Here are a few things about the making of soda and making of rock candy:
Both rock candy and soda are favored sugar products.
Rock candy can be made at home while soda is usually made in a factory unless you have one of those special soda-making machine at home.
Rock candy uses more sugar and corn syrup than soda.
Making soda does not a lot of time - once the ingredients are dissolved and the carbon dioxide added, it is done. Rock candy takes a long time for the rock sugar crystals to form.
Rock candy requires a stick or string for the crystal to form. Soda requires a bottle for the liquid to go in.