Year 7 Science · Unit 2 · Lesson 2
Challenge Worksheet
Learning Goals
Odd one out
In each set, circle the odd one out and give the reason. The rule for each set is given in brackets. There is one best answer in each set.
| Set | Circle the odd one out | Reason |
|---|---|---|
| 1 | (Energy taken in or given out) melting · evaporation · freezing · sublimation | |
| 2 | (Does it skip the liquid state?) a block of dry ice shrinking · frost forming on a windscreen · droplets forming on a cold drink can · the change called deposition | |
| 3 | (Is it evidence that particles move?) purple colour spreading through still water · the smell of perfume spreading across a room · pollen grains jiggling in water under a microscope · an ice cube keeping its shape in the freezer |
Find the error
Ella’s explanation of boiling a kettle
(1) As the water heats up, its particles move faster. (2) At 100 °C, bubbles of air form at the bottom of the kettle. (3) The temperature keeps rising while the water boils. (4) The steam is a new substance, because the particles have changed. (5) In the steam, the particles are far apart, with air in the spaces between them. (6) When the steam hits a cold window, it condenses and gives out energy.
1. Four of Ella’s six sentences contain one error each. For each wrong sentence, give its number and rewrite it correctly.
2. A block of dry ice at −78.5 °C is left on a bench at about 22 °C. It shrinks away to nothing and leaves no puddle. (a) Name this change. (b) Explain how the forces holding the block together, and the movement inside it, change as this happens.
Read the data: how fast does it spread?
Scenario
A class timed how long a colour or a smell took to spread 10 cm from where it started. The water and the air were kept still.
| Test | What spread | Through | Temperature (°C) | Time (min) |
|---|---|---|---|---|
| A | Purple colour | Still water | 10 | 25 |
| B | Purple colour | Still water | 60 | 8 |
| C | Smell of perfume | Still air | 20 | 1 |
Source: sample class results.
3. Compare tests A and B. Which one variable was changed? Use the particle model to explain the difference in time.
4. Ravi says: “Tests A and C prove that particles spread faster through a gas than through a liquid.” Is this a fair comparison? Explain, then say what Ravi should change to make it fair.
5. On cold, sunny days in the Snowy Mountains, some snow disappears without melting into puddles. A news report says “the snow just evaporates like water.” Is the report accurate? Explain the difference between evaporation and sublimation, and name the change that is happening.
6. A student says: “Particles can’t really be moving all the time. Nothing in a glass of still water looks like it is moving.” Evaluate this claim. Use at least three pieces of evidence from the lesson or this worksheet, and the particle rules.
Odd one out
1. Freezing: gives out energy; the others take it in. 2. Droplets on a cold can: condensation, gas to liquid; the others skip the liquid. 3. The ice cube: it shows particles held in place, not moving from place to place. [3 marks]
Find the error
1. Sentence 2: the bubbles are water vapour (gaseous water), not air (1). Sentence 3: the temperature stays at 100 °C while it boils (1). Sentence 4: steam is still water (H₂O); only the arrangement has changed (1). Sentence 5: the spaces between the particles are empty (1). Sentences 1 and 6 are correct. [4 marks]
Question 2
(a) Sublimation (1). (b) The particles gain energy from the room and vibrate faster (1); they break free of the forces holding them in place and move far apart as a gas (1). [3 marks]
Read the data: how fast does it spread?
3. The temperature of the water (1). At 60 °C the particles have more energy and move faster, so the colour spreads in 8 min instead of 25 min (1). [2 marks]
4. Not fair (1): A and C differ in what spread, what it spread through and the temperature, so the faster time cannot be put down to the gas alone (1). Test the same substance at the same temperature, changing only water or air (1). [3 marks]
Question 5
Not accurate (1). Evaporation is liquid to gas; sublimation is solid straight to gas (1). No puddles form, so the solid snow is turning straight into water vapour: sublimation (1). [3 marks]
Question 6
Marking guide: 1 judgement (wrong); 1 particles are too small to see; 1 each for three explained pieces of evidence (colour spreading through still water: diffusion; a smell spreading; Brown’s pollen grains jiggling as water particles hit them); 1 uses Table 1 (for example, the colour spread 10 cm in 25 min through water that was kept still). [6 marks]
Model answer: The claim is wrong. Particles are far too small to see, but we can see what their movement does. In Table 1, purple colour spread 10 cm through still water in 25 minutes with no stirring. A smell spreads across a room the same way. Robert Brown saw pollen grains jiggle in water because moving water particles hit them from every side. So particles are always moving.
Common error: thinking particles stop when nothing looks like it is moving.