Phase Changes

Phase Changes

Middle School

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Learning Objectives

• Describe what a phase change is and the role of heat energy.

• Differentiate between the six types of phase changes in matter.

• Explain how heat affects particles in a substance to cause a phase change.

• Interpret a heating curve to identify states of matter and phase changes.

Key Vocabulary

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Melting

The process where a solid turns into a liquid by absorbing heat from its surroundings.

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Freezing

The process where a liquid turns into a solid by releasing heat into its surroundings.

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Evaporation

The process where a liquid changes to a gas, occurring only at the liquid's surface.

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Condensation

The process where a gas changes into a liquid by releasing heat, causing particles to slow.

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Sublimation

The direct change of a substance from a solid to a gas without becoming a liquid.

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Deposition

The direct change of a substance from a gas to a solid without becoming a liquid.

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What Are Phase Changes?

Endothermic Process

• Heat is absorbed by the substance from its surroundings.

• Particles gain energy, move faster, and spread farther apart.

• This causes changes like melting (solid to liquid) or evaporation (liquid to gas).

Exothermic Process

• Heat is released from the substance into its surroundings.

• Particles lose energy, slow down, and move closer together.

• This causes changes like freezing (liquid to solid) or condensation (gas to liquid).

Multiple Choice

According to the text, what is the single most important factor involved in a phase change?

• Pressure

• Volume

• Heat

• Mass

Adding Heat: Endothermic Changes

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Melting

• Heating a solid makes its particles gain energy and vibrate more.

• The bonds holding the particles in fixed positions begin to weaken.

• The solid becomes a liquid, with particles that can move past each other.

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Vaporization

• Heating a liquid gives its particles enough energy to escape into the air.

• Boiling is when a liquid turns to gas throughout the entire liquid.

• Evaporation is when a liquid turns to gas only at the surface.

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Sublimation

• A solid can turn directly into a gas, skipping the liquid phase.

• This occurs when particles gain a large amount of energy very quickly.

• Dry ice, or solid carbon dioxide (CO₂), is a common example.

Multiple Choice

Which process is described as a solid turning directly into a gas?

• Melting

• Evaporation

• Sublimation

• Condensation

Removing Heat: Exothermic Changes

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Condensation

• ​Removing heat from a gas makes its particles lose their energy.

• ​​The particles slow down and move closer to form a liquid.

• ​An example is water droplets forming on a cold glass.

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Freezing

• ​Removing heat from a liquid causes its particles to slow down.

• ​​They arrange into a fixed pattern, creating a rigid solid structure.

• ​This is how liquid water turns into solid ice when frozen.

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Deposition

• ​A gas can turn directly into a solid, skipping the liquid phase.

• ​​This happens when a gas rapidly loses a significant amount of heat.

• ​Frost forming on a window is an example of deposition.

Multiple Choice

When enough heat is removed from a gas that it turns directly into a solid, like frost forming, which phase change has occurred?

• Freezing

• Deposition

• Sublimation

• Condensation

Properties of Matter States

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Solids

• Have a definite shape and a definite volume.

• Their particles are packed closely together in fixed positions.

• There is a strong attraction between the particles.

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Liquids

• Have a definite volume but no definite shape.

• Their particles are able to flow past one another.

• The attraction between their particles is weaker than solids.

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Gases

• Do not have a definite shape or definite volume.

• Their particles are spread far apart from each other.

• There is a very weak attraction between their particles.

Multiple Choice

Which state of matter is described as having a definite volume but not a definite shape?

• Solid

• Liquid

• Gas

• Plasma

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Understanding Heating Curves

Diagonal Lines

• ​The diagonal parts of the graph show a single state of matter: solid, liquid, or gas.

• ​​During this time, the temperature of the substance increases as more heat is added to it.

• ​The absorbed heat energy causes the particles within the substance to move faster, raising the temperature.

Flat Lines

• ​The flat parts of the graph indicate the substance is undergoing a phase change, like melting.

• ​​Throughout a phase change, the temperature of the substance remains constant, even as heat is added.

• ​All the added heat energy is used to break the bonds holding the particles together.

Multiple Choice

On a heating curve graph, what do the flat lines represent?

• A single state of matter

• A phase change is occurring

• The temperature is increasing

• The substance is cooling down

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Key Temperature Points

Melting & Freezing

• Melting point is the temperature at which a substance changes from a solid to a liquid.

• Freezing point is the temperature at which a substance changes from a liquid to a solid.

• The melting and freezing points of a substance are the same.

Boiling & Condensation

• Boiling point is the temperature at which a substance changes from a liquid to a gas.

• Condensation point is the temperature at which a substance changes from a gas to a liquid.

• The boiling and condensation points of a substance are the same.

Multiple Choice

What is the relationship between the boiling point and the condensation point of a substance?

• The boiling point is higher.

• The condensation point is higher.

• They are the same temperature.

• There is no relationship.

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Common Misconceptions

Misconception	Correction
Boiling and evaporation are the same.	Boiling is vaporization in the liquid; evaporation is only at the surface.
Temperature increases during a phase change when heat is added.	Temperature stays constant. Added energy breaks bonds between particles.
Cooling systems work by adding 'cold' to a space.	Cooling systems work by removing heat energy from a space.

Multiple Choice

Why does the temperature of water remain constant at 100°C while it is boiling?

• Because heat is no longer being added.

• Because the added energy is being used to break the bonds between water molecules.

• Because the water is turning into a solid.

• Because the thermometer is broken.

Multiple Choice

A heating system, like a furnace, transfers energy to a substance to raise its temperature. According to the text, how does a cooling system, like a refrigerator, work?

• It adds 'coldness' to the air inside.

• It destroys the heat inside the system.

• It transfers heat energy out of the substance or area.

• It stops the movement of all particles.

Multiple Choice

On a cold winter morning, you see frost on the outside of a window. This is an example of deposition. What phase transition happened to the water vapor in the air?

• It condensed into a liquid and then froze into a solid.

• It turned directly from a gas into a solid.

• It sublimated from a solid into a gas.

• It melted from a solid into a liquid.

Multiple Choice

If you were looking at a heating curve for an unknown substance, how could you determine its boiling point?

• Find the temperature at the start of the first flat line.

• Find the temperature at the start of the second flat line.

• Find the temperature at the highest point on the graph.

• Find the temperature at the end of the first diagonal line.

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Summary

• Heat energy affects particle movement and bonds, causing phase changes.

• Adding heat causes endothermic changes; removing heat causes exothermic changes.

• Heating curves show that temperature remains constant during a phase change.

• A substance’s melting and boiling points are unique physical properties.

Poll

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