1) They combust in oxygen to form carbon dioxide, water and heat. It is this heat which makes them good fuel sources.

e.g.

C₂H_{6 (g)} + 7/2 O_{2(g)  →} 2CO_2(g) + 3H₂0_(g) + heat

C₈H_20(g) + 13 O_2(g) →   8CO_{2 (g)} + 10H₂O_(g) + heat

2) They undergo substitution reactions with halogens in the presents of UV light.

CH_4(g) + Cl_{2(g)   →} CH₄Cl_(g) + HCl_(g)

The above reaction takes place first but there is eventually total substation of the Hydrogen atoms for Chlorine atoms. This reaction takes place through free radical substitution which you will learn as you move further in chemistry.

CH_4(g) + 4Cl_2(g) →   CCl_4(g) + 4HCl_(g)

Alkanes and some other homologous series which we will be introduced to later are referred to as hydrocarbons as they contain only hydrogen and carbon atoms.