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.
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:
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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 opposite of melting is called freezing (that is, changing from liquid to solid).
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.
The change from liquid to gas is called vaporisation and can happen in two ways:
The change of state from a gas to a liquid is called condensation and is the opposite of vaporisation.
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:
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.
Activity 6
Choose the correct option and find the relationship between global warming and the melting of polar ice caps.
Activity 7
Listen and say the process involved in the change of state.
Activity 8
At sea level, ice melts at 0 ºC. Why do you think we refer to sea level? Research and find out.
Activity 9
What is the relationship between the temperature at which a substance melts and the temperature at which the same substance freezes?
Activity 10
Does a liquid have to boil to change from liquid to gas?
Activity 11
Draw the cooling graph of water going from water vapour at 130 °C and cooling down to −10 °C.
Activity 12
Work out from the graph above what the aggregation state of water is at 50°C and at 125°C.
Activity 13
Describe in detail how you would carry out an experiment into the changes of state of water in the laboratory. Include all the materials you would need.
