Some properties of hydrocarbons depend on the size of their molecules. As molecules get larger, the boiling point increases, the viscosity increases (the liquid is thicker and flows more slowly), and the flammability decreases (the hydrocarbon is harder to ignite). So small molecules are easy to ignite and flow freely, while large molecules are thick, have high boiling points and ignite poorly. These properties influence how each hydrocarbon is used as a fuel.
To investigate this, you can compare different hydrocarbons by measuring how easily they flow and how easily they ignite, keeping the amounts and conditions the same.
Burning a hydrocarbon fuel releases energy. During combustion the carbon and hydrogen in the fuel are oxidised. In complete combustion, with plenty of oxygen, the products are carbon dioxide and water.
To balance an equation, balance the carbon first, then the hydrogen, then the oxygen. Propane burns like this: C3H8 + 5O2 → 3CO2 + 4H2O. Here 5 oxygen molecules react and 4 water molecules form, because the three carbon atoms make 3 CO2 and the eight hydrogen atoms make 4 H2O. Methane burns as CH4 + 2O2 → CO2 + 2H2O.