Fundamental chemical concepts
You should recall certain chemical formulas. You should also be able to write balanced symbol equations if you are given the formulas of the reactants and products.
Example: write the balanced equation for the reaction between hydrogen and oxygen to produce water.
Write the formulae for each substance:
H2 + O2 → H2O
Check for an unbalanced element, for example, O. Adjust the number of each atom or molecule needed, but never change a formula. We need two O atoms on each side:
H2 + O2 → 2H2O
Check for another unbalanced element. In this example, there are 2 H atoms on the left and 2 × 2 = 4 atoms on the right. So we need to double the number of hydrogen molecules:
2H2 + O2 → 2H2O