Chemistry in your cupboard

Content

Experimental work

The following are some suggestions for experimental work to support the above material.

If the water in your area is soft, samples of hard water can be made as follows (taken from the CLEAPSS Recipe Card):

Permanently hard water

Measure 0.45 g of calcium sulfate-2-water and add it to 1000 ml of water in a bottle and leave it overnight to dissolve.

Temporary hard water

Filter 130 cm³ of freshly-made calcium hydroxide solution. Bubble carbon dioxide though the solution so that calcium carbonate first precipitates and then, as more carbon dioxide is passed, re-dissolves to form calcium hydrogencarbonate. Dilute this clear solution to 1000 ml.

Demonstrating the mineral content of hard water

Evaporate a few drops of tap water, hard water and distilled (or de-ionised) water on a watch glass and note the deposits formed.

Testing water hardness

This can be done by adding soap solution to various water samples (full instructions).

Titration with EDTA

Instructions for testing water hardness by a titration with EDTA.

Note. Titration with EDTA is suitable for testing hardness of water. It cannot be used to test the softening ability of Calgon® as EDTA it is too strong a complexing agent and will move back the equilibrium, obtaining the same hardness result before and after addition of the water softener.

Demonstrating the threshold effect

Apparatus and chemicals

1 beaker 2 dm³

1.5 dm³ of distilled or de-ionised water

0.99 g of calcium chloride-2-water

3.5 g of potassium tripolyphosphate

Method

Using a magnetic stirrer

• fill the beaker with 1.5 dm³ of distilled or de-ionised water
• add 0.99 g of calcium chloride-2-water
• stir until the salt is completely dissolved
• add 1.5 g of potassium tripolyphosphate
• stir for 30 seconds, the solution will become opaque because of the formation of the insoluble salt of phosphate and calcium
• after 30 seconds add 2 g of potassium tripolyphosphate and keep stirring, the solution will turn back to clear