Phase Change

Phase Change

Middle School

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Learning Objectives

• Describe the states of matter based on particle arrangement and motion.

• Explain how thermal energy changes the kinetic energy and temperature of a substance.

• Model how energy transfer leads to phase changes like melting, freezing, and condensation.

• Define thermal energy, temperature, and heat, and describe how pressure affects phase changes.

Key Vocabulary

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Solid

In a solid, particles are packed tightly together and vibrate in fixed positions.

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Liquid

In a liquid, particles are close but can slide past one another, allowing it to flow.

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Gas

In a gas, particles are far apart and move around freely and quickly in all directions.

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Phase Change

A phase change is the process of matter transitioning from one state to another, like solid to liquid.

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Melting

Melting is the specific phase change when a substance transforms from a solid to a liquid.

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Freezing

Freezing is the specific phase change when a substance transforms from a liquid to a solid.

Key Vocabulary

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Evaporation

The process where a substance in a liquid state changes to a gaseous state.

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Condensation

The process where a substance in a gaseous state changes to a liquid state.

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Sublimation

The transition of a substance directly from the solid to the gas state.

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Deposition

The transition of a substance directly from the gas to the solid state.

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Kinetic Energy

The energy that an object possesses due to its motion, also known as movement.

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Thermal Energy

The total energy of the moving particles that make up an object or substance.

Key Vocabulary

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Temperature

Temperature is the measure of the average kinetic energy of the particles in a substance.

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Pressure

Pressure is the amount of force exerted by colliding particles on a given area.

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Heat

Heat is the transfer of thermal energy from a warmer object to a cooler one.

What Are the Three Phases of Matter?

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Solids

• ​Particles are tightly packed and held together by very strong bonds.

• ​​They can only vibrate in their fixed positions but do not move.

• ​Solids have a definite shape and a definite volume.

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Liquids

• ​The bonds between the particles in a liquid are much weaker.

• ​​Particles are still in contact but can slide past one another.

• ​Liquids can flow and take the shape of their container.

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Gases

• ​Particles in a gas are widely spaced with no effective bonds.

• ​​They move around freely and very rapidly in all directions.

• ​Gases have neither a definite shape nor a definite volume.

Multiple Choice

Which phase of matter is defined by having both a definite shape and a definite volume?

• Solid

• Liquid

• Gas

• Plasma

Multiple Choice

What is the primary reason that particles in a liquid can flow and take the shape of their container?

• The particles can slide past one another.

• The particles are held tightly in fixed positions.

• The particles have no effective bonds between them.

• The particles are widely spaced and move rapidly.

Multiple Choice

A substance is found to have particles that are widely spaced and move rapidly in all directions. Based on this information, what can be concluded about the substance?

• It has neither a definite shape nor a definite volume.

• It has a definite volume but not a definite shape.

• It has a definite shape but not a definite volume.

• It has both a definite shape and a definite volume.

Energy, Temperature, and Heat

• Thermal energy is the total energy of all particles in a system.

• Temperature measures the average motion of the particles.

• Heat is the transfer of thermal energy from a warmer to a cooler object.

Multiple Choice

What does the temperature of a substance measure?

• The average motion of the particles in a system

• The total energy of all particles in a system

• The transfer of energy between two objects

• The amount of space the particles occupy

Multiple Choice

Which statement best describes the process of heat?

• The total energy contained within an object

• The average speed of particles in an object

• The transfer of thermal energy from a warmer object to a cooler one

• The energy created when an object gets hotter

Multiple Choice

If a cold metal spoon is placed into a bowl of hot soup, what is the most likely outcome?

• Thermal energy will be transferred from the spoon to the soup.

• Thermal energy will be transferred from the soup to the spoon.

• The spoon will lose temperature without transferring energy.

• The soup will gain thermal energy from the air, not the spoon.

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Phase Changes: Melting and Freezing

Melting

• Adding thermal energy to a solid makes its particles vibrate faster and gain kinetic energy.

• The increased energy becomes strong enough to start breaking the fixed bonds between particles.

• Particles can then slide past one another, which turns the solid into a liquid.

Freezing

• Removing thermal energy from a liquid makes its particles slow down, decreasing their kinetic energy.

• The decrease in energy allows strong bonds to form and lock the particles together.

• The particles are now in a fixed position, which turns the liquid into a solid.

Multiple Choice

According to the principles of phase changes, what happens to the particles in a solid when thermal energy is added?

• They vibrate faster and start to break their fixed bonds.

• They slow down and form stronger bonds.

• They get farther apart but stop moving.

• They lose all of their kinetic energy.

Multiple Choice

How does the addition of thermal energy cause a solid to transform into a liquid?

• The particles have enough energy to break their fixed bonds and slide past one another.

• The particles lose energy, allowing them to move more freely.

• The particles slow down enough to lock into new positions.

• The particles stop vibrating, which turns the substance into a liquid.

Multiple Choice

If melting is caused by adding thermal energy to break bonds, what conditions are required for the reverse process of freezing to occur?

• Thermal energy is removed, which allows strong bonds to form between particles.

• Thermal energy is added, which allows strong bonds to form between particles.

• Thermal energy is removed, which causes the existing bonds to break.

• Thermal energy is added, which causes particles to slow down.

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Evaporation and Condensation

• Adding thermal energy causes a liquid’s particles to move much faster.

• These particles gain kinetic energy and escape into the air as a gas.

• Removing thermal energy from a gas makes its particles slow down.

• These slower particles then form bonds and become a liquid.

Multiple Choice

What process occurs when particles of a liquid escape into the air as a gas?

• Condensation

• Freezing

• Evaporation

• Melting

Multiple Choice

What is the relationship between thermal energy and the movement of particles during a phase change?

• Adding thermal energy increases particle speed, while removing it decreases particle speed.

• Adding thermal energy causes particles to stick together.

• Removing thermal energy causes particles to break their bonds.

• Thermal energy only affects the temperature of a substance, not its particles.

Multiple Choice

When water droplets form on the outside of a cold glass, what is the most likely explanation for what is happening to the particles?

• Gas particles in the air lose thermal energy to the cold surface, causing them to slow down, form bonds, and become a liquid.

• Liquid from inside the glass is leaking through to the outside.

• The cold surface causes the liquid particles to gain energy and move faster.

• The water particles are breaking their bonds and escaping as a gas.

Phase Change Temperatures & Special Cases

• Temperature stays constant during a phase change.

• Each pure substance has unique melting and boiling points.

• Sublimation is solid to gas; deposition is gas to solid.

Multiple Choice

What generally happens to the temperature of a pure substance as it is melting?

• It stays the same.

• It increases.

• It decreases.

• It increases and then decreases.

Multiple Choice

How is the process of deposition different from sublimation?

• Deposition is a gas changing to a solid, while sublimation is a solid changing to a gas.

• Deposition is a liquid changing to a solid, while sublimation is a solid changing to a liquid.

• Deposition involves heating a substance, while sublimation involves cooling it.

• Deposition happens only with water, while sublimation happens with any substance.

Multiple Choice

A student observes that pure Substance X melts completely at 100°C, while pure Substance Y melts completely at 115°C. What can be concluded from this observation?

• Substance X and Substance Y are different substances.

• Substance X and Substance Y are the same substance.

• Both substances will boil at the same temperature.

• Substance Y will melt faster than Substance X.

The Role of Pressure

• Gas particles hitting a surface cause pressure.

• Higher pressure on a liquid raises its boiling point.

• Lower pressure on a liquid lowers its boiling point.

Multiple Choice

What causes gas pressure?

• The temperature of the gas increasing.

• The liquid turning into a gas.

• Particles of gas hitting a surface.

• The total volume of the container.

Multiple Choice

Which statement correctly describes the relationship between pressure and the boiling point of a liquid?

• When pressure on a liquid increases, its boiling point decreases.

• When pressure on a liquid increases, its boiling point increases.

• The pressure on a liquid has no effect on its boiling point.

• Boiling point is only affected by the amount of liquid.

Multiple Choice

At high altitudes, the air pressure is lower. How does this affect the boiling point of water?

• The boiling point increases

• The boiling point decreases

• The boiling point stays the same

• Water cannot boil at high altitudes

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Common Misconceptions

Misconception	Correction
Heat and temperature are the same thing.	Temperature measures particle energy. Heat is the transfer of this energy.
Water vapor is visible, like steam or clouds.	Water vapor is an invisible gas. Visible steam is tiny liquid water droplets.
Freezing only happens when things get cold.	Freezing is a liquid turning solid, which can happen at very high temperatures.

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Summary

• A substance's state of matter depends on the arrangement and motion of its particles.

• Adding or removing thermal energy can change a substance's temperature or cause a phase change.

• During a phase change, temperature remains constant, and pressure can affect the boiling point.

• The six phase changes are melting, freezing, evaporation, condensation, sublimation, and deposition.

Poll

On a scale of 1-4, how confident are you about explaining how energy transfer causes phase changes?

• 1

• 2

• 3

• 4