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Science

Chemical equations

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When chemicals react with each other, different chemicals are made. One of the best ways to describe what is happening is by writing a chemical equation.

Word and symbol equations

Word equations

A chemical equation tells you which chemicals reacted together (the reactants) and the new chemicals that were made in the reaction (the products).

The simplest equation is a word equation. For example:

sodium + chlorine → sodium chloride

Symbol equations

A symbol equation gives more information about what is happening in the reaction:

2Na + Cl2 → 2NaCl

Each of the reactants and products is shown as a formula. This formula shows how many atoms of each element are present.

The formula for sodium is Na - the same as its symbol.

The formula for chlorine is Cl2, because the halogens exist as molecules of two atoms (diatomic molecules).

Each of the Group 1 halides has a formula with one symbol for the metal and one for the halogen. So, for sodium chloride the formula is NaCl.

The numbers in front of the formulae are there to balance the equation. This gives the same number of atoms of each element on each side of the equation.

For the exam you will be expected to know the following formulae:

  • hydrogen, H2
  • water, H2O
  • fluorine, F2
  • chlorine, Cl2
  • bromine, Br2
  • iodine, I2

Halides of Group 1 metals - for example:

NameSymbol
sodium chlorideNaCl
potassium bromideKBr
lithium iodideLiI

Hydroxides of Group 1 metals - for example:

NameSymbol
sodium hydroxideNaOH
potassium hydroxideKOH
lithium hydroxideLiOH

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