States of Matter: IGCSE Chemistry Exam Guide

States of matter is one of the shorter topics on the 0620 syllabus, but it underpins questions across every other topic. Particle theory explanations appear in questions about diffusion, dissolving, gas behaviour, reaction rates and equilibrium. Securing this topic early means marks gained across the entire paper. The full content is under states of matter.

The particle model: what the exam demands

Every particle-theory answer must include three ideas: arrangement, movement and spacing of particles. The exam penalises vague descriptions. Use this framework:

State	Arrangement	Movement	Spacing
Solid	Regular, fixed pattern	Vibrate about fixed positions	Close together, touching
Liquid	Irregular, no fixed pattern	Move around each other, slide past	Close together, touching
Gas	Random, no pattern	Move rapidly in all directions, random	Far apart, large spaces

When drawing particle diagrams, use equal-sized circles for a pure substance. The marks are awarded for the visual difference between the three states: regular vs irregular arrangement, and close vs far apart spacing.

Changes of state: the vocabulary that scores

The exam requires precise terms: melting, freezing, boiling, condensing, evaporating, subliming. The reverse processes must be paired correctly (melting and freezing, boiling and condensing).

The explain question asks why temperature stays constant during a change of state. The answer that scores:

“During melting (or boiling), the energy supplied is used to overcome the forces between particles (intermolecular forces) rather than to increase the kinetic energy of the particles. Since temperature is a measure of the average kinetic energy, the temperature remains constant until all the substance has changed state.”

The two marks are for: energy overcomes intermolecular forces (1), and kinetic energy does not increase so temperature stays constant (1).

Heating and cooling curves

A heating curve plots temperature against time as a substance is heated steadily.

• Rising sections: the substance is in one state and its temperature increases.
• Flat sections: a change of state is occurring. The first flat section (from solid to liquid) is the melting point; the second flat section (from liquid to gas) is the boiling point.

The exam asks you to label these sections, state what is happening to the particles, or identify the melting and boiling points from the graph. Read the temperature axis carefully; the flat sections tell you the exact melting and boiling points.

Cooling curves are the reverse: flat sections show freezing and condensing. The temperature is read the same way.

Diffusion

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration. The exam tests this with two classic experiments:

1. Bromine gas in a gas jar. Remove the cover slip and the brown colour spreads upwards. Explanation: bromine particles move randomly and spread out to fill the available space.
2. Ammonia and HCl in a tube. Cotton wool soaked in ammonia at one end and HCl at the other; a white ring of ammonium chloride forms closer to the HCl end. Explanation: ammonia molecules (Mr = 17) are lighter and diffuse faster than HCl molecules (Mr = 36.5), so the ring forms closer to the HCl end.

The key mark in the ammonia-HCl experiment is linking the position of the ring to the relative molecular masses and therefore the speeds of the molecules. “Ammonia is lighter so it moves faster” is the minimum.

Evaporation vs boiling: a favourite MCQ trap

Feature	Evaporation	Boiling
Temperature	Any temperature below boiling point	Fixed temperature (boiling point)
Where it occurs	Surface of the liquid only	Throughout the liquid (bubbles)
Speed	Slow, depends on surface area and temperature	Rapid at the boiling point
Energy source	Takes energy from surroundings (cooling effect)	Requires continuous heating

Paper 2 MCQs test this distinction directly. Paper 4 asks you to explain why evaporation causes cooling (the fastest-moving particles escape from the surface, lowering the average kinetic energy of the remaining particles, which lowers the temperature).

Gas pressure and volume (Extended)

Extended candidates need Boyle’s Law: at constant temperature, the pressure of a fixed mass of gas is inversely proportional to its volume. p1V1 = p2V2.

The exam gives you three of the four values and asks for the fourth. The common error: forgetting to keep units consistent (both pressures in the same unit, both volumes in the same unit).

The particle explanation: reducing the volume means particles hit the walls more frequently per unit area, increasing the pressure.

Worked exam question

Q (Paper 4): The diagram shows a glass tube. Cotton wool soaked in concentrated ammonia solution is placed at end A, and cotton wool soaked in concentrated hydrochloric acid is placed at end B. A white ring forms after a few minutes. (a) Name the white solid formed. (1) (b) Explain why the white ring forms closer to end B than to end A. (3)

Model answer: (a) Ammonium chloride (1). (b) Both ammonia and hydrogen chloride diffuse along the tube as gas molecules moving randomly (1). Ammonia has a lower relative molecular mass (17) than hydrogen chloride (36.5) (1), so ammonia molecules move faster and travel further in the same time, forming the ring closer to the HCl end (1).

The three marks are for: both gases diffuse (1), ammonia has a lower Mr (1), lighter molecules move faster so the ring is closer to the heavier gas (1). Stating “ammonia diffuses faster” without giving the reason (lower Mr) loses the second mark.

States of matter is a topic that responds quickly to focused revision because the content is finite and the question patterns repeat. If particle-theory explanations are costing marks in other topics, fixing the foundation here pays dividends across the whole paper. A trial lesson can identify exactly where the explanation structure is breaking down.