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activity: pH Testing

The pH scale is a way of measuring how acidic or alkaline a substance is.

We use indicators to show us the pH of a substance. Indicators like universal indicator change colour depending on whether a substance is acid or alkaline. Universal indicator is particularly useful because it changes to a range of different colours, so tells us how acidic, or how alkaline a substance is. If it changes red, orange or yellow (pH 1-6), it tells us the substance is acidic. A colour change of greenish blue, blue or purple (pH 7-14) indicates an alkali. If it turns green, then the substance is neutral (pH 7).

Most fish prefer water with a pH between 7 and 8. If the pH is outside the range 5 to 8.5 the water is seriously polluted with acid or alkali.

What you will need:

• 5 samples of water - tap water, distilled water and 3 other sources (this could be tap water from a different area, pond water, river water, sea water)
• narrow range pH paper
• several clean, dry test tubes
• test tube rack, or large beaker to hold the test tubes in

Click here for more information on collecting water samples.

What you will do:

1. Pour 1 cubic cm of a water sample into a test tube.
2. Add the pH paper and note down the colour change.
3. Pour the water sample away and throw the pH paper in the bin. Don't leave the pH paper in the sink!!

What do you observe?

You can record your results in a table like this:

Questions:

1. Did you find any pattern to your results? For instance, were the water samples mostly acidic, alkaline or neutral?
2. Why was it important to use dry test tubes for this experiment?

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