As temperature changes, rocks expand and contract. The rocks eventually break into
smaller pieces. Which process accounts for
this expansion and contraction of rocks?

Thermal heating and cooling

Explanation:

This process described results from thermal heating and cooling as a mechanism of mechanical weathering of rock bodies.

2h2o2--> 2h2o + o2

this reaction is of the spontaneous decomposition of hydrogen peroxide down into water and oxygen.

add 2 molecules of hydrogen peroxide and 2 molecules of water. since oxygen is naturally diatomic, the total number of atoms of each element is now the same on both sides of the equation so it is balanced.

this may not be entirely correct but let me know.