Vinegar
Baking powder is not a gas, but it does make a cake rise by releasing carbon dioxide into the batter through chemical reaction.
It's not the milk alone that makes a cake rise. If the recipe includes milk it probably also contains either baking powder and/or baking soda. This combination of a base (Baking soda or Baking Powder) and an acid (milk) causes a slight chemical reaction which causes the cake to rise.
the purpose of baking powder in a cake is to make it rise and not make it flat and if you put too much of baking powder your cake might burst in the oven/microwave what ever you use
Baking powder is a leavening agent, which is what causes the cake to rise. Too little baking powder will cause the cake to be tough and compact, and won't rise as expected.
baking powder
Self raising flour makes the cake rise, but if you don't have any you can use plain flour and baking powder which has the same effect. 225g plain flour and 4 teaspoons baking powder, will transform it into self raising flour.
It either helps whatever you're baking rise or make whatever you're baking soft and fluffy.
Baking Powder makes the cake rise because of its chemicals. It was a leavener and needed in baking. Why? It's maybe because it makes the baked product rise, and to have a good quality. I may not so good in baking, but I can baked. Leaveners such as: Baking Powder, Baking Soda, Yeast can make the cake rise.
it may not just be baking soda if it has baking soda baking powder salt and all-purpose flour in it that is just like using self rising flour sometimes it does not have to have baking powder in it for you to improvise self rising flour instead.
to make it rise and be less dense
Baking powder and yeast help in the rising of a bread or cake. If it is expired, then it won't be as "active" and therefore decreasing it's ability to rise which makes for a poor bread or cake.
It depends on the type of cake. If you are making a sponge cake where the mixing method involves whipping the eggs and sugar, and then folding in the flour and butter, then it will be fine. The whipping created bubbles which will rise in the oven - baking powder will give it additional rise, but it is not vital.