In this experiment the effect of dissolved salts, containing several different cations and anions, on the formation of a lather, is investigated.

Class practical

Hardness in water indicates an inability to form a lather with soap solution. The effect of various dissolved salts, containing several different cations and anions, on the formation of a lather, is investigated.

Lesson organisation

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 cm3 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.

Apparatus Chemicals

Eye protection

Each group of students will require:

Test-tubes, 8

Test-tube rack for 8 tubes

Beaker (100 or 150 cm3)

Dropping pipette

Bungs or corks, for test-tubes

Labels, for test-tubes

Soap solution in IDA (Industrially Denatured Alcohol) (HIGHLY FLAMMABLE, HARMFUL), 50 cm3 per group (Note 1)

Distilled or deionised water, 10 cm3 per group

The following solutions (all 0.1 M) in communal stock bottles, or beakers with dropping pipettes, 10 cm3 per group.

Sodium chloride

Calcium chloride

Magnesium chloride

Potassium nitrate(V)

Sodium sulfate(V)

Iron(II) sulfate(VI)

Magnesium sulfate(VI)

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, CaCl2(aq) - see CLEAPSS Hazcard.

Magnesium chloride solution, MgCl2(aq) - see CLEAPSS Hazcard.  

Potassium nitrate(V) solution, KNO3(aq) - see CLEAPSS Hazcard.   

Sodium sulfate(VI) solution, Na2SO4(aq) - see CLEAPSS Hazcard.  

Iron(II) sulfate(VI) solution, FeSO4(aq) - see CLEAPSS Hazcard and CLEAPSS Recipe Book. 

Magnesium sulfate(VI) solution, MgSO4(aq) - see CLEAPSS Hazcard.  

1 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.

Procedure

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 cm3 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.

Teaching notes

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 

Credits

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 July 2016