A control group is the comparison group that helps to "make sure your experiment works." A control group is separated from the rest of the experiment and nothing happens to it kinda like a controlled variable. Controlled variables are the variables in a experiment that remains the same for example a temperature, time, type of products, etc..
I am not sure about the first question but the temperature has an important role in this situation because as the temp goes up particles moves at a faster speed and spread out every where.
