AQA GCSE Chemistry (8462) — The rate and extent of chemical change
You can easily burn a piece of wood into ash and smoke, but you can never turn the smoke and ash back into wood. However, many chemical reactions do not work like this one-way street.
Chemists indicate that a reaction is reversible by using the symbol. This symbol consists of two half-headed arrows pointing in opposite directions.
You can control the direction of a reversible reaction by altering the conditions. For example, heating may drive a reaction forward, while cooling may drive it backward.
Heating ammonium chloride produces a chemical change known as thermal decomposition. The molecules break apart into two colourless gases: ammonia and hydrogen chloride.
Another classic reversible reaction involves copper(II) sulfate and water.
In equation-writing questions, always use the symbol instead of a standard arrow for reversible reactions, or you will be penalised.
In the decomposition of ammonium chloride, you must refer to as 'hydrogen chloride gas', not 'hydrochloric acid'.
When defining a reversible reaction, examiners look for the specific phrase that 'products can react to reform the original reactants'.
The reverse reaction of anhydrous copper(II) sulfate and water is commonly tested as the chemical test for water. Remember the specific observation: the white anhydrous powder turns blue.
Reversible reaction
A chemical reaction in which the products can react to reform the original reactants.
Forward reaction
The reaction moving towards the right in a chemical equation, where reactants form products.
Reverse reaction
The reaction moving towards the left in a chemical equation, where products reform the reactants.
⇌ symbol
The specific chemical symbol consisting of two half-headed arrows used to indicate that a reaction is reversible.
Thermal decomposition
The breakdown of a compound into two or more simpler substances using heat energy.
Hydrated salt
A salt containing water of crystallisation within its solid crystal structure.
Water of crystallisation
Water molecules that are chemically bonded and essential for the formation of the crystal structure of certain salts.
Anhydrous salt
A salt that does not contain any water of crystallisation.
Put your knowledge into practice — try past paper questions for Chemistry
Reversible reaction
A chemical reaction in which the products can react to reform the original reactants.
Forward reaction
The reaction moving towards the right in a chemical equation, where reactants form products.
Reverse reaction
The reaction moving towards the left in a chemical equation, where products reform the reactants.
⇌ symbol
The specific chemical symbol consisting of two half-headed arrows used to indicate that a reaction is reversible.
Thermal decomposition
The breakdown of a compound into two or more simpler substances using heat energy.
Hydrated salt
A salt containing water of crystallisation within its solid crystal structure.
Water of crystallisation
Water molecules that are chemically bonded and essential for the formation of the crystal structure of certain salts.
Anhydrous salt
A salt that does not contain any water of crystallisation.
