The hardness of temporarily and permanently hard water is tested using soap solution.

Class practical and demonstration

Temporarily hard water is made by bubbling carbon dioxide through limewater for some time. Temporarily and permanently hard water are boiled, have sodium carbonate added and are subjected to ion exchange. The hardness of the solutions, before and after, is tested using soap solution.

Lesson organisation

This experiment is designed as a combination of demonstration and class practical work. The making and boiling of temporarily hard water is best done on a larger scale than in a test-tube, and ion exchange columns are tedious to set up in quantity. Also, the students already have a bewildering array of colourless solutions with which to deal. Adding more might cause some to get very confused. With small groups, however, the preparation and boiling of temporarily hard water and the ion exchange could be done by the students.

If the suggested method here is used, the beakers of solutions should be labelled A to F and each should have a dropping pipette. Students should bring up their test-tube racks and move along the solutions, placing 1 cm depth of each solution in the corresponding test-tube.

Do not let them mix the dropping pipettes nor move the stock bottles around.

The demonstration plus the student practical will take about one hour.

Apparatus Chemicals

The teacher will require:

Eye protection

Beakers (500 cm3), 3

Bunsen burner

Tripod and gauze

Heat resistant mat

Vacuum filtration apparatus (Note 1)

Ion exchange apparatus (Note 2)

Gas generator for carbon dioxide (Note 3), or a CO2 cylinder

Each student or group of students will need:

Test-tubes, 9

Test-tube rack

Labels, for test tubes

Beaker (100 cm3)

Dropping pipette


The teacher will require:

Calcium sulfate solution, 300 cm3(Note 4)

Limewater (IRRITANT), 150 cm3

Sodium zeolite, about 5 g (Note 2)

Marble chips

Dilute hydrochloric acid, 2 M (IRRITANT)

Each student or group of students will need:

Solutions labelled as shown below, 100 cm3 each

A Temporarily hard water*

B Permanently hard water (Note 4)

C Temporarily hard water that has been boiled and filtered*

D Temporarily hard water that has passed through an ion exchange column*

E Permanently hard water that has passed through an ion exchange column*

F Distilled water (or deionised water)

* These are prepared by the teacher in the demonstration part of the experiment

Soap solution in IDA (Industrially Denatured Alcohol) (HIGHLY FLAMMABLE, HARMFUL), 10 cm3 (Note 5)

Sodium carbonate-10-water (IRRITANT), about 1 g

Refer to Health & Safety and Technical notes section below for additional information.

Health & Safety and Technical notes

Read our standard health & safety guidance

Wear eye protection throughout. 

Calcium sulfate, CaSO4.2H2O(s) - see CLEAPSS Hazcard.

Limewater (calcium hydroxide solution), Ca(OH)2(aq), (treat as IRRITANT) - see CLEAPSS Hazcard and CLEAPSS Recipe Book.

Sodium zeolite 

Sodium carbonate-10-water, Na2CO3.10H2O(s), (IRRITANT) - see CLEAPSS Hazcard

Marble chips (calcium carbonate), CaCO3(s) - see CLEAPSS Hazcard.

Hydrochloric acid, (IRRITANT) - see CLEAPSS Hazcard and CLEAPSS Recipe Book.  

1 Vacuum filtration apparatus. See CLEAPSS Laboratory Handbook.

2 Ion exchange apparatus. The sodium zeolite should be soaked in deionised water for 24 hours before use. (Dry resin would expand and crack the tube). A cotton wool plug should then be placed at the bottom of a tube with a tap* and the resin added (as a slurry) above the cotton wool. The resin must be kept covered in deionised water until the column is required. Some hard water is poured into the tube above the deionised water and the tap is opened. More hard water is added as the softened water is collected in a beaker below the tap.

* a burette will do, provided the tap is removable, allowing the cotton wool plug to be pushed out with a rod.

3 Gas generator for carbon dioxide. See Standard Techniques: Gas preparation, testing and collection.

4 Calcium sulfate solution. Stir a spatula or two of calcium sulfate dihydrate into distilled water until no more will dissolve (it is not very soluble). Allow to stand and decant off the clear, saturated solution. Dilute it with an equal volume of distilled or deionised water to make the stock solution of permanently hard water.

5 Soap solution in ‘ethanol’ (Industrial Denatured Alcohol, IDA – see CLEAPSS Hazcard, HIGHLY FLAMMABLE, HARMFUL) can be purchased or made up – Genuine liquid soap or soap flakes from which the liquid can be made, are increasingly difficult to obtain. Wanklyn’s and Clarke’s soap solutions should still be available from chemical suppliers. Lux soap flakes are ideal for making liquid soap if you can source them. Granny’s Original and other non-branded soap flakes work fine but need to be used in solution as soon as they are made. They do not form a stable emulsion and precipitate out overnight. Note that most liquid hand washes are based on the same detergents as washing-up liquids and do not contain soap. To obtain soap solution from soap flakes - dissolve soap flakes (or shavings from a bar of soap) in ethanol - use IDA (Industrial Denatured Alcohol) (HIGHLY FLAMMABLE, HARMFUL) - see CLEAPSS Recipe Book. Do not dissolve in water.


a Dilute about 150 cm3 of limewater with an equal volume of distilled or deionised water. Pass in carbon dioxide, taking care that the gas carries over no acid spray (from the reaction between the marble chips and the acid). A milky precipitate of calcium carbonate soon forms. Continue the passage of gas until all the precipitate dissolves, giving a solution of calcium hydrogencarbonate. This is temporarily hard water.

b Place about half of the temporarily hard water in a beaker and boil it for about 5 minutes. Filter, using vacuum filtration apparatus.

c Scrape some of the solid residue from b into a test-tube and add dilute hydrochloric acid. Fizzing should show that the solid is a carbonate (calcium carbonate).

d Boil about the same quantity of permanently hard water (to that used in b) in another beaker. Show that there is no precipitate. Allow the solution to cool until it is safe to handle.

e Set up two ion exchange columns containing sodium zeolite (see note 3). Pour about half of the temporarily hard water from a through one column and collect the solution in a beaker. Repeat with the other ion exchange column using an equal volume of permanently hard water.

Class experiments
a Set up six test tubes in a rack, labelled A – F, containing about 1 cm depth of

     A  Temporarily hard water
     B Permanently hard water 
     C Temporarily hard water that has been boiled and filtered
     D Temporarily hard water that has passed through an ion exchange column
     E Permanently hard water that has passed through an ion exchange column
     F Distilled or deionised water

b Collect 10 cm3 of soap solution in a small beaker.

c Add a drop of soap solution to tube A. Stopper the tube and shake it. If no lather (foamy bubbles) appear, add another drop, stopper and shake again. Continue until a lather appears that lasts for 5 seconds or longer. Count the number of drops that you have used. Note the appearance of the water in the test-tube.

d Repeat the procedure in c for tubes B to F.

e In another test-tube, take a fresh sample of any of one of the water samples that were ‘hard’ (that is, those that took a lot of soap to achieve a lather). Add half a spatula measure of sodium carbonate crystals to the test-tube and shake it. Observe the contents of the test-tube. Now repeat the procedure in c to see how many drops of soap solution are required to produce a lather. Note the new number of drops.

f Repeat the procedure in e for any other water samples that were ‘hard’.

Teaching notes

A and B should require a lot of drops of soap solution, while the others should not require many at all. A and B contain dissolved calcium salts that react with soap solution to form an insoluble ‘scum’ that should be seen as a white cloudiness in the tubes or as specks floating on the surface of the water:

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 hard water wastes soap as well as causing unsightly deposits on baths and showers.

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 limewater is reacted with carbon dioxide)

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

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

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

The calcium carbonate shows as a white cloudiness (precipitate) when the temporarily hard water is boiled. The water does not now contain any dissolved calcium salts, so it is no longer hard.

This solid calcium carbonate is ‘limescale’ that wastes energy if it forms in boilers and kettles and can be dangerous if it blocks pipes or washing machines.

Hard water of both types can also be softened by:

exchanging sodium ions for the calcium ions – these stay on the zeolite resin. This resin is a lattice with negative charges attached. These hold the positive ions. The attachment of the positive ions to the resin is reversible. The resin can be ‘regenerated’ by treating it with concentrated sodium chloride solution.

adding sodium carbonate. This precipitates out the calcium ions as insoluble calcium carbonate:

eg CaSO4(aq) + Na2CO3(aq) → CaCO3(s) + Na2SO4(aq)

The calcium carbonate is once again seen as a white cloudiness.

‘Bath salts’ often contain sodium carbonate (as well as perfume etc) and this softens the water.

‘complexing’ the calcium ions – ie adding large anions that form a ‘complex’ with the calcium ions and stop them reacting with soap to form scum. Water softeners such as ‘Calgon’ work this way. The chemistry of complex ions is beyond the intermediate level, however.

Health & Safety checked, 2016


This Practical Chemistry resource was developed by the Nuffield Foundation and the Royal Society of Chemistry.

© Nuffield Foundation and the Royal Society of Chemistry

Page last updated October 2015