Year 7 Science · Unit 2 · Lesson 6

Separation Techniques, Filtering and Evaporation

Challenge Worksheet

Name

Date

Class

Learning Goals

Plan the order of separation steps for a mixture of several parts, and justify it.
Use data from a polluted-creek model to show what filtration can and cannot remove.
Evaluate the claim that filtration is the best method for every mixture.

Warm Up

1. List the four property differences this lesson uses to pull mixtures apart. Beside each, name the technique that relies on it.

Recall 2 marks

2. Iron filings and chalk powder have been mixed together. Explain why adding water and filtering would not separate them. Suggest a better method and the property difference it uses.

Apply 3 marks

Your Turn

Find the errors

Jay’s plan

Jay’s mixture is iron filings, sand, salt and water. He wants each solid on its own, so he writes this plan:

1. Pour everything through filter paper. The salt and the sand will both be trapped on the paper.

2. Hold a magnet over the filtrate in the flask to pull out the iron filings.

3. Heat the filtrate as strongly as possible so that it boils away quickly.

3. Each step of Jay’s plan contains an error. For each step, explain what is wrong.

Analyse 3 marks

4. Write a correct plan for Jay, in order. Explain why the magnet step must come first.

Analyse 4 marks

Go Deeper

Read the data

Scenario: cleaning a model creek

A class makes a model polluted creek: a jar of water with soil, cooking oil, red food colouring and salt stirred in. They clean it in stages. After each stage they evaporate a 100 mL sample of the water and weigh the solid left behind.

Table 1. The model creek at each stage

Stage	What the water looks like	Solid left from 100 mL (g)
1. Just stirred	brown and cloudy, oily layer on top	not measured
2. Left still for an hour, oil skimmed off the top	pale brown, slightly cloudy, red tint	3.4
3. Filtered through filter paper	clear, red	3.0

Source: sample class results.

5. Calculate the percentage of the solid in the Stage 2 water that was still there after filtering. Give your answer to one decimal place.

Apply 2 marks

Answer %

6. The Stage 3 water is clear. Use Table 1 to explain why it is still not clean, and what the 3.0 g of solid is. Then explain why even water that left no solid at all could still make people sick, as the water from John Snow’s Broad Street pump did.

Analyse 3 marks

Show What You Know

Someone claims

“Filtering is always the best way to separate any mixture. If you put a mixture through fine enough filter paper, you can always get the parts out separately. Scientists should just use filtration for everything: it’s the simplest method and it doesn’t use heat, so it’s safer.”

7. Evaluate this claim. Include where the claim is supported, two cases where filtering fails with the reason for each, and how to pick a technique for an unknown sample.

Analyse 6 marks

Answer Key (for teachers)

Question 1

Particle size: filtration; boiling point: evaporation; solubility: dissolving then filtering; magnetism: a magnet. All four properties (1); a method for each (1). [2 marks]

Question 2

Neither dissolves, so both stay together on the paper (1). A magnet (1): iron is magnetic, chalk is not (1). [3 marks]

Find the errors: Question 3

Step 1: the salt has dissolved, so it passes into the filtrate (1). Step 2: the iron filings did not dissolve, so they are on the paper with the sand, not in the filtrate (1). Step 3: a hard boil spits hot salty water, burning people and losing salt; heat gently, stop before dry (1). [3 marks]

Question 4

Magnet to lift out the iron (1). Filter: sand is the residue (1). Evaporate the filtrate gently to leave the salt (1). Magnet first, or filtering traps iron and sand together on the paper (1). [4 marks]

Question 5

3.0 ÷ 3.4 × 100 (1) = 88.2% (1). [2 marks]

Question 6

It leaves 3.0 g per 100 mL and is red (1): dissolved salt and colouring passed through the pores (1). Bacteria such as cholera are too small to see and pass through filter paper (1). [3 marks]

Question 7

Marking guide: 1 supported part: filtration separates an insoluble solid from a liquid, e.g. sand from water; 2 any two mixtures it cannot separate, 1 each with reason (e.g. salt water: dissolved salt passes through; iron and chalk: both trapped); 2 method: 1 find the property that differs, 1 the order (magnetic? undissolved solid? dissolved?); 1 judgement: the claim is wrong. Accept: special membranes (reverse osmosis) can remove dissolved salt, but ordinary filter paper cannot. [6 marks]

Model answer: Filtration works for sand in water, as the grains are too big for the pores, but fails for salt water (dissolved salt passes through) and for iron filings in chalk (both stay on the paper). A scientist finds the property that differs, asking in order: magnetic? undissolved solid? dissolved? So the claim is wrong.

Common error: thinking finer paper would catch dissolved salt.