Why the liquid methane did not change phase before 2007

The liquid in the lake changed phase, either from liquid to gas (evaporated) or from liquid to solid (froze). Both of these changes involve a change in the freedom of movement of the molecules. As liquid, molecules of the lake moved around each other.

Explanation:

Before 2007 there wasnt enough energy to break the bond of attraction between the molecules keeping it at liquid but After 2007, the sun had transferred enough energy so that the kinetic energy of the methane molecules increased enough to break the attraction between them. The lake evaporated and the molecules started moving away from each other.

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