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• SkillCat Team

# HVAC Basic Science: Matter & Phase Changes

Updated: Jun 29

## In this unit we will define matter, atoms, and molecules. We will also learn about the chemical and physical properties of a substance. Skip to quiz!

### Matter and Atoms

The matter is anything that has weight and takes up space. Everything in the world is a matter, including us.

Every matter is made out of small units. These units are called atoms. There are many types of atoms. As of now, scientists have discovered 118 different types of atoms.

Atoms cannot be divided into something. Atoms are the smallest unit of matter. But, when two or more atoms are combined, they make molecules.

### Phases of Matter

Matter can be in three different phases:

• Solid,

• Liquid, and

• Gas

The phase of a matter depends on the arrangement of its molecules. Lets us look at how molecules are arranged in the three phases using an example.

In solid matter, the molecules are tightly packed together. The molecules stay in place and do not move around freely.

This means the shape and volume of a solid matter do not change until we do something to the matter, such as adding heat. Recall, volume is the amount of space a 3D object takes up.

Unlike solids, liquids do not have an exact shape. As we saw in the video, the molecules in a liquid move around while staying close together. This means the shape of the liquid is constantly changing with the molecules moving around. But the volume stays the same.

The molecules in a gas move around more than the molecules in a liquid. Similar to liquid, gas does not move in a definite shape because the molecules are moving around. But a gas also does not have a definite volume because gas is not a three-dimensional object.

If we change a matter from solid to liquid, will the mass of the matter stay the same? The law of conservation of matter states that matter cannot be created or destroyed during a phase change. Meaning, the mass of a matter states the same in all three phases.

The mass of a matter does not depend on its phase. This is because the amount of molecules of a matter stays the same in every phase. Recall, mass is the amount of matter something has.

Using the mass along with volume, we can determine another property of matter called density. Density measures how tightly compact the atoms inside the matter are. Let us look at how we can find the density of a matter.

To summarize, everything around us is a matter. All matter is made out of atoms. When two or more atoms are combined, they make a molecule.

Molecules of a solid are tightly packed together. Molecules in a liquid move around while staying close together. Molecules of a gas are further apart from each other and move freely.

## In this topic, we will look at how matter goes from one phase to another. Skip to quiz!

### Main Phase Changes Recall, there are three phases a matter can have.

1. Solid

2. Liquid, and

3. Gas or vapor

Phase changes are essential in our lives for many reasons. For example, it freezes our food and preserves it for a longer time. But how does a matter change its phase?

Recall, latent heat is thermal energy that changes a matters’ phase without changing the temperature of the matter. Let us look at how solid changes to liquid using heat.

### Melting Point

A solid matter becomes liquid when it is melting. The melting point is the temperature at which a solid matter becomes liquid.

Recall, in latent heat, the temperature stays constant during phase change. For example, the melting point temperature of the water is 32℉. When ice melts into water, the melting point temperature stays the same.

At the melting point, the compact molecules in a solid matter start to spread out. Recall, molecules that spread out are in the liquid phase. If we continue heating the liquid, it will reach the gas phase at boiling point.

### Boiling Point

The boiling point is the temperature when liquid matter becomes gas. Every matter has its boiling point depending on the properties of the matter.

The molecules of liquid move around freely at the boiling point. Recall, freely moving molecules are in the gas phase.

Similar to the melting point, at boiling point, the temperature stays constant because of latent heat. For example, the boiling point temperature of the water is 212℉. When water boils into gas, the boiling point temperature stays the same.

If the gas is heated further, the matter will pass its critical point. The critical point is the last point where we can tell gas and liquid apart. Beyond the critical point, the matter is gas and liquid at the same time. Let us see how that looks like with a demonstration.

After the critical point, matter can be liquid and gas at the same time. Say we don’t want to pass the critical point; how do we get gas to become liquid?

### Condensation

To turn gas into a liquid, we have to bring the gas molecules to a condensation point. Let us define condensation using few examples.

Condensation is when the energy in the gas molecules is reduced by decreasing the temperature. By reducing the energy, the freely moving gas molecules start getting closer to each other.

Recall that phase changes occurred when heat is added to the matter. But in condensation, gas is cooled to reduce the energy of the molecules and transform into liquid. If the liquid matter is cooled further, it will reach the freezing point.

The freezing point is the temperature when a liquid matter becomes solid. The freezing point and the melting point of a substance are usually at the same temperature.

### Pressure

So far, all the phase changes we have been talking about happens under constant pressure. Meaning, pressure does not change as the temperatures are changing. But, in the real world, pressure is not always constant.

Similar to heat, adding or removing pressure can also change phases. Let us see what happens to a matter when pressure is applied or removed.

If we increase the pressure, the melting and the freezing point will decrease. Meaning, at higher pressure, less heat is needed to melt and freeze a matter.

If we decrease the pressure of a matter, the melting point and freezing point increase. Meaning, at lower pressure, more heat is needed to melt and freeze a matter.

If we increase the pressure, the boiling and the condensation point also increase. This means, at higher pressure, more heat is needed to boil or condensate a matter.

If we decrease the pressure, the boiling and the condensation point also decrease. This means, at lower pressures, less heat is needed to boil or condensate a matter.

Say we have three different matter in the gas phase in a jar. How do we figure out the amount of pressure in the jar? Let’s take a look at it.

### Dalton’s Law

To find the pressure of the three gas mixtures, we use dalton’s law. As explained in the video, the law states that the pressure of each matter summed up will give us the amount of pressure in the jar.

In this unit, we learned that we could change the matters’ phase by increasing and decreasing the temperature and pressure. We also learned how to find the pressure of a gas mixture using Dalton’s law.

Question #1: Two or more molecules combine to form atoms.

1. True

2. False Two or more atoms combined to form molecules.

Question #2: At what phase do the molecules stay tightly packed?

1. Solid

2. Liquid

3. Gas

4. All of the above Molecules are tightly packed only in the solid phase.

Question #3: Which of the following is not a phase of a matter?

1. Solid

2. Liquid

3. Gas

4. Water A matter only has three phases: solid, liquid and gas.

Question #4: The melting point is the temperature of a matter where it turns from solid to Liquid.

1. True

2. False The definition of melting point is the temperature of a matter where it turns from solid to liquid.

Question #5: The boiling point is the temperature of a matter where it turns from liquid to vapor.

1. True

2. False The definition of boiling point is the temperature of a matter where it turns from liquid to vapor.

Question #6: Condensation happens when a solid cools down and becomes liquid.

1. True

2. False Condensation changes gas to liquid. This is because the temperature of the vapor decreases.

Question #7: The freezing point is when liquid matter becomes solid.

1. True

2. False At freezing point, liquid matter changes to solid.

Question #8: If we decrease the pressure of a matter, its melting point increases.

1. True

2. False Decreasing the pressure increases the melting point of a matter.

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

1. True

2. False Decreasing pressure decreases the boiling point of a matter.

Question #10: In a gas mixture, the total pressure is equal to the sum of the pressures of each gas.

1. True

2. False This is the Dalton’s law.