Answer:
Bubbles will form first in the hot water.*
A general rule in chemistry is that reactions proceed faster
at higher temperatures. Thus, the acid/base reaction which
produces the carbon dioxide bubbles you see, will happen
faster at a higher temperature.
*These results may depend upon the type of baking powder
you are using. There are actually three types: fast acting, slow
acting, and double-action. Fast acting works at room temperature,
as soon as the powder gets wet. Slow acting only works at oven
temperatures, and would probably not do much even in hot water.
And double-action works a little at both high and low temperatures.
