1.21 write word equations and balanced chemical equations to represent the reactions studied in this specification

To balance an equation, there has to be the same number of each atom/element on each side of the equation.

For example...
Aluminum + copper (II) oxide ----> Aluminum oxide + copper

      Al       +         CuO           ---->          Al₂O₃        +    Cu

on this side there is...                    on this side there is...

1 Al                                                      2 Al

1 Cu                                                     1Cu

1 O                                                       3 O



You need to make both sides even... there needs to be the same amount of each elements on both sides, so we need to balance it.

2Al + 3CuO ----> Al₂O₃ + 3Cu

on this side there is...                    on this side there is...

2 Al                                                      2 Al

3 Cu                                                     3 Cu

3 O                                                       3 O


Now the equation is balanced :)