# A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
I
igneous rock: Any rock formed by the cooling and crystallisation of magma or by the accumulation and consolidation of material such as ash.
immiscible liquids: Two liquids that do not mix.
inclination: The angle from the plane of an orbit with respect to the ecliptic plane.
indicator: A substance that is different colours in its acid and base forms.
inertia: The property of an object to resist changes in its motion. The inertia of an object depends on its mass.
inferior conjunction: The closest alignment of an inferior planet as it passes between Earth and the Sun.
inferior planet: A planet whose orbit is between Earth and the Sun-namely, Mercury and Mars.
infra-red radiation: Low-energy electromagnetic radiation emitted by warm objects.
insulator: A material that allows only a slight movement of electric charge within it
inter-molecular bond: A weak bond that connects molecules.
internal energy: The total amount of kinetic and potential energies of all the particles in an object.
intrusive igneous rock: Igneous rock formed under the surface of Earth.
ion: A positively or negatively charged atom or group of atoms.
ionic bond: The electrostatic attraction between oppositely charged ions.
ionic compound: A substance containing ions in a lattice structure of ionic bonds.
ionosphere: An upper layer of the atmosphere that contains charged particles.
isobar: A line on a weather map that indicates equal atmospheric pressure.
isotopes: Any two (or more) forms of the same element that have equal numbers of protons but different numbers of neutrons in the nuclei.