Heating curves are typically used to show the process of phase change. Heating curves represent the relationship between heat flow and temperature. The heat which flows into the material resolves itself as some combination of internal energy increase and work.

When heat is added to a solid below its melting point, the temperature begins to rise. Rising temperature means that the average kinetic energy of the particles is increasing (although with greater range of vibration along lines of intermolecular force, the electrostatic potential energy component is also increasing).

If the solid is heated at its melting point, the heating curve shows that the temperature remains constant until the solid has melted. The melting process requires energy. The enthalpy change of melting is called the heat of fusion, and does not represent an increase in the kinetic energy of the particles, hence no temperature increase. The heat flow is allowing the particles to escape from the rigid intermolecular binding of the solid state into the less tight arrangement of the liquid. When mutually attracting charged particles (the polarities which lead to intermolecular force) are moved apart from one another, the electrostatic potential energy of the system is increased. The heat flow at the melting point increases the internal energy. In other words, at the melting temperature, it is not the kinetic energy of the particles but the electrostatic potential energy between the particles that increases.

After all of the solid has melted, heating the liquid raises its temperature until the boiling point is reached. Head added to the liquid at its boiling point is absorbed as the heat of vaporization as the liquid boils at constant temperature. Similar to the heat of fusion, the internal energy increase brought about by the heat of vaporization is not increasing the average kinetic energy of the particles during the process of vaporization but the electrostatic potential energy increases as the particles are separated each from each other. Remember, though, that the heat flow is not only increasing the internal energy in vaporization. The phase change from liquid to gas is accompanied by a large change in the volume of the system, so as the heat flows in, it must also perform pressure-volume work.

After all the liquid has been vaporized, and the system is entirely gaseous, the addition of more heat then raises the temperature of the gas. At that point, the internal energy increase corresponding to heat flow is all dedicated to increasing particle kinetic energy.

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