In both the solid and liquid states, the relationship between heat and molecular motion is characterized by the kinetic theory of matter. According to this theory, heat energy is associated with the random motion of molecules within a substance.
In the solid state, molecules are tightly packed and held in fixed positions by intermolecular forces. However, they still possess thermal energy and undergo vibrational motion around their equilibrium positions. As heat is added to a solid, its temperature increases, causing the molecules to vibrate with greater amplitude. This increased molecular motion corresponds to an increase in the internal energy of the solid.
In the liquid state, molecules have more freedom of movement compared to solids. They are still attracted to each other but are not rigidly fixed in position. The intermolecular forces in liquids are weaker than those in solids. When heat is supplied to a liquid, the additional energy causes the molecules to move more rapidly. This results in an increase in the average kinetic energy of the molecules, leading to increased molecular motion and, subsequently, a rise in temperature.
It's important to note that in both solid and liquid states, the molecular motion is not a collective flow like in gases, but rather a random motion at the molecular level. The amount of molecular motion is directly related to the substance's temperature, with higher temperatures corresponding to greater molecular kinetic energy.
Overall, the relationship between heat and molecular motion in the solid and liquid states involves an increase in molecular vibrations or random movements as heat is added, resulting in a rise in temperature and an increase in the internal energy of the substance.