Transcript: Thermal equilibrium is an important physical principle. When two substances have unequal temperatures, thermal equilibrium means that they will tend to evolve to a situation of equal temperature. At a microscopic level we know that temperature denotes the instantaneous random speed of atoms or molecules in the substance. Hot substances have faster instantaneous motions of their atoms or molecules. So when ice is dropped into a glass of water the ice melts, and the water cools down which means that the instantaneous motions of the molecules in the ice have increased while the instantaneous motions of the molecules in the water have decreased as it heads towards thermal equilibrium.