5.02 Phase Changes

Boiling Point

Boiling is the process of transforming a liquid into a gas by adding heat. As heat is added, the particles in the liquid have a greater kinetic energy and move faster. Particles eventually have enough kinetic energy to escape the liquid phase and become a gas. The temperature at which a liquid boils, changing from a liquid into a gas, is called the boiling point.

Cooling Curves

Cooling curves are the opposite of heating curves. A cooling curve shows the temperature change as a substance loses energy and cools down. Cooling curves have the same horizontal sections that you saw in heating curves, where the phase changes from a gas to a liquid or from a liquid to a solid.

Freezing Point

Freezing is the process of transforming a liquid into a solid by the removal of heat, the reverse of the melting process. As the sample of liquid loses heat, the particles' movement slows down. The particles continue to move slower and slower until the attractive forces between them are able to hold the particles in a fixed position, transforming the liquid into a solid. The temperature at which a given substance transforms from a liquid into a solid is called its freezing point, which is the same temperature as the substance's melting point.

Did You Know?

Have you ever heard a weather report mention the dew point for that day? The dew point is the temperature at which the water vapor in the air will condense to a liquid. The dew point for a given day depends on the atmospheric pressure and other factors, so it may be different from day to day and season to season for your area. The dew point is associated with relative humidity, another term you have probably heard on the weather report. A high relative humidity means that the dew point is close to the current air temperature and there is a lot of moisture in the air.

Intro

Just as a substance's phase is determined by kinetic energy and intermolecular forces of attraction, a change between different phases of matter is caused by the transfer of energy (usually a gain or loss of heat). If you have ever noticed that a puddle of water in front of your house disappeared on a sunny day, that dew coated the grass in your yard after a cool summer night, or that a pond froze after several really cold days, then you have experienced examples of phase changes.

Melting Point

Melting is the process of a solid transforming into a liquid. When heat is added to a solid, the particles' kinetic energy increases, and they vibrate more and more violently. If enough heat is added, the attractive forces between the particles are not able to hold them together, and the substance melts. The temperature at which a given substance melts is called its melting point. Imagine a group of children are holding hands. If each of the children starts to jump around randomly, it becomes more difficult for them to hold onto each other. If the jumping becomes energized and violent enough, the children will not be able to hold on any longer. This is similar to what happens as the particles of a solid gain energy.

Types of Phase Changes:

Melting, Freezing, Boiling, and Condensing

Condensation Point

The process of transforming a gas into a liquid through the removal of heat is called condensing. This process occurs when the temperature of the gas is cooled enough for the particles to slow down and attract each other, forming a liquid. The temperature at which this phase change occurs is called the condensation point, the same temperature as the substance's boiling point. If you have ever had a cold glass of lemonade on a warm summer day, you may have noticed water droplets appearing on the outside of the glass. Your glass was not leaking; the water vapor from the surrounding air condensed to liquid when it was cooled on the surface of your glass. Condensation in the form of dew often appears on windows and grass in the morning because water vapor from the warm air of the sunny day was cooled to form a liquid during the night.

Heating Curves

This graph is called a heating curve because it represents the change in temperature as a substance under constant heat changes from a solid to a liquid to a gas. Solid -When the solid ice was heated, the temperature increased until it reached the substance's melting point. For water, the melting point is 0°C, and you may have noticed that the temperature stayed constant at 0°C until the entire sample melted. The heat that was being supplied by the Bunsen burner was used to melt the ice. Solid-Liquid Phase Change -When the energy being supplied in the form of heat is used for a phase change, the temperature of the sample always remains constant. This is represented by the horizontal sections in the heating curve. When a phase change is complete, the temperature begins to rise again. Liquid-Gas Phase Change -What does the second horizontal section of the heating curve represent? This flat section of the graph represents the substance's boiling point. When the liquid begins to boil, the temperature remains constant as the heat energy provided by the flame is used to move the particles farther apart to change the liquid to a gas. Once all of the particles are in the gas phase, the temperature will begin to rise again.