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# Review: Phase Changes

## 1. Changing Physical States

Recall that everything in the world around us is made of matter. There are three primary states of matter.

In order of increasing temperature, the three physical states are:

• Solid,

• Liquid, and

• Vapor, which is also known as gas.

This means that the state with the lowest temperature would be solids.

The state that a substance is in depends on both its temperature and pressure. If we change the pressure, temperature, or both, we can change the state of the substance.

For example, an ice cube is in solid form. If you were to put an ice cube in a pan and heat it up, its temperature will increase until it becomes a puddle of water. It is then in liquid form. This is an example of state change due to an increase in temperature.

In the same example, as you continue to heat up the water, it will increase in temperature until it bubbles up and becomes steam. The steam that leaves the pan is water in its vapor phase.

Decreasing temperature can also change the state of a substance. If we put water in the freezer, the liquid water turns to ice, which is solid. This is an example of phase change due to decreasing temperature.

We can also change the state of a substance by applying or removing pressure.

As mentioned in the video, increasing the pressure acting on a bubble changes the vapor to liquid. Decreasing the pressure acting on the bubble changes liquid to vapor.

## 2. Vaporization

Think of water. In liquids such as water, there are bonds between molecules that keep the molecules together. The bonds between molecules are what make liquids appear as tangible masses that we can see and recognize as liquids

Recall that temperature is a measure of how fast the molecules are moving in a substance, on average. The higher the temperature, the faster the molecules are moving inside the substance. For example, molecules in a hot coffee would be moving faster than an iced coffee.

When we add heat to a substance, we are increasing its temperature and making the molecules move faster until they overcome those bonds between them.

As temperature increases, the molecules overcome their bonds, and they no longer form a liquid. Instead, by overcoming those bonds, the molecules become water vapor. This is what is happening when we see steam coming up from a boiling pot.

The process of changing from a liquid to vapor is called boiling or vaporization. Remember, we need to add heat to a liquid for it to vaporize. In refrigeration systems, heat from indoor air is used to boil a liquid. When this heat is drawn from your house, this is what provides cooling.

## 3. Boiling Point

The boiling point is a temperature where a liquid changes phase to vapor. Each substance has its unique boiling point depending on its physical properties.

Recall that the boiling point of a substance depends on pressure. If we want to boil water at a higher pressure, the boiling point will be a higher temperature. This means it will take more heat to make the water change phase at a higher pressure.

The boiling point of pure water, for example, is 212 °F or 100 °C. That’s the temperature where water becomes vapor at a pressure of 1 atm, which is the standard pressure of the air around us at sea level.

Note that the boiling point will also be different if your substance is not pure water. For example, if there is salt or other minerals dissolved in the water, the boiling point will change.

If we are heating up pure water and the temperature doesn’t reach 212 °F, the water will not change to vapor. For example, if we heat up the pure water to 180 °F, the water becomes hot but it does not bubble. Bubbling is an indication of changing from liquid to vapor.

If the temperature of the water is close to 212 °F, you will see a few bubbles forming. This is because the bottom of the pan is in direct contact with the heat source, so it gets hot faster than the top of the pot. So the bubbles indicate that some molecules on the bottom have reached 212 °F and are changing phase.

The condensation point is the temperature at which vapor turns to liquid. Note that the condensation point is the same as the boiling point for pure substances. That means if you have pure water vapor at 250 °F and you let it cool, the water vapor will condense into liquid at 212 °F.

Both the boiling point and condensation point are properties of pure substances only. We will discuss the boiling temperature for mixtures when we discuss refrigerant blends.

## 4. Condensation

As the temperature of a substance is decreased, the substance also changes state. For our purposes, the state change we are most concerned with is condensation.

In the video, ice cubes are placed on top of the cup to remove heat from the air under the cup. Condensation occurs when the temperature is decreased, changing vapor to liquid. To reiterate, you need to remove heat from a substance to change state from vapor to liquid.

A decreasing temperature is the opposite of boiling. A decreasing temperature means a slower average speed of the substance. As molecules slow down, they start to form bonds with other molecules around them, forming a liquid.

We see this commonly on windows on a cold day. The outside air is much colder than the inside air, making the windows also cold. Since the cold window is in contact with warm indoor air, heat transfers from water vapor in the air to the cold windows, condensing the water vapor to liquid droplets.

## 5. Conclusion

In this chapter, we reviewed the three main states of matter. We also discussed boiling and condensation. Understanding how increasing and decreasing temperature is related to the different states of matter will help us understand how HVAC systems can provide cooling.

Question #1: Which of the following states has the highest temperature?

(Assume constant pressure)

1. Solid

2. Liquid

3. Vapor

4. Water

Vapor has the highest temperature of the three states.

Question #2: As the temperature of liquid increases, what phase does it turn into?

(Assume constant pressure)

1. Solid

2. Liquid

3. Vapor

4. Water

As the temperature of liquid increases, it turns into vapor. Think about water. As the water boils, it becomes steam, which is water vapor.

Question #3: If we increase the pressure acting on a liquid, what state can the substance change to?

(Assume constant temperature)

1. Solid

2. Liquid

3. Vapor

4. Water

If we increase the pressure acting on a liquid, the molecules will move closer and closer together. Eventually, if the pressure keeps increasing, the liquid can turn into a solid.

Question #4: For the same substance, the average speed of the molecules in a vapor is faster than the average speed of molecules in a liquid.

1. True

2. False

This is true. Recall that temperature is a measure of the average speed at which molecules are moving. The higher the temperature, the higher the speed of the molecules.

The temperature of vapor will be greater than the temperature of a liquid of the same substance. Since the temperature is higher, so is the average speed of the molecules.

Question #5: Boiling requires the addition of heat.

1. True

2. False

Adding heat will increase the temperature of the substance until it reaches its boiling point, which changes its state from liquid to vapor.

Question #6: The boiling point is the temperature of a substance where it turns from liquid to vapor.

1. True

2. False

This is the definition of the boiling point.

Question #7: The boiling point of most refrigerants is 212 °F.

1. True

2. False

The boiling point of water is 212 °F. Different substances generally have different boiling points. This means they will change from liquid to vapor at different temperatures. We choose substances with much lower boiling points to use as refrigerants. We’ll see why in the next module.

Question #8: Condensation happens when a gas or vapor cools down and becomes liquid.

1. True

2. False

This is true. Condensation is the state change from vapor to liquid. This is because the temperature of the vapor decreases.

Question #9: If we decrease the pressure of a substance, its boiling point decreases.

1. True

2. False

This is true. Decreasing pressure decreases the boiling point of a substance.

Question #10: You are boiling a pot of water with the lid on. As the water heats up and starts to boil, you notice that drops of water form on the bottom of the pot lid. This is an example of condensation.

1. True

2. False