Testing the hardness of water
Soap solution is used to measure the hardness of rain water, temporarily hard water and sea water. The effect of boiling both the hard water samples is investigated.
This is a student practical, where a lot of the preparation work has been done beforehand. It could be varied so that the students watched, or carried out themselves, the preparation of the solutions. This would require using real (or simulated) sea water, rather than mixing temporarily and permanently hard water. Also the temporarily hard water will really need to be boiled and cooled (as opposed to distilled water being substituted).
For younger, or less practically experienced students, consider providing the burettes already clamped and full of soap solution.
Students should bring their conical flasks to the stock bottles of A to E and use a dedicated measuring cylinder for each solution to obtain 10 cm3. With larger groups, consider telling different groups to start with a different letter.
The work as described will take about 45 minutes.
Soap solution (HIGHLY FLAMMABLE, HARMFUL), 75 cm3 per group (Note 1)
A supply of distilled or deionised water for rinsing their flask between experiments
Solutions as below, about 20 cm3 per group:
A Deionised water – labelled as Rain water
B A 50/50 mixture of temporarily and permanently hard water – labelled as Sea water
C Temporarily hard water (Note 2) – labelled as Temporarily hard water
D Deionised water – labelled as Boiled temporarily hard water
E Permanently hard water diluted 50/50 with deionised water (Note 3) – labelled as Boiled sea water
Refer to Health & Safety and Technical notes section below for additional information.
Measuring cylinders (10 cm3), 5 (one for each of A to E below)
Each group of students will need:
Conical flask (100 cm3)
Bung, to fit the conical flask
Burette and burette stand
Health & Safety and Technical notes
Wear eye protection throughout.
1 Soap solution in ‘ethanol’ (Industrial Denatured Alcohol, IDA – see CLEAPSS Hazcard) (HIGHLY FLAMMABLE, HARMFUL) can be purchased or made up – see CLEAPSS Recipe Book.
2 Dilute about 150 cm3 of limewater* (IRRITANT) with an equal volume of distilled water. Pass in carbon dioxide (see Standard Techniques: Generating, collecting and testing gases Generating, collecting and testing gases), taking care that the gas carries over no acid spray, whereupon calcium carbonate is soon precipitated. Continue the passage of gas until all the precipitate dissolves, giving a solution of calcium hydrogencarbonate. This is temporarily hard water.
* Limewater (calcium hydroxide solution) (IRRITANT) - see CLEAPSS Hazcard and CLEAPSS Recipe Book.
3 Stir a spatula or two of hydrated calcium sulfate - see CLEAPSS Hazcard - into some deionised water. Swirl to mix, allow to stand, then decant off the clear solution. This is permanently hard water.
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.
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 hydrogencarbonate)
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 hydrogencarbonates. These are unaffected by boiling.
Health & Safety checked May 2008
'Doc Brown' - a very readable summary of the above, and more!
Illinois State Water Survey (ISWS) - puts soap titrations into the context of the techniques used (or historically used) by water laboratories.
Page last updated on 31 July 2012