Substances on Earth can exist in one of four phases, but mostly, they exist in one of three: solid, liquid or gas. Learn the six changes of phase: freezing, melting, condensation, vaporization, sublimation and deposition.

## Changes of Phase

There are four states of matter in the universe: plasma, gas, liquid and solid.

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But, matter on Earth exists mostly in three distinct phases: gas, liquid and solid. A phase is a distinctive form of a substance, and matter can change among the phases. It may take extreme temperature, pressure or energy, but all matter can be changed.There are six distinct changes of phase which happens to different substances at different temperatures. The six changes are:

• Freezing: the substance changes from a liquid to a solid.

• Melting: the substance changes back from the solid to the liquid.
• Condensation: the substance changes from a gas to a liquid.
• Vaporization: the substance changes from a liquid to a gas.

• Sublimation: the substance changes directly from a solid to a gas without going through the liquid phase.
• Deposition: the substance changes directly from a gas to a solid without going through the liquid phase.

## Phase Change Diagram

Let’s take a look at phase change as it pertains to water. This will apply to all substances, but we are all most familiar with water.

Phase change is often shown in a diagram like the one below:

When a substance is in a solid state, it can absorb a lot of energy in the form of heat until it hits its melting point. Think about snow outside. Once snow hits the ground, it stays there, whether it is -50 degrees F outside or all the way up to 32 degrees F.

The snow can absorb energy all the way up until it hits its melting point of 32 degrees F. This is the diagonal line at stage I on the graph. Once a substance hits its melting point, it is a combination of solid and liquid for a while, as you can see by the flat line on the graph at stage II.At a certain point, all of the solid has turned into liquid. Now, the liquid can absorb energy in the form of heat for quite a while until it gets to its boiling point. Think about putting a pot of water on the stove and waiting for it to boil.

You start with the water at whatever temperature it came out of your tap, then you add heat for quite a while. The water gets hotter and hotter, absorbing more and more heat, but it is still all in the liquid stage. This is stage III on the graph.Eventually, after what seems like forever if you are hungry, the water gets to its boiling point.

At this point, the water starts to boil and turn into steam, or water vapor. For a long time, the water is in both liquid and vapor stage – stage IV on the graph. If you wait long enough, all the water will eventually turn into steam, and all of the liquid will be in a gaseous stage – stage V on the graph.

## Energy Involved in Phase Change

Now, here’s the weird thing about this diagram. Notice that at stage II and IV, when a substance hits its melting point and boiling point, no change in temperature happens. Energy is still being added to the substance, but the temperature doesn’t increase.

This is because at those points in the process, all the extra energy is being put into breaking the bonds between the molecules of the substance. This is also true at the other four changes of phase: freezing, evaporation, condensation and sublimation. Energy is either being used to break or form bonds and that is why the graph is flat at that point.Although we have talked mainly about water in this video, all substances have specific phase change temperatures. Also, freezing, melting and condensation points can change for different substances, depending on the pressure they are under.

If you try to boil water at a high altitude, you know that water boils at a lower temperature due to the lower pressure at that altitude.

## Lesson Summary

Substances on Earth can exist in one of six phases, but mostly they exist in one of three: solid, liquid or gas. There are six changes of phase that substances go through:

• Freezing: liquid to solid
• Melting: solid to liquid
• Condensation: gas to liquid
• Vaporization: liquid to gas
• Sublimation: solid to gas
• Deposition: gas to solid

## Learning Outcomes

Following this lesson, you’ll be able to:

• Define the six phase changes that substances can go through
• Provide examples of each of the six phase changes
• Explain why there are plateaus on a phase change diagram

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