How can hardness in water be removed?

In this experiment, students observe as the teacher prepares temporarily hard water by bubbling carbon dioxide through limewater. This water is then boiled to leave a solid residue, which can then be tested using hydrochloric acid, before boiling a sample of permanently hard water for comparison. Both samples are then subjected to ion exchange.

This initial demonstration produces a range of different samples which students can test for hardness using soap solution, before finally investigating the effect of adding sodium carbonate. Using their results, students can reflect on how effectively each treatment softens hard water.

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.

Equipment

Apparatus

For the teacher demonstration

• Eye protection
• Beakers, 500 cm³, x3
• Bunsen burner
• Tripod and gauze
• Heat resistant mat
• Vacuum filtration apparatus (see note 9 below)
• Ion exchange apparatus (see note 10)
• Gas generator for carbon dioxide (see note 11), or a CO² cylinder

For the student experiments

• Eye protection
• Test tubes, x9
• Test tube rack
• Labels, for test tubes
• Beaker, 100 cm³
• Dropping pipette
• Spatula

Chemicals

For the teacher demonstration

• Calcium sulfate solution, 300 cm³ (see note 12 below)
• Limewater (IRRITANT), 150 cm³
• Sodium zeolite, about 5 g (see note 10)
• Marble chips
• Dilute hydrochloric acid, 2 M (IRRITANT)

For the student experiments

• Solutions labelled as shown below, 100 cm³ each (A, C, D and E are prepared by the teacher during the demonstration):
  • A – temporarily hard water
  • B – permanently hard water (see note 13)
  • 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)
• Soap solution in IDA (industrially denatured alcohol) (HIGHLY FLAMMABLE, HARMFUL), 10 cm³ (see note 13)
• Sodium carbonate-10-water (IRRITANT), about 1 g

Health, safety and technical notes

• Read our standard health and safety guidance.
• Wear eye protection throughout.
• Calcium sulfate, CaSO₄.2H₂O(s) – see CLEAPSS Hazcard HC019B.
• Limewater (calcium hydroxide solution), Ca(OH)₂(aq), (treat as IRRITANT) – see CLEAPSS Hazcard HC018 and CLEAPSS Recipe Book RB020.
• Sodium zeolite 
• Sodium carbonate-10-water, Na₂CO₃.10H₂O(s), (IRRITANT) – see CLEAPSS Hazcard HC095A. 
• Marble chips (calcium carbonate), CaCO3(s) – see CLEAPSS Hazcard HC019B.
• Hydrochloric acid, (IRRITANT) – see CLEAPSS Hazcard HC047a and CLEAPSS Recipe Book RB043.  
• For vacuum filtration apparatus, see CLEAPSS Laboratory Handbook.
• 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 (a burette will do, provided the tap is removable, allowing the cotton wool plug to be pushed out with a rod) 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.
• See these standard techniques for generating, collecting and testing gases.
• 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.
• 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, and see CLEAPSS Recipe Book RB000. Do not dissolve in water.

Procedure

Demonstration

1. 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 (from the reaction between the marble chips and the acid). 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.
2. Place about half of the temporarily hard water in a beaker and boil it for about 5 minutes. Filter, using vacuum filtration apparatus.
3. Scrape some of the solid residue from step 2 into a test tube and add dilute hydrochloric acid. Fizzing should show that the solid is a carbonate (calcium carbonate).
4. Boil about the same quantity of permanently hard water (to that used in step 2) in another beaker. Show that there is no precipitate. Allow the solution to cool until it is safe to handle.
5. Set up two ion exchange columns containing sodium zeolite (see note 10 above). Pour about half of the temporarily hard water from step 1 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.

Student experiments

1. 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
2. Collect 10 cm³ of soap solution in a small beaker.
3. 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.
4. Repeat the procedure in step 3 for tubes B to F.
5. 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 step 3 to see how many drops of soap solution are required to produce a lather. Note the new number of drops.
6. Repeat the procedure in step 5 for any other water samples that were ‘hard’.

Teaching notes

Aand 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)

(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.