Water and its three states
Water is one of the most important substances on Earth. It exists in three physical states: solid (ice), liquid (water) and gas (water vapour or steam). When water is heated or cooled, it changes from one state to another without becoming a new substance. This is called a change of state, and it is a physical change.
When ice is heated it melts into liquid water. When liquid water is heated strongly it boils and turns into vapour. When vapour is cooled it condenses back into liquid, and when water is cooled enough it freezes into ice.
Key idea
Pure water melts (and freezes) at 0 °C and boils (and condenses) at 100 °C at normal atmospheric pressure. During a change of state the temperature stays constant even though heat is still supplied.
Other physical characteristics
Pure water is colourless, odourless and tasteless. It has a density of about 1 g/cm³ at 4 °C, which is why many objects float or sink in it. Water is also a very good solvent, dissolving many substances such as sugar and salt.
- Ice is less dense than liquid water, so ice floats.
- Water expands slightly when it freezes.
- Water can absorb and store a lot of heat.
Effect of impurities
When impurities such as salt are dissolved in water, the physical characteristics change. Impurities lower the melting point and raise the boiling point of water. This is why salty sea water freezes below 0 °C and boils above 100 °C.
Example
Adding salt to icy roads makes the ice melt at a temperature below 0 °C, helping to clear the ice in cold countries.