Matter changes states (from solid to liquid to gas and back) because of a change in its energy. Heat is the transfer of thermal energy. When a substance is heated, its particles absorb this energy and start to move or vibrate faster. When a solid's particles gain energy, they will start to overcome the forces holding them in a fixed position, allowing them to move freely and turn into a liquid. When a liquid's particles gain enough energy, they can escape completely, turning into a gas. When a substance cools down, its particles lose energy, slow down, and the attractive forces can pull them closer , causing a change back to a liquid or a solid. Heat is not electrons. It is a form of energy . But in metals, free electrons are the primary carriers of heat energy.

Ways matter changes states:

• Melting: The change from a solid to a liquid, by adding energy.

  
  

• Freezing: The change from a liquid to a solid, by removing energy.

  
  

• Boiling/Vaporization: The change from a liquid to a gas, by adding energy.

  
  

• Condensation: The change from a gas to a liquid, by removing energy.

  
  

• Sublimation: The change directly from a solid to a gas, without becoming a liquid.

  
  

• Deposition: The change directly from a gas to a solid, without becoming a liquid.