Investigating the action of heat on copper sulphate
A clean, dry boiling tube is weighed. About 20g of copper sulphate is placed in the tube which is then weighed again.
The apparatus is now set up as shown above and the tube heated gently.
A colourless liquid is seen to condense in the test tube and the copper sulphate changes from blue to white.
When there is no further change the apparatus is allowed to cool and the boiling tube re-weighed.
|Mass of empty boiling tube /g||75g|
|Mass of boiling tube + copper sulphate before heating /g||96g|
|Mass of boiling tube + copper sulphate after heating /g||91g|
|Mass of copper sulphate used /g||96-75 = 21g|
|Mass of water produced /g||96-91 = 5g|
The heat causes the blue copper sulphate (also known as hydrated copper sulphate) to decompose (or split apart) forming anhydrous copper sulphate and water.
Anhydrous copper sulphate is white in colour.
The liquid which condenses in the test tube is shown to be water 2 ways:
- It turns anhydrous cobalt chloride from blue to pink. This shows that it contains water.
- It boils at 100ºC. This shows that it is pure water.
When some water is added to the anhydrous copper sulphate two changes can be observed:
- i. the white powder goes blue again
- ii. it gets warm and starts to steam
Getting warm and changing colour are both signs that a chemical reaction is taking place.
A common test for water is to place it onto anhydrous copper sulphate and see if it turns blue.
Equation for the reaction:
Hydrated copper sulphate (blue) →← anhydrous copper sulphate (white) + water