Saturday, October 8, 2016

States of Matter and Phase Changes

Matter can be found as solids, liquids, gasses, plasmas, and even Bose-Einstein condensates. What makes each of the phases of matter different is how the molecules react to their kinetic energy and the strength of the molecular bonds. As kinetic energy (measured as temperature) goes up, the ability of the bonds to hold the molecules together is less and less, and eventually, the matter will change state—solids melt into liquids, liquids vaporize into gases, and gases ionize into plasmas. If kinetic energy goes down, the reverse happens.

phase of matter
A distinct form of any type of matter related to it molecular properties commonly including: solids, liquid, gas, or plasma. At any given temperature, any specific matter will be found in a particular phase.

As kinetic energy increases, matter will change from one phase to another. Likewise, as energy decreases, matter will also change phase. Each phase change has a particular name.

Melting: the process of changing from solid to liquid.

Freezing: the process of changing from liquid to solid.

Vaporization: the process of changing from liquid to gas.

Condensation: the process of changing from gas to liquid.

Sublimation: the process of changing from solid to gas.

Deposition: the process of changing from gas to solid.

Ionization: the process of changing from a gas to a plasma.

Recombination/deionization: the process of changing from plasma to gas.

Energy is either absorbed or released in a phase change. For example, for a solid to become a liquid, the energy that is absorbed results in the bonds holding the solid together to break. To reverse it, the liquid molecules must give up energy in order for the molecular bonds to "grab" them and force them into the rigid shape of a solid.

Whether the process takes in energy or gives off energy has a name:

Endothermic changes (or reactions) take energy IN. The process absorbs energy from it surroundings.

Exothermic changes (or reactions) give off energy—energy EXits from the system. The process releases energy into its surroundings.

The following phase changes are Endothermic (require energy to be added to the molecules to cause them to change state):
  • Melting
  • Vaporization
  • Sublimation
  • Ionization
The following phase changes are Exothermic (the molecules give up energy in order to "settle" into the new state):
  • Freezing
  • Condensation
  • Deposition
  • Recombination/deionization
For any substance, the changes in phase will happen at specific temperatures. These temperatures are known as the freezing/melting point and boiling point.

It is possible, however, for liquids to become gases, even though the liquid never reaches the boiling point. Water spilled on a table will eventually dry up, even though the air never reaches 100C around it. This is due to a process called evaporation.

Evaporation is the process that changes a substance from a liquid to a gas at temperatures below the substance's boiling point.

During a phase change, the temperature of a substance will not change. Rather the energy being transferred breaks or forms the molecular bonds. Only after (for example in the case of melting) all of the molecules of the solid have broken free and become liquid will the temperature begin to go up. For any substance, there is a specific amount of energy needed to break the bonds that hold the molecules in a particular shape or volume.

The amount of energy given off when a liquid becomes a solid (or taken in when a solid becomes a liquid) is called the heat of fusion.

The amount of energy given off when a gas becomes a liquid (or taken in when a liquid becomes a gas) is called the heat of vaporization.

Not all matter has a fixed melting point. Think about butter. It softens gradually from being hard in the refrigerator as it reaches room temperature.

Matter that does not have a fixed, set melting point is amorphous

Kinetic theory still applies, however. As the kinetic energy of the molecules increase, the bonds that hold the substance in shape and to a fixed volume are "stretched" and become lax. The substance becomes softer and will somewhat change shape, though not actually a liquid.

There are many examples of amorphous matter including glass and asphalt. 


Definitions and content from:

New Oxford American Dictionary
Physical Science Concepts in Action, Pearson

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