A, the principle of homemade soda: baking soda and citric acid (or tartaric acid) generate a large amount of carbon dioxide through a combined reaction, and carbon dioxide is dissolved in water in a large amount under pressure. When the pressure is reduced, a large amount of carbon dioxide will run out of the water. When drinking soda, the stomach does not absorb carbon dioxide, and carbon dioxide runs out of the mouth to take away a large amount of heat, which makes people feel cool, so it belongs to a chemical change, so A is correct.

B. Comparing hardness, hardness belongs to the physical properties of substances, and no new substances are generated, so B is wrong.

C, carbon black is prepared, and carbon dioxide and water are generated after complete combustion. When a cold plate is placed above the candle, the upper flame temperature is cold and incomplete combustion, and many amorphous substances remain to form carbon black. Therefore, it is a chemical change, so C is correct.

D, the principle of testing plastic wrap is to compare the remaining amount of water in three States. The evaporation of water does not produce new substances, so it belongs to physical changes, so D is wrong.

So choose a and C.