AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
As the pH decreases by one unit, the hydrogen ion concentration of the…
Full specification wording
Strong and weak acids (HT only): As the pH decreases by one unit, the hydrogen ion concentration of the solution increases by a factor of 10.
pH is a logarithmic scale: each decrease of one pH unit represents a tenfold increase in hydrogen ion concentration, [H⁺].
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 4 of 5
Study notes
From the checked teaching notes for this specification point.
The idea
pH is a logarithmic scale: each decrease of one pH unit represents a tenfold increase in hydrogen ion concentration, [H⁺]. For example, a solution of pH 3 has ten times the H⁺ concentration of a solution of pH 4, and one hundred times that of a solution of pH 5. Conversely, increasing pH by one unit divides the H⁺ concentration by 10. This relationship explains why small pH changes matter greatly in reactions and in living systems. To use it, compare pH values, find the difference, and raise 10 to that power: a fall of 2 pH units means 10² = 100 times the H⁺ concentration. The rule applies to whole-number pH values at this level.
What earns the marks
- A decrease of one pH unit corresponds to a tenfold increase in hydrogen ion concentration.
- A decrease of two pH units corresponds to a hundredfold increase, because 10² = 100.
- An increase of one pH unit corresponds to dividing the hydrogen ion concentration by 10.
- The relationship is logarithmic, so equal pH steps represent equal multiplicative changes in [H⁺].
- To compare two solutions, subtract the pH values and use the difference as the power of 10.
- The rule links the numerical pH scale directly to the concentration of H⁺ ions.
Common mistakes and how to put them right
Treating the change as additive, for example saying a fall of 2 pH units doubles the H⁺ concentration.
Put it right: The correct change is a factor of 10² = 100.
Reversing the direction, for example saying a pH decrease lowers the H⁺ concentration.
Put it right: A lower pH means a higher H⁺ concentration.
Applying the factor of 10 to the pH value itself rather than to the hydrogen ion concentration.
Exam tips
- Write the pH difference as a power of 10 before calculating, for example 10² = 100.
- Check the direction: lower pH means more H⁺ ions, so the factor should be greater than 1.
- Show your working clearly when comparing two pH values so the examiner can follow your reasoning.
Try the lesson
The start of the Strong and weak acids (HT only): As the pH decreases by one unit, the hydrogen ion concentration of the solution increases by a factor of 10. lesson, made from checked teaching notes for this specification point.
Have a go first
A solution has its pH decreased from 4 to 3. By what factor does the hydrogen ion concentration increase?
Listen
pH is a logarithmic scale: each decrease of one pH unit represents a tenfold increase in hydrogen ion concentration, [H⁺]. For example, a solution of pH 3 has ten times the H⁺ concentration of a solution of pH 4, and one hundred times that of a solution of pH 5. Conversely, increasing pH by one unit divides the H⁺ concentration by 10. This relationship explains why small pH changes matter greatly in reactions and in living systems. To use it, compare pH values, find the difference, and raise 10 to that power: a fall of 2 pH units means 10² = 100 times the H⁺ concentration. The rule applies to whole-number pH values at this level. A strong answer earns marks for these points. A decrease of one pH unit corresponds to a tenfold increase in hydrogen ion concentration. A decrease of two pH units corresponds to a hundredfold increase, because 10² = 100. An increase of one pH unit corresponds to dividing the hydrogen ion concentration by 10. The relationship is logarithmic, so equal pH steps represent equal multiplicative changes in [H⁺]. Exam tip: Write the pH difference as a power of 10 before calculating, for example 10² = 100.
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