States of matter

The three states of matter and their properties

A substance can exist as a solid, a liquid or a gas. The state it is in depends on how much energy its particles have. Adding energy by heating makes the particles move faster and loosens the forces between them; removing energy by cooling does the opposite.

The states are told apart by their properties. A solid has a fixed shape and a fixed volume because its particles are locked in position. A liquid has a fixed volume but takes the shape of its container, because its particles can slide past one another but stay close together. A gas has no fixed shape and no fixed volume: it fills whatever container it is in and can be squeezed into a smaller space.

Solids are difficult to compress and liquids are almost as difficult, but gases are easily compressed because there is a lot of empty space between their particles.

• Solid: fixed shape, fixed volume, cannot be compressed.
• Liquid: fixed volume, takes the shape of its container, cannot be compressed much.
• Gas: no fixed shape or volume, fills its container, easily compressed.
• All of these differences come from how far apart the particles are and how freely they move.

Particle arrangement, separation and motion

In a solid, the particles are very close together in a regular, ordered arrangement. Strong forces hold them in fixed positions, so they can only vibrate about a fixed point. This is why a solid keeps its own shape.

In a liquid, the particles are still close together but the arrangement is random, not regular. The forces between them are weaker, so the particles can slide past one another. This is why a liquid flows and takes the shape of its container but keeps a fixed volume.

In a gas, the particles are far apart with large spaces between them and the arrangement is random. The forces between the particles are negligible, so they move quickly and randomly in straight lines, colliding with each other and with the walls of the container.

Changes of state

A change of state is a physical change: the particles themselves do not change, only their arrangement and their energy. Melting is solid to liquid and freezing is liquid to solid. Boiling is liquid to gas and condensing is gas to liquid. Evaporation is also liquid to gas, but it happens only at the surface and at any temperature, whereas boiling happens throughout the liquid and only at the boiling point.

Sublimation is the change straight from solid to gas without passing through the liquid state. Dry ice, which is solid carbon dioxide, sublimes at room temperature, and iodine and ammonium chloride also sublime when they are heated.

During a change of state the temperature stays constant while the change is happening. The energy supplied is used to overcome the forces between the particles, not to increase their kinetic energy, so the thermometer reading does not rise.

Kinetic particle theory and heating curves

Kinetic particle theory says that all matter is made of particles in constant random motion, and that the temperature of a substance is a measure of the average kinetic energy of its particles. Heating transfers energy to the particles, so they move faster and the temperature rises.

A heating curve is a graph of temperature against time as a substance is heated steadily. The temperature rises while the substance is a solid, then stops rising at the melting point while the solid melts. It rises again as the liquid warms, stops again at the boiling point while the liquid boils, and rises once more when the substance is a gas.

The flat sections are the important part. While a substance is melting or boiling, the energy supplied is used to break the forces between the particles, so the particles gain potential energy rather than kinetic energy and the temperature stays constant. A cooling curve is the mirror image, with the flat sections at the freezing and condensing points.

Temperature, pressure and the volume of a gas

A gas can be compressed because there is a lot of empty space between its particles. Increasing the pressure on a gas pushes the particles closer together, so the volume decreases. If you double the pressure at constant temperature, the volume halves, because pressure and volume are inversely proportional.

Increasing the temperature of a gas at constant pressure gives the particles more kinetic energy, so they move faster, hit the walls harder and more often, and the volume increases. The volume of a gas is proportional to its temperature in kelvin (K), so to double the volume you must double the temperature in K, not the temperature in °C.

In terms of kinetic particle theory, the pressure of a gas is caused by its particles colliding with the walls of the container. More collisions per second, or collisions with more energy, means a higher pressure.

To convert a temperature to kelvin, add 273 to the temperature in °C, so 0 °C is 273 K and 25 °C is 298 K. The kelvin scale starts at absolute zero, -273 °C, where the particles have as little energy as possible.

Diffusion

Diffusion is the spreading out of the particles of a gas, or of a substance dissolved in a liquid, from a region of higher concentration to a region of lower concentration, as a result of their random motion. It happens on its own, without stirring, because the particles are always moving.

For example, if you open a bottle of a strong-smelling liquid at one end of a room, the smell reaches you at the other end a few moments later. The particles have diffused through the air: each particle moves in a random zig-zag path, and over time the particles spread evenly through the space.

Diffusion is faster when the particles have more kinetic energy, so it is faster at a higher temperature, and it is faster in a gas than in a liquid because gas particles are further apart and meet less resistance. It is also faster when the particles are smaller and lighter.

Diffusion and relative molecular mass

A gas with a lower relative molecular mass (Mr) diffuses faster than a gas with a higher Mr. At the same temperature the lighter particles move faster, so they spread out more quickly.

A standard demonstration uses a long glass tube with a plug of cotton wool soaked in ammonia solution at one end and a plug soaked in hydrochloric acid at the other. The ammonia gas (Mr = 17) and the hydrogen chloride gas (Mr = 36.5) diffuse towards each other and meet, forming a white ring of ammonium chloride.

The ring forms closer to the hydrochloric acid end, because the lighter ammonia molecules diffuse faster and travel further in the same time. The position of the ring is direct evidence that the rate of diffusion depends on relative molecular mass.