Year 7 Science · Unit 2 · Lesson 13
Challenge Worksheet
Learning Goals
1. Name each change of state, and say whether the particles gain or lose energy: (a) iodine crystals turning straight into a purple gas; (b) steam forming droplets on a cold window; (c) melted chocolate hardening in the fridge.
Find the error
A student wrote this summary of Unit 2. Five sentences contain one error each. Write “correct”, or rewrite the sentence so it is correct.
| # | Student’s sentence | Correct, or the fixed sentence |
|---|---|---|
| 1 | All matter is made of particles. | |
| 2 | In a solid, the particles are far apart and move around freely, which is why a solid keeps its shape. | |
| 3 | Elements and mixtures are both pure substances. | |
| 4 | Air is a compound, because it contains more than one element. | |
| 5 | Sand dissolves in water, so the two make a solution. | |
| 6 | Filter paper traps sand because the grains are too big to pass through it. | |
| 7 | Distillation separates two liquids because they have different melting points. |
2. Mei stirs blue copper sulfate crystals into hot water until no more will dissolve. She heats the solution gently until about half of the water has boiled off, then leaves it on a windowsill to cool. A day later, blue crystals have grown in the dish. (a) Using the word saturated, explain why crystals form. (b) Describe one way Mei could check that the crystals are still copper sulfate.
Read the data: weighing a solution
Scenario: where does the salt go?
Three groups each weighed a beaker of water, tipped in a weighed amount of salt, stirred until all of it had dissolved, and weighed the beaker again.
| Group | Before | Salt added | After |
|---|---|---|---|
| 1 | 250.0 | 10.0 | 260.0 |
| 2 | 248.5 | 15.0 | 263.4 |
| 3 | 251.2 | 20.0 | 271.2 |
Source: sample class results.
3. What mass should Group 2 have read after dissolving, if no mass was gained or lost? By how much does their reading differ from this?
Expected mass g
Difference g
4. What do the results in Table 1 show happens to salt when it dissolves? Explain using the particle model.
5. Leo says: “Group 2’s reading is 0.1 g too low, so a tiny bit of their salt was destroyed.” Evaluate Leo’s claim.
6. You are given an unlabelled white powder. It is salt, sugar or sand. Plan a series of at least three tests to identify it. For each test, say what you would observe and what it would tell you. Justify the order of your tests, state one limit of your evidence, and give one safety rule.
Question 1
(a) Sublimation, gain (1). (b) Condensation, lose (1). (c) Freezing, lose (1). Both parts per mark. [3 marks]
Find the error
1 correct. 2 close together, vibrating in place. 3 elements and compounds are pure. 4 air is a mixture: its gases are not chemically bonded. 5 sand is insoluble: a heterogeneous mixture. 6 correct. 7 boiling points. [5 marks: 1 per error fixed]
Question 2
(a) The solution is already saturated; boiling off water leaves more copper sulfate than the remaining water can hold (1); as it cools, even less copper sulfate can stay dissolved (1), so the extra particles join up as crystals (1). (b) Compare with the starting crystals: same blue colour and shape, and they dissolve to the same blue solution (1). [4 marks]
Teacher note: heating to dryness drives water out of the crystals, leaving white powder.
Question 3
Expected mass: 248.5 + 15.0 = 263.5 g (1). Difference: 0.1 g lower (1). [2 marks]
Question 4
The mass after equals water plus salt (1): the salt is not destroyed, its particles spread out between the water particles (1), so it could be got back by evaporating the water (1). [3 marks]
Question 5
Wrong: matter cannot be destroyed, and Groups 1 and 3 match exactly (1). 0.1 g is measurement error, such as grains left behind or a balance reading (1). [2 marks]
Question 6
Marking guide: 1 dissolve in warm water: sand does not dissolve; 1 heat gently: sugar melts (about 186 °C) and chars, salt is unchanged (melts at 801 °C); 1 evaporate: crystals come back, confirming; 1 order: dissolving rules out the most in one step; 1 limit: cannot rule out a fourth white solid; 1 safety: never taste it. [6 marks]
Model answer: First stir some into warm water, as it rules out most: if it stays on the bottom, sand. If it dissolves, heat a dry sample gently: melting, browning and charring means sugar; no change means salt. Then evaporate the solution to check the crystals come back. This only picks between the three given, not some other white solid. Never taste it.
Common error: measuring density by displacement; the powder dissolves in the water.