Describe the properties of acids and bases in terms of hydrogen and hydroxide ions, classify oxides, and use the pH scale and indicators to measure acidity and alkalinity

What this dot point is asking

SEAB wants you to describe acids and bases in terms of the ions they produce in water, to tell the difference between strong and weak acids, to use the pH scale and indicators to measure acidity, and to classify oxides as acidic, basic, amphoteric or neutral. These ideas underpin everything in the acids-and-salts topic, so the definitions must be exact.

The answer

Acids and the hydrogen ion

An acid is a substance that produces hydrogen ions, $\text{H}^+$, when dissolved in water. It is the hydrogen ion that gives acids their characteristic properties: a sour taste, a pH below 7, and the ability to react with metals, carbonates and bases. Common laboratory acids are hydrochloric acid $\text{HCl}$, sulfuric acid $\text{H}_2\text{SO}_4$ and nitric acid $\text{HNO}_3$.

Bases, alkalis and the hydroxide ion

A base is a substance that neutralises an acid, usually a metal oxide or hydroxide. An alkali is a base that is soluble in water, and it produces hydroxide ions, $\text{OH}^-$, in solution. Sodium hydroxide $\text{NaOH}$ and aqueous ammonia are alkalis; copper(II) oxide is a base but not an alkali because it is insoluble. Alkalis feel soapy, turn red litmus blue, and have a pH above 7.

Strong and weak acids

The strength of an acid is how completely it ionises in water, not how concentrated it is:

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

For the same concentration, a strong acid has a higher hydrogen-ion concentration and a lower pH than a weak acid.

The pH scale and indicators

The pH scale runs from 0 to 14 and measures how acidic or alkaline a solution is:

• pH less than 7 is acidic (the lower the number, the more acidic),
• pH equal to 7 is neutral,
• pH greater than 7 is alkaline (the higher the number, the more alkaline).

Indicators are dyes that change colour with pH. Litmus is red in acid and blue in alkali. Universal indicator gives a range of colours (red through orange and green to blue and purple) so you can read an approximate pH.

Classifying oxides

Oxides are classified by how they behave with acids and alkalis:

• Basic oxides are metal oxides that react with acids to form a salt and water (e.g. $\text{CuO}$, $\text{MgO}$).
• Acidic oxides are non-metal oxides that react with alkalis (e.g. $\text{CO}_2$, $\text{SO}_2$).
• Amphoteric oxides react with both acids and alkalis (e.g. $\text{Al}_2\text{O}_3$, $\text{ZnO}$).
• Neutral oxides react with neither (e.g. water, carbon monoxide).

Examples in context

Example 1. Testing soil pH for gardening. Gardeners measure soil pH with universal indicator or a meter because many plants grow best in slightly acidic to neutral soil. If the soil is too acidic, lime (a basic oxide or hydroxide) is added to raise the pH, a direct application of acid-base classification.

Example 2. Why vinegar is a mild acid. Vinegar contains ethanoic acid, a weak acid, so it is safe to taste and use in cooking even though it is acidic. A strong acid at the same concentration would have a much lower pH and would be far too corrosive, showing why strength matters as much as concentration.

Try this

Q1. State what all acids produce when dissolved in water, and give the pH range of an acidic solution. [2 marks]

• Cue. Acids produce hydrogen ions, $\text{H}^+$; an acidic solution has a pH below 7.

Q2. Classify magnesium oxide and carbon dioxide as acidic or basic oxides, giving a reason for each. [2 marks]

• Cue. Magnesium oxide is basic (a metal oxide that reacts with acids); carbon dioxide is acidic (a non-metal oxide that reacts with alkalis).

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

• Cue. The strong acid is fully ionised, giving a higher hydrogen-ion concentration, and a higher $\text{H}^+$ concentration means a lower pH.