Chemistry 2

Phase Changes

  1. Before this activity, gasses, almost exclusively, have been modeled. The model has assumed there is only an interaction between atoms when they collide. If this were true, then there would be no way for anything to exist in a liquid or solid state. Everything would be a gas! To model liquids and solids another set of rules needs be added to make the model more accurate:

    • There is an attractive force between all atoms.
    • This force is only significant when the atoms are very close together.
    • The attractive force also varies depending on the type of atoms.

     

  2. When atoms are moving around fast enough (meaning they are hot enough), they then bounce off of each other with enough force to overcome the attraction felt between them. If you cool them down enough, they slow down, having less kinetic energy, and start sticking together. They are unable to bounce off of each other because the attractive force holds them close. A substance in this state is called a liquid. The change from gas to liquid is called condensation, the opposite change, boiling.
  3. In the liquid state, atoms are not bouncing around freely, but they don’t form rigid structures either. The atoms have enough energy to flow by each other, continuously changing which atoms are next to each other. If the substance is cooled even more, the atoms will form a more ordered structure with little more than vibrational movement. Substances that form a more rigid, ordered structure are considered to be solids. The change from liquid to solid is called freezing, the opposite change, melting. The process of changing directly from solid to gas is called sublimation.
  4. Depending on the strength of the attractive force between atoms or molecules of a substance the temperature at which liquids and solids form will vary. For example, nitrogen molecules have a very weak attractive force, so nitrogen boils at -196°C. Water molecules have a much stronger attractive force between them so water boils at 100°C.
  5. Consider the process by which a substance melts and then boils. As a solid the atoms will be well ordered, vibrating around relatively fixed positions. When they are heated, they begin to vibrate even more, until, when the temperature gets high enough, the atoms start to slide by each other. At this temperature they still don’t have enough energy to completely break free of their attractions so the substance melts, becoming fluid, easily changing shape, with atoms still being held close together. If more heat is added, the atoms start to move even faster, eventually bouncing off each other with enough force to break free of the attraction, boiling, and becoming a gas.