Many of the ingredients used in the kitchen are chemicals, and almost all of the techniques used in cooking are concerned with chemical changes in the food we prepare.
The baking powder we use to make pancakes rise is a good example. Sodium bicarbonate powder supplies the carbon dioxide bubbles, and dry acid salts such as tartaric acid and monocalcium phosphate are used to release it by reacting with the bicarbonate.
These reactions takes place as soon as water is added to the dry powder. Other acid salts such as sodium aluminum sulfate are also added, since these only react at higher temperatures in the oven or on the griddle. This allows the batter to still rise after it has been sitting on the counter for a while.
A chemist knows that green vegetables turn an ugly drab color when the magnesium atom at the center of the chlorophyll molecule is replaced by a hydrogen atom. This can happen when green vegetables are heated, or when an acid is present.
Cooking for a shorter time, or avoiding acids like vinegar or lemon juice, can keep the colors bright.
Knowing what temperatures change the structure of the proteins in food can also be extremely helpful in cooking. Each of the proteins in an egg, for example, hardens at a different temperature. Keeping your egg at a temperature where all of the white hardens, but little or none of the yolk, allows you to have very tight control over how your eggs are cooked.
Controlling the temperature while meat cooks is important for the same reason. If you never allow your meat to reach the temperature where the meat proteins harden, you can avoid a tough cut, while allowing a high enough temperature to convert all of the tough connective tissue into soft gelatin.