Phase Changes

Phase Changes

Phase Changes That Require Energy

When ice gains enough energy, the hydrogen bonds that hold it together are disrupted enough to allow the molecules to move apart and enter the liquid phase This is called melting.

Phase Changes That Require Energy

During melting, the temperature of the ice-water does not increase. All the energy absorbed goes into breaking the bonds that hold the molecules together. The temperature at which the solid phase and the liquid phase can coexist is called the melting point. The melting point of ice is 0°C.

Multiple Choice

During melting, the temperature of the solid-liquid mixture remains constant.

• True

• False

Multiple Choice

The melting point of ice is

• 0°C

• 100°C

• 32°C

Phase Changes That Require Energy

When an ice cube melts, the temperature of the water produced remains constant. Once all of the ice has melted, additional energy added to the system increases the kinetic energy of the liquid molecules.

Phase Changes That Require Energy cont.

In liquid water, some molecules have more energy than others. Those molecules that have the energy required to overcome the forces of attraction escape from the liquid and enter the gas phase.

Phase Changes That Require Energy cont.

Vaporization is the process by which a liquid changes to a gas or vapor. When vaporization occurs only at the surface of a liquid, the process is called evaporation.

Multiple Choice

Evaporation occurs

• at the bottom of the liquid

• throughout the liquid

• at the surface of the liquid.

Phase Changes That Require Energy cont.

Evaporation takes place even in a sealed container. In a partially filled container, water vapor collects above the liquid and exerts pressure on the surface of a liquid. The pressure exerted by a vapor over a liquid is called vapor pressure.

Multiple Choice

The pressure exerted by a vapor over a liquid is called

• vapor pressure

• boiling point

• melting point

Phase Changes That Require Energy cont.

The temperature at which the vapor pressure of a liquid equals the external or atmospheric pressure is called the boiling point. At the boiling point, molecules throughout the liquid have enough energy to vaporize. Bubbles of vapor collect below the surface of a liquid and rise to the surface.

Multiple Choice

The temperature at which the vapor pressure of a liquid equals the external or atmospheric pressure is called the .

• melting point

• boiling point

• freezing point

Phase Changes That Require Energy cont.

Many substances have the ability to change directly from the solid phase to the gas phase. Sublimation is the process by which a solid changes directly to a gas without first becoming a liquid. Dry ice (CO₂) is an example of this process.

Multiple Choice

________________ is the process by which a solid changes directly to a gas without first becoming a liquid.

• Sublimation

• Deposition

• Condensation

Phase Changes That Release Energy

When enough energy has been removed, hydrogen bonds between water molecules keep the molecules fixed or frozen into set positions. Freezing is the reverse of melting. The freezing point is the temperature at which a liquid is converted into a crystalline solid. The freezing point of water is 0°C.

Multiple Choice

The freezing point of water is

• 32°C

• 0°C

• 100°C

Phase Changes That Release Energy

When a water vapor molecule loses energy, its velocity decreases. The water vapor molecule is more likely to form a hydrogen bond with another water molecule. The process by which a gas or vapor becomes a liquid is called condensation. Condensation is the reverse of vaporization.

Multiple Choice

Condensation is the reverse of

• vaporization

• melting

• freezing

Phase Changes That Release Energy cont.

The process by which a substance changes from a gas or vapor to a solid without first becoming a liquid is called deposition. Deposition is the reverse of sublimation. Frost is an example of deposition.

Multiple Choice

Deposition is the reverse of

• freezing

• sublimation

• melting

Phase Diagrams

Point B on the phase diagram of water, is the point where the three curves meet. It is called the triple point for water. The triple point is a point on a phase diagram that represents the temperature and pressure at which the three phases of a substance can coexist.

All six phase changes can occur at the triple point: freezing and melting; evaporation and condensation; sublimation and deposition. The diagram is broken into three areas (solid, liquid, and gas) where each phase can occur.

Multiple Choice

The triple point is where

• all three phase of matter can occur

• where solids and liquids occur

• where gases and liquids occur

Multiple Choice

What phase exists at 600 torr and 40°C?

• solid

• liquid

• gas

Multiple Select

What phase of phases exist at 4.6 torr and 0.01°C?

• solid

• liquid

• gas

End