States of Matter

A substance can exist as a solid, liquid or gas. Its particles do not become a different substance when its state changes. Instead, their energy, movement and arrangement relative to one another change. A change of state is therefore a physical change.

In a solid, particles are close together in a regular arrangement and vibrate about fixed positions. In a liquid, they remain close together but can move around and past one another. In a gas, they are much farther apart and move rapidly in random directions.

The word particle is deliberately general. Depending on the substance and its structure, the particles may be atoms, ions or molecules.

The temperatures at which states change

Melting is the change from solid to liquid; the reverse process is freezing. Both take place at the substance's melting point. Boiling is the change from liquid to gas throughout the liquid; the reverse process is condensing. Both take place at the boiling point.

This gives a simple way to predict a substance's state from supplied data:

• below its melting point, it is solid;
• between its melting point and boiling point, it is liquid;
• above its boiling point, it is gas.

For example, suppose a substance has a melting point of $-20\,^{\circ}\mathrm{C}$ and a boiling point of $80\,^{\circ}\mathrm{C}$. At $-40\,^{\circ}\mathrm{C}$ it is solid, at $25\,^{\circ}\mathrm{C}$ it is liquid, and at $100\,^{\circ}\mathrm{C}$ it is gas.

At exactly a melting or boiling point, a change of state can be taking place, so both states may be present. At the melting point, solid and liquid may coexist; at the boiling point, liquid and gas may coexist. In questions, use any further information about whether the substance is being heated, cooled or is part-way through the change.

Energy transfers during changes of state

Heating transfers energy to a substance. Its particles gain energy. During melting, this allows particles to leave their fixed positions and move past one another. During boiling, enough energy is supplied for particles to separate much more fully and form a gas.

Cooling transfers energy from a substance to the surroundings. The particles lose energy. During condensing, gas particles come closer together to form a liquid. During freezing, the particles become fixed in position in a solid arrangement.

The particles themselves do not melt, boil, expand or change size. It is the collection of particles—the material as a whole—that changes state. Melting point, boiling point and state are bulk properties, so an individual atom does not possess the bulk properties of the material made from many atoms.

Why different substances change state at different temperatures

Energy is needed to overcome attractions between particles. Stronger attractions require more energy to overcome, so substances with stronger forces between their particles generally have higher melting and boiling points.

The relevant attractions depend on the substance's bonding and structure. In a simple molecular substance, melting or boiling overcomes forces between molecules, not the strong covalent bonds holding the atoms together inside each molecule. In giant structures, the particles and attractions are different, so the strong ionic, covalent or metallic bonding can lead to much higher melting and boiling points.

This explains the connection between bonding and state changes: the particle model describes changes in arrangement and movement, while knowledge of bonding and structure explains why the required temperatures differ between substances.

Higher Tier: judging the simple particle model

A model is useful because it focuses attention on selected features. The sphere model clearly represents how particles are arranged and spaced in solids, liquids and gases. It can therefore help to describe melting, freezing, boiling and condensing. However, it must not be mistaken for a literal picture.

The model has several important limitations:

• It shows no forces between particles. This is a serious limitation when explaining state changes because forces determine how much energy is needed to separate or rearrange particles. Without forces, the model cannot by itself explain different melting and boiling points.
• It represents every particle as a sphere. Real particles may be atoms, ions or molecules, and molecules can have varied shapes and sizes.
• It represents particles as solid spheres. Atoms are not solid throughout; much of an atom is empty space.
• The spheres are inelastic. They are treated as rigid objects that do not deform, so the representation oversimplifies how real particles interact.

A strong evaluation does more than list these faults. It states what the model represents successfully and then explains how a particular simplification limits the conclusions that can be drawn from it.