Calcium, magnesium and iron(II) ions cause hardness in water, preventing a lather being formed with soap solution. In this experiment, students investigate the effect of various dissolved salts, containing several different cations and anions, on the formation of a lather. They then try to identify which ions are preventing lathering

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.

Equipment

Apparatus

  • Eye protections
  • Test tubes x8
  • Test tube rack for 8 tubes
  • Beaker, 100 or 150 cm3
  • Dropping pipette
  • Bungs or corks for test tubes
  • Labels for test tubes

Chemicals

  • Soap solution in IDA (Industrially Denatured Alcohol) (HIGHLY FLAMMABLE, HARMFUL), 50 cm3 per group (note 10)
  • 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)

Health, safety and technical notes

  • Read our standard health and safety guidance.
  • Wear eye protection throughout.
  • Sodium chloride solution, NaCl(aq) – see CLEAPSS Hazcard HC047b and CLEAPSS Recipe Book RB082.
  • Calcium chloride solution, CaCl2(aq) – see CLEAPSS Hazcard HC019A.
  • Magnesium chloride solution, MgCl2(aq) – see CLEAPSS Hazcard HC059b.
  • Potassium nitrate(V) solution, KNO3(aq) – see CLEAPSS Hazcard HC093.
  • Sodium sulfate(VI) solution, Na2SO4(aq) – see CLEAPSS Hazcard HC095B.
  • Iron(II) sulfate(VI) solution, FeSO4(aq) – see CLEAPSS Hazcard HC055B and CLEAPSS Recipe Book RB052.
  • Magnesium sulfate(VI) solution, MgSO4(aq) – see CLEAPSS Hazcard HC059b.
  • Soap solution in ‘ethanol’ (Industrial Denatured Alcohol, IDA – see CLEAPSS Hazcard HC040A, 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). Do not dissolve in water.

Procedure

  1. 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
  2. Collect about 50 cm3 of soap solution in a small beaker.
  3. Use a dropping pipette to add a 1 cm depth of soap solution to each test tube.
  4. Stopper each tube in turn and shake vigorously.
  5. Note which tubes contain a foamy lather and which do not.
  6. 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.