Lesson Video: Changes of State | Nagwa Lesson Video: Changes of State | Nagwa

# Lesson Video: Changes of State Science • First Year of Preparatory School

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In this video, we will learn how to describe and explain changes of state, and identify these changes through diagrams and heating curves.

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### Video Transcript

In this video, we will learn how to describe and explain changes of state and identify these changes through diagrams and heating curves.

The atoms and molecules that make up matter may have different speeds. Scientists have shown that atoms and molecules are generally in constant motion. In a solid, the atoms move very slowly. They vibrate and have relatively fixed positions. If a solid is heated, it gains thermal energy. This energy causes the molecules in the solid to move faster. If we heat a solid enough, we may cause it to change from the solid state to the liquid state. If we heat a liquid, its molecules will move faster. And if we heat it enough, it may change from the liquid state to the gas state. So, adding thermal energy changes the speed of molecules.

Adding thermal energy also changes the strength of the intermolecular attractions between molecules. In the word “intermolecular,” the prefix inter- means “between,” and “molecular” refers to molecules. So, intermolecular attractions are attractive forces between molecules.

Attractive forces are forces that pull molecules closer together. In a solid, the attractive forces between molecules are very strong. This keeps the molecules stuck in position and unable to move apart very much. In a liquid, the attractive forces between molecules are weaker than those in a solid. This means that the liquid molecules can move more freely, allowing a liquid to flow and take the shape of its container. Finally, in a gas, the attractive forces are the weakest. Gas molecules can move very freely, which is why they can fill the volume of any container they are in. In summary, we can see that as thermal energy is added to matter, the attractive forces between molecules get weaker.

Another property that changes when thermal energy is added is the intermolecular distance. Intermolecular distance is the distance between molecules. This distance changes when the state changes. In a solid, the molecules are relatively close together. In a liquid, molecules tend to be farther apart. In other words, the distance between molecules is longer. And in a gas, the distance between molecules is even longer.

Now that we know some of the properties of molecules that change when matter is heated, let’s discuss the changes of physical state that might result from heating. When we heat matter, we are adding thermal energy to the matter. Another way to think about this is that the matter is absorbing energy. Let’s say we heat a solid, and then it changes from a solid to a liquid state as it gains enough thermal energy. This change of state is called melting. Melting happens when a solid is heated to its melting point, which is the specific temperature at which a solid begins to melt.

So, what causes melting? Well, we already know that heating a solid makes its molecules move faster. As the molecules move faster, the attractive forces between the molecules get weaker. This causes the molecules to get farther apart. In other words, the molecules move away from their positions. When all of the molecules in the solid have undergone these changes, the solid will have changed into a liquid. We can see in our cartoon of a liquid here that the molecules in the liquid are less ordered than the molecules in the solid. This makes sense because the liquid molecules are more free to move about.

Now, if we heated a liquid, a different change of state tends to happen. If a liquid absorbs enough thermal energy, its state will ordinarily change from liquid to gas. This change of state is called vaporization. Vaporization happens when a liquid is heated to its boiling point, which is the specific temperature at which a liquid begins to boil. Heating the liquid makes its molecules move even faster. As the molecules move faster, the attractive forces between the molecules get weaker, and the distance between the molecules gets much larger. When all of the molecules in the liquid have undergone these changes, the liquid will have changed to a gas.

In certain situations, a solid may go through a change of state called sublimation. During sublimation, a solid changes directly from a solid to a gas state. Melting is not part of the process, so there is no intermediate liquid stage.

Now that we know the changes of state that can happen when matter is heated, let’s talk about cooling matter. When matter cools, it loses thermal energy. We can say that the matter releases energy to the surrounding environment. If we cooled a gas enough, its state would change from a gas to a liquid state. The name of this change of state is condensation. How does condensation happen? Well, as the gas molecules lose thermal energy, they begin to move more slowly. The attractions between the slower molecules get stronger. These stronger attractions pull the molecules closer together, and the gas turns into a liquid.

Now, if we cooled a liquid, a different change of state tends to happen. If a liquid loses enough thermal energy, it ordinarily changes from the liquid to solid state. The name of this change of state is freezing. During freezing, the liquid molecules slow down even more, and the attractions between the molecules get very strong. These strong forces pull the molecules very close together, and the solid forms. We can see in our cartoon here that the molecules in a solid are much more ordered than in a liquid or a gas.

Now, let’s turn our discussion to density and mass. During a change of state, the density of a substance tends to change. Density is defined as the amount of mass per unit volume. We can think of density as a measure of compactness or how compact a substance is. When matter absorbs thermal energy and changes state, it tends to become less dense. In general, when a substance goes through melting, vaporization, or sublimation, its density decreases. The blue boxes drawn here are all the same size. We notice that in the solid the blue box holds the most molecules, the liquid holds less, and the gas holds the least. This is because, in general, solids are the most dense, liquids are less dense, and gases are the least dense state of matter.

As we might think, when matter releases thermal energy and changes state, it tends to become more dense. For example, when a gas condenses to form a liquid, it becomes more dense. When a liquid freezes, it also becomes more dense. Deposition is the name of the change when a gas turns directly from a gas to a solid state without the intermediate liquid stage. Even though density changes when matter changes its state, mass does not. Mass remains constant. Mass is defined as the quantity of matter in an object.

Let’s say we have some ice in a beaker, and its mass is 70.0 grams. We then allow the ice to melt completely into liquid water. If we used a balance to find the mass of the water, we would see that it did not change. The mass of the water is the same as the ice before melting. It is still 70.0 grams. This is because we have the same number of molecules of water before and after melting. In fact, as long as we have a closed system, the mass of a substance will not change when its state changes. If we use a closed container in our experiment, then no molecules can escape, and the mass remains the same.

Now, we’re ready to discuss the types of graphs that scientists use to keep track of changes in the temperature of matter over time. The graph we will look at first is called a heating curve. This type of graph also shows the state of a substance as it gains thermal energy. The 𝑦-axis of the graph represents temperature, and the 𝑥-axis represents increasing energy. But scientists also label the 𝑥-axis with time. The graph shown here is the heating curve for water. Let’s start at the bottom-left part of the graph.

This part of the graph represents solid ice. As we heat the ice, its temperature increases, which is why the graph has a positive slope here. The melting point of ice is zero degrees Celsius. When the temperature of the ice is zero degrees Celsius, it begins to melt. The ice does not change instantly from a solid to a liquid as soon as it reaches its melting point. We must continue to heat the ice for a period of time. During this time, both ice and water are present. While the ice is melting, the temperature remains constant, which is why the graph is flat here. The heat that is absorbed or released by a substance as its state changes at a constant temperature is called latent heat. So, the heat that is absorbed or gained by a substance as it melts at a constant temperature is called the latent heat of melting.

Now that the ice has melted completely, the next section of the graph represents liquid water. If we heat the liquid water, its temperature will rise. This is why the graph has a positive slope here. If we heat water for long enough, it will reach its boiling point, which is 100 degrees Celsius. At 100 degrees Celsius, water begins to vaporize. But this change does not happen all at once. We will need to continue to add heat to convert all the liquid water into a gas. Both water and water vapor will be present as we are heating. While vaporization is happening, the temperature remains constant. The heat absorbed by water as it vaporizes at a constant temperature is the latent heat of vaporization.

The last section of the graph represents heating the gas form of water, which is known as water vapor or steam. As we heat the steam, its temperature increases. So, this part of the graph again has a positive upward slope.

Now that we’ve discussed a heating curve, let’s take a look at a cooling curve. A graph that shows the temperature and state of a substance as it loses thermal energy is called a cooling curve. Let’s start at the upper-left part of the graph. The first section of the graph represents cooling a gas. As we cool the gas, its temperature decreases. When the temperature of the gas reaches the boiling point, then the gas begins to change to a liquid. This change of state is called condensation and is represented by the first flat part of the graph.

The next part of the graph represents cooling the liquid. While the liquid is cooled, its temperature decreases. When the temperature of the liquid reaches the freezing point, then the liquid begins to change to a solid. This change of state is called freezing and is represented by the second flat part of the graph.

The last part of the graph represents cooling the solid. While the solid is cooled, its temperature decreases.

During condensation and freezing, heat is being released, but the temperature of the matter is not changing. Heat that is released when the state of a substance changes at constant temperature is called latent heat.

Before we summarize what we’ve learned, let’s take a look at a question.

Which of the following is not the name for a change of state? (A) Melting, (B) freezing, (C) dissolution, (D) vaporization, (E) condensation.

Matter can have a change of state if it gains or loses enough thermal energy. Let’s begin by looking at the answer choices. Melting and vaporization are both changes of state that can happen when matter gains thermal energy. On the other hand, freezing and condensation are changes of state that can happen when matter loses thermal energy. Melting is the change of matter from the solid to liquid state.

Since we are looking for the word that is not the name for a change of state, we can eliminate answer choice (A). Freezing is the change of matter from the liquid to solid state. So, it cannot be the correct answer. Vaporization is the change of matter from the liquid to gas state. And condensation is the change of matter from the gas to liquid state. Since these two options are the names of changes of state, we can eliminate them.

This leaves only dissolution, which must be the correct answer. Dissolution is the process where a solute dissolves in a solvent. Dissolution is not the name of a change of state. Therefore, the correct answer is answer choice (C), dissolution.

Now, let’s summarize our learning by going over some key points. Substances can change state when they absorb or release thermal energy. The properties of molecules change as they gain or lose thermal energy. Melting, vaporization, and sublimation are changes of state that can happen when matter absorbs thermal energy. Melting is the change from the solid to liquid state, and vaporization is the change from the liquid to gas state. Sublimation is the change from solid directly to gas with no intermediate liquid phase.

Condensation and freezing are changes of state that can happen when matter loses thermal energy. Condensation is the change from the gas to liquid state, and freezing is the change from the liquid to solid state. Density changes during a change of state, but mass does not. Latent heat is the energy absorbed or released by matter as its state changes at a constant temperature.

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