Year 7 Science • AC9S7U05 • Homework
Particle Theory, States of Matter & Diffusion
Explain observations by linking particle arrangement, motion, spacing, energy and attraction to the properties of solids, liquids and gases.
Part A
5 × 2-mark questions
Describe how particles are arranged in a solid compared with a gas. [2]
Explain what happens to particle motion when heat is added to a substance before a change of state. [2]
State one difference between the effect of particle attractions in a liquid and in a gas. [2]
Explain why the gas state of a substance is usually much less dense than its liquid state. [2]
Define diffusion in terms of particle movement and concentration. [2]
Part B
5 × 5-mark questions
Use a particle model to describe what changes when a solid melts and then becomes a gas. Include particle arrangement, motion, energy and attraction. [5]
Explain how absorbing and releasing heat energy affect particles during melting, freezing, evaporation and condensation. [5]
Compare solids, liquids and gases in terms of shape, volume and compressibility, and explain these differences using particle theory. [5]
A syringe filled with air compresses easily, but a syringe filled with water barely compresses. Explain this observation and link it to particle spacing and compressibility. [5]
Explain diffusion in liquids and gases. Compare their rates and explain why food colouring spreads faster in warm water than cold water. [5]
Part C
Enrichment — 8–10 marks
Analyse how particle theory explains the behaviour of substances across solids, liquids and gases. In your response, connect particle motion and energy to state, explain how attraction and spacing influence density and compressibility, explain how particle models represent state changes, use diffusion as evidence that particles move, and discuss one real-world application such as thermal expansion, evaporation or gas pressure.
Teacher / self-check
Answers and marking guidance
- 2: solid particles are close together and held in fixed average positions; gas particles are widely separated and move freely through the container.
- 2: particles gain kinetic energy and average motion/vibration increases; particles themselves do not become larger.
- 2: liquid particles remain close because attractions still have a strong effect, while in a gas the particles are much farther apart and attractions have much less effect on their motion.
- 2: the same mass occupies much more volume in the gas because particles are much farther apart, so mass per unit volume is lower.
- 2: continual random particle motion produces a net spread from higher concentration toward lower concentration.
- 5: award for solid arrangement and vibration; energy gain during melting; liquid particles moving past one another; further energy gain to gas; gas particles widely separated/free-moving while substance identity is preserved.
- 5: heating increases average particle energy; cooling decreases it; melting/evaporation require energy; freezing/condensation release energy; correct particle-motion/arrangement link.
- 5: accurate comparison of shape, volume and compressibility for all three states plus particle explanations.
- 5: gas has large average gaps that can be reduced; liquid particles are already close; particles do not shrink; connect to observed compression; use clear scientific vocabulary.
- 5: define diffusion; random motion; net high-to-low spread; gas generally faster because of greater spacing/freedom; warmer liquid generally diffuses faster because average kinetic energy is greater.
- 8–10: reward integrated reasoning across arrangement, motion, energy, attraction, properties, diffusion and a real-world application; highest responses also identify that particle diagrams are simplified models rather than literal scale drawings.