Extracting Copper from Copper Carbonate In a crucible, we added copper carbonate and the total weight of both is 37.04g. After, we heated the crucible with the copper carbonate. It was easy to judge when the reaction had taken place as the colour of the substance had changed from the light green of copper carbonate to the black of copper oxide.After we observed this, we weighed the copper oxide and we notified the result which was 36.42g. We should have heated and weighed the crucible again, but we weren't able because it was broken(due to temperature change). But theoretically, which would be approximately the value of the second weighing?I'm curious because after searching on the Internet, I have seen both increase and reduction of the remaining mass.
The repeated heatings were just to ensure that you had oxidized all of the copper carbonate to copper oxide. If you heated it a second time and the mass decreased, that would just mean that some of the copper carbonated had not been oxidized by the first heating. Then you would just keep heating until the mass quit decreasing, meaning that you had definitely oxidized all of it.  

So the answer to you question is simply that the mass theoretically should not have decreased at all with a second heating, unless some of the copper carbonate had not been oxidized the first time.