Freezing food
Carbon dioxide, in liquid or gaseous form, is used by the food industry for chilling, quick freezing, and refrigeration during food transport. Cryogenic freezers produce lower temperatures than domestic models. Liquid carbon dioxide is sprayed onto the food in a cryogenic freezer. When this happens, about half of the liquid carbon dioxide becomes a solid and about half of it becomes a gas. Heat is drawn out of the food to change the solid carbon dioxide into gaseous carbon dioxide, a change of state called sublimation. This causes most of the freezing effect, but the presence of the cold gaseous carbon dioxide also helps.
Fire extinguishers
The inert properties of carbon dioxide make it ideal for use in fire extinguishers. The carbon dioxide puts out the fire by cooling it and excluding air. A typical extinguisher contains liquid carbon dioxide under pressure. When the carbon dioxide emerges from the horn, it changes to a very cold mixture of solid and gaseous carbon dioxide. A fire will be extinguished if one side of the 'fire triangle' is removed: heat, oxygen or fuel. A carbon dioxide fire extinguisher removes the heat and the oxygen. Burning magnesium ribbon will continue to burn when placed in carbon dioxide, forming magnesium oxide and carbon. However, carbon dioxide does not usually support combustion, so it is used in fire extinguishers.
Carbonation
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 gives drinks a sharper, crisper taste.
Medicine
Carbon dioxide is used in keyhole surgery. Keyhole surgery allows surgeons to operate without the need to make large incisions in their patients. It allows for a quicker recovery than would be experienced after normal surgery. Carbon dioxide gas is injected into the abdomen to expand the space inside, giving the surgeon sufficient room to manipulate their instruments inside the patient. It may also be added to oxygen supplies to act as a respiratory stimulant, so helping patients breathe more deeply.
