Explain why cool rock material sinks when convection takes place in the mantle?

This is because its density increases. In a liquid, the density decreases, allowing it to stay on top of another substance. The liquid cools as it rises away from the core, and gets closer to the crust. The loss of temperature makes it more dense than the liquid.

CH4 + 2O2 = CO2 + 4H20
(First one) seems to be balanced correctly I might be wrong the way the numbers look are kind of confusing. You just have to count and make sure there’s the same number of atoms on each side