U

Universal indicator     This is a mixture of indicators which shows the strength of an acid or alkali as well as simply showing which it is. By comparison with a colour chart, a pH number can be selected. pH numbers less than 7 show an acid (the smaller the number, the stronger the acid) and pH numbers bigger than 7 show an alkali (the larger the number, the stronger the alkali).

Unreactive     A substance for which reactions are difficult. Unreactive substances come at the bottom of a reactivity series. A typical unreactive metal is gold. Iodine is a typical unreactive non-metal.

Unsaturated     An unsaturated hydrocarbon is a substance that contains only carbon atoms and hydrogen atoms and it must contain at least one carbon-carbon double bond. Alkenes are unsaturated hydrocarbons. So are alkynes.

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