why must the CuO be previously heated in the test for carbon and hydrogen?

Question

the procedure to my experiment is:
-2 grams of PREVIOUSLY HEATED CuO and a corn-grain amount of sugar were grinded in a mortar.
-The mixture was transferred in a test tube and covered with a rubber stopper with bent glass tubing.
-The test tube was attached to an iron stand in an inclined position, and the other end of the glass tubing was immersed in 5ml Lime water.
-The test tube with the grinded mixture was heated with a bunsen burner.

another question regarding this are:
1. which of the observed changes in the experiment indicate the presence of a) carbon b) hydrogen?
2.what is the role of CuO in the reaction?
3. why should the delivery tube be removed first from the limewater before the flame is put off?

Answer 1

CuO is being used as an oxidising agent. It will convert carbon in your sugar to carbon dioxide, and hydrogen to water.

I think you heat the CuO to get rid of any water sticking to it. Then if you see water condensing in your tubing, you will know that there was hydrogen in your sample.

Both carbon and hydrogen in your sample will reduce CuO to metallic copper. Hydrogen will generate water. Carbon will generate carbon dioxide, which will precipitate calcium carbonate from your lime water.

You remove the tube from the limewater before you turn off the Bunsen, because otherwise limewater would be sucked back into your test-tube as it cooled down, and water hitting the hot test-tube would shatter it.