We can use a titration to get an approximate measure of the hardness of water samples. Watch the animation.

Here are results obtained by a student who tested equal volumes (25cm³) of water obtained from different areas across the UK.

With each water sample, the student used a burette to find the volume of soap solution needed to give a permanent lather. This gives a measure of the hardness of the water sample.

Area of origin of sample Burette readings (cm³) Volume of soap solution to give permanent lather (cm³)
Start Finish
Dŵr Cymru 0.1 12.4 12.3
Thames Water 15.3 36.5
Severn Trent 17.5 10.5
South West 12.8 4.1

Calculate the missing values and answer the question.

Suggest what else the student should do before drawing any firm conclusions about whether the water in these areas is 'hard' or 'soft'.

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