State symbols are used to show the physical state of reactants and products in a reaction, at the reaction temperature. This means the temperature at which the reaction takes place.
State symbols are shown in subscript, just like when we use numbers to show how many atoms are in a molecule such as diatomic oxygen (O₂).
There are four state symbols to learn for GCSE chemistry:
(s) – solid
(l) – liquid
(g) – gas
(aq) – aqueous
Three of these symbols should be familiar as they are the common states of matter – solids, liquids and gases. Read on to find out what aqueous means.
Aqueous is a state of being dissolved in water and uses the state symbol (aq) which originated from the Latin word for water – Aqua. This word can often be found at number one on the ingredients lists of inedible items like shampoos and conditioners.
Water is an example of a compound comprised of hydrogen and oxygen bonded covalently.
State symbols are often used when dealing with acids and alkalis because the reactions can cause changes in state. For example, when we react metal oxides with acids we get the following general reaction:
metal oxide + acid → salt + water
These are known as neutralisation reactions because the acid is neutralised, making water. Look at the following reaction between magnesium oxide and sulfuric acid. Note the change in state from solid and aqueous to aqueous and liquid.
MgO(s) + H₂SO₄(aq) → MgSO₄(aq) + H₂O(l)
Water is always given the state symbol (l) when a reaction takes place at room temperature.
The reaction temperature determines the state and so the following rules will help you to understand which state symbols to use:
Take a look at our quizzes on Acids below to find out more about using state symbols and neutralisation reactions.
