Carbon dioxide puts the fizz into soft drinks, such as cola and lemonade.
It is injected into the drink under high pressure. When the drink is opened, pressure is released and dissolved carbon dioxide comes out of the solution, creating bubbles.
When a gas dissolves in water, an equilibrium forms between the free gas and the dissolved gas. More gas will dissolve if the pressure is increased. Carbon dioxide is injected into drinks in a high pressure 'mixing bowl' before packaging in cans or bottles. When the drink is opened, the pressure returns to atmospheric pressure. The position of equilibrium changes and dissolved carbon dioxide comes out of the solution, forming bubbles. Carbon dioxide forms a weak acid called carbonic acid when it dissolves in water. This give drinks a sharper, crispier taste.