Testing water hardness
Tap water in some parts of the country is very pure and is said to be ‘soft’. It easily makes a lather with soap.
Water from other parts may contain various dissolved impurities and is described as ‘hard’ water. Temporary hardness may be removed by boiling, but permanent hardness survives the boiling process. In this practical activity, water hardness can be measured by finding out the volume of soap solution required to form a permanent lather with a known volume of water.
What to record
Record the volume of soap needed to produce a permanent lather with each type of water. Note any difference between the appearance of the samples after the addition of soap solution.
Volume of soap required to produce 3Water type a permanent lather /cm
Temporary hard water
Seawater, boiled then cooled (permanent
Temporary hard water, boiled then cooled
What to do
1. Collect a conical flask and bung. Check the bung is a good fit.
3 2. Measure 10 cmof water sample into a conical flask using a measuring cylinder.
33. Using the burette add 1 cm of soap solution to the water. Stopper the flask and shake vigorously. If no lather is 3produced, add another 1 cm of soap solution. Continue in this way until a permanent lather (one that lasts for 30
seconds) is obtained. Record the volume of soap solution needed to produce a permanent lather. 4. Repeat this procedure for the other water samples.
Wear eye protection.
1. Is the rainwater hard or soft?
2. Is seawater hard or soft?
3. Does seawater contain temporary hardness, permanent hardness or both?