What is matter?

Air, water and a metal coin look very different, but all three have mass and take up space. They are matter. Chemistry explains their properties by looking at the particles they contain.

Define matter

Matter is anything that has mass and occupies space (has volume).

Key ideas

• Matter is made of tiny particles: atoms, molecules, or ions.
• Solids, liquids, and gases are different states of matter with different particle arrangements.
• State symbols distinguish solids (s), liquids (l) and gases (g). The symbol (aq) means dissolved in water, rather than a fourth state of matter.
• Light transfers energy; heat is energy transferred because of a temperature difference. Neither is a substance.

Develop the particle explanation

Matter has mass and volume

Names for particles

We use three useful names for the particles in matter:

• An atom is the smallest particle of an element that can take part in a chemical reaction.
• A molecule is a separate group of two or more chemically joined atoms with no overall charge, such as O₂ or H₂O.
• An ion is an atom or group of atoms with an overall electric charge. For a single atom, losing electrons forms a positive ion; gaining electrons forms a negative ion.

For the structure of atoms (protons, neutrons, electrons), see Atomic Structure.

Compare solids, liquids and gases

The state depends on particle arrangement, movement and attractions between particles. Develop the explanation in Kinetic Particle Theory and Changes of State.

Crystalline solid

Motion: fixed positions; vibrate

Property: fixed shape and volume

Liquid

Motion: move past one another

Property: fixed volume; takes container shape

Gas

Motion: rapid random motion

Property: fills container; compressible

Basic comparison:

For a given substance, the solid and liquid usually have much higher densities than its gas. Do not assume that every solid is denser than its liquid: ice is less dense than liquid water. Shape and volume in the table describe a sample at unchanged temperature and pressure.

Read state symbols

State symbols show whether a substance is solid, liquid, gaseous or dissolved in water:

• (s): solid
• (l): liquid
• (g): gas
• (aq): aqueous (dissolved in water)

Example: NaCl(s) → Na + (aq) + Cl-(aq).

Misconceptions to check

• Saying “heat is matter” or “light is matter”. They are energy, not substances.
• Mixing up atom and molecule (e.g., calling O₂ an atom).
• Writing “(aq) means liquid”. Wrong: (aq) means dissolved in water.
• Forgetting that gases have volume (they occupy the whole container) so they are matter.

Build a clear explanation

Worked examples

Modelled example 1

Distinguishing Particles

Core

Problem

A sample of gas consists of pairs of oxygen atoms joined together. Classify the particles as atoms, ions or molecules.

Study the worked solution

1. Identify what each particle contains

   Method

   Recognise that each particle contains two oxygen atoms joined together.

   Reason

   The number of atoms and whether they are chemically joined determine whether the particle is a single atom or a molecule.

   Working

   Each particle is represented by O₂, not a single O atom.

2. Apply the particle definition

   Method

   Classify each pair as a molecule.

   Reason

   A molecule is a separate, uncharged group of two or more chemically joined atoms.

   Working

   O₂ is a molecule of the element oxygen.

3. Exclude the nearby categories

   Reason

   The particle contains more than one atom and no charge is shown.

   Working

   It is neither a single atom nor an ion.

Guided practice 2

Matter or Not Matter

About 4 min

Problem

A student says, “Air is not matter because you cannot see it.” Explain why this is wrong using the definition of matter.

Apply both parts of the definition

Decisive reason

Air has mass and occupies spaceAir is sometimes visibleAir is a form of energy

Check my attempt

Write the corrected explanation

Hints

Hint 1: recall the definition

Matter is tested by mass and occupied space, not by visibility.

Hint 2: use observable evidence

A balloon shows that air takes up space. Think about adding air to a rigid container whose outside volume stays the same to test its mass.

View solution step by step

1. Replace the incorrect test

   Method

   Ignore whether air can be seen and apply the definition of matter.

   Reason

   Visibility is not a defining property of matter.

   Working

   The required tests are mass and volume.

2. Apply both tests

   Method

   State that air has mass and occupies space.

   Reason

   Air fills a balloon, showing that it occupies space. Adding air to a sealed, rigid container increases the mass measured on a balance without changing the container’s outside volume.

   Working

   Therefore, air is matter even though it is invisible.

Common misconception 3

Compressibility

Find and correct the mistake

Learner response

A student explains that gases can be compressed easily because gas particles are smaller than solid particles. Locate the first error and replace it with a particle-arrangement explanation.

Choose the property that changes during compression

Correct explanation

Gas particles are smallerGas particles have much larger spaces between themGas particles always move faster

Check my attempt

View solution step by step

1. Locate the first error

   Method

   Reject particle size as the cause of different compressibility.

   Reason

   The particle model explains state properties through arrangement, spacing and motion; a substance’s particles do not become smaller merely because it is gaseous.

   Working

   The wrong response changes particle size instead of particle spacing.

2. Compare the available space

   Method

   Contrast widely spaced gas particles with closely packed solid particles.

   Reason

   Compression pushes particles closer by reducing the empty space between them.

   Working

   Gases have substantial empty space to reduce; solids have very little.

3. State the corrected conclusion

   Working

   Gases are easily compressed because their particles are far apart, while solids are not easily compressed because their particles are already closely packed.

Examiner practice 4

State Symbols

2 marks

Examination question

State what (aq) means in a chemical equation and give one correctly written example. [2 marks]

Define the symbol and write a formula

Write your two-mark answer

View solution step by step

1. Define the state symbol

   1 mark

   Method

   State that the substance is dissolved in water.

   Reason

   Aqueous describes a solute in water; it is not another label for a pure liquid.

   Working

   (aq) means aqueous, or dissolved in water.

2. Write a valid example

   1 mark

   Working

   HCl(aq) represents hydrogen chloride dissolved in water.

Self-mark with the mark scheme

Compare your response with each mark point. Select a point only when your response contains that evidence.

Self-mark the definition and the correctly written state-symbol example separately.

Mark points

1. 1 mark: States that (aq) means dissolved in water or aqueous solution.
2. 1 mark: Gives a chemically valid formula carrying the (aq) state symbol.

Calculate my mark

Challenge 5

Molecule vs Ion

Minimal support

Symbolic classification transfer

Classify each particle as an atom, molecule or ion: He, H₂O and Na⁺.

Use both atom count and charge

He

AtomMoleculeIon

H₂O

AtomMoleculeIon

Na⁺

AtomMoleculeIon

Check my attempt

Hints

Hint 1: look for charge first

A displayed positive or negative charge identifies an ion.

Hint 2: count joined atoms next

Helium consists of single neutral atoms; water consists of separate groups of joined atoms. Use what you know about each substance’s structure.

View solution step by step

1. Classify helium

   Method

   Identify He as an atom.

   Reason

   It is one neutral helium particle represented by a single element symbol.

   Working

   He: atom.

2. Classify water

   Method

   Identify H₂O as a molecule.

   Reason

   Each water particle is a separate, uncharged group of chemically joined hydrogen and oxygen atoms.

   Working

   H₂O: molecule.

3. Classify the sodium particle

   Method

   Identify Na⁺ as an ion.

   Reason

   The positive charge shows that it is a charged particle.

   Working

   Na⁺: ion.

Next step

Try Mind Stretcher 1 independently.

Try these independently

Mind stretcher 1: “Occupies Space” ExplanationExtension

Question: A student claims “a gas has no volume because it spreads out”. Explain why spreading out does not mean it has no volume.

Show Answer

A gas does have volume: it occupies space and fills its container. That is why it is matter.

Mind stretcher 2: Classification TrapExtension

Question: Solid sodium chloride contains a regular arrangement of oppositely charged particles. The particles become mobile when it melts, and the molten substance conducts electricity. Are these particles molecules, neutral atoms or ions? Explain.

Show Answer

It contains ions: positively charged sodium ions and negatively charged chloride ions. Neutral atoms and molecules have no overall charge. When molten, the ions can move and carry electric charge. You will develop this explanation in Ionic bonding.

Practise and check