wrapped in paper so the filings can be pulled off easily; (d) a separating funnel, running off the lower water layer.

Warning

Common mistake: thinking that filtering muddy water makes it safe to drink. A filter paper removes mud but lets dissolved substances and many germs pass through. Filtered water still needs boiling or treatment with chlorine.

Tip

Before you choose a method, ask: Which part dissolves? Which part is magnetic? Which is denser? Which boils first? The answer names the method.

Cognito: Separation Techniques: Filtration, Evaporation, Crystallisation (5 min). Note when the video stops heating and why.

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Practice

What is meant by (a) residue, (b) filtrate?
Why is a filter paper folded so that one side of the cone has three layers?
Describe how to obtain dry salt from a mixture of salt and sand.
Why is sugar obtained by crystallisation rather than by evaporating to dryness?
A student separates 25.0 g of a sand and salt mixture and recovers 16.5 g of sand. Assuming none was lost, what percentage of the mixture was salt?
Suggest a method to separate (a) paraffin (kerosene) from water, (b) maize grains from maize flour.
Farmers winnow beans by tossing them on a tray in the wind. Explain which property of the chaff this uses.

Show answersHide answers

(a) the insoluble solid left in the filter paper (b) the liquid that passes through the paper.
The three layers give the cone strength where the glass rod rests against it, so the wet paper does not tear.
Add water and stir to dissolve the salt; filter to remove the sand; evaporate the filtrate until crystals appear, then let the rest dry.
Sugar breaks down (caramelises and chars) if heated to dryness; crystallisation lets the crystals form at a lower temperature.
Salt $= 25.0 - 16.5 = 8.5\,\mathrm{g}$ ; percentage $= \frac{8.5}{25.0} \times 100\% = 34\%$ .
(a) a separating funnel (they are immiscible) (b) sieving (the grains are bigger than the flour particles).
The chaff is much lighter (less dense) than the beans, so the wind blows it away while the beans fall back onto the tray.

Sources and credits

Separation process (the properties used by filtration, evaporation, crystallisation and separating funnels), Wikipedia contributors, Wikipedia, licensed under CC BY-SA 4.0. Adapted by Ganzaa.
Image: Salt mining in Lake Katwe, Uganda 16.jpg by Sandra Aceng, via Wikimedia Commons, licensed under CC BY-SA 4.0.
Video: GCSE Chemistry - Separation Techniques - Filtration | Evaporation | Crystallisation, Cognito, embedded from YouTube.
The diagrams, worked examples, Practice items and exercises were written and drawn by Ganzaa.

This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1Salt from a salty mud

2Experiment: separating salt from sand

3Crystallisation

4More ways to separate mixtures

Salt from a salty mud

Experiment: separating salt from sand

Crystallisation

More ways to separate mixtures