Solids, Liquids and Gases

Three boxes of dots, one mark each: that is how Papers 1 and 2 usually test this subtopic, and it still produces wrong answers every series. On Papers 3 and 4 the same content turns into 2-3 mark “describe the arrangement and movement of particles” questions, where one missing strand (usually energy) costs the mark. This page covers everything the states of matter pillar summarises, at full answer depth.

The three states compared (Core)

Cambridge wants three things about each state: how the particles are arranged, how they move, and how much energy they have. Learn the table below as a unit, as exam answers are lifted straight from it.

	Solid	Liquid	Gas
Arrangement	Closely packed, regular pattern (lattice)	Close together but irregular	Far apart, random
Movement	Vibrate about fixed positions	Slide past each other; move randomly	Move rapidly and randomly in all directions
Energy	Least	More than solid	Most
Shape and volume	Fixed shape, fixed volume	Takes shape of container, fixed volume	Fills container, no fixed volume
Compressibility	Cannot be compressed	Cannot be compressed (noticeably)	Easily compressed

The macroscopic properties follow from the particle picture. A solid keeps its shape because particles cannot leave their fixed positions. A gas can be compressed because the particles are far apart with empty space between them; the particles themselves do not shrink. That last clause matters: mark schemes reject “the particles get squashed”.

Drawing particle diagrams

A question may give you an empty box and ask for the particles in a liquid. The rules: solid, circles touching in rows; liquid, circles still mostly touching but jumbled with no regular pattern, filling the box from the bottom; gas, circles widely spaced and scattered through the whole box (around 5-8 circles). The classic error is drawing a liquid with gaps everywhere, which examiners read as a gas. Liquid particles are nearly as close together as solid particles, which is why liquids are also almost incompressible.

Explaining properties with particles (Core, extended in Supplement)

Core candidates state the differences. Supplement candidates explain them through kinetic particle theory: particles in constant motion, with the forces of attraction between particles strongest in solids and weakest (negligible) in gases. Pressure of a gas comes from particles colliding with the container walls; raise the temperature and the particles move faster, collide harder and more frequently, so pressure rises. Decrease the volume and the same number of particles hit the walls more frequently, so pressure rises again.

The state of a substance at room temperature comes from its melting and boiling points. Given data, you decide: below its melting point, solid; between melting and boiling point, liquid; above its boiling point, gas. Room temperature in 0620 questions is taken as 20-25 °C. This data-reading skill reappears in changes of state questions with heating curves.

Worked exam question

The melting point of substance X is −7 °C and its boiling point is 59 °C. (a) State the physical state of X at 20 °C. (1) (b) Describe the arrangement and movement of the particles in X at 20 °C. (2) (c) Explain why X cannot be compressed at 20 °C. (1)

Model answer: (a) Liquid, as 20 °C lies between −7 °C and 59 °C. (b) The particles are close together but arranged irregularly (1); they slide past each other / move randomly over short distances (1). (c) The particles are already (very) close together, so there is (almost) no space between them to push them closer.

Mark-by-mark: (a) needs the deduction from both data points, not a guess. (b) splits cleanly into one arrangement mark and one movement mark. An answer giving only “close together” scores 1 of 2. (c) the mark sits on “particles already close / no space between particles”; “liquids are incompressible” restates the question and scores zero.

The mistakes that cost marks

1. Describing liquid particles as “far apart”. They are close together; only the regular arrangement is lost. This single error turns a liquid description into a gas description.
2. Giving arrangement but not movement (or vice versa) in a 2-mark describe question. Count the marks; give one distinct point per mark.
3. Saying particles “expand” or “get bigger” when heated. Particles gain energy and move more or further apart; the particles themselves never change size.
4. Answering “because it is a solid” when asked to explain a property. Circular answers score zero; the explanation must be about particles.

How examiners want it phrased

Typical student wording	Accepted mark-scheme wording
”Solid particles are stuck together"	"Particles are closely packed in a regular arrangement and vibrate about fixed positions"
"Liquid particles float around"	"Particles are close together, irregularly arranged, and slide past one another"
"Gas particles fly everywhere"	"Particles are far apart and move rapidly and randomly"
"Gases squash easily because they’re soft"	"Gases can be compressed because there are large spaces between the particles”

The Malaysia note

Malaysian students in international schools meet this in the first weeks of Year 10, then rarely see it again until revision, and it shows in May/June scripts when “describe the particles in a liquid” earns one mark out of two. SPM-stream textbooks teach the same “jirim” particle model, so transfer students rarely struggle with content, only with 0620’s strand-by-strand marking. A free 1-hour trial lesson is the quickest way to find out whether your answers are losing strands like this.

Test yourself

Work through all three without looking back at the page. Each answer stays hidden until you click it.

Q1 (2 marks). The melting point of substance Q is −101 °C and its boiling point is −34 °C. State the physical state of Q at −50 °C and at 25 °C.

Show answer

• At −50 °C: liquid, as the temperature lies between the melting point (−101 °C) and the boiling point (−34 °C) [1] • At 25 °C: gas, as the temperature is above the boiling point [1]

Q2 (2 marks). Describe the arrangement and movement of the particles in a gas.

Show answer

• Arrangement: particles are far apart and randomly arranged [1] • Movement: particles move rapidly and randomly in all directions [1]

Q3 (2 marks). A sealed syringe contains air. Explain why the air can be compressed by pushing the plunger, but a syringe filled with sand cannot be compressed.

Show answer

• Gas particles are far apart, so there are large spaces between them into which the particles can be pushed [1] • The particles in a solid are already close together / touching, so there is no space to push them closer [1]