Testing Hardness of water

Procedure

a Collect about 75 cm³ of soap solution in a small beaker.

b Set up a burette and, using the small funnel, fill it with soap solution.

c Use a measuring cylinder to measure out 10 cm³ of one of the samples of water from the list below into a conical flask.

     – A Rain water

     – B Sea-water

     – C Temporarily hard water

     – D Boiled temporarily hard water 

     – E Boiled sea-water

d Read the burette. Add 1 cm³ of soap solution to the water in the conical flask. Stopper the flask and shake it. If a lather appears that lasts for 30 seconds, stop and read the burette.

e If no lather forms, add another 1 cm³ of soap solution. Shake the flask. Repeat the process until a lather forms that lasts for 30 seconds. Read the burette.

f Rinse out the flask with distilled water. Repeat the experiment with 10 cm³ of another water sample, until you have tested them all. Make a note of the volumes of soap solution that were needed in each case to produce a lather.

g From your experiments, decide:

which water samples are ‘soft’ and why

whether sea water contains permanent hardness, temporary hardness or a mixture of both.

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Teaching notes

Sample A will require very little soap solution. This shows that rainwater is soft. It has effectively been distilled (and like distilled water, it will contain dissolved carbon dioxide but no salts).

Sample D will also require very little soap. This shows that temporarily hard water can be softened by boiling (see theory below).

The other samples will require more soap but E will require less than B, showing that sea water contains both temporary and permanent hardness.

The volumes of soap solution needed give a measure of the relative hardness of the various samples. With more able groups, it might be worth considering that rainwater is completely soft, so that the volume of soap required here is just the amount required to get a lather, not to overcome hardness. This volume should be subtracted from the other volumes before the relative hardnesses are compared.

Hard water contains dissolved calcium (or magnesium) salts that react with soap solution to form an insoluble scum that should be seen as a white cloudiness in the tubes:

calcium salt(aq) + sodium stearate (soap)(aq) → calcium stearate (scum)(s) + sodium salt(aq)

Only when all the calcium ions have been precipitated out as scum will the water lather. Thus the volume of soap solution measures the amount of hardness.

Temporarily hard water is defined as that which can be softened by boiling. The reactions by which it is made here are:

Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)
(Calcium carbonate is the ‘milkiness’ that forms when lime water is reacted with carbon dioxide)

CaCO₃(s) + CO₂(g) + H₂O(l) → Ca(HCO₃)₂(aq) (calcium hydrogen carbonate)

This reaction also occurs when rain water (containing dissolved carbon dioxide) flows over limestone rocks. On boiling, the reaction is reversed, softening the water:

Ca(HCO₃)₂(aq) → CaCO₃(s) + CO₂(g) + H₂O(l)

Permanently hard water contains calcium or magnesium salts other than the hydrogen carbonates. These are unaffected by boiling

 

For Further details:

http://www.rsc.org/learn-chemistry/resource/res00000426/testing-the-hardness-of-water?cmpid=CMP00005922