(1) Environmental factors such as temperature and pressure changes may also lead to experimental errors, so control experiments should be designed. Compared with the above-mentioned device, the control experimental device changed the yeast culture solution in the balloon to cooled yeast culture solution (or culture solution without yeast) after heating and boiling, and observed the change of liquid level in the beaker.

(2) Because the amount of oxygen consumed by aerobic respiration of yeast is equal to the amount of carbon dioxide released, the liquid level in the beaker remains unchanged when the yeast only breathes aerobically. The products of aerobic respiration of yeast are only carbon dioxide and water, and the products of anaerobic respiration are alcohol and carbon dioxide, so if you want to confirm whether the yeast in the balloon breathes aerobically, you can detect whether the fermentation broth contains alcohol.

(3) If the experimental device is used to explore the suitable temperature for anaerobic respiration of yeast, yeast can breathe anaerobic and cannot inject oxygen into the balloon; The experimental variables (independent variables) are different temperatures, and the volume of carbon dioxide produced by anaerobic respiration is different at different temperatures. Therefore, the reaction variable (dependent variable) is represented by the rise of the liquid level in the beaker.

So the answer is:

(1) Replace the yeast culture solution in the balloon with the cooled yeast culture solution (or the culture solution without yeast) after heating and boiling.

(2) Yeast only has aerobic respiration? alcohol

(3)① Do not inject oxygen into the balloon.

② Take several identical devices and put them in constant temperature water bath pots at different temperatures? The liquid level in the beaker rises at different temperatures