������̳

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, the usual one-way arrow is not used. Instead, two arrows are used, each with just half an arrowhead (⇌). The top one points right, and the bottom one points left.

For example:

ammonium chloride ⇌ ammonia + hydrogen chloride

NH₄Cl(s) ⇌ NH₃(g) + HCl(g)

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.

This slideshow shows a reversible reaction involving white anhydrous copper(II) sulfate and blue hydrated copper(II) sulfate. The equation for this is:

anhydrous copper(II) sulfate + water ⇌ hydrated copper(II) sulfate

CuSO₄(s) + 5H₂O(l) ⇌ CuSO₄·5��₂O(s)

Note that the removal of water from hydrated copper(II) sulfate requires heat, and so is an endothermic process. The reverse process (the addition of water to anhydrous copper(II) sulfate) is an exothermic process. The amount of energy absorbed in the forward process is exactly the same as the amount of energy released in the reverse process.

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.

A similar reversible reaction takes place between anhydrous cobalt(II) chloride (which is blue) and water to produce hydrated cobalt(II) chloride (which is pink). Heating pink hydrated cobalt(II) chloride makes it turn blue.