Hey!
A control is something that is not subject to the independent variable (independent variables = the one that is adjusted), so that the changes observed in the experiment can be attributed TO that independent variable.
The control for the heat of combustion experiment would either be:
- just a spirit burner (not ignited) with the same amount of contents as the one you're actually using in the experiment to show that the water is being heated by using up the fuel inside the spirit burner (and not something else), or
- the same amount of water that you're using in the experiment in a similarly built calorimeter (that isn't subject to heat) to show that the water is being heated by the fuel inside the spirit burner, and not because the water heats itself up automatically, even without a heat source
There may be more, but these 2 are the ones I can think of at the moment.
Hope this helps! Good luck with your studies!