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What makes a substance acidic or alkaline, and how does the pH scale measure it?

Describe the properties of acids and bases, use the pH scale and indicators, and explain neutralisation

A focused answer to the N(A) Chemistry outcome on acids and bases. The properties of acids and alkalis, the pH scale and indicators such as litmus and universal indicator, and what happens during neutralisation.

Generated by Claude Opus 4.88 min answer

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  2. The answer
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What this dot point is asking

The syllabus wants you to describe the properties of acids and bases (alkalis), to use the pH scale and indicators such as litmus and universal indicator, and to explain neutralisation. These ideas run through the rest of the module, since making salts depends on acids reacting with bases. The key idea is that acids and alkalis are opposites that cancel each other, and the pH scale measures where a solution sits between them.

The answer

Properties of acids

An acid is a substance that forms hydrogen ions, H+\text{H}^{+}, when dissolved in water. Acids:

  • have a pH below 7,
  • turn blue litmus red,
  • taste sour (never tasted in the lab),
  • react with metals, carbonates, and bases.

Common acids are hydrochloric acid, sulfuric acid, and nitric acid.

Properties of bases and alkalis

A base is a substance that reacts with an acid to neutralise it. A base that dissolves in water is called an alkali, and it forms hydroxide ions, OH−\text{OH}^{-}. Alkalis:

  • have a pH above 7,
  • turn red litmus blue,
  • feel soapy.

Common alkalis are sodium hydroxide and aqueous ammonia.

The pH scale and indicators

The pH scale runs from about 00 to 1414 and tells you how acidic or alkaline a solution is:

  • below 7 is acidic (the lower, the more acidic),
  • exactly 7 is neutral, like pure water,
  • above 7 is alkaline (the higher, the more alkaline).

An indicator changes colour with pH. Litmus is red in acid and blue in alkali. Universal indicator gives a range of colours, red for strong acid, green for neutral, and purple for strong alkali, so it gives an approximate pH value.

Neutralisation

Neutralisation is the reaction between an acid and a base to form a salt and water. The hydrogen ions from the acid react with the hydroxide ions from the alkali:

H++OH−→H2O\text{H}^{+} + \text{OH}^{-} \rightarrow \text{H}_2\text{O}

As an acid is added to an alkali, the pH falls toward 77.

Examples in context

Example 1. Treating an acidic stomach. Indigestion is caused by too much stomach acid. Antacid tablets contain a base such as magnesium hydroxide that neutralises the excess acid, raising the pH back toward a comfortable level. This is neutralisation used in medicine.

Example 2. Correcting acidic soil. Farmers spread lime, a base, on soil that is too acidic for crops. The base neutralises the acid in the soil, raising the pH so that plants grow better. Choosing how much lime to add is a real use of the pH scale.

Try this

Q1. State the colour of litmus paper in (a) an acid and (b) an alkali. [2 marks]

  • Cue. In an acid litmus is red; in an alkali litmus is blue.

Q2. A solution has a pH of 22. State whether it is acidic, neutral, or alkaline, and name the ion responsible. [2 marks]

  • Cue. It is acidic (pH below 77); the ion responsible is the hydrogen ion, H+\text{H}^{+}.

Q3. Write the ionic equation for the neutralisation of an acid by an alkali. [1 mark]

  • Cue. H++OH−→H2O\text{H}^{+} + \text{OH}^{-} \rightarrow \text{H}_2\text{O}.

Exam-style practice questions

Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Original4 marksA student tests three solutions with universal indicator. Solution A gives a red colour, solution B gives green, and solution C gives purple. (a) State the approximate pH of each. (b) State which solution is neutral and which is most strongly alkaline.
Show worked answer →

(a) Red is a low pH, about pH 11 to 22 (strongly acidic). Green is pH 77 (neutral). Purple is a high pH, about pH 1313 to 1414 (strongly alkaline).

(b) Solution B (green, pH 77) is neutral. Solution C (purple, high pH) is the most strongly alkaline.

What markers reward: red as a low acidic pH, green as pH 77 neutral, purple as a high alkaline pH, and correctly naming B as neutral and C as most alkaline.

Original3 marksHydrochloric acid is added slowly to sodium hydroxide solution until the mixture is neutral. (a) Name this type of reaction. (b) State what the pH does as the acid is added. (c) Name a suitable indicator to show when the mixture is neutral.
Show worked answer →

(a) The reaction is neutralisation.

(b) The pH starts high (alkaline) and falls toward 77 as the acid is added, reaching 77 when the mixture is neutral.

(c) A suitable indicator is litmus (turns from blue to red at the end) or universal indicator (green at neutral); a single indicator such as methyl orange or phenolphthalein is also acceptable.

What markers reward: naming neutralisation, the pH falling from high toward 77, and a sensible indicator.

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