The pH Scale

Use the pH scale and indicators to measure how acidic or alkaline a solution is.

About 3 min read
  • Describe the pH scale
  • Identify acidic, neutral and alkaline solutions
  • Use universal indicator
Before you start

You know that acids and alkalis are chemical opposites, and that some household substances contain them.

The pH scale

The pH scale shows how acidic or alkaline a solution is. It usually runs from 0 to 14. A solution with pH 7 is neutral. Solutions with a pH below 7 are acidic, and solutions with a pH above 7 are alkaline. Lemon juice is acidic, pure water is neutral, and bleach is strongly alkaline.

Definition: pH

A number that shows how acidic or alkaline a solution is.

Definition: indicator

A substance that changes colour depending on the pH of a solution.

Measuring pH

Universal indicator shows many colours. Strong acids turn it red, weak acids turn it orange or yellow, neutral solutions turn it green, and alkalis turn it blue or purple. A pH meter gives a more exact number than matching a colour by eye.

Identify a solution

Universal indicator turns purple in a cleaning solution. What can you say about it?

Purple is at the alkaline end of the colour chart, which matches about pH 13.

Answer: It is a strong alkali.

Lower does not mean weaker

Lower pH means more strongly acidic. pH 1 is much more acidic than pH 5.

Exam focus
Say it like this“pH 3 is acidic because it is below 7.”
Not like this“pH 3 is low so it is weak.” — A low pH shows a strong acid, not a weak one.
Common mistakeThinking that a high pH means a stronger acid. High pH values are alkaline.

Practice

Is pH 11 acidic, neutral or alkaline?
Alkaline, because it is above 7.
What pH is a neutral solution?
pH 7.
What colour does universal indicator turn in a strong acid?
Red.
Going further
A student adds alkali to an acid. The pH rises from 2 to 7. Explain what happened.
The alkali neutralised the acid, so the solution became less acidic until it was neutral.

Quick check

pH quick check

Which pH shows the strongest acid?

The further below 7 a solution is, the more strongly acidic it is.

What colour is universal indicator in a neutral solution?

The middle of the colour chart matches pH 7, between acids and alkalis.

Why is a pH meter better than indicator paper?

It shows a measured number and avoids guessing between similar colours.
Key takeaways
  • The pH scale usually runs from 0 to 14.
  • Below 7 is acidic, 7 is neutral and above 7 is alkaline.
  • Indicators change colour with pH.
  • pH meters give more exact readings.
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