The ingredients in a fire extinguisher vary based on its application, or class of fire extinguisher. The United States categorizes fire extinguishers into classes A,B,C,D and K, each of which contains different ingredients to put out different types of fires.
There are different types of chemical compounds and/or ingredients in different classes of fire extinguishers; some ingredients can be used for multiple types of fires. It's best to always read the labels on the fire extinguisher to understand its classification and ingredients list.
Dry chemicals, such as mono ammonium phosphate (A,B,C), sodium bicarbonate (B,C), potassium bicarbonate (B,C) are typically nonconductive and used on solid, liquid or electrical fires; most dry chemicals require minimal to immediate cleanup afterwards.
Wet chemicals, such as potassium acetate-based agents, are used solely for the class K (kitchen) type of fires. Class K fires are oils and fats that can be extremely flammable or produce combustible vapors and the wet chemical provides a foam-like discharge to engulf the flammable materials.
Water and carbon dioxide are typically used for only class A fires: ordinary solid combustibles that can be put out with water and are typically reduced to ash.
Metal powders, such as copper or sodium chloride and sand, are very effective in putting out class D fires, which are comprised of combustible metals. Combustible metals, when on fire, react violently to water, so a special type of powdered metal should be used to combat these types of fires. Powdered copper works well on fires caused by lithium, while sodium chloride works best for fires caused by uranium, powdered aluminum, magnesium, potassium and sodium.