Why is energy required for a substance to change phase/state?

Expert Answers

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Energy is required for a substance to change phase or state because the atoms in the substance must move faster.  Every substance is made of atoms or molecules.  They are constantly moving.

Think of it just like when someone gives you sugar.  Sugar is energy to the human body.  When you get sugar you move faster, right?  Think of a whole room full of people who all got a shot of sugar at one time.  They would start moving faster and faster.  Would the room be enough to contain them?  No they would leave so that they could have more room.

The same thing happens when you add energy in the form of heat to a substance.  The atoms or molecules move faster and faster until they change into the next state of matter...from a solid to a liquid or from a liquid to a gas.

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