What does the pH scale measure?

The pH scale measures the concentration of hydrogen ions (H+) in a solution.

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What pH values are associated with acids and alkalis?

  • Acids have pH values below 7.
  • Alkalis have pH values above 7.


A pH of 7 is considered neutral, neither acidic nor alkaline.

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What is the formula to calculate the pH of a solution from the concentration of H+ ions?

pH = -log10[H+(aq)]

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What is the formula to calculate the concentration of H+ ions in a solution from its pH?

[H+(aq)] = 10-pH

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How does the concentration of H+ ions change as the pH of a solution decreases?

As the pH of a solution decreases by one unit, the concentration of H+ ions increases by a factor of 10.


For example, if pH decreases from 7 to 6, [H+] increases from 10-7 to 10-6 mol dm-3.

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What are the steps involved in calculating the pH of a strong base from the concentration of OH- ions in solution?

The steps to calculate the pH of a strong base from [OH-] are:

  1. Rearrange the ionic product of water to find [H+]:

[H+]=[OH−]Kw​​

  1. Use [H+] to find pH:

pH = -log10[H+]

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In what way do the relative concentrations of H+ and OH- ions determine whether a solution is acidic, neutral, or alkaline?

  • Acidic solution: [H+] > [OH-].
  • Neutral solution: [H+] = [OH-].
  • Alkaline solution: [H+] < [OH-].

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Why does water have a neutral pH?

Water has a neutral pH of 7 because it each molecule ionises to produce equal concentrations of H+ and OH- ions. This is shown by the equation:

H2O(l) ⇌ H+(aq) + OH-(aq).

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State and explain how the ionic product of water (Kw) changes when temperature is increased.

The ionic product of water (Kw) increases with temperature.


The equilibrium reaction for the ionisation of water, H2O(l) ⇌ H+(aq) + OH-(aq), is endothermic. As temperature rises, the equilibrium position shifts to the right and Kw increases.

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