Describe the properties of acids and bases, use the pH scale and indicators, and write the products of acid reactions with metals, bases and carbonates

What this dot point is asking

SEAB wants you to describe the properties of acids and bases, to use the pH scale and indicators to classify a solution, and to write down what is produced when an acid reacts with a metal, a base or a carbonate. The central idea is that acids and alkalis are opposites, and the pH scale measures where a solution sits between them.

The answer

Acids and bases

• An acid is a substance that forms an acidic solution with a pH below $7$. Common examples are hydrochloric acid, sulfuric acid and nitric acid. Acids taste sour and turn blue litmus red.
• A base is a substance that reacts with and neutralises an acid. A base that dissolves in water is called an alkali, and its solution has a pH above $7$. Alkalis turn red litmus blue and feel soapy.
• A neutral solution, such as pure water, has a pH of exactly $7$.

The pH scale and indicators

The pH scale runs from about $0$ to $14$ and tells you how acidic or alkaline a solution is:

• pH below $7$: acidic (the lower the number, the stronger the acid),
• pH equal to $7$: neutral,
• pH above $7$: alkaline (the higher the number, the stronger the alkali).

Universal indicator changes colour across the scale: red and orange for acids, green for neutral, and blue and purple for alkalis. Litmus is a simpler indicator: red in acid, blue in alkali.

The three reactions of acids

Acids react in three patterns you must learn:

• acid plus metal gives a salt plus hydrogen,
• acid plus base (or alkali) gives a salt plus water (this is neutralisation),
• acid plus carbonate gives a salt plus water plus carbon dioxide.

A salt is the compound formed when the hydrogen of an acid is replaced by a metal.

Testing for the gases

Two of these reactions give off a gas, and you must know how to test for each:

• hydrogen (from acid plus metal): hold a lighted splint at the mouth of the tube. Hydrogen burns with a squeaky pop.
• carbon dioxide (from acid plus carbonate): bubble the gas through limewater. Carbon dioxide turns the limewater milky (cloudy).

These quick tests let you confirm which reaction has taken place from the gas produced.

Strong and weak, concentrated and dilute

Be careful not to mix up two different ideas. The pH tells you how strong an acid is (how readily it produces acidic particles), while concentration tells you how much acid is dissolved in the water. A strong acid has a low pH; a concentrated acid simply has a lot of acid in a small volume. An acid can be strong but dilute, or weak but concentrated.

Examples in context

Example 1. Treating indigestion. Too much hydrochloric acid in the stomach causes discomfort. Indigestion tablets contain a base such as a carbonate or hydroxide that neutralises the excess acid, forming a salt and water and raising the pH back towards neutral.

Example 2. Correcting acidic soil. Farmers add lime (a base) to soil that is too acidic for crops. The neutralisation reaction raises the soil pH towards $7$, which lets a wider range of plants grow well.

Try this

• Cue. State the pH of a strong alkali. A high value, around pH $13$ or $14$.
• Cue. Name the products of hydrochloric acid plus sodium carbonate. A salt (sodium chloride), water, and carbon dioxide.
• Cue. State the colour of universal indicator in a neutral solution. Green.