Making Sherbet

Explanation

The bicarbonate of soda has locked up inside it a gas called carbon-dioxide, the bicarbonate of soda is an alkali so will react with an acid. As it does so the carbonate part becomes unstable and decomposes to water and carbon-dioxide.

A conventional acid is a solution with lots of free H⁺ ions these are stabilised by a water molecule making H₃O⁺ which means that Citric Acid dry is just a molecule that could loose one of it's Hydrogens quite easily, you have to make it wet to turn it into a real acid. This is why the sherbert only fizzes when it gets wet.

Real world

Baking powder, that you use to make cakes rise and is in self raising flour,  works on exactly the same principle except that tartaric acid is often used rather than citric acid, plus a second acid that only becomes active at high temperatures such as calcium aluminum phosphate. So as soon as you mix the baking powder with the wet mixture it starts to release carbon-dioxide and when you heat it up you get a second release of gas while it is cooking.