3.5 The boiling points of liquids can be used to separate mixtures

3.5

Learning intentions

By the end of this topic, you will be able to: • describe the processes of evaporation, distillation and crystallisation • explain how different boiling points can be used to separate mixtures.

Key ideas

• The different particles in a mixture will often have different boiling points. • The substance with the lowest boiling point will evaporate first. • When a solvent evaporates, it will leave behind crystallised solute. • If the evaporated solvent is cooled it will condense into a liquid in a process called distillation.

Figure 1 Water will evaporate from a mixture of salt and water, leaving behind salt crystals.

crystallisation a separation technique used with evaporation to remove a dissolved solid from a liquid; after the liquid has been evaporated the solid remains, often in the form of small crystals

distillation a technique that uses evaporation and condensation to separate a solid from the solvent in which it has dissolved

The boiling points of liquids can be used to separate mixtures

Evaporation and crystallisation When water in a saucepan is heated, it will quickly start to boil. This means the liquid evaporates: it becomes a gas. Every substance has a different volatility. This means they evaporate at different temperatures. Table 1 shows the boiling points of some common liquids. The different boiling points of liquids can be used to separate them in a mixture.

A mixture of water and turpentine can be easily separated because the water will evaporate first. This means the water will become a gas (water vapour) and move away from the turpentine. Eventually only turpentine will be left behind.

Evaporation can also be used to separate the parts of a solution. Salt evaporates at 1414°C. When a mixture of salt and water is heated, the water evaporates first, leaving behind the salt crystals (Figure 1). This process of evaporating the solvent (the water) and leaving behind the solute (salt) is called crystallisation.

Distillation What if we want to keep the substance that has the lowest boiling point? Collecting drinkable water from sea water is difficult if all the water evaporates into the air. Distillation is a way of collecting the gas that evaporates from a mixture and cooling it down so that it becomes a liquid again (Figures 2 and 3). This cooling down of a gas into a liquid is called condensation. DRAFT

Table 1 Boiling points of common liquids

Liquid

Water

Boiling point (°C) 100

Alcohol 78

Petrol

Olive oil

Tar

Turpentine 95

300

30

160 Figure 2 Whisky production uses distillation.

Distillation was a process used by many First Nations Australians to separate the medicinal oils from eucalyptus plants (Figure 4). When the leaves are placed over a fi re, the heat causes the leaves to break apart and the water and eucalyptus oil to be released into the air. If someone is sick, they might lean over the steam to breathe in the oil. This could ease their breathing. If the steam is collected, it will contain a mixture of oil and water which can be separated in a slower distillation process that uses the different boiling points of the two liquids.

Figure 4 Eucalyptus oil was distilled from eucalyptus leaves by First Nations Australians to use as a medicine.

Clamp

Condenser

Stand

Flask Solution Gauze mat Tripod stand Bunsen burner 3.5 Check your learning Retrieve 1 Recall the difference between evaporation and crystallisation. Comprehend 2 Identify an example of a mixture that could be separated by evaporation and crystallisation. Explain why distillation may not be appropriate. 3 Alcohol boils at 78°C, while water boils at 100°C.

In a mixture of alcohol and water, explain which liquid would be the fi rst collected through distillation. (HINT: The word ‘explain’ means you need to provide a reason why.) 4 Identify the separation technique that is being conducted in Figure 5.

Test tube Stand Water in Water out Distillate Figure 3 Equipment set-up for distillation DRAFT Figure 5 Conducting a separation technique

Apply

5 Create a scientifi c diagram of the equipment set-up that could be used to produce pure water from sea water by distillation. 6 Propose how you would answer question 5 without using the science equipment found in a laboratory.

Quiz me Complete the Quiz me to check how well you’ve mastered the learning intentions and to be assigned a worksheet at your level.