The process of diffusion causes molecules to move from low to high or high to low concentration’s until a state of blank is reached

The word that replaces blank is "equilibrium"

Explanation:

Equilibrium is where both sides have an equal concentration of the certain molecule.

before, trains worked using the steam engine. thus, it uses heat so it is crucial to apply heat balance. the steam engine absorbs energy during the stage of boiling water. so, this is the endothermic reaction. when the steam is used to work the piston, it has to release this energy through running the train. the excess energy is released as an exhaust to the chimney. this is the exothermic process because it releases energy to the surroundings.