Alkenes and alkanes can be differentiated by their chemical structure and reactivity, specifically by the presence or absence of a carbon-carbon double bond.

Both alkenes and alkanes are types of hydrocarbons, meaning they consist solely of hydrogen and carbon atoms. However, they exhibit distinct differences in the types of bonds that link the carbon atoms. Alkanes are characterized by single bonds between carbon atoms, whereas alkenes contain at least one carbon-carbon double bond. This variation in bonding leads to differences in their chemical properties and reactivity.

Alkanes, often referred to as saturated hydrocarbons, follow the general formula $C_nH_{2n+2}$. This indicates that for every carbon atom present, there are $2n + 2$ hydrogen atoms. For instance, examples of alkanes include methane ($CH_4$), ethane ($C_2H_6$), and propane ($C_3H_8$).

Conversely, alkenes, known as unsaturated hydrocarbons, adhere to the general formula $C_nH_{2n}$. This means that for every carbon atom, there are precisely $2n$ hydrogen atoms. Examples of alkenes include ethene ($C_2H_4$), propene ($C_3H_6$), and butene ($C_4H_8$).

A notable difference between alkenes and alkanes lies in their reactivity. Alkenes are generally more reactive due to the presence of the carbon-carbon double bond, which can readily break and form new bonds. This makes alkenes more inclined to participate in chemical reactions. In contrast, alkanes are relatively stable and less reactive, owing to their single bonds.

A common method for distinguishing between alkenes and alkanes is the bromine water test. When bromine water is added to an alkene, the solution changes from brown to colorless as the bromine reacts with the carbon-carbon double bond. In contrast, when bromine water is introduced to an alkane, no reaction occurs, and the solution retains its brown color. This test serves as a straightforward and effective means of differentiating between alkenes and alkanes.