Phase Changes

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The Phase Diagram

At high pressure and low temperature, a substance is in a solid phase. At low pressure and high temperature, a substance is in a gas phase. Liquid phase is in between these two.

The line seperate the phases correspond to points at which the substance con eist simultaneously in both phases at equilibrium. The normal freezing point is the temperature at which solid-liquid line crosses the 1 atomphere pressure line. The normal boiling point is the temperature at which the liquid gas equilibrium line crosses the 1 atmosphere pressure line.

The triple point is the temperature and pressure at which all three phases can exist simultaneously in equilibrium

The critical point is the temperature beyond which the moelcules of a substance have too much kinetic energy to stick together and form a liquid.

Phase changes

Solid --> liquid = Melting

liquid --> solid = Freezing

liquid --> gas = Eavaporation

gas --> liquid = Condensation

soiid --> gas = sublimation

Heat of Fusion

The heat of fusion = the energy must put into a solid so it melts. This energy is needed to overcome the the forces holding the solid together. Alternatively, the heat of fusion is the heat given off by a substance when it freezes The intermolecular forces within a solid are more stable and therefore have lower energy than the forces within a liquid, so energy is released in the freezing process.

Heat of Vaporization

The heat of vaporization is the energy that must be put into liquid so it vaporizes. This energy is needed to overcome the forces holding the liquid together. Alternatively, the heat of vaporization is the heat given off by a substance when it condenses. As heat is added to a substance in equilibrium, the temperature of the substance can increase or the substance can change phases, both changes cannot occur simultaneously.

The phase diagram for water

In the phase diagram of substance other than water, the solid-liquid equilibrium line slopes upward. IN the phase diagram of water, the solid-liquid equilibrium line slopes downward. What this means is that when pressure is increased, a normal substances will change from liquid to solid, but water will change from solid to liquid. Moreover, hydrogen bonds form a lattice structure when water freezes. This forces the moelcules to remain farther apart in ice than in water, making the solid phase less dense than the liquid phase. That is why ice is less dense than the water

The Heating Curve

Substance starts as a solid. AS heat is added, the KE of the substance increases and the temperature increases. When the temperature reaches the melting point, the line flattened out. At this point, during the phase changes from solid to liquid, all added heat goes toward overcoming the intermolecular forces in the solid; therefore, there is no increase in kinetic energy during the phase change and the temperature stays the same.

The length of the horizontal line at the melting point will be propertional to the heat of fusion and the length of the line at the boiling point will be proportional to the heat of vaporization. Note that the heat of vaporization is larger than the heat of fusion That's because many more intermolecular forces must be overcome in vaporization than in melting. Also, since its easier to change the temperature of substance with a low specific heat, a graph whose temperature curve has a steep slope will indicate a substance that has a low specific heat.