States of Matter: IGCSE Chemistry 0620

States of Matter is Topic 1 of the 0620 syllabus and the first place students throw away easy marks. It appears every series: 2-4 multiple choice questions on Paper 1 or 2, plus a structured question on Paper 3 or 4 worth 3-5 marks. Examiner reports repeat the same complaint year after year: students describe what they see (“the ice melts”) instead of what the particles do (“particles gain energy and break free from fixed positions”). That gap between everyday language and particle-level language is where the marks sit.

Solids, liquids and gases (Core)

The mark scheme wants three properties for each state, described at particle level:

State	Arrangement	Motion	Separation
Solid	Regular, fixed positions	Vibrate about fixed positions	Touching, closely packed
Liquid	Random, irregular	Move and slide past each other	Touching, close together
Gas	Random	Move fast in all directions	Far apart

From these three columns you can explain every macroscopic property they ask about. Solids have a fixed shape because particles cannot leave their positions. Liquids flow because particles move past each other while staying in contact. Gases are compressible because there is empty space between particles. The particles themselves do not shrink.

One MCQ trap appears repeatedly: a diagram shows particles close together but randomly arranged. That is a liquid, not a “compressed gas”. Arrangement and separation together identify the state.

Kinetic particle theory (Core, with Supplement extensions)

The kinetic particle model says matter is made of small particles in constant motion, and the energy of that motion increases with temperature. Three lines of reasoning come up in exams:

Temperature and particle energy. Heat a substance and the particles gain kinetic energy and move (or vibrate) faster. Cool it and they lose energy and slow down. Write it exactly that way: “particles gain energy and move faster” is the accepted wording.

(S) Gas pressure. Extended candidates explain pressure as particles colliding with the walls of the container. Raise the temperature and particles move faster, so they collide with the walls more frequently and with more force, so pressure increases. Decrease the volume and the same number of particles hit the walls more frequently, so pressure increases again. The scoring word is collisions; answers that never mention particles hitting the walls score zero.

(S) Limitations of the model. The simple model treats particles as solid spheres with no forces between them. Real particles attract each other and have their own volume, which is why the model breaks down for gases at high pressure or low temperature.

Changes of state (Core)

You need six names: melting, freezing, boiling, condensing, evaporation and sublimation. Two distinctions earn marks.

Boiling versus evaporation trips up half of each cohort. Boiling happens at one fixed temperature, throughout the liquid, with bubbles of vapour forming. Evaporation happens at any temperature below the boiling point, only at the surface, and only the most energetic particles escape. If a question asks why a puddle disappears on a 30 °C day in Malaysia, the answer is evaporation. Water does not need to reach 100 °C.

Heating and cooling curves are the other reliable mark source. The flat sections occur at the melting and boiling points. Why flat? Energy supplied is used to overcome the forces of attraction between particles rather than to increase their kinetic energy, so the temperature does not rise while two states coexist. That sentence, almost word for word, is the mark scheme. Cooling curves run the same logic in reverse: energy is released as bonds between particles form, holding the temperature steady at the freezing point.

Pure substances melt and boil at sharp, fixed temperatures. Mixtures melt and boil over a range, which is how Paper 6 asks you to judge purity, and why this topic links straight into purification and separation methods.

Diffusion (Core, with Supplement extension)

Definition first: diffusion is the spreading out of particles from a region of higher concentration to a region of lower concentration, due to the random motion of the particles. Both halves are needed for 2 marks: the direction (high to low concentration) and the cause (random particle motion). “The smell spreads through the room” describes the observation, not the chemistry.

The classic experiment is the ammonia and hydrogen chloride tube. Cotton wool soaked in concentrated ammonia solution at one end, concentrated hydrochloric acid at the other, and a white ring of ammonium chloride forms where the gases meet, closer to the HCl end.

(S) Why closer to the HCl end? Ammonia (Mr = 17) has a lower relative molecular mass than hydrogen chloride (Mr = 36.5), so its molecules move faster at the same temperature and travel further in the same time. The general rule: the lower the relative molecular mass, the faster a gas diffuses. Quote the two Mr values in your answer. Examiners reward the comparison, not the bare claim. Calculating those Mr values is covered under stoichiometry.

Temperature matters too: at higher temperatures particles have more kinetic energy, move faster, and diffusion speeds up.

Worked exam question

A sealed gas syringe contains 50 cm³ of air at 25 °C. The syringe is warmed to 80 °C while the plunger is held in place. Explain, using the kinetic particle model, why the pressure inside the syringe increases. [3]

Model answer: The particles gain kinetic energy and move faster (1). They collide with the walls of the syringe more frequently (1) and each collision exerts more force on the walls (1), so the pressure increases.

Mark-by-mark: the first mark is the energy statement: “gain energy and move faster”, not “the air gets hotter”. The second mark needs collision frequency with the walls. The third needs collision force or energy. Cambridge mark schemes list frequency and force as separate marking points; students who write “they hit the walls more” without saying harder or more frequently as distinct ideas cap themselves at 2.

The mistakes that cost marks

1. Describing the substance instead of the particles. “The ice gets warmer and turns to water” earns nothing. Every explanation in this topic must name particles, their energy, their motion or their arrangement.
2. Saying particles “expand” or “get bigger” when heated. Particles stay the same size; the spaces between them increase. This single error appears in examiner reports almost every series.
3. Confusing evaporation with boiling. Writing “the water boils away” for a puddle drying at room temperature loses the mark. Only evaporation fits.
4. Flat heating-curve sections explained as “the heat stops”. Energy is still being supplied; it goes into overcoming forces of attraction between particles instead of raising temperature.
5. Diffusion answers that omit “random motion”. Direction alone (high to low concentration) is half the definition and half the marks.

How to phrase it for full marks

Student wording	Mark-scheme wording
”The solid melts into a liquid"	"Particles gain energy and break free from their fixed positions"
"Heat makes the gas push harder"	"Particles move faster and collide with the walls more frequently and with more force"
"The smell travels across the room"	"Particles spread from higher to lower concentration by random motion"
"Ammonia is lighter so it wins"	"Ammonia has a lower Mr (17 vs 36.5), so its molecules travel faster"
"The temperature pauses"	"Energy supplied overcomes forces of attraction between particles, so temperature stays constant”

The pattern: name the particles, state the energy change, state the consequence. Train that sentence shape and this topic becomes a guaranteed mark bank. Command words matter here too: “state” wants one line, “explain” wants the particle-level cause, as the 0620 exam format guide breaks down paper by paper.

The Malaysia note

Malaysian students sit 0620 in the May/June or October/November series, and most international schools in KL, Penang and JB teach this topic in the first term of the two-year course, which means it is two years cold by exam day. It feels easy, so it never gets revised, and then a 3-mark kinetic theory explanation goes begging. Students coming from a KSSM Sains background also carry over Bahasa-style phrasing (“the particles vibrate vigorously”) that is fine, but tend to skip the energy statement that earns the first mark. A free 1-hour trial lesson with one of our Chemistry specialists usually starts with a topic like this one (easy chemistry, exam-grade phrasing), so you see exactly how mark-scheme training works before paying anything.

Every sub-topic in States of Matter

• Solids, Liquids and Gases
• Kinetic Particle Theory
• Changes of State
• Diffusion