A solution with pH 7 is neutral. Aqueous solutions of acids have pH values…

The idea

The pH scale runs from 0 to 14 and tells you how acidic or alkaline an aqueous solution is. A solution with pH 7 is neutral, for example pure water at room temperature. Aqueous solutions of acids have pH values below 7: the lower the number, the more acidic the solution, so pH 1 is more acidic than pH 5. Aqueous solutions of alkalis have pH values above 7: the higher the number, the more alkaline the solution, so pH 13 is more alkaline than pH 9. The scale is continuous, so a solution with pH 6.5 is weakly acidic. You can measure pH approximately using universal indicator, which changes colour across the range, or more precisely with a pH meter. Knowing the pH allows you to classify a solution as acidic, neutral or alkaline.

What earns the marks

• States that pH 7 is neutral, using pure water as a familiar example.
• States that aqueous acidic solutions have pH values below 7, and that lower values indicate greater acidity.
• States that aqueous alkaline solutions have pH values above 7, and that higher values indicate greater alkalinity.
• Uses the pH scale to classify a solution as acidic, neutral or alkaline from a given pH value.
• Recognises that the pH scale is continuous, so non-integer values such as pH 6.5 are possible and meaningful.
• Links the use of universal indicator to obtaining an approximate pH value, and a pH meter for a precise measurement.

Common mistakes and how to put them right

• Thinking that pH 7 is acidic or alkaline rather than neutral.

  Put it right by linking pH 7 to pure water and neutral solutions.

• Believing that a higher pH always means a stronger acid.

  Put it right by stating that acids have pH below 7 and that lower values mean greater acidity.

• Assuming pH values must be whole numbers.

  Put it right by explaining that the scale is continuous and can include decimals such as pH 6.5.

Exam tips

• When asked to classify a solution, quote the pH value and state whether it is below, equal to or above 7 before naming it acidic, neutral or alkaline.
• If a question gives a colour from universal indicator, match it to the correct region of the pH scale rather than guessing a single number.
• Use comparative language such as more acidic or more alkaline when comparing two pH values, and justify it with the numerical values.