Particle Model and Changes of State

Matter represented as particles

Solids, liquids and gases are three states of matter. The particle model explains these states by representing a substance as many tiny particles. Depending on the substance, these may be atoms, molecules or ions. Simple diagrams use circles or spheres to stand for them.

The important differences between states are how the particles are arranged, how far apart they are and how they move. A substance can change state without becoming a different chemical substance.

How particles explain the three states

In a solid, particles are close together in a regular arrangement. They vibrate about fixed positions rather than moving freely from place to place. This gives a solid a fixed shape and volume.

In a liquid, particles are still close together, but their arrangement is irregular. They can move past one another, so a liquid flows and takes the shape of its container. Because the particles remain close together, a liquid has a nearly fixed volume and is difficult to compress.

In a gas, particles are far apart and randomly arranged. They move rapidly in all directions, spreading through the available space. A gas therefore has neither a fixed shape nor a fixed volume. It can be compressed because there are large gaps into which its particles can move.

When reading a particle diagram, look first at the spacing and arrangement. Close, ordered particles represent a solid; close, irregular particles represent a liquid; widely separated particles represent a gas. Movement needs arrows or an accompanying explanation: a still diagram alone cannot show particles vibrating or moving.

Changing state transfers energy

Heating transfers energy to a substance. Away from a change of state, this generally makes its particles move or vibrate faster and raises its temperature. To change from solid to liquid, or liquid to gas, energy is needed to overcome attractions between particles.

Melting changes a solid into a liquid. Particles leave their fixed positions and become able to move past one another. Freezing is the reverse: energy is transferred to the surroundings and the particles become held in fixed positions.

Boiling changes a liquid into a gas throughout the liquid, at its boiling point. Particles become widely separated and move freely. Evaporation also changes liquid into gas, but happens at the surface and can occur below the boiling point. Condensation changes gas into liquid as energy is transferred to the surroundings and particles come closer together. Some substances can also change directly from solid to gas; this is sublimation.

For a pure substance at a fixed pressure, melting and freezing occur at the same temperature. Boiling and the reverse change occur at the boiling point. During melting or boiling, transferred energy is used to overcome attractions rather than raise the temperature, so the temperature stays constant until that change is complete.

Why substances change state at different temperatures

Particles attract one another, but these attractions are not equally strong in every substance. Stronger attractions require more energy to overcome, so substances with stronger attractions generally have higher melting and boiling points.

For molecular substances, the attractions between molecules are called intermolecular forces. Melting overcomes enough of these attractions to allow molecules to move past one another. Boiling separates the molecules much more completely. It does not break the chemical bonds within each molecule.

This connects a visible measurement—a melting or boiling temperature—to what is happening between particles. Saying that a substance has a high boiling point is not itself an explanation: the explanation is that strong attractions require a larger energy transfer to separate its particles.

Physical changes and chemical changes

A change of state is a physical change: no new substance forms. Ice, liquid water and water vapour all contain water molecules. Their spacing, arrangement and movement change, but their chemical identity does not. Cooling water vapour can turn it back into liquid water.

A chemical change produces new substances. The particles of the reactants are rearranged into chemically different products. This is more than a change in how the original particles are spaced or how quickly they move. Although changes of state are readily reversed by heating or cooling, the decisive distinction is whether new substances form.

A particle does not have the properties of the whole substance

A bulk property belongs to many particles acting together. A single particle does not have all the properties of a visible sample. For example, a piece of copper conducts electricity, but an individual copper atom does not itself have the electrical conductivity of that piece.

Similarly, one water molecule cannot be described as a tiny drop of liquid water or a tiny piece of ice. Solid, liquid and gas describe the collective arrangement and behaviour of many particles. When ice melts, the molecules do not themselves melt.

Using temperature data to predict a state

Compare the given temperature with a substance's melting and boiling points:

• Below the melting point: solid.
• Between the melting and boiling points: liquid.
• Above the boiling point: gas.

Suppose three substances have the following transition temperatures at the same fixed pressure:

At 20°C, A is a liquid because 20°C lies between −20°C and 60°C. B is a solid because 20°C is below its melting point of 40°C. C is a gas because 20°C is above its boiling point of 10°C.

At exactly a transition temperature, temperature alone may not identify a single state. For example, at A's melting point of −20°C, solid and liquid can coexist while melting or freezing takes place. These comparisons apply at the pressure for which the data were given.

Limitations of the sphere model — Higher Tier

Representing particles as inelastic spheres, like tiny rigid bowling balls, helps show arrangement and spacing. However, it leaves out important features needed to explain changes of state.

The spheres do not show the forces of attraction between particles. Simply moving balls apart cannot explain why energy must be transferred to change state, or why different substances have different melting and boiling points. The model needs the additional idea that attractions differ in strength.

The spheres also do not accurately represent the sizes and shapes of different particles or the relative spaces between them. Real substances contain different kinds of particles and have different structures; they are not all collections of identical solid balls. The model is therefore useful for describing broad differences between states, but is not a complete account of the particles or the energy involved.

Particle arrangements and movement

Large gaps make gases compressible. Liquids and solids are difficult to compress.

Changes of state

• Solid → liquid: melting; reverse: freezing.
• Liquid → gas: boiling or evaporation; reverse: condensation.
• Solid → gas directly: sublimation.
• Melting and boiling need energy to overcome attractions; freezing and condensation transfer energy to the surroundings.
• Stronger attractions → more energy needed → generally higher melting and boiling points.
• During a pure substance's change of state at fixed pressure, temperature stays constant.

Essential distinctions

• Physical change: same substance; particle identity unchanged.