3. CHANGES OF STATE
You have probably seen how an ice cube melts when you take it out of the freezer. The only thing needed is a heat supply (we might not realise it but the temperature in the room is a heat supply because the room temperature is higher than the temperature in the freezer).
On the other hand, if we want to make ice cubes, we put water in an ice-cube tray and leave it in the freezer for a while, to cool it down.
In both cases, water is going from one state of matter to another; what we call a change of state.
A change of state is a change in the state of aggregation of matter without changing its chemical composition (the substance doesn't change).
Given that the state of aggregation of matter depends on the arrangement of its particles, changes of state imply a modification of this arrangement but not of the type of particles (so the substance is still the same).
All substances can exist in each of the three states of matter; they can all change from one to another if the temperature changes. However, there are some substances that we only know in one state, for example, oxygen as a gas.
To observe oxygen in a liquid or solid state, we need temperatures below −183°C for liquid oxygen and below −218°C for solid oxygen. Such low temperatures are very rare but it's possible to reach them in laboratories.
This diagram shows the process involved in the changes of state in matter:
3.1. The temperature at which changes of state occur
3.1.1. Change from solid ⇔ liquid
The change of state from solid to liquid is called melting. The temperature of the melting point is not the same for all substances. For example, ice (frozen water) melts at 0°C at sea level but gold melts at 1063°C.
The temperature at which a solid melts and becomes a liquid is called the melting point and it's different for every substance.
The opposite of melting is called freezing (that is, changing from liquid to solid).
The temperature at which a substance changes to a solid is the same as its melting point and is called the freezing point.
For example, if we put a thermometer in a glass of water and we leave it by a window on a very cold day, the water will not turn to ice until the temperature is 0°C. Remember that this is also the temperature at which ice cubes melt.
3.1.2. Change from liquid ⇔ gas
The change from liquid to gas is called vaporisation and can happen in two ways:
- Evaporation: occurs at any temperature on the surface of the liquid. Thanks to evaporation we can dry our clothes in summer and in winter.
You can test how evaporation works by putting a small amount of perfume into a glass and leaving it outside. After a few days, all the perfume will have disappeared due to evaporation.
- Boiling: occurs at a fixed temperature for every substance in the whole mass of the liquid vigorously. We can seen this at home when we heat up (boil) water for cooking.
The boiling point of a substance is different for every substance. For example, water boils at 100°C at sea level, but gold boils at 2857°C.
The temperature at which a substance boils is called the boiling point. It is different for every substance.
The change of state from a gas to a liquid is called condensation and is the opposite of vaporisation.
3.2. A heating curve
Gas vs vapour
We talk about gas to refer to substances in that state in the environment, like oxygen or nitrogen. However, we use the word vapour to refer to the gaseous state of a substance that we normally find as a solid or liquid in the environment, like water vapour.
If we heat ice cubes at −20°C up to 130°C and represent the change in temperature over time on a graph, we would expect to see the line go up little by little. However, we will see that the temperature on the thermometer remains stable at two different points in the process:
- When it reaches 0°C, the transformation point from ice to liquid (melting).
- When it reaches 100°C, the transformation point from liquid to vapour (vaporisation).
It's curious to see that while these changes of state are happening, the temperature stays the same and doesn't go up again until the transformation has happened entirely (until there is no ice left in the first case and no water left in the second). Remember that the temperature for each change of state is different for every substance.
The graph shown above is known as a heating curve. If we cooled something down rather than heating it up, we would get a similar graph going down instead of up, called a cooling curve.
- Every change of state happens at a different temperature for each substance.
- The temperature of a change of state and its opposite change is the same.
- Graphs that represent temperature versus time are called heating curves or cooling graphs.