Changes of State

Changes of State

Middle School

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Learning Objectives

• Develop a model for how thermal energy affects particle motion and matter states.

• Describe particle arrangement and motion in solids, liquids, and gases.

• Relate thermal energy, particle kinetic energy, and temperature.

• Describe phase changes like melting, freezing, vaporization, condensation, and sublimation.

Key Vocabulary

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Particle Motion

The constant, random movement of atoms and molecules that make up all forms of matter.

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State of Matter

The distinct physical form that matter can exist in, most commonly as a solid, liquid, or gas.

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Kinetic Energy

This is the specific type of energy an object possesses simply as a result of its motion.

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Thermal Energy

Represents the total kinetic energy of all the individual atoms or molecules within a substance.

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Temperature

A direct measure of the average kinetic energy of the particles that are inside a substance.

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Heat

The transfer of thermal energy from a warmer object to a cooler one due to this difference.

Key Vocabulary

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Melting

Melting is the change of state from a solid to a liquid, typically by applying heat.

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Freezing

Freezing is the process that causes a substance to change from a liquid to a solid.

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Vaporization

Vaporization is the change of state when a substance in a liquid form becomes a gas.

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Condensation

Condensation is the process where a substance in a gaseous state turns back into a liquid.

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Sublimation

Sublimation is the unique process where a solid substance turns directly into a gas without melting first.

Modeling Matter: Solids, Liquids, and Gases

• Solid particles are closely packed and vibrate in fixed spots.

• Liquid particles are close but can move and slide past each other.

• Gas particles are far apart and move randomly and quickly.

Multiple Choice

In which state of matter are particles packed closely together and vibrate in fixed positions?

• Solid

• Liquid

• Gas

• Plasma

Multiple Choice

How does the movement of particles in a liquid compare to the movement of particles in a solid?

• Liquid particles can slide past one another, while solid particles cannot.

• Solid particles are far apart, while liquid particles are close together.

• Liquid particles move randomly, while solid particles are stationary.

• Solid particles are smaller than liquid particles.

Multiple Choice

Imagine a substance in a clear, sealed container. If you could see its particles, you would observe them moving rapidly and randomly, with large empty spaces between them. What conclusion can you draw about this substance?

• The substance is a gas because its particles are far apart and moving quickly.

• The substance is a liquid because its particles can move around.

• The substance is a solid because it is made of particles.

• The substance is a liquid because its particles are moving randomly.

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Thermal Energy vs. Temperature

Temperature

• ​Temperature measures the average kinetic energy of the particles in a substance.

• ​​It indicates how hot or cold something is, but not its total energy.

• A high temperature means particles are moving very quickly on average.

Thermal Energy

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

• ​​It depends on the temperature, number of particles, and the material.

• A large pot of cool water can have more thermal energy than a hot cup.

Multiple Choice

What is the primary difference between temperature and thermal energy?

• Temperature measures the average kinetic energy of particles, while thermal energy is the total energy of all particles.

• Temperature is the total energy of particles, while thermal energy is the average kinetic energy.

• Temperature depends only on the number of particles, while thermal energy depends on their speed.

• Temperature and thermal energy are two different terms for the exact same measurement.

Multiple Choice

How can a large pot of cool water have more thermal energy than a small, hot cup of water?

• Because thermal energy is determined by the total number of particles, not just their average speed.

• Because cool water is always denser than hot water.

• Because the pot is a better insulator than the cup.

• Because large volumes of water cannot get hot.

Multiple Choice

Consider a massive iceberg and a small, steaming hot teacup. Which statement accurately compares them?

• The iceberg has a lower temperature but greater thermal energy than the teacup.

• The teacup has a higher temperature and greater thermal energy than the iceberg.

• The iceberg has a higher temperature but less thermal energy than the teacup.

• The teacup and the iceberg have the same thermal energy because they are both made of water.

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The Effect of Adding and Removing Energy

Adding Energy

• Adding thermal energy to a substance can increase the kinetic energy of its particles.

• This increase in particle movement raises the overall temperature of the substance.

• The added energy can also be used to change the state of matter, like melting ice.

Removing Energy

• Removing thermal energy from a substance can decrease the kinetic energy of its particles.

• This decrease in particle movement lowers the overall temperature of the substance.

• The removal of energy can also cause a change of state, like freezing water.

Multiple Choice

What is the direct effect of adding thermal energy to a substance?

• The kinetic energy of its particles increases.

• The substance immediately becomes a gas.

• The overall temperature of the substance decreases.

• The particles stop moving completely.

Multiple Choice

Why does adding or removing thermal energy cause a substance's temperature to change?

• Because the speed of particle movement determines the substance's temperature.

• Because temperature is only affected by changes in the state of matter.

• Because adding energy always causes a substance to melt.

• Because removing energy makes particles larger.

Multiple Choice

If a glass of liquid water is placed in a freezer, which statement best describes the process that occurs?

• Thermal energy is removed from the water, causing its particles to slow down and the water to freeze.

• Thermal energy is added to the water, causing its particles to speed up and the water to freeze.

• The temperature of the water drops, but the kinetic energy of its particles increases.

• The water freezes first, which then causes the thermal energy to be removed.

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From Solid to Liquid and Back: Melting and Freezing

Melting

• Melting is the change of state when a solid substance turns into a liquid.

• This happens when the substance absorbs thermal energy, causing its particles to break free from their fixed positions.

• For a pure substance, this occurs at a specific temperature known as the melting point.

Freezing

• Freezing is the change of state when a liquid substance becomes a solid.

• This process happens when the substance loses thermal energy, causing its particles to slow down.

• The temperature for this change is the freezing point, which is the same as the melting point.

Multiple Choice

What do the processes of melting and freezing describe?

• A change of state between solid and liquid

• A process that creates new substances

• A change in the color of a substance

• A process that only happens to water

Multiple Choice

What is the key difference in how thermal energy affects particles during melting versus freezing?

• Melting involves absorbing thermal energy, while freezing involves losing it.

• Melting makes particles slow down, while freezing makes them break free.

• Melting only happens at high temperatures, and freezing only at low ones.

• Melting changes a substance's mass, while freezing does not.

Multiple Choice

A pure substance is observed to melt at a temperature of 10°C. What can be concluded about this substance?

• The substance will also freeze at 10°C.

• The substance will freeze at a lower temperature, like 0°C.

• The substance will lose thermal energy when it melts.

• The substance's particles will speed up when it freezes.

From Liquid to Gas: Vaporization and Pressure

• Vaporization is the process of a liquid turning into a gas.

• Boiling occurs throughout a liquid, while evaporation happens only at the surface.

• At higher altitudes, lower pressure causes liquids to boil at lower temperatures.

Multiple Choice

What is vaporization?

• The process of a gas changing into a liquid.

• The process of a liquid changing into a gas.

• The process of a solid changing into a liquid.

• The process of a liquid freezing into a solid.

Multiple Choice

What is the key difference between boiling and evaporation?

• Boiling happens only at the surface, while evaporation occurs throughout the liquid.

• Boiling happens throughout the liquid, while evaporation occurs only at the surface.

• Boiling is a cooling process, while evaporation is a heating process.

• Boiling only happens to water, while evaporation can happen to any liquid.

Multiple Choice

If you were trying to boil water on top of a high mountain, what would you expect to happen to its boiling temperature?

• It would boil at a higher temperature than at sea level.

• It would boil at the exact same temperature as at sea level.

• It would boil at a lower temperature than at sea level.

• It would take much longer to start boiling.

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Other State Changes: Condensation and Sublimation

Condensation

• ​Condensation is the change of state from a gas to a liquid, the reverse of vaporization.

• ​​This occurs when gas particles lose enough thermal energy, causing them to slow down and clump together.

• ​A common example is water vapor forming droplets of liquid water on the outside of a cold glass.

Sublimation

• ​Sublimation is the change of a substance directly from a solid to a gas, skipping the liquid state.

• ​​This happens when surface particles of a solid gain enough energy to break free and become a gas.

• ​A well-known example of this process is solid dry ice turning directly into carbon dioxide gas.

Multiple Choice

Which process describes the change of a substance from a gas to a liquid?

• Condensation

• Sublimation

• Vaporization

• Freezing

Multiple Choice

How do the energy requirements for condensation and sublimation differ?

• Condensation involves a loss of thermal energy, while sublimation involves a gain of energy.

• Condensation turns a solid into a gas, while sublimation turns a gas into a liquid.

• Condensation only happens to water, while sublimation happens to all substances.

• Condensation skips the liquid state, while sublimation creates a liquid.

Multiple Choice

Why does sublimation happen in certain substances like dry ice?

• Because the solid melts completely into a liquid first

• Because the particles slow down and settle into a liquid

• Because the surface particles gain enough energy to break free and become a gas

• Because the solid absorbs water from the air

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Common Misconceptions

Misconception	Correction
Heat and temperature are the same.	Temperature is average particle energy. Heat is the transfer of this energy.
Boiling is the only way liquids become gases.	Evaporation also turns liquids to gas, but only at the surface.
Adding heat must always increase the temperature.	During a state change, temperature stays constant as particle arrangement changes.
The visible steam from boiling water is a gas.	Visible ‘steam’ is tiny liquid water droplets. True water vapor is invisible.

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Summary

• The arrangement and motion of particles determine a substance's state: solid, liquid, or gas.

• Adding or removing thermal energy changes a substance's temperature or causes a phase change.

• The temperature of a substance does not change during a change of state.

• Lowering the pressure on a liquid lowers its boiling point.

Poll

On a scale of 1-4, how confident are you about explaining the different changes of state?

• 1

• 2

• 3

• 4