Water on Earth

Why the Oceans Do Not Freeze

Outline:

II. What happens when water freezes

III. What happens to salt when it is dissolved in water

IV. How this effects the freezing point of water

1. Introduction  -  The oceans do not freeze (except in extreme polar areas) because of the high concentration of salt that they contain. Salt causes water to freeze at a lower temperature than normal. In fact, any substance dissolved in water lowers its freezing point. Why is this?
    
2. What happens when water freezes?  -  When water freezes - goes from the liquid state to the solid state - its molecules go from a disorganized state to an organized state.

   A. Each water molecule (H2O) is made up of two hydrogen atoms and one oxygen atom, arranged like this:
   In this picture, the oxygen atom is red and the hydrogens are blue.
   B. In liquid water, the molecules, while they are held together and form a cohesive mass, are in a fairly disordered state and are free to move and tumble around one another, so they exist in a random state, something like this:
   In this picture, oxygen is red and hydrogen is white (the white dashed lines represent weak attractive forces between the molecules).
   C. When water freezes to a solid, molecular motion slows down enough that the molecules become permanently fixed in an orderly arrangement called a crystal, like this:
   Notice that the molecules are arranged in a regular pattern; in fact, they form hexagons.
   It is important to understand that when the temperature is lowered, energy is removed from the molecules, they slow down, and are thus enabled to stick together.

3. What happens to salt when it is dissolved in water?  -  The individual particles that make up salt (they are called ions) arrange themselves around the water molecules. In doing so, they shield the water molecules from interactions among themselves, making it less likely that they will find each other and form the regular hexagon structure of ice.
    
4. How this effects the freezing point of water  -  The result is that the water molecules have a harder time coming together to form their regular hexagons.  The water molecules have to be slowed down even more in the presence of salt in order to form a solid.  So you have to go to a lower temperature in order to freeze water when it has salt dissolved. 
   

   Now look at this diagram: At the normal ocean temperature the pure water is below its freezing point so it is frozen.  But the salt water is above its freezing point, so it is still liquid.