States of Matter

The One Thing

The properties of solids, liquids and gases, together with changes of state and diffusion, are explained by the spacing, arrangement, movement and energy of their particles. Heating or cooling changes particle energy and may alter the state without forming a new substance.

Reactions, Processes and Experiments

Key Terms

• Particle model: Matter is made of tiny particles that are constantly moving; their spacing, arrangement and movement differ in solids, liquids and gases.
• Solid: A state with a fixed shape and fixed volume. Its particles are closely packed in an ordered arrangement and vibrate about fixed positions.
• Liquid: A state with a fixed volume but no fixed shape. Its particles are close together but can move past one another.
• Gas: A state with no fixed shape or volume. Its particles are far apart, move rapidly and randomly, and are easily compressed.
• Kinetic energy: The energy associated with particle movement. Increasing temperature generally increases the average kinetic energy of particles.
• Melting: The change from solid to liquid when particles gain enough energy to move out of their fixed positions.
• Freezing: The change from liquid to solid when particles lose energy and become arranged in fixed positions.
• Boiling: The change from liquid to gas throughout the liquid at a specific temperature called the boiling point.
• Evaporation: The change from liquid to gas at the surface of a liquid, occurring at temperatures below the boiling point.
• Condensation: The change from gas to liquid when particles lose energy and come closer together.
• Sublimation: The direct change from solid to gas without passing through the liquid state.
• Deposition: The direct change from gas to solid without passing through the liquid state.
• Diffusion: The net movement of particles from a region of higher concentration to a region of lower concentration due to random motion.
• Concentration gradient: The difference in concentration between two regions; a larger gradient usually produces faster diffusion.
• Brownian motion: The irregular movement of visible particles caused by collisions with rapidly moving invisible particles.
• Pressure: The force per unit area caused by gas particles colliding with the walls of their container.
• Latent heat: Energy absorbed or released during a change of state without a change in temperature.
• Density: Mass per unit volume, calculated using density = mass / volume, or ρ = m/V.
• Absolute zero: 0 K, equivalent to approximately −273 °C; particles still have quantum motion, but this is the lowest theoretical temperature.
• Boiling point: The specific temperature at which boiling occurs; boiling occurs when vapour pressure equals external pressure.
• Heating or cooling curve: A graph showing temperature against time. Flat sections represent changes of state, while sloping sections represent temperature changes within one state.

Easily Confused

• Boiling and evaporation: Boiling occurs throughout a liquid at a specific temperature, whereas evaporation occurs only at the surface and can occur below the boiling point.
• Melting and dissolution: Melting is a change of state from solid to liquid; no dissolution process is described here.
• Diffusion and Brownian motion: Diffusion is the net movement from higher to lower concentration, whereas Brownian motion is the irregular movement of visible particles caused by collisions.
• Temperature and latent heat: Temperature measures average kinetic energy, whereas latent heat changes particle separation and attractive forces during a change of state without changing temperature.
• Solid, liquid and gas: Solids have fixed shape and volume, liquids have fixed volume but no fixed shape, and gases have neither fixed shape nor fixed volume.
• Pressure and force: Pressure is force per unit area, given by pressure = force / area, or p = F/A.
• Celsius and kelvin: Temperature in kelvin is the temperature in degrees Celsius + 273; absolute zero is 0 K, approximately −273 °C.

What Gets Asked

• Explain the properties of solids, liquids and gases using particle spacing, arrangement and movement. Marks are lost by describing macroscopic properties without linking them to particle behaviour.
• Identify or describe a change of state, including melting, freezing, boiling, evaporation, condensation, sublimation and deposition. The key distinction is whether the change occurs throughout the liquid, at the surface, or directly between solid and gas.
• Interpret a heating or cooling curve. Flat sections represent changes of state, whereas sloping sections represent temperature changes within one state.
• Explain why temperature remains constant during melting, boiling, freezing and condensation. Energy changes particle separation and attractive forces rather than the average kinetic energy.
• Explain factors affecting evaporation and diffusion. Evaporation depends on temperature, surface area, air movement and humidity; diffusion depends on concentration gradient, travel distance, temperature and the state of matter.
• Calculate or apply pressure, density and temperature relationships using pressure = force / area, or p = F/A; density = mass / volume, or ρ = m/V; and temperature in kelvin = temperature in degrees Celsius + 273.