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5.1.3Atomic Structure and the Periodic Table

The pH Scale and Neutralisation

5 min read 5 parts Lesson 1 of 10 Unit quiz

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Section 1 of 5The pH Scale

5.1.3 The pH Scale and Neutralisation

The pH scale measures how acidic or alkaline a solution is, with pH 7 as neutral.

Acidic
  • Solutions with a pH below 7 are acidic, and the lower the pH the stronger the acid tends to be.
Neutral
  • A solution with pH 7, such as pure water, is neither acidic nor alkaline.
Alkaline
  • Solutions with a pH above 7 are alkaline, and the higher the pH the stronger the alkali.
Universal indicator
  • A mixture of indicators that gives an approximate pH by matching its colour to a colour chart.

Section 2 of 5Ions in Acids, Alkalis and Neutralisation

Neutralisation works because the hydrogen ions from the acid and hydroxide ions from the alkali combine.

1
Acids produce hydrogen ions, H⁺, in aqueous solution, which is what makes a solution acidic.
2
Alkalis produce hydroxide ions, OH⁻, in aqueous solution, which is what makes a solution alkaline.
3
When an acid and an alkali are mixed, H⁺ ions and OH⁻ ions react together to form water.
4
Because water is formed, the mixture becomes neutral, with a pH of 7, when the reactants are exactly balanced.

Section 3 of 5Simplest Ionic Equations

Simplest Ionic Equations
Net ionic equation
  • Every acid-base neutralisation can be summarised as H⁺(aq) + OH⁻(aq) → H₂O(l).
Spectator ions
  • Ions that appear unchanged on both sides, such as Na⁺ or SO₄²⁻, are cancelled out to give the simplest ionic equation.
Soil treatment
  • Farmers add lime or quicklime to acidic soil to neutralise it and raise the pH, which helps crops grow.
State symbols
  • The ionic equation should include state symbols, such as (aq) and (l), as these are often asked in exams.

Section 4 of 5Worked Example

Question Write the balanced symbol equation for copper(II) hydroxide reacting with sulfuric acid to form copper(II) sulfate and water, then write the simplest ionic equation.

  1. Balanced symbol equationCu(OH)₂ + H₂SO₄ → CuSO₄ + 2H₂O.
  2. Full ionic equationCu(OH)₂(s) + 2H⁺(aq) + SO₄²⁻(aq) → Cu²⁺(aq) + SO₄²⁻(aq) + 2H₂O(l).
  3. Cancel the spectator ions Cu²⁺ and SO₄²⁻, then simplify to H⁺ + OH⁻ → H₂O.

Answer Simplest ionic equation: H⁺(aq) + OH⁻(aq) → H₂O(l)

Section 5 of 5Exam Checklist & Tips

5.1.3 – Key Points for the Exam

  • pH A scale from 0 to 14 that shows how acidic or alkaline a solution is.
  • Neutral A solution with a pH of 7, where the concentrations of H⁺ and OH⁻ ions are equal.
  • Spectator ion An ion that is present on both sides of an equation and takes no part in the reaction.
  • Universal indicator An indicator that changes through many colours to show the approximate pH of a solution.

Common Mistakes

  • Saying pH 7 is always produced by any mixture of acid and alkali, when the amounts must exactly neutralise each other.
  • Writing the ionic equation with spectator ions still included, rather than cancelling them out.
  • Stating that a higher pH means a more acidic solution.
  • Leaving out the (aq) and (l) state symbols in the ionic equation.
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