Many reactions, such as burning fuel, are irreversible - they go to completion and cannot be reversed easily. Reversible reactions are different. In a reversible reaction, the products can react to produce the original reactants again.
When writing chemical equations for reversible reactions, we do not use the usual one-way arrow. Instead, we use two arrows, each with just half an arrowhead - the top one pointing right, and the bottom one pointing left. For example:
ammonium chlorideammonia + hydrogen chloride
ammonia + hydrogen chloride
The equation shows that ammonium chloride (a white solid) can break down to form ammonia and hydrogen chloride. It also shows that ammonia and hydrogen chloride (colourless gases) can react to form ammonium chloride again.
The animation below shows a reversible reaction involving white anhydrous copper(II) sulfate and blue hydrated copper(II) sulfate, the equation for which is:
anhydrous copper(II) sulfate + water hydrated copper(II) sulfate
The reaction between anhydrous copper(II) sulfate and water is used as a test for water. The white solid turns blue in the presence of water.
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