What are the main differences between solids, liquids, and gases in terms of particle arrangement?

In solids, particles are tightly packed in a fixed, orderly arrangement, allowing them to maintain a definite shape and volume. In liquids, particles are close together but not in a fixed position, which allows them to flow and take the shape of their container while maintaining a constant volume. In gases, particles are far apart and move freely, filling the volume of their container and having neither a definite shape nor a fixed volume. These differences are fundamental to understanding the states of matter in the Cambridge IGCSE Chemistry curriculum.

How does temperature affect the states of matter?

Temperature plays a crucial role in changing the states of matter. When a solid is heated, its particles gain energy and vibrate more vigorously, eventually overcoming their fixed positions and transitioning into a liquid, a process known as melting. Further heating causes the liquid particles to gain enough energy to break free from their bonds and become a gas, a process called evaporation or boiling. Conversely, cooling a gas can cause it to condense into a liquid, and further cooling can freeze the liquid into a solid. Understanding these processes is essential for Cambridge IGCSE Chemistry students.

What is the process of sublimation and which substances commonly undergo this change?

Sublimation is the process where a solid changes directly into a gas without passing through the liquid state. This occurs when the particles of a solid gain enough energy to overcome their intermolecular forces and enter the gaseous state. Common substances that undergo sublimation include dry ice (solid carbon dioxide) and iodine. Sublimation is an important concept in the study of states of matter for Cambridge IGCSE Chemistry students.

Why do gases have lower densities compared to solids and liquids?

Gases have lower densities than solids and liquids because their particles are much farther apart. In a gas, particles are spread out and occupy a larger volume, resulting in fewer particles per unit volume compared to solids and liquids, where particles are closely packed. This difference in particle arrangement and spacing is a key concept in understanding the properties of gases in the Cambridge IGCSE Chemistry curriculum.

How do pressure changes affect gases differently than solids and liquids?

Gases are highly compressible, meaning their volume can change significantly with pressure changes. When pressure is applied to a gas, its particles are forced closer together, reducing its volume. In contrast, solids and liquids are much less compressible because their particles are already closely packed. This behavior of gases under pressure is explained by the gas laws, which are an important part of the Cambridge IGCSE Chemistry syllabus.

Why is "Confusing evaporation with boiling" a common mistake in Solids, liquids and gases?

Why it happens: Both are liquid → gas. How to avoid it: Evaporation: surface, any temperature. Boiling: throughout, at the boiling point. Source: 0620/42 — recurring

Why is "Saying particles 'expand' or 'get bigger' when heated" a common mistake in Solids, liquids and gases?

Why it happens: Conflating thermal expansion with particle behaviour. How to avoid it: Particles do NOT change size. They move FASTER and (for solids/liquids) further apart on average.

Why is "Stating 'particles hit the walls more' without 'frequency'" a common mistake in Solids, liquids and gases?

Why it happens: Imprecise language. How to avoid it: Use 'frequency of collisions' AND 'change in momentum at each collision'.

Why is "Using Celsius in $p_{1}V_{1}/T_{1} = p_{2}V_{2}/T_{2}$" a common mistake in Solids, liquids and gases?

Why it happens: Forgetting to convert. How to avoid it: Gas-law T must be in KELVIN: T(K) = (C) + 273.

States of MatterCambridge IGCSE Chemistry 0620 Extended

Solids, liquids and gases

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Solids, Liquids and Gases — Cambridge IGCSE 0620 Chemistry Extended (2026)

Particle arrangement, motion and energy in the three states. Plus the names of the six changes of state and what happens to particles during each.

What you’ll learn

Mapped to the Cambridge IGCSE 0620 syllabus (2026-2028).

1.1 — Describe the arrangement and motion of particles in solids, liquids and gases.
1.1 — Explain changes of state in terms of particle motion and forces.
1.1 — Recall the names of the six changes of state.

Particles in solids, liquids and gases

Spacing and freedom increase from solid to liquid to gas.

Solid.

Particles in a regular, repeating pattern (lattice).
Vibrate about FIXED positions; cannot move past each other.
Strong forces between particles.
Fixed shape AND volume.

Liquid.

Particles still close together but in NO regular pattern.
Move past each other freely (slide / flow).
Weaker forces than in a solid.
Fixed volume; takes the shape of its container.

Gas.

Particles far apart (about $10\times$ liquid spacing).
Move quickly in random directions.
Almost no forces between particles.
No fixed shape OR volume — fills its container completely.

Spacing and freedom of motion increase from solid to liquid to gas.

Cambridge tip. When asked to describe the particles in a state, mention BOTH arrangement AND motion. Many students lose marks by giving only one.

Worked qualitative. A liquid is heated from $20\degree\text{C}$ to $80\degree\text{C}$ . What happens to the particles?

They gain kinetic energy.
They move faster.
They overcome more of the intermolecular forces.
The liquid expands slightly (particles take up more average space).

Solid: regular lattice, fixed positions.
Liquid: close but flowing.
Gas: far apart, fast, random.
Always state arrangement AND motion.

Changes of state

Six changes; sublimation jumps solid→gas directly.

The six changes of state.

From	To	Process	Energy direction
Solid	Liquid	Melting	Energy IN
Liquid	Solid	Freezing (solidifying)	Energy OUT
Liquid	Gas	Boiling / evaporation	Energy IN
Gas	Liquid	Condensation	Energy OUT
Solid	Gas	Sublimation	Energy IN
Gas	Solid	Deposition	Energy OUT

Energy IN to break bonds; energy OUT when bonds re-form.

Six named changes of state; sublimation and deposition skip the liquid stage entirely.

Sublimation examples.

Solid carbon dioxide ("dry ice") sublimes at $-78\degree\text{C}$ at atmospheric pressure.
Iodine sublimes when heated gently — purple vapour rises straight from the crystals.

During a change of state, the temperature stays CONSTANT even though energy continues to flow in (or out). The energy is breaking (or forming) intermolecular forces, NOT speeding up particle motion. Heating curve has FLAT plateaus during melting and boiling.

Cambridge wording.

Melting (solid → liquid).
Freezing OR solidifying (liquid → solid).
Boiling (rapid liquid → gas at boiling point); evaporation (slow surface escape, any temperature below boiling point).
Condensing (gas → liquid).
Sublimation (solid → gas, direct).
Deposition (gas → solid, direct — sometimes still called desublimation in older texts; Cambridge prefers "deposition").

Six changes — memorise names.
Energy IN: melt, boil, sublime.
Energy OUT: freeze, condense, deposit.
Sublimation: solid → gas (e.g. iodine, dry ice).
Constant $T$ during phase change.

How it’s examined

States of matter appear most years on Paper 4 (4-6 marks): describe particle arrangement and motion, identify a phase change, link to a heating curve. Examiner reports flag two errors: (i) saying particles 'stop moving' in a solid (they vibrate), (ii) using sublimation for liquid→gas (that's boiling/evaporation; sublimation skips the liquid).

Sources: Cambridge IGCSE Chemistry 0620 syllabus 2026-2028 (1.1); 0620/42 Oct/Nov 2024 — Q1 (states, changes of state); 0620 Examiner Reports 2022-2024. Last reviewed 2026-05-06.

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Step-by-step worked examples — Solids, liquids and gases

Step-by-step solutions to past-paper-style questions on solids, liquids and gases, written exactly the way a tutor would explain them at the board.

1Compare the three states
Core• states
Question
For each of solid, liquid and gas, state the arrangement of particles, motion, and the strength of forces.
Step-by-step solution
1. Step 1
  Solid: regular close-packed; vibrate in fixed positions; strong forces.
2. Step 2
  Liquid: close but disordered; slide past each other; moderate forces.
3. Step 3
  Gas: far apart, random; fast motion; very weak forces (negligible in ideal gas).
Answer
See descriptions above.
2Name the six changes of state
Core• Adapted from 0620/22 May/Jun 2024 Q1• changes of state
Question
Name each of the six changes of state.
Step-by-step solution
1. Step 1
  Solid → liquid = melting. Liquid → solid = freezing.
2. Step 2
  Liquid → gas = boiling/evaporation. Gas → liquid = condensation.
3. Step 3
  Solid → gas = sublimation. Gas → solid = deposition.
Answer
Melting, freezing, boiling/evaporation, condensation, sublimation, deposition.
3Why heating a sealed gas raises pressure
Extended• explain
Question
Explain in particle terms why the pressure of a gas in a sealed container rises when heated.
Step-by-step solution
1. Step 1
  Heating raises the average kinetic energy of particles → faster motion.
2. Step 2
  More frequent collisions with walls AND each collision transfers more momentum.
3. Step 3
  Force per unit area (pressure) on the walls increases.
Answer
Faster particles → more frequent and more forceful collisions → higher pressure.
Examiner tip
Examiners credit BOTH 'frequency of collisions' AND 'momentum/force per collision'.
4Boyle's law for gases
Extended• pV
Question
A fixed mass of gas occupies $200\,\text{cm}^{3}$ at $1.0 \times 10^{5}\,\text{Pa}$ . The volume is reduced to $50\,\text{cm}^{3}$ at constant temperature. Find the new pressure.
Step-by-step solution
1. Step 1
  $p_{1}V_{1} = p_{2}V_{2}$ at constant $T$ .
  $p_{2} = \dfrac{(1.0 \times 10^{5})(200)}{50} = 4.0 \times 10^{5}\,\text{Pa}$
Answer
$4.0 \times 10^{5}\,\text{Pa}$

Key Formulae — Solids, liquids and gases

The formulae you need to memorise for solids, liquids and gases on the Cambridge IGCSE 0620 paper, with every variable defined in plain English and a note on when to use it.

Boyle's law
$p_{1}V_{1} = p_{2}V_{2}$
When to use
Fixed mass of gas at constant temperature.

Key Definitions and Keywords — Solids, liquids and gases

Definitions to memorise and the exact keywords mark schemes credit for solids, liquids and gases answers — sharpened from recent examiner reports for the 2026 0620 sitting.

Changes of state
Examiner keyword
Melting (s → l), freezing (l → s), evaporation/boiling (l → g), condensation (g → l), sublimation (s → g), deposition (g → s).
Melting point
Examiner keyword
Temperature at which a pure solid melts to form a liquid. Sharp for pure substances; range for mixtures.
Evaporation
Examiner keyword
Liquid → gas at the surface, at any temperature below boiling. Cools the remaining liquid.
Boiling
Examiner keyword
Liquid → gas throughout the bulk at a fixed temperature (the boiling point).
Kinetic-particle model
Examiner keyword
All matter is made of particles in constant motion; explains state changes, diffusion and gas pressure.

Common Mistakes and Misconceptions — Solids, liquids and gases

The traps other students keep falling into on solids, liquids and gases questions — taken from recent Cambridge IGCSE 0620 examiner reports and mark schemes — and how to avoid them.

✕Confusing evaporation with boiling
0620/42 — recurring
Why it happens
Both are liquid → gas.
How to avoid it
Evaporation: surface, any temperature. Boiling: throughout, at the boiling point.
✕Saying particles 'expand' or 'get bigger' when heated
Why it happens
Conflating thermal expansion with particle behaviour.
How to avoid it
Particles do NOT change size. They move FASTER and (for solids/liquids) further apart on average.
✕Stating 'particles hit the walls more' without 'frequency'
Why it happens
Imprecise language.
How to avoid it
Use 'frequency of collisions' AND 'change in momentum at each collision'.
✕Using Celsius in $p_{1}V_{1}/T_{1} = p_{2}V_{2}/T_{2}$
Why it happens
Forgetting to convert.
How to avoid it
Gas-law $T$ must be in KELVIN: $T(\text{K}) = \theta(\degree\text{C}) + 273$ .

Practice questions

Exam-style questions with step-by-step worked solutions. Try one before checking the method.

Past paper style quiz

Get a report showing which sub-topics you've nailed and which ones still need work.

Video lesson

Short walkthrough of the concepts students most often get stuck on.

Solids, liquids and gases — frequently asked questions

The things students keep getting wrong in this sub-topic, answered.

Solid.

Particles in a regular, repeating pattern (lattice).
Vibrate about FIXED positions; cannot move past each other.
Strong forces between particles.
Fixed shape AND volume.

Liquid.

Particles still close together but in NO regular pattern.
Move past each other freely (slide / flow).
Weaker forces than in a solid.
Fixed volume; takes the shape of its container.

Gas.

Particles far apart (about $10\times$ liquid spacing).
Move quickly in random directions.
Almost no forces between particles.
No fixed shape OR volume — fills its container completely.

Spacing and freedom of motion increase from solid to liquid to gas.

Cambridge tip. When asked to describe the particles in a state, mention BOTH arrangement AND motion. Many students lose marks by giving only one.

Worked qualitative. A liquid is heated from $20\degree\text{C}$ to $80\degree\text{C}$ . What happens to the particles?

They gain kinetic energy.
They move faster.
They overcome more of the intermolecular forces.
The liquid expands slightly (particles take up more average space).

From

Process

Energy direction

Solid

Liquid

Melting

Energy IN

Liquid

Solid

Freezing (solidifying)

Energy OUT

Liquid

Gas

Boiling / evaporation

Energy IN

Gas

Liquid

Condensation

Energy OUT

Solid

Gas

Sublimation

Energy IN

Gas

Solid

Deposition

Energy OUT

Solids, liquids and gases

Short Study Notes in the form of Slides

Short Study Notes — Solids, liquids and gases

Detailed Notes

What you’ll learn

How it’s examined

1Compare the three states

2Name the six changes of state

3Why heating a sealed gas raises pressure

4Boyle's law for gases

Boyle's law

Changes of state

Melting point

Evaporation

Boiling

Kinetic-particle model

✕Confusing evaporation with boiling

✕Saying particles 'expand' or 'get bigger' when heated

✕Stating 'particles hit the walls more' without 'frequency'

✕Using Celsius in p1V1/T1=p2V2/T2p_{1}V_{1}/T_{1} = p_{2}V_{2}/T_{2}p1​V1​/T1​=p2​V2​/T2​

Practice questions

Past paper style quiz

Assess your understanding

Video lesson

Solids, liquids and gases — frequently asked questions

Solids, liquids and gases

Short Study Notes in the form of Slides

Short Study Notes — Solids, liquids and gases

Detailed Notes

What you’ll learn

How it’s examined

1Compare the three states

2Name the six changes of state

3Why heating a sealed gas raises pressure

4Boyle's law for gases

Boyle's law

Changes of state

Melting point

Evaporation

Boiling

Kinetic-particle model

✕Confusing evaporation with boiling

✕Saying particles 'expand' or 'get bigger' when heated

✕Stating 'particles hit the walls more' without 'frequency'

✕Using Celsius in p1V1/T1=p2V2/T2p_{1}V_{1}/T_{1} = p_{2}V_{2}/T_{2}p1​V1​/T1​=p2​V2​/T2​

Practice questions

Past paper style quiz

Assess your understanding

Video lesson

Solids, liquids and gases — frequently asked questions

✕Using Celsius in $p_{1}V_{1}/T_{1} = p_{2}V_{2}/T_{2}$

✕Using Celsius in $p_{1}V_{1}/T_{1} = p_{2}V_{2}/T_{2}$