What it shows
Crystals of potash alum contain water locked in their structure. On heating the water is driven off as steam and the crystal puffs up into a white powder that weighs much less than before.
The science
Potash alum is KAl(SO4)2.12H2O. Its formula mass is 474, of which 12 water molecules make 216, so water makes about 45.5 per cent of the mass. When heated above about 90 to 100 degrees C the crystal first dissolves in its own water of crystallisation, boils, swells into a foam and finally leaves dry powder called burnt alum. Weighing before and after shows how much water was lost. The water of crystallisation is part of the chemistry of the crystal and not just wetness on the surface. Ammonium alum gives a slightly different value, so check what the shop sells.
Materials
Cost: under Rs 50. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- 3 to 5 g of potash alum (phitkari) crystals
- A clean steel spoon or a small tin lid
- A kitchen gas stove (adult use only)
- A scale reading to 0.01 g or a jeweller’s scale
- A pair of tongs
- A dry plate to cool the sample
- A calculator
Safety
For this project: An adult must do all the heating. Use tongs, because the metal gets very hot. Keep your face away from the steam and the puffed powder. Do not taste the alum, and wash hands afterwards.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Weigh the clean dry spoon or lid and note the mass.
Add 3 g of alum crystals to the spoon and weigh again to find the exact mass of crystals.
Place the spoon on a low flame, held with tongs by an adult. The crystals melt and bubble as water is given off.
Continue heating gently until the foam stops rising and a dry white mass remains, for about 10 minutes.
Place the spoon on a dry plate and let it cool for a few minutes. Do not leave it long, because burnt alum slowly takes water back from the air.
Weigh the spoon with the white powder and calculate the mass of the powder.
Find the mass of water lost, then the percentage lost, and compare with the theoretical 45.5 per cent.
Add three drops of water to the cooled powder and note whether it feels warm. Write what you see.
What you should see
The sample loses a mass that is near 45 per cent of its starting mass. Your measured percentage, compared with 45.5 per cent, shows how complete the heating was.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What is water of crystallisation?
Water molecules that are part of the crystal structure in a fixed number for each unit of the salt.
2Why does the crystal appear to melt before it dries?
It dissolves in its own water of crystallisation which is released first.
3How many water molecules are in potash alum?
Twelve for each KAl(SO4)2.
4What is the use of burnt alum?
It is used as an astringent in some medicines and to stop small bleeding when applied by a trained person.
Take it further
Do the same with copper sulphate crystals, noting the blue to white colour change, if your teacher can supervise it in a lab.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
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