Burn some gas on a gas stove. Heat some chalk (make sure it is calcium CARBONATE, CaCO3, NOT calcium SULFATE, CaSO4. If you heat calcium sulfate, it will decompose to form calcium oxide and sulfur trioxide, which is a poisonous gas. Blackboard chalk is calcium sulfate. Heat your calcium carbonate directly on the flame until it becomes red hot. Do this for about 2-3 minutes.