12.1.2Particle Model
Solids, Liquids and Gases — Edexcel GCSE Combined Science 1SC0 - Physics Revision Notes & Topical Questions
10 min read 8 parts Lesson 3 of 13
Part 1 of 8Key ideas
Key ideas
A pure substance melts and boils at fixed, sharp temperatures; impurities lower the melting point and raise the boiling point, and cause melting over a range.
Increasing temperature increases the average kinetic energy of particles, so they move faster.
(Supplement) Increasing the temperature of a gas at constant volume increases its pressure: particles move faster and collide with the walls more often and with more force.
(Supplement) Increasing the pressure on a gas at constant temperature decreases its volume: the particles are pushed closer together.
(Supplement) Increasing the temperature of a gas at constant pressure increases its volume.
(Supplement) Rate of diffusion: lower Mr → faster diffusion. Example: NH3 (Mr = 17) diffuses faster than HCl (Mr = 36.5).
Diffusion is faster at higher temperature because particles have more kinetic energy.
Water
Water melting point 0 °C, boiling point 100 °C at standard atmospheric pressure.
A substance is a solid at room temperature (about 25 °C) if its melting point is above 25 °C; liquid if m.p. is below and b.p. is above 25 °C; gas if b.p. is below 25 °C.
Part 2 of 8Learning objectives
1
Describe the properties and particle structure of solids, liquids and gases.
2
Describe and explain changes of state, including heating and cooling curves.
3
Explain the effect of temperature and pressure on the volume of a gas (Supplement).
4
Describe diffusion and explain the effect of relative molecular mass on its rate (Supplement).
Part 3 of 8Essential knowledge
1
All matter is made of particles (atoms, molecules or ions) that are in constant motion.
2
In a solid, strong forces hold particles in fixed positions in a regular lattice; they can only vibrate, so the solid has a fixed shape and volume.
3
In a liquid the particles are still close together (so fixed volume, hard to compress) but can move past each other, so a liquid flows and takes the shape of its container.
4
In a gas the particles are far apart with negligible forces between them, moving rapidly and randomly, so a gas spreads out to fill any container and can be compressed.
5
When a solid is heated, the particles vibrate more; at the melting point they have enough energy to break out of the fixed lattice and the solid melts.
6
When a liquid is heated its particles move faster; at the boiling point they have enough energy to overcome the attractive forces completely and escape as a gas throughout the liquid.
7
Evaporation happens at any temperature, only at the surface; it is faster at higher temperature, larger surface area and with air movement.
8
Heating curves show sloping sections (temperature rising within one state) and flat sections (changes of state, energy used to overcome forces). Cooling curves are the reverse, with flat sections at condensation and freezing.
9
(Supplement) Gas volume increases with temperature at constant pressure because faster-moving particles hit the walls harder and more often, pushing them outward; at constant temperature, increasing pressure squashes particles closer so volume decreases.
10Diffusion evidence
Diffusion evidence a purple crystal of potassium manganate(VII) in water slowly spreads colour throughout; bromine vapour spreads through air in a gas jar.
11
(Supplement) In a long tube with cotton wool soaked in concentrated ammonia at one end and concentrated hydrochloric acid at the other, a white ring of ammonium chloride forms nearer the HCl end because NH3 (Mr 17) diffuses faster than HCl (Mr 36.5).
12Core
Diffusion is quicker in gases than in liquids: gas particles move much faster and have far more space between them, so they spread out and mix more rapidly. Diffusion needs no energy input, but it speeds up as temperature rises.
Part 4 of 8Process & sequence
- Place a sample of a solid (e.g. stearic acid) in a boiling tube with a thermometer and heat it gently in a water bath.
- Record the temperature every 30 seconds while stirring gently.
- Note that the temperature rises (solid particles vibrate more) until the melting point is reached.
- During melting the temperature stays constantenergy is being used to overcome forces between particles.
- Once all the solid has melted the temperature rises again as the liquid particles gain kinetic energy.
- Plot temperature (y-axis) against time (x-axis); the horizontal section gives the melting point.
- For a cooling curve, let the liquid cool and record temperature; the flat section is the freezing point.
Heating curve of a pure solidwrite these steps in this order; each step is a marking point.
- Solid heatedtemperature rises as particles vibrate more.
- Melting point
- temperature constant while solid and liquid coexist;
- energy overcomes forces.
- temperature constant while solid and liquid coexist;
- Liquid heatedtemperature rises as particles move faster.
- Boiling point
- temperature constant;
- energy lets particles escape as gas.
- temperature constant;
- Gas heatedtemperature rises again as particles gain kinetic energy.
Part 5 of 8Worked examples
Liquid
Substance X has melting point –7 °C and boiling point 59 °C. What is its state at 25 °C and at 80 °C?
- At 25 °C: 25 °C is above the melting point (–7 °C) but below the boiling point (59 °C), so X is a liquid.
- At 80 °C: 80 °C is above the boiling point, so X is a gas.
Closer to the HCl end
(Supplement) In a 100 cm tube, ammonia and hydrogen chloride are released at opposite ends at the same time. Explain where the white ring of ammonium chloride forms.
- Mr(NH3) = 14 + 3(1) = 17;
- Mr(HCl) = 1 + 35.5 = 36.5.
- NH3 has the lower Mr, so its molecules move faster and diffuse further in the same time.
- The ring (NH3 + HCl → NH4Cl) therefore forms closer to the HCl end (roughly 40 cm from the HCl end).
Part 6 of 8Summary
- Solids have a regular, closely packed arrangement and particles only vibrate.
- Melting, boiling and evaporation absorb energy; condensing and freezing release energy.
- Temperature stays constant during a change of state.
- Diffusion is the net movement from high to low concentration due to random particle motion.
- Gases with lower Mr diffuse faster.
Part 7 of 8Keep going
Solids, liquids & gases IGCSE Chemistry 0620: what to do next
These Solids, liquids & gases IGCSE Chemistry 0620 notes covered the key ideas, worked examples and exam technique for this topic. Check your Solids, liquids & gases IGCSE Chemistry 0620 understanding with the practice questions below, then reread any Solids, liquids & gases IGCSE Chemistry 0620 point you were unsure about before moving on.
Solids, liquids & gases IGCSE Chemistry 0620: revision checklist
- Revise the main Solids, liquids & gases IGCSE Chemistry 0620 definitions until you can write them from memory.
- Practise Solids, liquids & gases IGCSE Chemistry 0620 exam questions under timed conditions.
- Link each Solids, liquids & gases IGCSE Chemistry 0620 idea to a diagram or a worked example.
- List the common Solids, liquids & gases IGCSE Chemistry 0620 mistakes and how to avoid them.
- Explain one Solids, liquids & gases IGCSE Chemistry 0620 idea out loud as if teaching a friend.
- Summarise Solids, liquids & gases IGCSE Chemistry 0620 in five key points on a revision card.
- Practice: Solids, liquids & gases IGCSE Chemistry 0620 practice questions
- Next lesson: 1.2 Kinetic theory — 0620 Notes
- Course: Cambridge IGCSE Chemistry 0620 — all units
- Official syllabus: Cambridge IGCSE Chemistry 0620 syllabus
Part 8 of 8Compare: Compare
| Property | Solid | Liquid | Gas |
|---|---|---|---|
| Arrangement | Regular, closely packed (lattice) | Irregular, close together | Random, far apart |
| Motion | Vibrate about fixed positions | Move/slide past each other | Move quickly and randomly in all directions |
| Shape | Fixed | Takes shape of container | Fills whole container |
| Volume | Fixed | Fixed | Not fixed |
| Compressibility | Not compressible | Almost incompressible | Easily compressed |
| Energy of particles | Lowest | Higher | Highest |
| Change | Name | Energy change |
|---|---|---|
| Solid → liquid | Melting | Energy absorbed (endothermic) |
| Liquid → gas | Boiling / evaporation | Energy absorbed (endothermic) |
| Gas → liquid | Condensation | Energy released (exothermic) |
| Liquid → solid | Freezing | Energy released (exothermic) |
On a heating curve the temperature stays constant during melting and boiling because the energy supplied is used to overcome attractive forces between particles, not to raise the kinetic energy.
Key terms
Command terms
Command terms
State / Name
Give a short answer only; no explanation is needed (1 mark = 1 fact).
Define
Give the precise meaning. Learn the syllabus definitions word-for-word: every keyword in the definition is a marking point.
Describe
Say what happens / what the data show, with key features and figures. Do not give reasons.
Explain
Give reasonswhat happens AND why/how, linked with “because”, “so”, “therefore”.
Suggest
Apply knowledge to an unfamiliar situation; any sensible, scientifically-based answer can score.
Compare
Give similarities AND differences, each as a direct comparison (“X is … whereas Y is …”).
Outline
Set out the main points briefly.
Calculate
Work out from given facts; show working, give the unit and the requested significant figures.
Identify
Name/select from given information.
Predict
Give an expected result without a full justification.
Evaluate
Judge the importance or quality of the evidence/method, with reasons.
Exam technique
Keywords the mark scheme looks for in this lesson
Keywords the mark scheme looks for in this lesson
Solid · Liquid · Gas · Melting · Boiling · Evaporation · Condensation · Freezing · Kinetic particle theory · Diffusion · Gas pressure · Relative molecular mass (Mr) and diffusion
Learn each definition in the Key words tab exactly; a missing keyword in a definition usually loses the mark.
How to get full marks (examiner guidance)
1
Use the exact scientific term and learn syllabus definitions word-for-word.
2
Write “rate of … increases”, not “… increases”.
3
Say “concentration gradient”, never just “gradient”; name particles precisely (atoms, molecules, ions) — never “it”.
4
Give units with every numerical answer and use the significant figures asked for; show all working.
5
Compare = a similarity AND a difference; describe = what happens; explain = why it happens.
6
Repeats let you identify anomalies and calculate a mean — they do not make an experiment “valid”.
Words to avoid
1
the particles expand when heatedthe particles move faster and further apart; the particles themselves do not change size.
2
particles in a solid do not moveparticles in a solid vibrate about fixed positions.
3
heat rises in a gasparticles gain kinetic energy and move faster.
4
diffusion is movement from high to low concentrationdiffusion is the NET movement from high to low concentration due to random motion of particles.
5
the bonds break when water boilsthe attractive forces between molecules are overcome (covalent bonds within molecules do not break).
6
heavier gases diffuse fastergases with lower relative molecular mass diffuse faster.
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