Year 7 Science · Unit 2 · Lesson 6

Separation Techniques, Filtering and Evaporation

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Learning Goals

Put the steps of a filtration in order and explain the safety rules for evaporation.
Calculate the mass of salt recovered by evaporation, and explain a poor result.
Link Aboriginal and Torres Strait Islander separation techniques to the property each uses.

Warm Up

Order the steps: filtering sandy water

Number the steps from 1 (first) to 7 (last).

Step number	What you do
	Wait while the water drips through the paper into the flask.
	Fold a circle of filter paper into a cone.
	Lift out the filter paper and leave the sand on it to dry.
	Pour the sandy water down a glass stirring rod into the funnel.
	Put on safety glasses before you handle any equipment.
	Rinse the sand left on the paper with a little clean water.
	Open the cone inside the funnel, and sit the funnel in the neck of a conical flask.

1. When salt water is evaporated, which substance is the solvent and which is the solute? Which one do you keep at the end?

Recall 2 marks

Your Turn

Cause and effect: a solar salt pond

Complete the chain. Each box is caused by the one before it. Write in the empty boxes.

Apply 2 marks

Seawater is let into a shallow pond at a salt works on a hot, windy day.

A white crust of salt is left on the floor of the pond, ready to harvest.

2. Name one safety rule you must follow when you evaporate salt water over a Bunsen burner, and explain the reason for it.

Apply 2 marks

Go Deeper

Read the data

Scenario: how much salt is in it?

Three groups evaporate 100 g water samples and weigh the dry salt left. Groups 1 and 2 use the same seawater: Group 1 heats gently, stopping when a little liquid is left; Group 2 boils hard until dry. Group 3 gently heats salt-lake water.

Table 1. Mass readings for each group (g)

Group	Empty basin	Basin + water sample	Basin + dry salt	Mass of salt
1 (gentle)	52.0	152.0	55.5	3.5
2 (hard boil)	48.0	148.0	50.1	2.1
3 (gentle)	50.0	150.0	58.6

Source: sample class results.

3. Calculate Group 3’s mass of salt and write it in Table 1. About how many times more salt is in the salt-lake water than in seawater (use Group 1)?

Apply 2 marks

Answer times

4. Groups 1 and 2 used the same seawater, yet Group 2 recovered less salt. Explain what happened to the missing salt, and how Group 2 should change its method.

Analyse 2 marks

Show What You Know

5. Aboriginal and Torres Strait Islander peoples separated mixtures precisely long before laboratory funnels existed. Choose two of winnowing, leaching and straining. For each, explain what it separates, which property difference it uses and how, and name a method from this lesson that uses the same property.

Analyse 6 marks

Answer Key (for teachers)

Order the steps

Reading down the table as printed, the correct step numbers are 5, 2, 7, 4, 1, 6, 3. In order: safety glasses; fold the paper into a cone; cone in funnel, funnel in flask; pour down a stirring rod; wait while the water drips through; rinse the sand; lift out the paper and dry the sand.

Question 1

Water is the solvent, salt the solute (1). You keep the salt; the water leaves as vapour (1). [2 marks]

Cause and effect

Box 1: the Sun heats the water and it evaporates; wind carries the vapour away (1). Box 2: the salt does not evaporate, so it stays in the pond and the water left gets saltier (1). [2 marks]

Question 2

One rule (1) with its reason (1): heat gently and stop before dry, as a hard boil spits hot salty water; use tongs, as the basin is hot; wear safety glasses against splashes. [2 marks]

Question 3

58.6 − 50.0 = 8.6 g (1). 8.6 ÷ 3.5 ≈ 2.5 times (accept 2 to 3 times) (1). [2 marks]

Question 4

The hard boil spat droplets of salty water out of the basin, carrying salt away (1). Heat gently and stop when a little liquid is left (1). [2 marks]

Question 5

Marking guide: per technique, 1 what is separated, 1 property and how, 1 matching method. Winnowing: seed from light chaff; density: tossed in a coolamon, the breeze carries the lighter chaff away; like skimming oil, which floats. Leaching: soluble poison from ground nardoo spores or cycad seeds; solubility: running water dissolves and washes the toxin away; like dissolving salt out of salt and sand. Straining: twigs and grit from water; particle size: woven grass or reeds trap the bits; like filtration. [6 marks]

Model answer: Straining separates twigs and grit from water using particle size: woven reeds let water through but trap the large bits, like filtration. Leaching separates a poison from ground nardoo spores using solubility: the toxin dissolves in running water and is washed away, like dissolving salt away from sand.

Common error: saying the salt evaporates too.