Page 32 - Big Ideas Year 10 sciPAD 2025

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32 SCIPAD - BIG IDEAS - BOOK 2
BASIS OF CHEMICAL REACTIONS
Indicators and the pH Scale
Scientists need a way of identifying acids and bases without having to taste them (for obvious
reasons!) or undertaking time-consuming chemical tests. They discovered a number of natural
dyes that change colour depending on whether they are placed in an acid or base. These dyes
are called indicators. We use two common types of indicator in the school laboratory - litmus
and Universal indicator.
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
Litmus Indicator
Litmus indicator solution turns red in acidic
solutions and blue in alkaline solutions. Litmus
usually comes in a paper form. Blue litmus
paper will turn red in an acidic solution and
remain blue when added to an alkali solution.
Litmus does not tell you how concentrated
the solution is.
Universal Indicator
Universal indicator shows us how strongly acidic
or alkaline a solution is. Universal indicator has
many different colour changes, from red for
strong acids to dark purple for strong bases. In
the middle, neutral pH 7 is indicated by green.
Neutral solutions are neither acidic nor basic.
1. Outline the purpose of an indicator. 

2. If a student tested a solution with blue litmus paper and the paper remained blue, what
could this indicate about the solution? 

The pH Scale
The pH scale is used to measure how acidic or basic a solution is. The scale ranges from 0 to 14.
Acids have a pH less than 7. The lower the pH the more acidic a solution is. Bases have a pH
greater than 7. The higher the pH the more basic a solution is. Solutions which are neither acidic
nor basic are neutral and have a pH of 7.
1. Complete the following paragraph about the pH scale.
The scale is used to how acidic or basic a
solution is. A very strong acid will have a pH of and a strong base
will have a pH of . Solutions such as pure water have a pH of 7,
meaning they are .
2. Using coloured pencils, complete the diagram below by colouring in the various colours for
Universal indicator at various pH.
3. Match the correct description to each pH number.
pH 1 • • slightly basic
pH 5 • • neutral
pH 7 • • strongly basic
pH 9 • • slightly acidic
pH 13 • • strongly acidic
4. Describe the colour of the following solutions if Universal indicator was added.
(a) Saliva is a weak acid with a pH of about 6. 
(b) Toothpaste is a weak base with a pH around 9. 
(c) Vinegar has a pH between 2.5 and 3.5. 
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