Acids and Bases: Quick Revision

A brief note on Acids and Bases. Read it before a test. Try each question at the end before you open its answer.

Indicators

Onion, vanilla and clove oil lose their smell in a base: they are olfactory indicators.

What acids react with

• Acid + metal $\rightarrow$ salt + hydrogen. Hydrogen burns with a pop.
• Acid + carbonate or hydrogencarbonate $\rightarrow$ salt + water + carbon dioxide. Carbon dioxide turns lime water milky.
• Acid + base $\rightarrow$ salt + water: neutralisation.
• Metal oxides are basic; non-metal oxides are acidic.

Ions in water

Acids give $\text{H}^+$ (as $\text{H}_3\text{O}^+$) in water; alkalis, the bases that dissolve, give $\text{OH}^-$. Strong acids split fully, weak ones a little. Always add acid to water, never water to acid.

A worked example

What volume of hydrogen comes from $0.6\ \text{g}$ of magnesium with plenty of hydrochloric acid? One mole of gas fills about $24\,000\ \text{cm}^3$.

1. Equation. $\text{Mg} + 2\text{HCl} \rightarrow \text{MgCl}_2 + \text{H}_2$.
2. Moles. $\dfrac{0.6\ \text{g}}{24\ \text{g/mol}} = 0.025\ \text{mol}$ of magnesium, so $0.025\ \text{mol}$ of hydrogen.
3. Volume. $0.025 \times 24\,000\ \text{cm}^3 = 600\ \text{cm}^3$.

Practice with new numbers

A program to try

The program lists three reactions of acids, and works out the worked example.

Mistakes to avoid

• Tasting or touching to test. Use an indicator.
• Thinking every base is an alkali. Only the ones that dissolve are.
• Thinking anything with hydrogen is an acid. It must give $\text{H}^+$ ions in water.
• Pouring water into concentrated acid. It boils and spits.

Check yourself

1. Which gas is given off when an acid reacts with a hydrogencarbonate?

2. Which ion makes a solution basic?

3. Name the indicator that turns yellow in a base.

4. Is sodium oxide an acidic or a basic oxide?