Salt, Ice and the Coldest Mixture

Notes

Here are some notes that I use when I introduce the Salt and Ice lab to students. A visual aid that I find useful for this discussion can be found at General Chemistry Online by Fred Senese.

• The melting point of water is 0°C for pure water.
  • At 0°C the solid is in equilibrium with the liquid.
  • Solid particles that vibrate too fast break loose and join the liquid phase.
  • Liquid particles that hit the surface and lose their speed cause more vibrations for nearby solid particles but stick and join the solid phase.
  • These two types of events happen at the same rate at 0°C for water. That is the meaning of equilibrium: just as many molecules of the solid become molecules of the liquid and vice versa.
  • Above 0°C more solid molecules become liquid than vice versa.
  • Below 0°C more liquid molecules become solid than vice versa.
• For solutions (homogeneous mixtures of a solute and a solvent, like salt dissolved in water) the melting point is less than 0°C.
  • The molecules in the solid become liquid at the same rate in a solution at 0°C as they do in pure water at 0°C.
  • The molecules in the liquid become solid at a lower rate in a solution at 0°C than they do in pure water at 0°C.
    • The same number of particles hit the surface of the ice per unit time but fewer of them are particles of water.
    • Particles of the solute do not stick to the ice.
    • As a result, the freezing rate (the sticking rate of the liquid molecules) goes down.
• The temperature drops when salt is added to ice.
  • Temperature is a measure of the average speed of a collection of molecules.
  • The faster the average speed of the molecules is, the higher the temperature. The slower the average speed, the lower the temperature.
  • Heat is energy that can be moved from place to place. When heat enters a substance it makes the molecules move faster: this is why heating some increases its temperature. When heat leaves a substance it makes the molecules move more slowly. When the molecules move more slowly on average for the whole collection of molecules then the temperature decreases.
  • Making molecules in the solid vibrate fast enough to come loose requires energy (heat).
  • This heat comes from surrounding molecules of the solid and liquid. When fast-moving molecules strike the molecules in the solid they can give them enough energy to break free. A similar example is if you strike a billiard ball that is standing still with a fast-moving cue ball. The cue ball slows down and the other ball speeds away.
  • This sets up a situation in which feedback affects the process and complicates it:
    1. The solid melts more.
    2. The solid and surrounding liquid gets colder.
    3. The solid melts less.