Heating in a crucible

Question 1

when is this method used

Answer 1

for measuring mass loss in various thermal decomposition reactions and also for mass gain when reacting magnesium in oxygen

Question 2

what’s the method for the water of crystallisation in calcium sulphate crystals , being removed as water vapour by heating

Answer 2

• weigh an empty clean dry crucible and lid
• add 2g of hydrated calcium sulphate to the crucible and weigh again
• heat strongly with a bunsen for a couple of minutes
• allow to cool
• weigh the crucible and contents again
• heat crucible again and reweigh until you reach a constant mass
  • to ensure reaction is complete

Question 3

what’s the reaction that shows water of crystallisation in calcium sulphate crystals can be removed as water vapour by heating

Answer 3

CaSO4.xH2O(s) => CaSO4(s) + xH2O(g)

Question 4

why are large amounts of hydrated calcium sulphate , such as 50g not used

Answer 4

The decomposition is likely to be incomplete

Question 5

why does the crucible need to be dry

Answer 5

a wet crucible will give an inaccurate result .
It would cause mass loss to be too large as the water would be lost when heating

Question 6

why is a lid used in the crucible

Answer 6

lid improves accuracy of the experiment as it prevents loss of solid from the crucible but should be loose fitting to allow gas to escape

Question 7

why should small amounts of solid such as 0.1g not be used

Answer 7

the percentage uncertainties in weighing will be too high