A Dictionary of Chemistry by Law Jonathan; Rennie Richard; & Richard Rennie

Author:Law, Jonathan; Rennie, Richard; & Richard Rennie [Law, Jonathan & Rennie, Richard]
Language: eng
Format: epub
Publisher: Oxford University Press, Incorporated
Published: 2020-01-30T00:00:00+00:00

https://www.webelements.com/hassium/

• Information from the WebElements site

Haworth projection A simple perspective drawing of the cyclic structure of *monosaccharides devised by the English chemist Sir Norman Haworth (1883–1950) in 1929. The molecule is viewed slightly from above, with thicker lines used to represent bonds between those atoms that are closer to the viewer. This representation is not completely realistic, as the molecules represented are non-planar. The Haworth projection has been used extensively to demonstrate the structures of sugars. See also fischer projection.

hazchem code (emergency action code, EAC) A code designed to be displayed when hazardous chemicals are transported or stored in bulk. It is used to help the emergency services to take action quickly in any accident. The code consists of a number followed by one or two letters. The number indicates the type of substance to be used in treating the accident (e.g. stream of water, fine spray, foam, dry agent). The first letter indicates the type of protective clothing needed along with information about the possibility of violent reaction on whether the substance should be contained or diluted. The second letter, where it exists, is letter E, indicating that people have to be evacuated from the neighbourhood of the incident. In the UK, the code is usually displayed as part of a panel, which includes an international UN number for the substance, a telephone number for specialist advice, the company name, and a symbol indicating the danger (e.g., a skull and crossbones for toxic substances).

HCFC (hydrochlorofluorocarbon) See halocarbons.

h.c.p. Hexagonal close packing. See close packing.

headspace The space above a sample held in a sealed container. Headspace analysis is used in forensic science to investigate the volatile constituents of a sample.

heat capacity (thermal capacity) The ratio of the heat supplied to an object or specimen to its consequent rise in temperature. The specific heat capacity is the ratio of the heat supplied to unit mass of a substance to its consequent rise in temperature. The molar heat capacity is the ratio of the heat supplied to unit amount of a substance to its consequent rise in temperature. In practice, heat capacity (C) is measured in joules per kelvin, specific heat capacity (c) in J K−1 kg−1, and molar heat capacity (Cm) in J K−1 mol−1. For a gas, the values of c and Cm are commonly given either at constant volume, when only its *internal energy is increased, or at constant pressure, which requires a greater input of heat as the gas is allowed to expand and do work against the surroundings. The symbols for the specific and molar heat capacities at constant volume are cv and Cv, respectively; those for the specific and molar heat capacities at constant pressure are cp and Cp.

heat engine A device for converting heat into work. Engines usually work on cycles of operation, the most efficient of which would be the *Carnot cycle.

heat of atomization The energy required to dissociate one mole of a given substance into atoms.

heat of combustion The energy liberated when one mole of a given substance is completely oxidized.

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