What causes a phase change?

The single most important factor involving a phase change is heat.

Adding Heat

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Adding Heat

When heat is added to a substance, heat energy is absorbed by that substance.

That heat energy excites the particles.

Excited particles move faster, and the bonds between them break. This leads to a phase change.

Adding Heat to a Solid - Melting

When heat is added to a solid, the particles are more excited, and the bonds between them are less rigid.

The particles can now move freely past one another.

The solid is now a liquid.

Adding Heat to a liquid - Evaporating

When heat is added to a liquid, the particles are more excited, and they move even further apart.

The particles now have a great amount of energy, can move at great speeds, and are farther apart.

Boiling vs. Evaporation.

Boiling occurs when all the molecules in a liquid have enough energy to vaporize.

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Evaporation occurs slowly, and only the molecules at the surface of the liquid contain enough energy to vaporize.

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Both boiling and evaporation are types of vaporization.

Adding Heat to a solid - Sublimation

When enough heat is added to a solid substance, it can have enough energy to skip the liquid phase (melting), and vaporize directly into a gas.

A common example of this is dry ice, or solid Carbon Dioxide.

Melting and Evaporation are Endothermic.

Endo = into

Thermic = thermal or "heat"

Endothermic means putting heat into, or adding heat.

Removing Heat

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Removing Heat

When heat is removed from a substance, heat energy is take away from that substance.

When there is less energy in a substance, the particles slow down.

When particles slow down, they move closer together.

Removing Heat from a Gas - Condensation

When heat is removed from a gas, energy is taken away.

Less energy in gas = particles slow down.

When particles slow down, they move closer together, or condense.

This leads to the formation of water droplets.

Removing Heat from a Liquid - Freezing

When heat is removed from a liquid, energy is taken away.

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Less energy in liquid = particles slow down.

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When particles slow down, they move closer together, or freeze.

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This leads to the strong attraction of water molecules in ice, forming a rigid solid structure.

Removing Heat from a Liquid - Freezing

When heat is removed from a liquid, energy is taken away.

Less energy in liquid = particles slow down.

When particles slow down, they move closer together, or freeze.

This leads to the strong attraction of water molecules in ice, forming a rigid solid structure.

Removing Heat from a gas - Deposition

When enough heat is removed from a solid substance, it can lose enough energy to skip the liquid phase (melting), and freeze directly into a solid.

A common example of this is when water vapor transforms directly into ice without becoming a liquid, a process that often occurs on windows during the winter months.

Key Points

Energy is required to change the phase of a substance, like the energy to break the bonds between molecules in a block of ice so it may melt.

During a phase change energy my be added or subtracted from a substance.

Particles change their position based on the amount of energy that is added, or removed. The change in position results in a phase change.

Your Assignment

Copy down and fill out the graph in the next slide using this Quizizz lesson. When finished, take a picture, and upload it to Google Classroom under this assignment. Turn it in.