Baking powder is one part baking soda (bicarbonate of soda) to two parts cream of tartare (tartaric acid).
The very thing that makes baking powder work also makes it unstable and gives it a short shelf life. Eventually the two chemicals cancel each other out. When this happens the baking powder is dead and anything made when using it will not rise.
Purchasing baking soda and cream of tartare separately and making your own baking powder is a good way of increasing the shelf life and always ensures you have a good, workable baking powder otherwise you will have to use it frequently before its shelf life expires. By making your own you can make smaller quantities than you can buy from the supermarket.
If you are looking for information to help you with baking or cake making you might like to take a look at: