Question

What is the reason that causes octane to be a liquid at room temperature but methane to be a gas at the same temperature?

Answer 1

one simple reason: the number of carbons, molecular structure.

Answer 2

Yeah, just to elaborate a bit, it's just like what makes a few rocks easier to lift than a huge boulder? It's more stuff. Except instead of the energy being used in your arms to lift the rock, it's heat. If you give a methane molecule some heat it takes less to break it free from the liquid state and turn into gas while octane takes much more heat, so it will remain liquid unless you add more to turn it into vapor.