Which ions cause hardness in water?

Read our standard health & safety guidance

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 and chemicals

Each group of students will require:

Eye protection

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

Access to:

Soap solution (Highly flammable, Harmful), 50 cm³ per group (see note 1)
Distilled or deionised water, 10 cm³ per group

The following solutions (all 0.1 mol dm^–3) in communal stock bottles, or beakers with dropping pipettes, 10 cm³ per group.
Sodium chloride (Low hazard)
Calcium chloride (Low hazard at this concentration)
Magnesium chloride (Low hazard)
Potassium nitrate (Low hazard at this concentration)
Sodium sulfate (Low hazard)
Iron(II) sulfate (Low hazard at this concentration)
Magnesium sulfate (Low hazard)

Technical notes

Sodium chloride (Low hazard) Refer to CLEAPSS Hazcard 47B and Recipe card 63
Calcium chloride (Low hazard at concentration used) Refer to CLEAPSS Hazcard 19A
Magnesium chloride (Low hazard) Refer to CLEAPSS Hazcard 59B
Potassium nitrate (Low hazard at concentration used) Refer to CLEAPSS Hazcard 82
Sodium sulfate (Low hazard) Refer to CLEAPSS Hazcard 98B
Iron(II) sulfate (Low hazard at concentration used) Refer to CLEAPSS Hazcard 55B and Recipe card 40
Magnesium sulfate (Low hazard) Refer to CLEAPSS Hazcard 59B

1 Soap solution in ‘ethanol’ (Industrial Denatured Alcohol, IDA – Refer to CLEAPSS Hazcard 40A) (Highly flammable, Harmful) can be purchased or made up – Refer to CLEAPSS Recipe card 60.

Procedure

HEALTH & SAFETY: Wear eye protection throughout.

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, May 2008

Web Links

www.gcsescience.com/f3.htm extends the theory above and includes chemical equations

(Website accessed July 2008)