Question

You are cleaning when you find two jars, all you can read on the labels are 'Ca' and 'O'. You surmise that one is plaster of Paris, CaSO4, and the other is crushed chalk, CaCO3. You want to keep the plaster of Paris. Describe how you can positively identify them using kitchen chemicals.

Answer 1

Chalk powder is calcium carbonate, which is insoluble in water and plaster of Paris is CaSO4, both are salt.

From kitchen chemicals, we can take vinegar or lemon juice for differentiation between calcium carbonate and plaster of paris.

A very little amount of both will take in separate test tube of any small pot and then following procedure will be carried out:

We can take vinegar (dilute acetic acid) and then 1/2 ml of vinegar is added separately in both chalk powder and as well as plaster of Paris.

out of two pot or test tube, in a one T.T., brisk effervescence will come out it is indicated that this one will be CaCO3 hence 2nd one will be Plaster of Paris.

Reaction:

CaCO3 + 2CH3COOH (CH3COO)₂Ca + CO2 (gas bubbled) + H2O

But CaSO4 + CH3COOH does not react and not liberate CO2 gas.

We can be checked by lemon juice, acidic nature same response like vinegar.