Chemical Reactions Vocabulary

Decomposition

A chemical reaction that breaks down compounds into simpler products. (For example, salt (NaCl) is decomposed into sodium (Na) and chlorine gas (Cl2).)

Reactant

A chemical substance that is present at the start of a chemical reaction. Located on the left side of a chemical equation.

Molecule

A group of atoms bonded together (example: H2O)

Coefficient

A number in front of a chemical formula in an equation that indicates how many molecules or atoms of each reactant and product are involved in a reaction.

Subscript

A number written slightly below and to the right of a chemical symbol that shows how many atoms of an element are in a compound.

Element

A pure substance made of only one kind of atom

Compound

A substance made up of atoms of two or more different elements joined by chemical bonds

Product

A substance produced in a chemical reaction. Located on the right side of a chemical equation.

Endothermic

any process which requires or absorbs energy from its surroundings, usually in the form of heat.

Exothermic

Chemical Reaction in which energy is primarily given off in the form of heat

Single Replacement

a reaction in which an element reacts with a compound to form a new compound and a different element. (For example, aluminum (Al) can react with hydrochloric acid (HCl) to form aluminum chloride (AlCl3) and hydrogen gas (H2).)

Double Replacement

a reaction in which two compounds exchange elements or molecules with one another. (For example, sodium sulfide (Na2S) and hydrochloric acid (HCl) can react to form salt (NaCl) and hydrogen sulfide (H2S).)

Combustion

a chemical reaction in which a fuel burns in a gas, usually oxygen. (Many combustion reactions involve a carbon-based fuel, such as methane (CH4), burning in oxygen gas (O2) to produce carbon dioxide (CO2) and water (H2O). )

Synthesis

a chemical reaction in which two or more reactants form a single product. (For example, hydrogen (H2) combines with oxygen (O2) to form water (H2O).)

Law of Conservation of Mass

the law that states that mass cannot be created or destroyed in ordinary chemical and physical changes. This law states that the total mass of the reactants in a chemical reaction must equal the total mass of the products in that reaction.