Reversible and Irreversible reactions:

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 pointing right, and the bottom one pointing left.

For example:

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.

Equilibrium

If a chemical reaction happens in a container where one or more of the reactants or proucts can escape, you have an open system. If a chemical reaction happens in a container where none of the reactants or products can escape, you have a closed system. Reversible reactions that happen in a closed system eventually reach equilibrium.

At equilibrium, the concentrations of reactants and products do not change. But the forward and reverse reactions have not stopped - they are still going on, and at the same rate as each other.

Imagine walking the wrong way on an escalator - at the same speed as the escalator but in the opposite direction. Your legs are still walking forwards, and the escalator continues…