H

Heat conductor (thermal conductor)     A substance that can allow heat to pass through it easily. Metals are good heat conductors. The bottom of a frying pan is made of metal so that the heat from the cooker can reach the food very easily.

High-alloy steel     Some steels contain up to 25% of other elements. Stainless steel is an example of a high alloy steel (it contains large amounts of nickel and chromium).

High melting point    Substances with high melting points have strong attractions between their particles.Metals (in general) have high melting points and so do ionic compounds and macromolecules.

Humus     This is a component of soil. Humus is the mixture of rotting vegetation and animal remains. A compost bin will provide a source of humus for a gardener.

Hydrocarbon     A hydrocarbon is a chemical that contains ONLY hydrogen atoms and carbon atoms. The main source of hydrocarbons is crude oil. This is mostly alkanes (hydrocarbons with only single bonds). The hydrocarbons in crude oil are separated by fractional distillation into groups that have similar boiling points.

Hydrogen     This gas is produced when a metal reacts with an acid. Hydrogen gas has a very low density. It burns with a squeeky pop. This is the test for hydrogen.

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