Changing States

Navigate the knowledge tree: 🌿 Skills ➡ Life Processes

Physical Changes

The three states of matter are solids, liquids and gases. When you heat or cool a substance, a change of state may occur. As a substance is heated, particles gain thermal energy and therefore gain kinetic energy. As a substance is cooled, particles lose thermal energy and therefore lose kinetic energy. These are physical change, where the state of a substance changes, but not what the actual substance is.

A substance changes its state at a specific temperature, unique to the substance involved. For example, lead turns from a solid to a liquid at 327°C; methylated spirits turns from a liquid to a gas at 78°C; mitrogen turns from a gas to a liquid at -196°C.

When a substance is changing state, the temperature always stays constant - you will see this very clearly in the heating and cooling curves further below. The following diagram shows the names given to the various changes of state.

When heated, particles making up a solid, liquid or gas move about more and take up more room, so the substance expands. Cooling makes substances shrink.

This can cause problems in large buildings and bridges (e.g. the Auckland Harbor Bridge is loosely connected to allow for its expansion in warm temperatures),

But this physical change can also be useful, e.g. in thermometers, which work through the liquid inside changing volume when it is heated or cooled.

Melting Point & Boiling Point

All substances have different melting and boiling points. Examples for some common substances are:

Melting and boiling points are also useful in categorising and comparing different substances and for determining if a substance is pure or not. Substances containing impurities melt ina different temperature range several degrees below what the melting point should be. For example, pure water melts at 0°C

Solid - Liquid Changes

Changes in state are caused by particles of matter gaining or losing thermal energy.

As a solid is heated, the particles gain thermal energy and move more quickly (more kinetic energy). This expands the substance. When they have sufficient energy to overcome the forces holding them in fixed places, they break away from their solid structure and move around past each other as a liquid. The solid has now melted into a liquid. Melting is the change of state from a solid to a liquid. The temperature at which the solid substance melts is called its melting point (MP). The melting point of ice is 0°C.

When a liquid cools the reverse occurs. It loses thermal energy, so the particles slow down until they cannot move past each other. Eventually, the forces between the particles hold them in fixed places and the liquid freezes (solidifies). Solidification or freezing is the change of state from a liquid to a solid. The temperature at which a liquid becomes a solid is called its freezing point (FP). The temperature at which a substance freezes is the same as its melting point temperature. The freezing point of water is 0°C.

Liquid - Gas Changes

When a liquid is heated, the particles gain thermal energy and increase their movement (more kinetic energy). The more energetic particles escape the attrative forces of other liquid particles and become gas particles. This occurs either through evaporation or boiling. With evaporation, liquid particles slowly gain energy to become gas particles. With boiling, the change occurs rapidly.

The temperature at which the liquid substance boils is called its boiling point (BP). The boiling point of water is 100°C.

When gas particles cool and lose thermal energy, they slow down (less kinetic energy) and are overcome by the attractive forces between them. The gas condenses to form a liquid in which the particles are moving close together. The emperature at which a gas becomes a liquid is the same as its boiling point (100°C).

Boiling vs. Evaporation

There are two ways in which a liquid can change into a gas - boiling and evaporation. And they are distinct from each other.

As explained above, if enough thermal energy is added to a liquid, it will start to boil. Boiling is a rapid change from a liquid to a gas state and occurs at a fixed temperature, the boiling point (BP). Water at standard air pressure boils at 100°C to produce steam.

But liquids will also change slowly to a gas state if left open to the air. This process occurs at everyday temperatures (or cooler than the boiling point) and is called evaporation.

When evaporation is occurring, individual particles in the liquid gain enough thermal energy to escape from the surface of the liquid to become gas particles. When water evaporates, it forms water vapour.

Solid - Gas Changes

Some substances (e.g. dry ice) do not have a liquid state, but change straight from a liquid to a gas state when heated.

Sublimation is the change directly from a solid to a gas, when thermal energy is added. The particles in the solid gain thermal energy and move faster until they can break away from their solid structure and move apart as a gas. The temperature at which a substance sublimes is the sublimation point. Solid carbon dioxide, or 'dry ice' has a sublimation point of -78°C, so at this temperature it changes directly from a solid to a gas.

When the same substance is cooled, the reverse occurs. As the particles lose energy, they change directly from a gas to a solid - reverse sublimation (or deposition).

So gases can result from boiling, evaporation or sublimation.

Heating and Cooling Curves

When substances are heated, the particles absorb thermal energy which is converted into kinetic energy.

Heating a solid causes its particles to vibrate more. As the temperature increases, they vibrate so much that the solid expands until the structure breaks and the solid melts.
On further heating, the now liquid substance expands more and some particles at the surface gain sufficient energy to overcome the forces between molecules and evaporate.
When the boiling point temperature is reached, all the particles gain enough energy to escape and the liquids boil.

These changes in state can be shown on a graph called a heating curve. Note on the heating curve that during a change of state, the extra energy does not cause the substance's temperature to rise. Instead, the particles become much freer in their movement.

When substances are cooled, the particles lose thermal energy and therefore lose kinetic energy.

Cooling a gas causes its particles to lose kinetic energy. As the temperature decreases, the particles move less and less freely until the particles become more closely packed and forces between molecules become stronger.
On further cooling, the substance becomes a liquid.
When the freezing point temperature is reached, the particles have lost so much thermal energy that bonds between molecules hold the particles tightly together. Particles can vibrate but cannot move freely.

These changes in state can be shown in a graph called a cooling curve. Note on the cooling curve below that during a change of state, the loss of energy does not cause the substance's temperature to fall. Instead, the particles become much less free in their movement.