The chemical equation

A chemical equation describes the changes that occur during a chemical reaction. Instead of writing an equation using words, for example:

1 part of solid zinc & 2 parts of hydrochloric acid gives 1 part of zinc chloride & 1 part of hydrogen gas

we express it as:-

1Zn(s)+2HCl(aq)\rightarrow 1ZnCl_2(aq)+1H_2(g)

or simply (by omitting the number 1, which is quite redundant)

Zn(s)+2HCl(aq)\rightarrow ZnCl_2(aq)+H_2(g)

In general, a chemical equation has the form:


    1. the upper-case letters represent elements, ionic formula or molecular formula (each entity is known as a chemical species);
    2. the lower-case letters (known as stoichiometric coefficients) represent the relative portions of chemical species in the reaction;
    3. the letters in brackets indicate the physical state of the elements or molecules (s for solid, l for liquid, g for gas and aq for aqueous, i.e. a species dissolved in water); and
    4. a single-arrow or double-arrows symbol is included depending on whether the chemical reaction is non-reversible or reversible respectively.

Note that the ratio of stoichiometric coefficients for the chemical species in the zinc-acid chemical equation above is 1:2:1:1. Hence, we can say that one mole of zinc reacts with two moles of hydrochloric acid to give one mole zinc chloride and one mole hydrogen gas.


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