Particle model of solids, liquids and gases

A material contains many particles. The particle model represents them as small solid spheres so that their arrangement and movement are easier to picture. The model helps explain why the same material behaves differently as a solid, liquid or gas.

Particle arrangement and movement

• Solid: The particles are close together in a fixed arrangement. They vibrate about fixed positions but do not move past one another. A solid therefore keeps its shape.
• Liquid: The particles are close together but are not fixed in place. They move past one another, so a liquid can flow and take the shape of its container.
• Gas: The particles are far apart and move freely in all directions. A gas spreads out to fill its container.

Energy and changes of state

When energy is transferred to a material, its particles can move more and their arrangement can change. A solid can melt to form a liquid, and a liquid can become a gas. During a change of state, the transferred energy changes the particle arrangement rather than necessarily increasing the particles’ speed.

When energy is transferred out of a material, its particles can move less and their arrangement can change. A gas can condense to form a liquid, and a liquid can freeze to form a solid.

Particles and bulk properties

Properties such as flowing or keeping a shape describe the behaviour of the material as a whole. They arise from how many particles are arranged and move together; a single atom or particle does not itself have these bulk properties.

States of matter and changes of state

Matter can exist as a solid, liquid or gas. A substance can change from one state to another when it is heated or cooled. The substance itself remains the same; its physical state changes.

Melting and boiling points

A substance changes state at characteristic temperatures:

• At its melting point, a solid melts to form a liquid, or a liquid freezes to form a solid.
• At its boiling point, a liquid boils to form a gas, or a gas condenses to form a liquid.

These temperatures describe when the bulk material changes state. The melting and boiling points used must be for the pressure being considered.

Predicting the state of a pure substance

At a given pressure, compare the substance’s temperature with its melting and boiling points:

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

At a melting or boiling point, the two states involved can coexist while the substance changes state.

For example, at standard atmospheric pressure, pure water has a melting point of $$0,^{\circ}\mathrm{C}$$ and a boiling point of $$100,^{\circ}\mathrm{C}$$. It is solid at $$-5,^{\circ}\mathrm{C}$$, liquid at $$25,^{\circ}\mathrm{C}$$ and a gas at $$110,^{\circ}\mathrm{C}$$.

Limitations of the simple particle model

The simple particle model represents matter as tiny solid spheres. It helps explain basic ideas about how particles are arranged and move in different states, and what happens during changes of state. However, the spheres are a simplified representation: real particles are not literally tiny solid balls, and they do not all have the same basic shape.

Forces missing from the model

The model does not show the forces between particles. These forces help hold particles together. During melting or boiling, energy is needed to overcome some of the attractions between particles so that they can move more freely. Without forces, the simple model cannot fully explain why these changes require energy.

Forces between particles differ from one substance to another. This affects how much energy is needed to separate the particles and helps explain why substances have different melting and boiling points.

The model is therefore useful for describing basic features of states and state changes, but it cannot fully show what particles are really like or explain all their behaviour.

Changes of state

A substance can change between the solid, liquid and gas states. A change of state is a physical change: the substance remains the same substance, but the arrangement and motion of its particles change.

In a solid, particles are held in fixed positions and vibrate. In a liquid, particles are close together but can move past one another. In a gas, particles move freely and are much farther apart.

Melting and freezing

Melting is the change from solid to liquid. As the substance gains energy, its particles move more and can overcome enough of the forces holding them in fixed positions. They can then move past one another as a liquid.

Freezing is the change from liquid to solid. The particles lose energy and move less. They become held in fixed positions, although they continue to vibrate.

Boiling and evaporation

Both boiling and evaporation change a liquid into a gas. In each case, particles gain enough energy to separate from the liquid and move freely as gas particles.

• Boiling happens throughout the liquid, forming bubbles of gas within it.
• Evaporation happens at the surface, where particles escape from the liquid into the gas state.

Condensation

Condensation is the change from gas to liquid. Gas particles lose energy and slow down. They come closer together and form a liquid, in which the particles can still move past one another.