Teaching Science Glossary



oblate: Having an equatorial diameter greater than the polar diameter.
occultation: The hiding of one astronomical body by another, such as the occultation of a star by the Moon.
oceanic crust: Earth’s crust underneath the oceans.
oceanic ridge: The place where plates separate and sea floor spreading occurs.
ohm: The unit of electrical resistance.
ohmmeter: A device that measures the electrical resistance of a load.
Olber’s paradox: The observation that the sky is dark at night contrasted to a simple argument that shows the sky should be uniformly bright if the universe was infinite.
Oort cloud: The hundreds of millions of comets surrounding the solar system in a sphere with a radius of approximately 50 000 AU (astronomical units).
opacity: The ability of a substance to block light.
opaque material: Material that does not let light pass through it.
open circuit: A break in the conducting path of an electric circuit.
open universe: The version of big bang theory where the universe will expand forever.
opposition: The passage of a planet through the point directly opposite the Sun on the other side of Earth.
orbital: A region within a shell that can contains a maximum of two electrons.
organic compounds: Compounds related to living things. However, this definition can be extended to any compound containing carbon, excluding carbonates and hydrogen carbonates.
outer shell electron: An electron that is located in the outer shell or energy level of an atom. Also called a ‘valence electron’.
oxidant: A substance that causes another substance to undergo an oxidation reaction; an oxidant accepts one or more electrons from the substance it oxidises.
oxidation: A reaction in which a substance loses one or more electrons and increases its oxidation number.
oxidation number: Numbers that are assigned to elements to determine if a reaction can be classified as a redox reaction.
ozone: A material made up of molecules that contain three oxygen atoms bonded together.
ozone layer: The region of the atmosphere that contains concentrations of ozone.

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