How can hardness in water be removed?

Read our standard health & safety guidance

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 and chemicals

The teacher will require:

Eye protection

Beakers (500 cm³), 3
Bunsen burner
Tripod and gauze
Heat resistant mat
Vacuum filtration apparatus (see note 1)
Ion exchange apparatus (see note 2)
Gas generator for carbon dioxide (see note 3), or a CO₂ cylinder
Calcium sulfate solution (Low hazard), 300 cm³ (see note 4)
Limewater (Irritant), 150 cm³

Sodium zeolite (see note 2), about 5 g
Dilute hydrochloric acid, 2 mol dm^–3 (Irritant at this concentration)

Each student or group of students will need:

Test-tubes, 9
Test-tube rack
Labels, for test tubes
Beaker (100 cm³)

Dropping pipette
Spatula

Access to:

Solutions labelled as shown below (Low hazard), 100 cm³ each

A Temporarily hard water*
B Permanently hard water (see 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 (see note 5)
Sodium carbonate-10-water (Irritant), about 1 g

Technical notes

Calcium sulfate (Low hazard) Refer to CLEAPSS Hazcard 19B
Limewater (calcium hydroxide solution) (Irritant) See CLEAPSS Hazcard 18A and
Recipe card 15
Sodium zeolite (Low hazard)
Sodium carbonate-10-water (Irritant) Refer to CLEAPSS Hazcard 95A
Hydrochloric acid, (Irritant at concentration used) Refer to CLEAPSS Hazcard 47A and Recipe Card 31

1 Vacuum filtration apparatus. See CLEAPSS Laboratory Handbook sections 13.7.7, 10.6.4 and 10.6.5.

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 – 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.

Demonstration

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, whereupon 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)0

(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 and Safety checked, March 2008

Web Links

www.britishwater.co.uk/water_treatment/water_softening.aspx deals with general aspects of hard water and discusses ion-exchange water softening.

www.docbrown.info/page01/AqueousChem/AqueousChem.htm#1c contains ‘Doc Brown’s’ usual racy and accurate summary of the chemistry of the topic.

(Websites accessed July 2008)