A baker added a white powder while baking a cake to make it soft and spongy. Identify the main ingredients in the white powder. Explain the function of each ingredient. When the powder is heated during baking what is the reaction taking place?

The white powder which is added in caked while baking is baking powder. It is prepared with baking soda, i.e. sodium hydrogen carbonate, along with a mild acid like tartaric acid. Baking soda, being base, is bitter, and tartaric acid is sour in taste. When the mixture is added, neutralization takes place, neutralizing the bitter and sour effects.
Baking soda + Tartaric acid - Baking powder
Baking soda is mainly for producing effervesces, and tartaric acid is used for neutralizing the bitter effect of sodium bicarbonate.

$2NaHC{O}_{3}Heat\to N{a}_{2}C{O}_3+C{O}_2\uparrow +H_{2}O$