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Lesson Explainer: Changes of State Science

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

All molecules move, but they do not all move at the same speed. Molecules move about relatively slowly when they are at a low temperature and faster when they are at a higher temperature. They move faster and more freely as the temperature increases because they gain thermal energy.

Let us consider some water molecules. They barely move at all in a block of ice at a temperature lower than 0C. However, they move much faster as the temperature increases. They move fast enough at 0C to change from the solid to the liquid state. The term melting describes a solid substance gaining enough thermal energy to turn into a liquid.

Figure 1: A block of ice melting as it gains thermal energy.

Definition: Melting

Melting is the change of matter from the solid state to the liquid state when it gains enough thermal energy.

Vaporization is a similar process to melting. Vaporization happens when a liquid substance gains enough thermal energy to turn into a gas. The molecules of a gaseous substance tend to move very fast since they tend to have more thermal energy than the molecules of a liquid. Water gains enough thermal energy at 100C to vaporize and turn into a gaseous substance called steam.

Definition: Vaporization

Vaporization is the change of matter from the liquid state to the gas state when it gains enough thermal energy.

Thermal energy also affects the space between molecules. A substance tends to get more distantly separated particles as it gains thermal energy and its state changes. There is, for example, relatively little intermolecular space between the molecules of liquid water. However, there is a lot of distance between the molecules of gaseous steam.

Figure 2: How thermal energy affects the properties of liquid water.

Example 1: Using Figures to Describe What Happens When a Solid Substance Melts

Three states of matter are shown in containers A, B, and C. What change will occur during the process of melting?

Answer

Thermal energy tends to affect the speed of molecules and the distance between them. A substance has compact and slowly moving particles when it is solid at a low temperature. It tends to get more intermolecular space and faster-moving particles as it melts and then vaporizes.

System A has compact and slow-moving particles, while system B has more intermolecular space and fast-moving particles. System C also has fast-moving particles and the greatest intermolecular distance. System A must represent a solid substance, while system B represents a liquid substance. System C must be for a gaseous substance, as it has the greatest intermolecular distance.

System A must turn into system B during melting, as melting happens when a solid substance gains thermal energy and turns into a liquid. The correct answer to this question is A to B.

Almost all substances get a lower density at higher temperatures because intermolecular space tends to increase with temperature. Thus, substances are usually less compact at higher temperatures. They tend to become less dense as they melt or vaporize. Oxygen ordinarily becomes hundreds of times less dense when it changes from liquid to gas.

Figure 3: How thermal energy ordinarily affects the density and intermolecular space of a substance.

So, a substance tends to get faster-moving and more distantly separated particles as it gains thermal energy and changes state. The strength of its intermolecular forces will ordinarily also decrease as it gains thermal energy and changes from one state to another. Its particles tend to be further apart at high temperatures, and intermolecular forces are weaker over longer distances. So, intermolecular forces are ordinarily weaker at high temperatures.

Example 2: Representing How Intermolecular Forces Change as Molecules Change from the Liquid State to the Gaseous State

In the diagram below, the attractive forces between two molecules in a liquid are indicated by an arrow. The larger the arrow, the stronger the attractive force.

What will these arrows look like if the liquid undergoes a change of state to a gas?

Answer

A liquid will ordinarily get more intermolecular space if it gains thermal energy and becomes a gas. It will also tend to get weaker forces between its particles. Intermolecular forces decrease during vaporization because they depend on distance. They tend to be weaker over longer distances.

The particles of a liquid are relatively close. So, there tend to be strong forces between them. The particles of a gas are further apart. So, there tend to be relatively weak forces between them. Choice A must be the correct answer. The molecules of choice A have less substantial intermolecular forces than the molecules of the liquid.

A substance can also change state when the temperature drops. Its molecules release thermal energy as the temperature drops, and they become more compact and move less rapidly. Condensation happens when a gas loses enough thermal energy and turns into a liquid. Condensation can occur when hot air hits a cool pane of glass and loses thermal energy.

Figure 4: Droplets of water forming on cold window panes through condensation.

Definition: Condensation

Condensation is the change of matter from the gas state to the liquid state when it loses enough thermal energy.

Freezing is a similar process to condensation. Freezing happens when the temperature decreases enough, and a liquid turns into a solid substance. The particles of a liquid tend to lose thermal energy when they freeze and form a solid. They usually become more compact and move less rapidly when they lose thermal energy and freeze.

Definition: Freezing

Freezing is the change of matter from the liquid state to the solid state when it loses enough thermal energy.

So, substances can change state when the temperature changes, and they either gain or lose thermal energy. Sublimation is one of the most unusual phase change processes, as it significantly affects the speed of molecules and the forces between them.

Sublimation happens when a solid substance gains thermal energy and turns into a gas. The solid substance does not first melt and then vaporize. It turns into a gas without going through a liquid stage. Molecules tend to become significantly faster and less compact when they sublime. Molecules also tend to experience weaker forces of attraction when they sublime because they end up being further apart.

Figure 5: The sublimation of ice into water vapor.

Definition: Sublimation

Sublimation is the change of matter from the solid state to the gas state with no intermediate liquid stage.

Example 3: Identifying Which Term Is Not for a Change of State Process

Which of the following is not the name for a change of state?

  1. Melting
  2. Freezing
  3. Dissolution
  4. Vaporization
  5. Condensation

Answer

Change of state processes can happen as a substance gains or loses thermal energy. Melting and vaporization can occur as molecules gain thermal energy. Condensation and freezing ordinarily take place when a material loses thermal energy.

Dissolution is not a change of state process. Dissolution is the process where a solute dissolves in a solvent and forms a solution. We can use this information to determine that choice C, dissolution, is the correct answer.

Some physical properties stay the same when a substance transforms from one state to another. Mass is one property that is not different after a phase change process. A solid material, for example, does not suddenly weigh more or less if it becomes a liquid or a gas. Density might change with phase, but mass does not.

Figure 6: The mass of some ice cubes before and after they melt.

Example 4: Determining the Mass of Ice in a Container after It Melts

In an experiment, a sample of a solid is placed inside a sealed container and then weighed on a mass balance. The container is then heated until all the solid has changed into a liquid. The container is then immediately placed back onto the mass balance. What will the mass balance read?

Answer

Mass is one physical property that does not change during a phase change process. Blocks of ice have the same mass before and after they melt and turn into water. So, the mass of the container should be the same before and after the experiment. The container should be twenty-six grams when it has ice or liquid water. The correct answer is, therefore, 26 g.

A substance does not usually change state the moment the temperature reaches its melting or boiling point. It ordinarily spends some time at its melting or boiling point before its state changes. Ice, for example, does not instantly melt if the temperature rises to 0C.

It takes time for all the solid-phase molecules to gain enough energy and melt. The latent heat of melting is the energy gained by a substance as it reaches its melting point and all of its molecules change from the solid to the liquid state. Similarly, the latent heat of vaporization is the energy gained by a substance as all of its molecules vaporize and change from the liquid state to the gas state.

Definition: Latent Heat of Melting

It is the energy gained by a substance as it melts at a constant temperature.

Definition: Latent Heat of Vaporization

It is the energy gained by a substance as it vaporizes at a constant temperature.

Figure 7: The temperature and structure of water as it gains thermal energy.

Definition: Latent Heat

It is the energy absorbed or released by a substance as its physical state changes at a constant temperature.

Scientists can make heating curves to represent the state of a substance as it gains thermal energy. Heating curves are informative while also being relatively simple and easy to understand. They ordinarily have the temperature of the substance on the 𝑦-axis and time on the 𝑥-axis. Latent heat increases at the points where a heating curve is flat.

Figure 8: A heating curve.

Example 5: Identifying the Term for Energy Absorbed When a Solid Melts at a Constant Temperature

The graph below shows how the temperature of a solid changes over time when heated. The part of the graph marked X indicates the point at which a change of state from a solid to liquid occurs. At this point, the temperature remains constant despite there being continuous heating. What name is given to the heat required for a change of state to occur?

  1. Phase heat
  2. Specific heat
  3. Excess heat
  4. Latent heat
  5. Sensible heat

Answer

Latent heat is the energy absorbed or released by a substance as its physical state changes at a constant temperature. The graph is for the melting of a solid. So, label X represents the latent heat of melting. We can use this information to determine that choice D, latent heat, is the correct answer.

Cooling curves represent the state of a substance as it loses thermal energy. They are similar to heating curves and have the temperature on the 𝑦-axis and time on the 𝑥-axis. A cooling curve also has flat sections, just like a heating curve. The flat sections describe times when the latent heat of the substance changes.

Figure 9: A cooling curve.

Let us summarize what we have learned in this explainer.

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 is the change of matter from the solid state to the liquid state when it gains enough thermal energy.
  • Vaporization is the change of matter from the liquid state to the gas state when it gains enough thermal energy.
  • Condensation is the change of matter from the gas state to the liquid state when it loses enough thermal energy.
  • Freezing is the change of matter from the liquid state to the solid state when it loses enough thermal energy.
  • Sublimation is the change of matter from the solid state to the gas state with no intermediate liquid stage.
  • Mass does not change when a substance changes from one state to another.
  • Latent heat is the energy absorbed or released by a substance as its physical state changes at a constant temperature.
  • Heating graphs represent the state of a substance as it gains thermal energy.
  • Cooling graphs represent the state of a substance as it loses thermal energy.

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