AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
The pH scale and neutralisation: use the pH scale to identify acidic or alkaline solutions.
The pH scale runs from 0 to 14 and shows how acidic or alkaline a solution is.
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The pH scale runs from 0 to 14 and shows how acidic or alkaline a solution is. A pH below 7 means the solution is acidic; the lower the pH, the more acidic the solution. A pH of exactly 7 means the solution is neutral, as in pure water. A pH above 7 means the solution is alkaline; the higher the pH, the more alkaline the solution. For example, a solution of pH 2 is acidic, a solution of pH 7 is neutral, and a solution of pH 12 is alkaline. The pH scale is related to the concentration of hydrogen ions, H⁺: acidic solutions have a higher concentration of H⁺ than neutral or alkaline solutions, while alkaline solutions have a lower concentration of H⁺.
What earns the marks
- The pH scale runs from 0 to 14 and is used to classify solutions as acidic, neutral or alkaline.
- A pH value below 7 indicates an acidic solution, and a lower pH value indicates a more acidic solution.
- A pH value of exactly 7 indicates a neutral solution, such as pure water.
- A pH value above 7 indicates an alkaline solution, and a higher pH value indicates a more alkaline solution.
- Acidic solutions have a higher concentration of hydrogen ions, H⁺, than neutral or alkaline solutions.
- The pH scale can be used to compare the relative acidity or alkalinity of two solutions from their pH values.
Common mistakes and how to put them right
Thinking that a higher pH number means a stronger acid.
Put it right: The opposite is true, because a lower pH means a more acidic solution.
Treating pH 7 as acidic or alkaline.
Put it right: pH 7 is neutral, and only values below 7 are acidic while values above 7 are alkaline.
Assuming the pH scale ends at 14 in all contexts.
Put it right: The scale commonly runs from 0 to 14, but values outside this range are possible for very concentrated solutions.
Exam tips
- Quote the pH value and then state the classification, for example pH 3 is acidic and pH 10 is alkaline.
- When comparing two acids, refer to the lower pH value as the more acidic solution rather than saying it has a higher acidity number.
- Link pH to the concentration of H⁺ ions to show understanding beyond simple colour changes.
Try the lesson
The start of the The pH scale and neutralisation: use the pH scale to identify acidic or alkaline solutions. lesson, made from checked teaching notes for this specification point.
Have a go first
A solution has a pH of 3. Is the solution acidic or alkaline?
Listen
The pH scale runs from 0 to 14 and shows how acidic or alkaline a solution is. A pH below 7 means the solution is acidic; the lower the pH, the more acidic the solution. A pH of exactly 7 means the solution is neutral, as in pure water. A pH above 7 means the solution is alkaline; the higher the pH, the more alkaline the solution. For example, a solution of pH 2 is acidic, a solution of pH 7 is neutral, and a solution of pH 12 is alkaline. The pH scale is related to the concentration of hydrogen ions, H⁺: acidic solutions have a higher concentration of H⁺ than neutral or alkaline solutions, while alkaline solutions have a lower concentration of H⁺. A strong answer earns marks for these points. The pH scale runs from 0 to 14 and is used to classify solutions as acidic, neutral or alkaline. A pH value below 7 indicates an acidic solution, and a lower pH value indicates a more acidic solution. A pH value of exactly 7 indicates a neutral solution, such as pure water. A pH value above 7 indicates an alkaline solution, and a higher pH value indicates a more alkaline solution. Exam tip: Quote the pH value and then state the classification, for example pH 3 is acidic and pH 10 is alkaline.
See it
Key words
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pH scale
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Acidic solution
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Neutral solution
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Alkaline solution
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See every definition
- pH scale
- Runs from 0 to 14 and shows how acidic or alkaline a solution is.
- Acidic solution
- Has a pH below 7, and the lower the pH, the more acidic the solution.
- Neutral solution
- Has a pH of exactly 7, as in pure water.
- Alkaline solution
- Has a pH above 7, and the higher the pH, the more alkaline the solution.
Label the diagram
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1Box 1
2Box 2
3Box 3
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