After this lesson, you should be able to...
Materials exist as solids, liquids, or gases depending on the strength of the intermolecular forces (IMFs) between particles. Since the presence of IMFs reduce the energy of the system (forming IMFs releases energy), to "break" or overcome these attractive forces between particles one must put energy into the system. Phase changes such as fusion (melting) and vaporization require an input of energy. Enough energy, in the form of heat, must be put into the system to overcome the IMFs tightly holding the particles in the solid phase. Once the particles are in the liquid phase, they have enough kinetic energy to overcome some of the IMFs and can move freely past one another. The weaker IMFs and increased molecular movement gives liquids their fluidity. Even more energy is required to go from a liquid to a gas because the differences between the liquid and gas phases are greater than the difference between the liquid and solid states. In the gas phase, the particles have little to no IMFs.