Solids, Liquids and GasesCambridge IGCSE Chemistry: Revision notes
Section 1
What are the distinguishing properties of solids, liquids and gases?
The three states of matter have clearly distinct physical properties that allow us to identify and classify them:
| Property | Solids | Liquids | Gases |
|---|---|---|---|
| Shape | Fixed shape; maintains own shape | Takes shape of container | Takes shape of container |
| Volume | Fixed volume | Fixed volume | No fixed volume; expands to fill container |
| Density | High density | High density (slightly less than solids) | Very low density |
| Compressibility | Cannot be compressed | Cannot be compressed (incompressible) | Easily compressed |
| Particle motion | Vibrate in fixed positions | Move freely throughout liquid | Move rapidly and randomly |
| Surface tension | N/A | Present (molecules attracted to each other) | Absent |
These properties directly result from how the particles are arranged and how much energy they possess. Understanding these distinguishing features is essential for identifying unknown substances and predicting their behaviour.
Examiners often ask you to identify which state of matter is being described. Always justify your answer using at least two properties (e.g. 'It is a gas because it has no fixed shape AND no fixed volume').
Section 2
How do particle structure and arrangement differ between states of matter?
The kinetic particle theory explains the properties of each state by describing how particles are arranged and how they move:
Solids:
- Particles are tightly packed in a fixed, regular arrangement (crystalline structure)
- Particles are held in place by strong forces of attraction
- Particles vibrate about fixed positions but do not move from place to place
- Very little space between particles
Liquids:
- Particles are loosely packed and in random arrangement
- Particles are held together by forces of attraction, but these are weaker than in solids
- Particles move freely throughout the liquid, sliding past one another
- More space between particles than in solids, but still close together
Gases:
- Particles are very far apart with random arrangement
- Negligible forces of attraction between particles (we can usually ignore them)
- Particles move rapidly, randomly and continuously in all directions
- Most of the volume occupied by a gas is empty space
The degree of particle separation and freedom of movement directly determines the physical properties observed (shape, volume, compressibility).
Think of particles in a solid as dancers who hold hands in a fixed formation, liquid particles as a crowd moving around a room, and gas particles as birds flying randomly throughout the sky.
Students often say gas particles have 'no forces of attraction' between them. Technically, forces do exist but are negligible compared to kinetic energy, so we ignore them in calculations.
Section 3
What happens during changes of state?
Changes of state occur when energy is supplied to or removed from a substance, causing particles to gain or lose kinetic energy. There are five key changes of state:
Melting: Solid → Liquid
- Heat energy is supplied to the solid
- Particles vibrate more vigorously until forces of attraction are broken
- Fixed structure breaks down; particles can now move freely
- Temperature remains constant during melting (at the melting point)
Freezing: Liquid → Solid
- Heat energy is removed from the liquid
- Particles slow down and settle into fixed positions
- Forces of attraction re-establish a regular structure
- Temperature remains constant during freezing (at the freezing point)
Boiling: Liquid → Gas (at a specific temperature)
- Heat energy is supplied rapidly
- Particles gain enough energy to overcome all forces of attraction
- Particles escape from the liquid as gas throughout the liquid (bubbles form)
- Temperature remains constant during boiling (at the boiling point)
Evaporation: Liquid → Gas (at any temperature)
- Only the most energetic particles at the surface escape
- Occurs continuously at any temperature, not just at boiling point
- Slower than boiling and occurs only from the surface
Condensing: Gas → Liquid
- Heat energy is removed from the gas
- Particles slow down and forces of attraction re-establish
- Particles form a liquid as they come closer together
- Temperature remains constant during condensation (at the condensation point)
All these changes are reversible — the substance can return to its original state if energy is supplied or removed appropriately.
Ice at –10°C is heated. The temperature rises to 0°C (solid warming). At 0°C, the ice melts but temperature stays at 0°C until all ice becomes water. Then temperature rises again to 100°C. At 100°C, water boils; temperature stays at 100°C during boiling.
When answering about changes of state, always specify the direction (e.g. 'melting is solid to liquid') and explain at the particle level using energy and forces of attraction.
Section 4
How do heating and cooling curves show changes of state?
Heating curves (and cooling curves) graphically show how temperature changes as a substance is heated uniformly. The curve has distinct features that reveal what is happening at the particle level:
On a heating curve:
- Upward sloping section (solid warming): Temperature increases steadily; particles vibrate more vigorously but remain in fixed positions
- Horizontal plateau (melting): Temperature remains constant; all energy goes into breaking forces of attraction and changing structure from solid to liquid
- Upward sloping section (liquid warming): Temperature increases again; liquid particles move faster and further apart
- Horizontal plateau (boiling): Temperature remains constant; all energy goes into overcoming forces of attraction as particles escape to gas phase
- Upward sloping section (gas warming): Temperature increases; gas particles move even more rapidly
Key observations:
- The length of each plateau indicates the amount of energy required for that change of state (latent heat)
- Melting and freezing occur at the same temperature (melting point = freezing point)
- Boiling and condensation occur at the same temperature (boiling point = condensation point)
- The slope of the sloping sections relates to the specific heat capacity of the substance
Cooling curves show the reverse process: temperature decreases, with the same horizontal plateaus at freezing and condensation, but in reverse order.
Examiners frequently ask you to interpret heating curves. Always explain horizontal sections in terms of energy breaking forces of attraction, and sloping sections in terms of particles gaining kinetic energy.
Students often think temperature continues to rise during melting or boiling. Remember: during a change of state, temperature is constant even though energy is being added; all energy goes into changing state, not temperature.
Section 5
How do temperature and pressure affect gas volume?
The volume of a gas changes predictably when temperature or pressure changes. These relationships are explained by the kinetic particle theory:
Temperature and volume (at constant pressure):
- When temperature increases, gas volume increases
- When temperature decreases, gas volume decreases
- This relationship is directly proportional (if absolute temperature doubles, volume doubles)
- This is known as Charles's Law: V ∝ T (at constant pressure)
Explanation: Higher temperature means particles move faster and with more kinetic energy. They strike container walls more frequently and with greater force, pushing outward. The volume must increase to allow these more energetic particles to move about.
Pressure and volume (at constant temperature):
- When pressure increases, gas volume decreases
- When pressure decreases, gas volume increases
- This relationship is inversely proportional (if pressure doubles, volume halves)
- This is known as Boyle's Law: P ∝ 1/V (at constant temperature)
Explanation: Increasing pressure pushes particles closer together. Since temperature is constant, particles have the same average kinetic energy, but in a smaller space. The volume must decrease to accommodate the increased external pressure.
Both relationships combined can be expressed as: PV ∝ T or PV = kT (where k is a constant)
These relationships only apply to ideal gases — real gases follow these laws closely at room temperature and pressure, but deviate at very high pressures or very low temperatures.
A balloon contains 1 litre of gas at 20°C and 1 atm pressure. If heated to 40°C at constant pressure, volume increases to approximately 1.07 litres (using absolute temperature: 293K to 313K). If instead compressed to 2 atm at 20°C, volume decreases to 0.5 litres.
Always use absolute temperature (Kelvin) when calculating gas problems with Charles's Law. Convert °C to K by adding 273.
Must Know
- Three states of matter have distinct properties: solids have fixed shape and volume; liquids have fixed volume but variable shape; gases have neither fixed shape nor volume and are easily compressed
- Particle theory explains properties: solids have tightly packed particles in fixed positions; liquids have loosely packed particles that move freely; gases have very far apart particles moving rapidly and randomly
- Changes of state are reversible: melting/freezing (solid ↔ liquid), boiling/condensation (liquid ↔ gas), and evaporation (gradual surface evaporation at any temperature)
- During changes of state, temperature remains constant because energy goes into breaking or forming forces of attraction, not into increasing kinetic energy
- Heating and cooling curves show plateaus (horizontal sections) at melting and boiling points; the length of these plateaus indicates the latent heat required
- Gas volume increases with temperature (Charles's Law: V ∝ T) and decreases with pressure (Boyle's Law: P ∝ 1/V) because particle kinetic energy and collision frequency change
That's the notes covered.
Carry on to the next subtopic.