Chemical Reactions

Chemical formula

a symbolic representation of an element or compound. Chemical formulas use subscripts and parentheses to denote the number of atoms in a molecule of the substance. Examples of chemical formulas include NaCl (table salt), H2O (water), and Ca(OH)2 (calcium hydroxide).

Mole

a unit amount of substance. A mole of a substance has the same number of particles as 12.0 grams of carbon-12. The SI symbol for the mole is "mol."

Molecular mass

(M) the mass of a molecule of a substance, as measured in universal mass units (u).

Conservation of matter

Conservation of matter a scientific law that states that the total amount of matter in a closed system remains constant. A chemical equation satisfies conservation of matter if it is balanced.

Decomposition

Decomposition a chemical reaction in which a single substance is broken down into two or more products. For example, salt (NaCl) can be decomposed into sodium (Na) and chlorine gas (Cl2).

Chemical equation

a symbolic representation of a chemical reaction. In a chemical equation, reactants are shown on the left, and products are shown on the right. For example, the chemical equation Na + Cl2 NaCl describes the reaction of sodium (Na) and chlorine gas (Cl2) to form table salt. In a balanced chemical equation, there are the same numbers of each type of atom on each side of the equation: 2Na + Cl2 2NaCl is balanced because there are two sodium atoms and two chlorine atoms on each side of the equation.

Coefficient

a number that multiplies a term in an equation. In a chemical equation, the coefficients indicate the number of each type of molecule. For example, 6H2O means that there are six water molecules.

Chemical reaction

a process in which one or more substances are transformed into others. In a chemical reaction, bonds between atoms are broken and new bonds are formed, joining atoms into different combinations. No atoms are created or destroyed in a chemical reaction.

Molecule

a stable particle made of two or more atoms. A water molecule (H2O) is made of two hydrogen atoms and one oxygen atom.

Product

a substance that is formed in a chemical reaction.

Reactant

a substance that takes part in a chemical reaction.

Combustion

a chemical reaction in which a fuel is burned. Most examples of combustion involve the burning of a hydrocarbon in oxygen, producing water and carbon dioxide. For example, methane burns in oxygen to form water and carbon dioxide: CH4 + 2O2 2H2O + CO2

Single replacement

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

Double Replacement

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

Combination

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

Subscript

a number in a chemical formula representing the number of atoms of a particular element in one molecule of the compound. For example, the subscript "2" in H2O indicates that there are two hydrogen atoms in a water molecule.

Molar mass

the mass of one mole of a substance. The molar mass of an element or compound in grams is equal to the atomic mass of the atom or molecule of which it is composed. For example, the atomic mass of an oxygen molecule (O2) is 32 universal mass units. The molar mass of oxygen gas is 32 grams.

Avogadro's number

the number of atoms or molecules in a mole of a substance. Avogadro's number is equal to 6.0221415 × 1023.