Water was heated in the vessel. What can you say about her internal energy?

The internal energy of the water increased during heating. The heat energy transferred to the water when heated is converted into kinetic energy of the movement of water molecules, they begin to move faster. Internal energy is the sum of the kinetic energy of the movement of molecules and the energy of their interaction.
According to the basic equation of the molecular kinetic theory, the kinetic energy of molecules is determined by the formula: Ek = 3 * k * T / 2, where k is the Boltzmann constant, T is the absolute temperature.
According to the first law of thermodynamics, the transferred amount of heat is used to increase its internal energy and to perform work: Q = ΔU + A. Since water does not do work, the entire amount of heat goes to change the internal energy.