Changes of State Matter on Earth can exist in any of these states – gas, liquid, or solid – and can change from one state to another. Listed below are possible changes of state.

Changes of State and Equilibrium Changes of state = phase change (solid to liquid, etc.). A phase is any part of a system that has uniform composition and properties. By system, we mean the particular part of the universe that we have decided to study, such as a mixture undergoing a chemical reaction in a beaker. Literally everything else in the universe besides the system constitutes the system’s surroundings.

Separating the system from its surroundings is a boundary, which might be real (like the walls of a beaker) or imaginary (like the imaginary envelope between the atmosphere’s troposphere and the stratosphere). We have an insulated system or closed system when energy cannot cross the system’s boundary in any direction.

Equilibrium is a dynamic condition in which two opposing changes occur at equal rates in a closed system.

To illustrate equilibrium, let’s look at a liquid-vapor equilibrium in a closed system. Remember a closed system is one in which energy cannot cross the system’s boundary in any direction. First let’s define condensation. Condensation is the process by which a gas changes to a liquid. A gas in contact with its liquid or solid phase is often called a vapor.

Though water exists mainly as a liquid at room temperature and pressure, water in an open container evaporates over a period of time until it all “disappears”. You may have noticed that in a closed system, such as a sealed bottle of water, water does not appear to evaporate.

When placed in a closed container, water does evaporate until the air in the container is saturated with water vapor. When the air is saturated with water vapor, the molecules in the vapor condense to a liquid as fast as the liquid evaporates, and the two processes (evaporation and condensation) continue at equal rates. This is called an equilibrium.

The evaporation and condensation are proceeding at the same rate, so there is no net change. In a closed container, the pressure (force per unit area on a surface) due to the water vapor reaches a maximum value (for a given temperature) called vapor pressure. The pressure exerted by a vapor in equilibrium with its corresponding liquid at a given temperature is called the equilibrium vapor pressure of the liquid.

Increasing the temperature of a liquid increases the average kinetic energy of the liquid’s molecules (increases the number of molecules in the vapor phase).

Boiling Point The temperature at which a substance turns from liquid to vapor. Thought question: Is the boiling point of a liquid the same in every location? Boiling point is directly related to atmospheric pressure, and indirectly related to altitude! The lower the atmospheric pressure is, the lower the boiling point.

Remember, as the temperature of water increases, the vapor pressure increases. When the vapor pressure equals the atmospheric pressure on the liquid, the liquid will boil. At high altitudes, the boiling points of liquids is lower than at sea level. A liquid boils when the vapor pressure equals the atmospheric pressure. (a) At 70 o C and 760 mmHg the atmospheric pressure is greater than the vapor pressure of the liquid. (b) At the boiling point (100 o C for water), the vapor pressure matches the atmospheric pressure. Bubbles of vapor form inside the liquid, and it boils. (c) At higher altitudes, the atmospheric pressure is less. Thus the liquid boils at a lower temperature.

In Denver, Colorado water will boil at about 94 o C. Do not confuse boiling with cooking. Cooking pasta in Denver is a slower process because the water is a lower temperature. Also, realize that water boiling rapidly is no hotter than water boiling slowly. The temperature of the water remains constant during the boiling process. And, the temperature of a boiling liquid never rises above its boiling point. No matter how much heat is applied, the liquid only boils faster, not hotter.

Boiling At the boiling point, all of the energy absorbed is used to evaporate the liquid, and the temperature remains constant as long as the pressure does not change. If the pressure above the liquid being heated is increased, the temperature of the liquid will rise until the vapor pressure equals the new pressure and the liquid boils once again.

We often use graphs to show the correlation between vapor pressure and boiling. The graph below is the vapor pressure curve for four different substances. Using this graph we can find the “Normal boiling point” for any substance by observing its boiling point at standard pressure (101.3 kPa, 1 atm, 760 mmHg). For example the “normal boiling point” of substance A would be 35°C. What would the normal boiling of substance D be? 100°CBased on this information, what do you think substance D might be?

Line D represents water. If the atmospheric pressure in a flask is lowered to 70 kPa, water would boil at what temperature?

Boiling vs. Evaporation Occurs below the boiling point, only at the surface of the liquid. Occurs at the boiling point, throughout the entire liquid. Normal Boiling Point: the temperature at which vapor pressure of a liquid is equal to atmospheric pressure.

Freezing and Melting The physical change of a liquid to a solid is called freezing. Freezing involves a loss of energy in the form of heat by the liquid. In the case of a pure crystalline substance, this change occurs at a constant temperature.

Freezing and Melting Melting, the reverse of freezing also occurs at constant temperature.

Freezing and Melting Points At what temperature does water freeze? At what temperature does ice melt? (Freezing point) (Melting point)

Sublimation and Deposition The change of state from a solid directly to a gas is known as sublimation. The reverse process is called deposition, the change of state from a gas directly to a solid.

Phase Diagrams A phase diagram is a graph of pressure versus temperature that shows the conditions under which the phases of a substance exist. Also reveals how the states of a system change with changing temperature or pressure.

The triple point indicates the temperature and pressure conditions at which the solid, liquid, and vapor of the substance can coexist at equilibrium. The critical point indicates the critical temperature and critical pressure.

The critical temperature is the temperature above which the substance cannot exist in the liquid state. The critical pressure is the lowest pressure at which the substance can exist as a liquid at the critical temperature.