Chemical Reactions and the Conservation of Matter

In a chemical reaction the starting substances — the reactants — are rearranged into new substances, the products. We write this with an arrow that means becomes:

A word equation names them: magnesium $+$ oxygen $\rightarrow$ magnesium oxide.

Signs that a chemical (not merely physical) change has happened:

• a new substance with new properties appears;
• there is a colour change, a gas is given off, or a solid forms in a clear liquid;
• energy is released or absorbed (the container gets hot or cold);
• it is usually difficult to reverse.

Melting ice, dissolving sugar and boiling water are all physical changes: no new substance, and easy to reverse. Rusting iron, burning wood and baking bread are chemical changes.

The law of conservation of matter. In a chemical reaction atoms are only rearranged — never created and never destroyed. So the total mass of the products always equals the total mass of the reactants.

This is why a chemical equation must be balanced: the same number of each kind of atom on both sides.

Worked example — balance $\text{Mg} + \text{O}_2 \rightarrow \text{MgO}$.

Count first: on the left, 1 Mg and 2 O. On the right, 1 Mg and 1 O. The oxygen does not match.

1. Put a 2 in front of $\text{MgO}$ to fix the oxygen: $\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}$. Now O is 2 and 2. But Mg is 1 on the left and 2 on the right.
2. Put a 2 in front of $\text{Mg}$: $2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}$.

Check: Mg is 2 and 2; O is 2 and 2. Balanced.

Worked example — the masses. $12\ \text{g}$ of magnesium burns completely in $8\ \text{g}$ of oxygen. No atoms are lost, so