Year 7 Science · Unit 2 · Lesson 4
Challenge Worksheet
Learning Goals
1. What is a pure substance, and what lab measurement tells a chemist that a sample is pure?
True or false? Fix the false ones
Circle T or F. If a statement is false, correct it on the line.
1. Gold and distilled water are both pure substances, even though gold is an element and water is not.
Correct it:
2. Weak tea and strong tea can be made from the same two ingredients in different amounts, because tea is a mixture.
Correct it:
3. Tap water is a pure substance because you cannot see anything else in it.
Correct it:
4. Salt water boils at one fixed temperature, just as pure water does.
Correct it:
5. In salt water, the salt particles are unchanged; they are spread out among the water particles.
Correct it:
2. Draw a particle diagram of distilled water and one of salt water. Use a different symbol for each kind of particle and add a key.
Find the mistakes
A student wrote this answer
“Salt water is a pure substance because it looks the same everywhere: if you scoop it from the top or the bottom of the glass, it looks identical. It also tastes salty all the way through, which proves all the particles are the same. My dad buys ‘pure sea salt’ at the shops, so I know salt is pure. Because it is uniform, salt water must be a pure substance.”
3. The answer contains three different errors. For each one, quote the error and explain the correct science.
Read the data
Scenario: the scoop test, with a balance
Noah tests two mixtures by taking scoops from the top, middle and bottom of each container. He evaporates each 10 g scoop of salt water and weighs the salt left behind. He picks the raisins out of each 50 g scoop of muesli and weighs them.
| Mixture | Scoop size | What was weighed | Top | Middle | Bottom |
|---|---|---|---|---|---|
| Salt water in a jug | 10 g | salt left after evaporating | 0.35 | 0.35 | 0.35 |
| Muesli in a tub | 50 g | raisins picked out | 4 | 9 | 14 |
Source: sample class results.
4. Work out the percentage by mass of salt in Noah’s jug.
Answer %
5. Use Table 1 to decide which mixture is homogeneous and which is heterogeneous. Suggest why the three muesli scoops are so different.
6. A chemist counts a sample as pure only if it boils at one fixed, sharp temperature. Describe what the thermometer would show if you boiled seawater from Bondi Beach, and explain whether the seawater passes the test.
7. Mateo says: “Only elements, like gold, can be pure. Water can never be pure, because it is made from two different elements.” Evaluate Mateo’s claim, using the chemistry meaning of “pure” and evidence from the lesson.
Question 1
Matter made of only one kind of particle (1). It melts and boils at one fixed, sharp temperature, found with a thermometer (1). [2 marks]
True or false
1. True. 2. True. 3. False: tap water has dissolved minerals; it is a homogeneous mixture. 4. False: salt water has no single boiling point; its temperature keeps rising. 5. True.
Question 2
Distilled water: every particle the same (1). Salt water: two kinds, salt spread evenly among water particles (1). A key names each symbol (1). [3 marks]
Find the mistakes: Question 3
1 each: “looks the same everywhere” means homogeneous, not pure; “tastes salty” shows salt particles as well as water, two kinds; “pure sea salt”: shops use pure loosely, and pure salt in water still makes a mixture. [3 marks]
Question 4
0.35 ÷ 10 × 100 (1) = 3.5% (1). [2 marks]
Question 5
Salt water is homogeneous: 0.35 g in every scoop (1). Muesli is heterogeneous: 4 g to 14 g of raisins (1). The raisins are not spread evenly (e.g. heavier pieces settle) (1). [3 marks]
Question 6
It starts boiling above 100 °C (1) and keeps climbing, so there is no single boiling point (1). It fails: as water leaves, the rest gets saltier, the sign of a mixture (1). [3 marks]
Question 7
Marking guide: 1 judgement (wrong); 1 pure means every particle is the same kind; 1 distilled water is all H₂O, so pure though a compound; 1 gold is pure as every atom is gold: sameness, not simplicity; 1 pure water boils at a fixed 100 °C; 1 contrast: salt water boils over a rising range. [6 marks]
Model answer: Mateo is wrong: pure means every particle is the same. Every particle of distilled water is H₂O, so it is pure though a compound; gold is pure as every atom is gold. Pure water boils at a fixed 100 °C; salt water, two kinds of particle, boils over a rising range.
Common error: treating “pure” as “element”.