(1), yeast is active and needs suitable temperature. If the water temperature is too high, its activity will be reduced or killed. Therefore, using warm water instead of hot water will make the dough temperature too high, which is not conducive to the life of yeast.

(2) Hypothesis can be confirmed or denied. For example, this topic can assume that temperature will affect the life of yeast, or temperature will not affect the life of yeast.

(3) This experiment is about fermentation. The yeast in the bottle breaks down glucose and produces carbon dioxide. Carbon dioxide is a gas, so bubbles will appear and balloons will expand. This experiment belongs to exploratory experiment, and the control group needs to be set up.

So the answer is: (1) The change of hot water makes the dough too hot, which is not conducive to the life of yeast.

(2) Temperature will affect the life of yeast.

③ carbon dioxide; The control group was absent during the experiment.