KCIO3 → KCI + O2
First you have to count all the atoms of each element in both sides of the reaction, in this case:
Left side: there is one K atom, one Cl atom and 3 O atoms
Right side: there is one K atom, one Cl atom and 2 O atoms
K and Cl are balanced but O isn't
To balance O you can add a coefficient to O2 in the right side of the equation like this:
KCIO3 → KCI +3 O2
Now you have 6 oxygen atoms in the right side but only 3 in the left side, so you can add a coefficient to KClO3:
2KCIO3 → KCI +3 O2
And finally, Balance K and Cl again by adding a 2 as a coefficient for KCl
2KCIO3 → 2KCI +3 O2
Now both sides have the same amount of atoms of each element (2 K atoms, 2 Cl atoms and 6 O atoms)
