What ions cause hardness in water?
in water indicates an inability to form a Hardness
with lather . The effect of various dissolved soap solution , containing several different salts and cations , on the formation of a lather, is investigated. anions
This can be done as a class practical, with the students working in pairs or larger groups.
Students need first to label eight test-tubes, or they could each investigate distilled water plus three or four others - making sure they had at least one calcium or magnesium salt and at least one sodium or potassium salt.
They then need to be carefully organised to collect the solutions from the stock bottles. It would be best for them to bring their test-tube racks (or single test-tubes) to the stock bottles. Full pipettes should not be carried around, nor should the stock bottles be moved.
They can each then collect a small beaker containing about 50 cm
3 of soap solution. They use a dropping pipette to add this to the distilled water and the salt solutions.
This experiment should take about 30 minutes.
Each group of students will require:
Test-tube rack for 8 tubes
Beaker (100 or 150 cm
Bungs or corks, for test-tubes
Labels, for test-tubes
Soap solution in IDA (Industrially Denatured Alcohol) (HIGHLY FLAMMABLE, HARMFUL), 50 cm
3 per group (Note 1)
Distilled or deionised water, 10 cm
3 per group
The following solutions (all 0.1 M) in communal stock bottles, or beakers with dropping pipettes, 10 cm
3 per group.
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.
Sodium chloride solution, NaCl(aq) - see CLEAPSS Hazcard and CLEAPSS Recipe Book.
Calcium chloride solution, CaCl
2(aq) - see CLEAPSS Hazcard.
Magnesium chloride solution, MgCl
2(aq) - see CLEAPSS Hazcard.
Potassium nitrate(V) solution, KNO
3(aq) - see CLEAPSS Hazcard.
Sodium sulfate(VI) solution, Na
2SO 4(aq) - see CLEAPSS Hazcard.
Iron(II) sulfate(VI) solution, FeSO
4(aq) - see CLEAPSS Hazcard and CLEAPSS Recipe Book.
Magnesium sulfate(VI) solution, MgSO
4(aq) - see CLEAPSS Hazcard.
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 Set up eight labelled test-tubes, each containing 1 cm depth of one of the following:
distilled or deionised water
sodium chloride solution
calcium chloride solution
magnesium chloride solution
potassium nitrate solution
sodium sulfate solution
iron(II) sulfate solution
magnesium sulfate solution
b Collect about 50 cm 3 of soap solution in a small beaker.
c Use a dropping pipette to add a 1 cm depth of soap solution to each test-tube.
d Stopper each tube in turn and shake vigorously.
e Note which tubes contain a foamy ‘lather’ and which do not.
f Try to work out which ions are preventing the lathering.
The calcium, magnesium and iron(II) ions cause ‘hardness’, that is they stop the lathering that should be apparent in the distilled water and the other test-tubes. Intermediate students should be able to track the cause of hardness down to these cations and say that the anions make no difference.
Hardness of water is usually attributed to calcium and magnesium salts but any cation with two or more charges can cause it (eg iron(II) here). The cation will form a precipitate (scum) with soap, eg:
calcium chloride + sodium stearate (soap) → calcium stearate (scum) + sodium chloride
This scum wastes soap and can cause abrasion to clothes, as well as causing unsightly deposits round baths and showers.
Temporarily hard water, which contains the hydrogencarbonates (‘bicarbonates’) of calcium and magnesium, releases the carbonates of these metals on boiling (limescale). This can ‘fur up’ kettles, boilers and pipes, which wastes energy and can be dangerous if the flow of water is impeded.
Health & Safety checked, July 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
- extends the theory above and includes chemical equations
Page last updated July 2016