Copper sulphate (CuSO4) is a soluble salt that forms when sulphuric acid (H2SO4) reacts with copper oxide (CuO). It has two forms – hydrated copper sulphate and anhydrous copper sulphate.
Hydrated copper sulphate (CuSO4.5H2O) has five water molecules chemically bound to it giving it a crystal texture and a vivid blue colour. Heating the hydrated copper sulphate drives away the water in a thermal decomposition reaction leaving powdery white anhydrous copper sulphate behind.
Adding water back to the anhydrous copper sulphate makes it hot (exothermic) and turn blue. This reaction is used as the chemical test for the presence of water.
I would not recommend that you touch anhydrous copper sulphate. It can react with the water in your skin, get hot and burn you. Copper sulphate is also quite toxic so should be handled as little as possible.
To grow a large crystal of copper sulphate you first need to dissolve as much copper sulphate as possible into hot distilled water. Distilled water should have no impurities in it ensuring that a good quality crystal will grow. When no more copper sulphate will dissolve you have made a saturated solution. Filter this mixture to remove any undissolved copper sulphate.
Leave the solution for the solvent (water) to slowly evaporate. You may want to cover it with a cloth to make sure that no dust falls into the mixture. Within 48 hours you should have some small crystals that you can use as a seed for growing a larger crystal. Select one and tie it to a fine thread. Filter the solution again to remove any other crystals that have formed and suspend the single seed crystal in the saturated solution once more.
Leave this mixture to evaporate slowly, checking on the growth of the crystal regularly. Remove any crystals that form on the thread and filter the solution to keep it clean. Your original crystal should increase in size over the course of several weeks. For a really big crystal make sure that the original saturated solution is clean and that the evaporation happens very slowly.
This video was made by someone whose first language is not English but the process is very well demonstrated nonetheless.
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Questions…
- What type of reaction is sulphuric acid reacting with copper oxide?
- What is a saturated solution?
- What two physical changes are required to make distilled water?
- Name another metal salt that changes colour when it thermally decomposes. What colour change does it do?
- What would have happened if he had tried to grow the crystal in a less than saturated solution?
- Why did he coat the crystals in clear nail varnish (lacquer)?
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