Reactions with Oxygen, Acids and Bases

Four families of reaction cover most of what you meet in Grade 9.

1. Metal $+$ oxygen $\rightarrow$ metal oxide. Magnesium burns with a blinding white flame: $2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}$. Iron does the same thing slowly, and we call it rust. Metal oxides that dissolve in water make the solution basic (pH above 7).

2. Non-metal $+$ oxygen $\rightarrow$ non-metal oxide. Carbon burns to carbon dioxide, $\text{C} + \text{O}_2 \rightarrow \text{CO}_2$; sulfur burns to sulfur dioxide, $\text{S} + \text{O}_2 \rightarrow \text{SO}_2$. Non-metal oxides dissolve in water to make the solution acidic (pH below 7). This is exactly how acid rain forms downwind of coal-fired power stations on the Mpumalanga Highveld: sulfur in the coal burns to $\text{SO}_2$, which dissolves in cloud droplets.

3. Metal $+$ acid $\rightarrow$ salt $+$ hydrogen gas. Drop zinc into hydrochloric acid and it fizzes: $\text{Zn} + 2\text{HCl} \rightarrow \text{ZnCl}_2 + \text{H}_2$. Hold a burning splint at the mouth of the test tube and the escaping hydrogen makes a squeaky "pop" — the standard test for hydrogen.

4. Acid $+$ base $\rightarrow$ salt $+$ water. This is neutralisation: $\text{HCl} + \text{NaOH} \rightarrow \text{NaCl} + \text{H}_2\text{O}$. An antacid tablet neutralises excess stomach acid; a farmer spreads lime on acidic soil to bring the pH back towards 7.

Worked example — write the word equation for magnesium and hydrochloric acid.

This is family 3: metal $+$ acid. The salt is named from the metal and what remains of the acid (hydrochloric acid gives chlorides):

The pH scale runs from 0 to 14: below 7 is acidic, exactly 7 is neutral, above 7 is basic.