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What makes a solution acidic or alkaline, and what happens when they react?

Acids and bases: the pH scale and the effect of dilution, forming acids and alkalis from oxides, neutralisation and naming salts, spectator ions, and titration calculations.

An SQA National 5 Chemistry answer on acids and bases, covering the pH scale and dilution, how non-metal and metal oxides form acids and alkalis, neutralisation and naming salts, spectator ions, and titration calculations using the mole.

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  1. What this key area is asking
  2. The pH scale
  3. The effect of dilution
  4. Forming acids and alkalis
  5. Neutralisation and naming salts
  6. Spectator ions
  7. Worked example: a titration calculation
  8. Examples in context
  9. Try this

What this key area is asking

The SQA wants you to use the pH scale and explain the effect of dilution, describe how acids and alkalis form from dissolving oxides, write neutralisation reactions and name the salts, identify spectator ions, and carry out a titration calculation. It pulls together the mole work from the previous key area and is a reliable source of extended-answer marks.

The pH scale

The effect of dilution

Adding water to a solution always moves its pH towards 7:

  • Diluting an acid lowers the concentration of hydrogen ions, so the pH rises towards 7.
  • Diluting an alkali lowers the concentration of hydroxide ions, so the pH falls towards 7.

Dilution never carries an acid above 7 or an alkali below 7; it only approaches neutral.

Forming acids and alkalis

Neutralisation and naming salts

The name of the salt has two parts:

  • The first part comes from the metal (or ammonium): sodium, calcium, magnesium.
  • The second part comes from the acid: hydrochloric acid gives a chloride, sulfuric acid gives a sulfate, nitric acid gives a nitrate.

So hydrochloric acid plus sodium hydroxide gives sodium chloride, and sulfuric acid plus magnesium oxide gives magnesium sulfate.

Spectator ions

In every acid-alkali neutralisation the real reaction is the same:

H+(aq)+OH−(aq)→H2O(l)\text{H}^+(aq) + \text{OH}^-(aq) \rightarrow \text{H}_2\text{O}(l)

The metal ion from the alkali and the negative ion from the acid are the spectators.

Worked example: a titration calculation

Examples in context

Acid-base chemistry runs through daily life. Indigestion remedies contain a base such as magnesium hydroxide that neutralises excess stomach acid, forming a harmless salt and water. Farmers add lime (calcium oxide or calcium carbonate) to neutralise acidic soil so crops grow better. Titration is the standard laboratory way to check the exact concentration of an acid or alkali, used in food testing and quality control, and it relies entirely on the mole and the balanced equation.

Try this

Q1. State the pH range of an alkaline solution and name the ion responsible. [2 marks]

  • Cue. Greater than 7; the hydroxide ion, OH−\text{OH}^-.

Q2. Name the salt formed when sulfuric acid reacts with sodium hydroxide. [1 mark]

  • Cue. Sodium sulfate (plus water).

Q3. 30 cm330 \text{ cm}^3 of acid is neutralised by 0.0030 mol0.0030 \text{ mol} of alkali in a 1 to 1 reaction. Calculate the concentration of the acid. [2 marks]

  • Cue. Moles of acid =0.0030= 0.0030; C=0.0030/0.030=0.10 mol l−1C = 0.0030 / 0.030 = 0.10 \text{ mol l}^{-1}.

Exam-style practice questions

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

SQA N5 2019 style3 marksDescribe what happens to the pH of an acidic solution and an alkaline solution as each is diluted with water, and state the pH of a neutral solution.
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Markers reward the direction for the acid, the direction for the alkali, and the neutral value.

As an acidic solution is diluted with water, the concentration of hydrogen ions falls, so the pH rises towards 7 (but does not go above 7).

As an alkaline solution is diluted with water, the concentration of hydroxide ions falls, so the pH falls towards 7 (but does not go below 7).

A neutral solution has a pH of 7. The key idea is that adding water always moves the pH towards 7 from whichever side it started on.

SQA N5 2021 style4 marksIn a titration, 20.0 cm cubed of sodium hydroxide solution was exactly neutralised by 25.0 cm cubed of 0.10 mol per litre hydrochloric acid. The equation is HCl + NaOH gives NaCl + H2O. Calculate the concentration of the sodium hydroxide solution in mol per litre.
Show worked answer →

A 4 mark answer needs the moles of acid, the mole ratio, the moles of alkali, and the concentration.

First find the moles of acid, converting the volume to litres:

n(HCl)=C×V=0.10×25.01000=0.0025 moln(\text{HCl}) = C \times V = 0.10 \times \frac{25.0}{1000} = 0.0025 \text{ mol}

The equation shows a 1 to 1 ratio of HCl to NaOH, so the moles of sodium hydroxide are also 0.0025 mol.

Then find the concentration of the alkali using its own volume in litres:

C(NaOH)=nV=0.002520.0/1000=0.00250.020=0.125 mol l−1C(\text{NaOH}) = \frac{n}{V} = \frac{0.0025}{20.0/1000} = \frac{0.0025}{0.020} = 0.125 \text{ mol l}^{-1}

The usual error is mixing up which volume goes with which substance; always pair the moles of a substance with its own volume.

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