Bonding, structure and properties of matter: states of matter and changes of state

What you'll learn

This topic examines how particles behave in solids, liquids and gases, and what happens during state changes. You'll explore the arrangement and movement of particles in each state, understand energy changes during heating and cooling, and interpret state change diagrams. These concepts underpin much of physical chemistry and link directly to topics like separating mixtures and energy calculations.

Key terms and definitions

State of matter — the physical form a substance takes (solid, liquid or gas), determined by the arrangement and movement of its particles

Melting point — the temperature at which a solid changes to a liquid; a pure substance has a sharp, specific melting point

Boiling point — the temperature at which a liquid changes to a gas throughout the bulk of the liquid, not just at the surface

Sublimation — the direct change from solid to gas without passing through the liquid state (e.g. solid carbon dioxide/dry ice)

Latent heat — the energy required to change state without changing temperature; energy is used to overcome forces between particles rather than increase kinetic energy

Intermolecular forces — forces of attraction between molecules; these must be overcome during melting and boiling

Kinetic energy — the energy associated with movement; particles in all states possess kinetic energy, which increases with temperature

Physical change — a change in state where no new substances are formed and the change is reversible (e.g. melting ice)

Core concepts

The three states of matter: particle arrangement and movement

The behaviour of any substance depends on how its particles are arranged and how freely they can move.

Solids:

Particles are arranged in a regular, fixed pattern
Particles vibrate about fixed positions but cannot move from place to place
Strong forces of attraction hold particles close together
Solids have a fixed shape and volume
Cannot be compressed (particles already tightly packed)
High density compared to gases

Liquids:

Particles are close together but can move past one another
No fixed arrangement; particles are randomly distributed
Moderate forces of attraction between particles
Fixed volume but no fixed shape (takes the shape of its container)
Virtually incompressible
Density usually slightly lower than the solid form (water is an exception)

Gases:

Particles are far apart with no regular arrangement
Particles move rapidly and randomly in all directions
Very weak forces of attraction between particles (negligible at GCSE level)
No fixed shape or volume; gases fill their container completely
Easily compressed
Low density compared to solids and liquids

Changes of state and energy transfer

State changes are physical changes because no new chemical substances are formed. The process is reversible, and the substance retains its chemical properties.

The six state changes:

Melting (solid → liquid): particles gain enough energy to overcome some forces of attraction and begin moving past each other
Freezing (liquid → solid): particles lose energy; forces of attraction pull particles into fixed positions
Boiling/evaporation (liquid → gas): particles gain sufficient energy to overcome all intermolecular forces completely
Condensing (gas → liquid): particles lose energy; forces of attraction pull particles closer together
Sublimation (solid → gas): particles gain enough energy to overcome all forces without passing through liquid state
Deposition (gas → solid): the reverse of sublimation

Energy considerations:

When a substance is heated:

Temperature rises as particles gain kinetic energy and move faster
At the melting or boiling point, temperature remains constant
During the state change, energy supplied breaks intermolecular forces rather than increasing particle speed
Once the state change completes, temperature rises again

When a substance cools:

Temperature falls as particles lose kinetic energy
At boiling or melting point, temperature plateaus
Energy is released as new intermolecular forces form
Temperature resumes falling once the state change completes

State change graphs and interpretation

A heating curve shows how temperature changes as a substance is continuously heated.

Key features of heating curves:

Sloping sections:

Temperature increases
Kinetic energy of particles increases
Particles move faster
No state change occurring
Steeper gradient indicates substance heats quickly (low specific heat capacity)

Horizontal sections (plateaus):

Temperature remains constant despite continued heating
Energy is being used to overcome intermolecular forces
State change is occurring
First plateau = melting point
Second plateau = boiling point
Length of plateau relates to the strength of intermolecular forces

Cooling curves show the reverse process with the same principles:

Horizontal sections occur at the same temperatures (boiling and melting points)
Energy is released as intermolecular forces form
Temperature only decreases when state change is complete

Limitations of the simple particle model

The particle model used at GCSE level has limitations that you should recognise:

No forces shown: the model doesn't explicitly show the forces between particles, though these are crucial for determining state and properties
Particle size: diagrams show particles as solid spheres, but real particles (atoms, molecules, ions) have different sizes and the space between them is much larger than shown in proportion
Perfect spheres: real particles aren't perfect spheres — molecules have specific shapes determined by their bonding
No representation of particle movement: static diagrams cannot show the constant, random motion of particles

Despite these limitations, the model successfully explains and predicts the properties of matter in different states and behaviour during state changes.

Factors affecting melting and boiling points

Different substances have different melting and boiling points because of variations in the strength of forces between particles.

Stronger intermolecular forces:

Higher melting and boiling points
More energy needed to overcome forces
Examples: water has relatively high boiling point due to hydrogen bonding

Weaker intermolecular forces:

Lower melting and boiling points
Less energy needed for state change
Examples: noble gases have very low boiling points

Pure substances vs mixtures:

Pure substances melt and boil at specific, sharp temperatures
Impure substances and mixtures melt over a range of temperatures
This principle is used to test purity in practical chemistry

Predicting and explaining physical properties

Using particle theory, you can predict and explain observable properties:

Why solids maintain their shape: Strong forces hold particles in fixed positions; particles can only vibrate, not move past each other.

Why liquids flow: Forces between particles are strong enough to keep particles close but weak enough to allow movement past each other.

Why gases fill containers: Very weak forces between widely spaced particles; particles move independently in all directions.

Why gases are easily compressed: Large spaces between particles can be reduced; in solids and liquids, particles are already close together.

Why heating causes expansion: Particles gain kinetic energy and move faster, requiring more space; average separation between particles increases.

Worked examples

Example 1: Interpreting a heating curve

Question: A student heated a pure substance and plotted temperature against time. The graph showed temperature rising from 20°C to 80°C, then remaining constant at 80°C for 3 minutes, then rising again to 120°C, where it remained constant for 5 minutes before rising further.

(a) What is the melting point of this substance? [1 mark]

(b) What is the boiling point of this substance? [1 mark]

Answers:

(a) 80°C [1]

(b) 120°C [1]

(c) Energy supplied is being used to overcome forces of attraction [1] between the particles/molecules [1] rather than increasing the kinetic energy of the particles [1].

Alternative wording: "to break intermolecular forces" would also gain the first mark.

Example 2: Describing particle behaviour

Question: Describe the arrangement and movement of particles in liquid water and in water vapour (steam). [4 marks]

Answer:

In liquid water: particles are close together [1] with a random arrangement/no fixed pattern [1]; particles can move past each other [1].

In water vapour: particles are far apart [1] with a random arrangement; particles move rapidly in all directions [1].

Mark scheme notes: Need clear comparison. "Touch each other" insufficient for "close together". Must specify random arrangement for both states.

Example 3: Application to Caribbean context

Question: In Trinidad, naphthalene (mothballs) is used to protect clothes. Naphthalene has a melting point of 80°C. At room temperature (28°C), naphthalene is a solid, yet mothballs gradually disappear without leaving liquid residue.

(a) What process causes the mothballs to disappear? [1 mark]

(b) Explain this process in terms of particles and energy. [3 marks]

Answers:

(a) Sublimation [1]

Accept: "the solid changes directly to gas"

(b) Particles on the surface of the solid [1] gain sufficient energy (from surroundings/room temperature) [1] to overcome all the forces holding them in the solid and escape as gas particles [1].

Must reference: surface particles, gaining energy, and overcoming forces.

Common mistakes and how to avoid them

Confusing melting and dissolving: melting is a state change (ice to water); dissolving is when a solute mixes with a solvent. Melting doesn't require a second substance. Always check the context — does the question mention adding water or heating?
Thinking particles expand when heated: individual particles don't change size when heated. They move faster and the space between particles increases. Write "particles move further apart" not "particles expand."
Stating particles "gain heat": particles gain energy, not heat. Heat is the transfer of energy. Use precise terminology: "particles gain kinetic energy" or "energy is transferred to the particles."
Forgetting particles move in solids: all particles move. Solid particles vibrate in fixed positions — they don't become stationary. This is a common error in describing solids.
Misidentifying state change plateaus: the first (lower temperature) plateau is always melting; the second (higher temperature) plateau is always boiling. Don't confuse these — temperature alone indicates which process occurs.
Ignoring the significance of constant temperature: when temperature remains constant during state change, explicitly state that energy is overcoming intermolecular forces. This is worth marks and demonstrates understanding of latent heat.

Exam technique for "Bonding, structure and properties of matter: states of matter and changes of state"

"Describe" questions: provide observable characteristics without explanation. For particle arrangement, state whether regular/random, close/far apart, and how they move. Expect 1 mark per distinct point. Example: "Describe the arrangement of particles in a solid" requires at least two points (regular pattern, close together).
"Explain" questions: link observations to particle theory using causal language ("because," "therefore," "so"). Always reference forces between particles and energy changes. These are typically worth 2-3 marks. Structure: state what happens + explain why in particle terms + link to forces/energy.
Graph interpretation questions: identify plateaus as state changes, read values from axes accurately, and describe what's happening during different sections. Don't just read values — explain their significance (e.g., "80°C is the melting point because temperature is constant while state changes").
Extended response (6 marks): organise answers by state or by feature (arrangement, then movement, then forces). Use comparison language ("whereas," "in contrast," "however"). Cover all three states systematically. Quality of written communication is assessed — use scientific terminology correctly and write in clear sentences.

Quick revision summary

Matter exists as solid, liquid or gas depending on particle arrangement and movement. Solids have fixed, regular arrangement with vibrating particles; liquids have particles close together moving past each other; gases have widely-spaced, rapidly-moving particles. State changes are physical changes requiring energy to overcome intermolecular forces. During state changes, temperature remains constant as energy breaks or forms forces rather than changing kinetic energy. Heating curves show these plateaus at melting and boiling points. Stronger intermolecular forces result in higher melting and boiling points.