Final answer:
Methane is a gas at room temperature because it is a small hydrocarbon, while octane is a liquid at room temperature because it is a larger hydrocarbon with stronger intermolecular forces.
Step-by-step explanation:
Hydrocarbons, such as methane and octane, have different states at room temperature due to their molecular structure and size. Methane is a small hydrocarbon with only one carbon atom and four hydrogen atoms, making it a gas at room temperature. Octane, on the other hand, is a larger hydrocarbon with eight carbon atoms and eighteen hydrogen atoms, resulting in a higher molecular weight and stronger intermolecular forces, which allows it to exist as a liquid at room temperature.