What Is Matter?
Matter: made of atoms, molecules or ions, and the particle-model properties of solids, liquids and gases (shape, volume, compressibility).
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The core idea
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Learning objectives
- describe the solid, liquid and gaseous states of matter and explain their interconversion in terms of the kinetic particle theory and of the energy changes involved.
Chemistry starts with one filter: is it matter or not? If it has mass and takes up space, it is matter. If it does not (like light), it is not.
1. Definition
Matter is anything that has mass and occupies space (has volume).
2. 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 in equations tell you the state: (s), (l), (g), (aq).
- Do not confuse matter with energy (e.g., light/heat).
3. Detailed Explanations
- Matter has mass and occupies space (volume).
- Matter particles can be atoms, molecules, or ions.
- State symbols: (s), (l), (g), (aq) where (aq) = dissolved in water.
A. Matter vs Not Matter
| Example | Matter? | Reason |
|---|---|---|
| Air in a syringe | Yes | Has mass and occupies space |
| Water | Yes | Has mass and occupies space |
| A metal coin | Yes | Has mass and occupies space |
| Light from a torch | No | It is energy, not a substance |
| Heat | No | It is energy transferred, not a substance |
B. Particles in Matter
Matter is made up of tiny discrete particles:
- An atom is the smallest particle of an element that can take part in a chemical reaction.
- A molecule is two or more atoms chemically joined together (e.g., O₂, H₂O).
- An ion is a charged particle formed when an atom (or group of atoms) loses or gains electrons.
An electron is a tiny particle with a negative charge. Losing electrons makes a positive ion; gaining electrons makes a negative ion.
For the structure of atoms (protons, neutrons, electrons), see Atomic Structure.
C. States of Matter (Solids, Liquids, Gases)
The state depends on how the particles are arranged and how strongly they attract each other. The particle explanation is covered properly in Kinetic Particle Theory and Changes of State.
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:
| Property | Solid | Liquid | Gas |
|---|---|---|---|
| Shape | Fixed | Not fixed (takes the shape of the container) | Not fixed (takes the shape of the container) |
| Volume | Fixed | Fixed | Not fixed (occupies the entire container) |
| Density | High | Moderate to High | Low |
| Compressibility | Incompressible | Almost incompressible | Highly compressible |
D. State Symbols in Equations
State symbols are part of mark-scheme chemistry language:
- (s): solid
- (l): liquid
- (g): gas
- (aq): aqueous (dissolved in water)
Example: NaCl(s) → Na + (aq) + Cl-(aq).
4. Common Mistakes
- 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.
5. Exam Tips
If asked “What is matter?”, write: “Matter has mass and occupies space (has volume).”
In equations, include state symbols when asked. “Aqueous” must be written as (aq), not “(l)”.
6. Worked Examples
Modelled example 1
Distinguishing Particles
Problem
Study the worked solution
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.Apply the particle definition
Method
Classify each pair as a molecule.Reason
A molecule contains two or more atoms chemically joined together.Working
O₂ is a molecule of the element oxygen.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
Problem
Apply both parts of the definition
Hints
Hint 1: recall the definition
Hint 2: use observable evidence
View solution step by step
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.Apply both tests
Method
State that air has mass and occupies space.Reason
Trapped air fills a balloon and contributes to its measured mass.Working
Therefore, air is matter even though it is invisible.
Common misconception 3
Compressibility
Learner response
Choose the property that changes during compression
View solution step by step
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.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.State the corrected conclusion
Working
Gases are easily compressed because their particles are far apart, while solids are not because their particles are already closely packed.
Examiner practice 4
State Symbols
Examination question
Define the symbol and write a formula
View solution step by step
Define the state symbol
1 markMethod
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.Write a valid example
1 markWorking
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.
Challenge 5
Molecule vs Ion
Symbolic classification transfer
Use both atom count and charge
Hints
Hint 1: look for charge first
Hint 2: count joined atoms next
View solution step by step
Classify helium
Method
Identify He as an atom.Reason
It is one neutral helium particle represented by a single element symbol.Working
He: atom.Classify water
Method
Identify H₂O as a molecule.Reason
Its hydrogen and oxygen atoms are chemically joined and no overall charge is shown.Working
H₂O: molecule.Classify the sodium particle
Method
Identify Na⁺ as an ion.Reason
The positive charge shows that it is a charged particle.Working
Na⁺: ion.
7. Mind Stretchers
Mind stretcher 1: “Occupies Space” ExplanationExtension
Question: A student claims “a gas has no volume because it spreads out”. Correct this using one mark-scheme point.
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: A substance is made of positive and negative particles arranged in a lattice and it conducts electricity when molten. Is it made of molecules, atoms, or ions? Explain.
Show Answer
It is made of ions. A lattice of positive and negative particles describes an ionic compound, and conduction when molten is due to mobile ions.
8. Quiz
Ready to check your understanding? Try the interactive quiz, then review any questions you missed.