# 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
H
half-equation: An equation that shows either the oxidation or reduction process in a redox reaction.
half-life: The time it takes for half the amount of a radioactive material to disintegrate.
hardness: The resistance of a mineral to abrasion. A measure of a substance’s ability to resist scratching or denting.
hazardous chemical: A commonly used chemical that has the potential to harm humans or the environment.
Hazchem Code: A code that identifies a specific type of hazardous chemical.
heat: The amount of energy that is transferred when a hot object is placed near a colder object.
heat energy: A form of electromagnetic energy given off by hot objects. Atoms or molecules that are moving in a material have heat energy. Heat energy is the kinetic energy of atoms or molecules.
hectopascal (hPa): Equivalent to 100 Pa (pascals). The unit of air pressure.
heliocentric: Sun-centred.
hertz (Hz): The unit of frequency. 10 Hz means ten repetitions per second.
Hertzprung-Russell diagram: A graph of temperature or colour versus luminosity or brightness for a group of stars.
heterogeneous substance: A mixture of substances such that the constituent particles are easily identified in the resulting substance (e.g. concrete).
homogeneous substance: A mixture of substances such that the particles present are well mixed, even on a microscopic level (e.g. sea water).
humidity: The amount of water vapour in the air.
hydrogen bonds: A strong electrostatic attraction between molecules containing hydrogen and molecules containing nitrogen, oxygen and fluorine.
hydrophilic substances: ‘Water-loving’ substances. They are attracted to water.
hydrophobic substances: ‘Water-hating’ substances. Their attraction to water molecules is too weak for them to dissolve.
hydrosphere: The water environment of Earth.
hygrometer: A device that measures humidity.