chemistry

Pure Substance

A substance that cannot be separated into simpler components by physical methods.

Element

A pure substance consisting of only one type of atom.

Empirical formula

The simplest whole number ratio of atoms in a compound

Compound

A pure substance consisting of two or more different elements chemically bonded together.

Mixture

A non-chemical combination of two or more substances.

Homogeneous Mixture

A mixture of uniform composition where all components are in the same phase.

Heterogeneous Mixture

A mixture of non-uniform composition where components can be easily identified.

Freezing Point

The temperature at which a liquid changes to a solid by the removal of heat.

Melting Point

The temperature at which a solid, given sufficient heat, becomes a liquid.

Boiling Point

The temperature at which the vapor pressure of a liquid equals the external (atmospheric) pressure.

Diffusion

The movement of atoms or molecules from an area of high concentration to an area of low concentration.

Brownian Motion

The random movement of microscopic particles suspended in a liquid or gas.

Isotopes

Atoms of the same element having the same number of protons but different numbers of neutrons.

Relative Atomic Mass

The mass of a particle on a scale where an atom of Carbon-12 has a mass of 12.

1st Ionisation Energy

The energy needed to remove one electron from an atom in the gas phase.

Electron Affinity

The energy released when an electron is added to a neutral atom in the gaseous state to form a negative ion.

Electronegativity

A measure of the tendency of an atom to attract a bonding pair of electrons.

Chemical Bond

A mutual attraction between two atoms resulting from the simultaneous attraction between their nuclei and the outer electrons.

Valence Electrons

The electrons in the highest energy level of an atom.

Physical Change

A change in which no new substances are formed and the process is often easily reversible.

Chemical Change

A change in which new chemical substances are formed and the process is not easily reversible.

Law of Conservation of Mass

The total mass in a closed system is conserved during a chemical reaction.

Law of Constant Proportions

A chemical compound always contains the same elements in the same fixed proportion by mass.

Exothermic Reaction

A reaction that releases energy (usually as heat).

Endothermic Reaction

A reaction that absorbs energy.

Mole

The amount of substance having the same number of particles as there are atoms in 12 g of carbon-12.

Molar Mass (M)

The mass in grams of one mole of that substance.

Avogadro's Constant (N_A)

The number of particles (6,02 \times 10^{23}) in one mole of a substance.

Molar Gas Volume (V_m)

The volume occupied by one mole of a gas (22,4 \text{ dm}^3 at STP).

Concentration (c)

The amount of solute per unit volume of solution.

Dissociation

The process in which solid ionic crystals are broken up into ions when dissolved in water.