Key Chemistry Definitions: Elements, Compounds, and Laws

Posted on Mar 22, 2025 in Chemistry

Key Chemistry Definitions

Pure Substance: Homogeneous matter that cannot be broken down through simple physicochemical processes.
Element: A pure substance that cannot be broken down into simpler substances.
Compound: A substance made of two or more different elements.
Chemical Formula: Expresses the qualitative and quantitative composition of a given substance. The number in subscript form indicates the number of atoms.
Ionic Species: A formula that takes a superscript indicating the net electrical charge of that ion.
Unit Formula: An atom or molecule that indicates the formula of the substance.
Structural Formula: Used in carbon compounds to show how atoms join together.
Empirical Formula: An expression that represents the simplest ratio of atoms present in a chemical compound.
Molecular Formula: A chemical formula that indicates the number of different atoms in the molecule.
Atomic Mass Unit (amu): It is 1/12th of the mass of an atom of carbon-12 (¹²C).
Relative Atomic Mass: Indicates how many times greater the mass of an atom is than the atomic mass unit.
Molecular Mass: The sum of the masses of the atoms that compose a molecule.
Mole (mol): The amount of substance that has as many elementary entities (atoms, molecules, or other particles) as there are atoms in exactly 12g of the isotope carbon-12 (¹²C).
Molar Mass: The mass of one mole of that substance.

Boyle’s Law: At constant temperature, the volume occupied by a gas sample is inversely proportional to the applied pressure. pV = constant.
Charles’s Law (Gay-Lussac’s Law): At constant pressure, a gas expands when heated and contracts when cooled proportionately. V / T = constant.
Avogadro’s Law: At equal pressure and temperature, equal volumes of two different gases contain the same number of molecules. V / n = constant.
Molar Volume: The volume occupied by one mole of that substance.
Dalton’s Law: In a nonreactive gas mixture, the total pressure is the sum of the pressures exerted by each gas if it were alone in the mixture.

Solvent: The substance in greater proportion in a mixture.
Solute: The substance in a lesser extent.
Solubility: The maximum amount of solute that can be dissolved in 100g of water at a given temperature.
Saturation: A saturated solution contains the maximum possible amount of solute for a given volume of solvent.
Percentage Mass: Grams of solute per 100g of solution.
Percentage Volume: Defined similarly to percentage by mass, but refers to the mass of solute per 100mL of solution.
Molarity: Defined as the amount of solute in moles per liter of solution.
Normality: Defined as the number of equivalents of solute per liter of solution.
Molality: Defined as the amount of solute in moles per kilogram of solvent.
Mole Fraction: Defined as moles of solute divided by moles of solute plus moles of solvent.
Dilution: The process of adding water to a solution to decrease its concentration and increase its volume.
Limiting Reagent: The reactant that is completely consumed in a chemical reaction, thus determining or limiting the amount of product formed.