Phase changes

The states of matter depend on the of its intermolecular forces (IMF) and kinetics energies (KE) of its particles -either molecules, ions, or atoms. The chemical identities of the particles in a liquid determine the types and strength of intermolecular attraction possible.

The change of energy in a system determines the phase changes. To change from a certain state to another state of matter, we must add enough energy to overcome the intermolecular forces that holding the particles together.

The different phase changes are described. The change of phase from solid to gas is called sublimation, and from gas to solid deposition. From solid to liquid is called melting or fusion and from liquid to solid freezing. From liquid to gas it is called vaporization and from gas to liquid, condensation. Changes from solid to liquid, solid to gas and liquid to gas, are endothermic processes, which means that energy needs to be added. The reverse processes are endothermic processes, which means that energy is released from the substance

Figure 1. The phase changes.

Melting The energy required to break the intermolecular forces and increase the motion of the particles during solid to liquid transition is called heat of fusion, or enthalpy of fusion (ΔHfus). In chemistry, melting is known as fusion.

Vaporation The degree of particle movement keeps increasing as the temperature rises. The motion of the particles becomes more rapid and spread apart as the particles move into gaseous states. At this point, even less intermolecular forces keep the molecules together. The transition from liquid to gas is called evaporation. The energy involved in this phase changes is called heat of vaporization, or enthalpy of vaporation (ΔHvap).

Sublimation Some solids can skip the liquid phase and change to the gaseous state directly. The transition from solid to gas is called sublimation. The energy required in sublimation is called heat of sublimation, or enthalpy of sublimation (ΔHSub).

Generally, the heat of sublimation is a lot larger than the heat of vaporation, with the heat of fusion as the smallest value. This is because all the intermolecular forces in the gaseous states are severed, while in liquid some are intact, and in solid most are unbroken. It requires more energy to sever all intermolecular forces between the particles.

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