Indicators and the pH scale
Solutions can be acidic, alkaline or neutral:
- we get an solution when an acid is dissolved in water
- we get an solution when an alkali is dissolved in water
- solutions that are neither acidic nor alkaline are
Pure water is neutral, and so is petrol.
An is a substance that changes colour when it is added to acidic or alkaline solutions. You can prepare homemade indicators from red cabbage or beetroot juice - these will help you see if a solution is acidic or alkaline.
Litmus and universal indicator are two indicators that are commonly used in the laboratory.
Litmus
Litmus indicator solution turns red in acidic solutions and blue in alkaline solutions. It turns purple in neutral solutions.
is usually more reliable, and comes as red litmus paper and blue litmus paper. The table shows the colour changes it can make.
| Red litmus | Blue litmus | |
| Acidic solution | Stays red | Turns red |
| Neutral solution | Stays red | Stays blue |
| Alkaline solution | Turns blue | Stays blue |
| Acidic solution | |
| Red litmus | Stays red |
| Blue litmus | Turns red |
| Neutral solution | |
| Red litmus | Stays red |
| Blue litmus | Stays blue |
| Alkaline solution | |
| Red litmus | Turns blue |
| Blue litmus | Stays blue |
Notice how we say 'stays red'. This is better than saying 'nothing' or 'stayed the same', because it tells us the colour we actually see.
Universal indicator and the pH scale
Universal indicator is supplied as a solution or as . It is a mixture of several different indicators. Unlike litmus, universal indicator can show us how strongly acidic or alkaline a solution is, not just that the solution is acidic or alkaline.
This is measured using the , which is a continuous coloured number scale measuring acids from below zero to alkali's above fourteen.
Universal indicator has many different colour changes, from red for strongly acidic solutions to dark purple for strongly alkaline solutions. In the middle, neutral pH 7 is indicated by green.
When you use universal indicator paper, you get more accurate results if you only put a small spot of the test solution on the paper, and then leave the colour to develop for about 30 seconds before comparing it with the colour chart.
These are the important points about the pH scale:
- neutral solutions are pH 7 exactly
- acidic solutions have pH values less than 7
- alkaline solutions have pH values more than 7
- the closer to pH 0 you go, the more strongly acidic a solution is
- the closer to pH 14 you go, the more strongly alkaline a solution is
