when matter are mixed together there is a tendency for chemical reaction to take place especially if they are chemically active. so it does  not mean that mixing of matter will always cause chemical reaction just like mixing components of halo halo in a container all ingredients are matter but when mixed they don't react.

when matter is exposed to heat its molecules will absorb heat allowing them to  move, but the movement will vary according to the materials state or phase, for gases as the molecules are exposed to heat it creates great pressure because its molecules will move far from each other; for liquid as the molecules are exposed to heat, the bonds holding their molecules together are broken allowing evaporation to take place, and as for solid negligible movement will take place since it is rigid due to strong force of molecular attraction but ins some cases like metals gold for example it will undergo melting.