Final answer:
The phase of a substance such as methane depends on temperature and pressure. High pressure would likely be the reason methane remained in the liquid phase before 2007, as it prevents the particles from separating enough to change into a gaseous state, overriding the effects of temperature.
Step-by-step explanation:
The question is asking why liquid methane did not change phase before 2007. The phase of a substance is heavily dependent on two macroscopic variables: temperature and pressure. Substances exist as solids, liquids, or gases at specific temperatures and pressures due to the balance between the energy of the particles and the intermolecular forces acting between them.
If we look at a phase diagram, which shows the state of a substance at different temperatures and pressures, we see that a substance such as methane will only change phase under certain conditions. A high pressure tends to favor the liquid or solid phase because it allows particles to be pushed closer together, making it harder for them to separate and enter the gas phase. Therefore, if methane was under high pressure, it would remain as a liquid even if its temperature was at a point where it might otherwise expect to be a gas. On the other hand, given methane's low boiling point, at lower pressures and temperatures, it's more likely to be in a gaseous state.
Since the question suggests that methane remained liquid before 2007, it is likely due to high pressure conditions that allowed it to stay in that state despite its low temperature.