Describe acids as sources of hydrogen ions and bases as proton acceptors, distinguish strong and weak acids, and describe the characteristic reactions of acids with metals, carbonates and bases

What this dot point is asking

SEAB wants you to define acids and bases, tell apart strong and weak acids, recognise alkalis as soluble bases, and describe the three characteristic reactions of acids (with metals, with carbonates and with bases). This topic is the foundation of the whole acids and salts section and supplies reactions you will use to prepare salts and to identify substances.

The answer

What an acid is

An acid is a substance that produces hydrogen ions ($\text{H}^+$) when dissolved in water. It is these hydrogen ions that give acids their characteristic properties: a sour taste, a pH below $7$, and the ability to turn blue litmus red. Common laboratory acids are hydrochloric acid ($\text{HCl}$), sulfuric acid ($\text{H}_2\text{SO}_4$) and nitric acid ($\text{HNO}_3$).

What a base and an alkali are

A base is a substance that neutralises an acid, accepting hydrogen ions. Bases are usually metal oxides or metal hydroxides. A base that is soluble in water is called an alkali; it produces hydroxide ions ($\text{OH}^-$) in solution, has a pH above $7$, and turns red litmus blue. Sodium hydroxide and aqueous ammonia are alkalis; copper(II) oxide is a base that is not an alkali because it is insoluble.

Strong and weak acids

This is a favourite distinction:

• A strong acid is fully ionised in water: every molecule releases its hydrogen ion. Hydrochloric, sulfuric and nitric acids are strong.
• A weak acid is only partially ionised in water: at any moment only a small fraction of the molecules have released a hydrogen ion. Ethanoic acid (in vinegar) and carbonic acid are weak.

At the same concentration, a strong acid has a higher hydrogen ion concentration than a weak acid, so it has a lower pH and reacts faster. Note that "strong" (degree of ionisation) is not the same as "concentrated" (amount dissolved).

The three characteristic reactions of acids

Acids react in three signature ways, each worth learning with its products:

1. Acid + metal gives a salt + hydrogen. For example, magnesium + hydrochloric acid gives magnesium chloride + hydrogen. You see effervescence and the metal dissolves.
2. Acid + carbonate gives a salt + water + carbon dioxide. For example, calcium carbonate + hydrochloric acid gives calcium chloride + water + carbon dioxide. You see effervescence (the gas turns limewater milky).
3. Acid + base (or alkali) gives a salt + water. This is neutralisation, for example hydrochloric acid + sodium hydroxide gives sodium chloride + water.

Examples in context

Example 1. Indigestion remedies. Antacids contain bases such as magnesium hydroxide or calcium carbonate that neutralise excess stomach acid. The carbonate version fizzes as it gives off carbon dioxide, while both reduce the acidity, a direct everyday use of acid-base reactions.

Example 2. Acid rain attacking limestone. Rain made acidic by dissolved gases reacts with limestone (calcium carbonate) buildings and statues, giving a soluble salt, water and carbon dioxide, so the stone slowly wears away. This is the acid-carbonate reaction happening on a large scale in the environment.

Try this

Q1. State what all acids produce when dissolved in water. [1 mark]

• Cue. Hydrogen ions ($\text{H}^+$).

Q2. Write a word equation for the reaction of zinc with dilute hydrochloric acid. [2 marks]

• Cue. Zinc + hydrochloric acid $\rightarrow$ zinc chloride + hydrogen.

Q3. Explain why a strong acid has a lower pH than a weak acid of the same concentration. [2 marks]

• Cue. A strong acid is fully ionised, so it has a higher concentration of hydrogen ions, and more hydrogen ions means a lower pH.