1. Describe dipole-dipole forces.
2. Describe London dispersion forces and relate their strength to other types of intermolecular attraction.
3. Explain how a hydrogen bond is different from other dipole-dipole forces and how it is responsible for many of water's properties.
This is brief video (2 minutes) explaining dipole-dipole intermolecular attraction:
London dispersion forces are the weakest intermolecular forces, typically between nonpolar molecules. Electrons, being negatively charged, repel each other -- with this separation, a temporary separation of charge occurs, or an induced dipole.
An extra-strong dipole-dipole attraction is hydrogen bonding, where the very small hydrogen atom bonded to a highly electronegative atom results in a greater separation of charge and results is a stronger dipole. Examples of hydrogen bonding include the sugars in molasses, the bonds holding together the strands in DNA (strong enough to hold the two strands together but weak enough to "unzip" for replication), and water.
Here is a brief explanation -- I like how it compares the boiling point differences. Many textbooks have a similar graph to what is shown here:
Its high boiling point temperature ensures that it remains water through a large temperature range. It has a relatively high specific heat -- the temperatures at locations by the ocean are more moderate and fluctuations are not so great as other locations -- that's one reason beach living is so desired.
The hydrogen bonding of water gives it surface tension, which allows for some insects to actually "skate" on the surface.
Here is a very brief video (4 minutes) that provides a simple summary of water's unique properties:
(1) The student would examine the surface tension -- we used the paper clips to poke at the "skin" on top of the water.
(2) The class would have a contest to see who could put the most drops of water on the head of a penny -- as you add drops, they would accumulate and a large water drop would collect on top of the penny.
(3) The students would try to float a paper clip on the surface of the water -- it is possible, just lay the paper clip flat on the water -- one way is to place the paper clip on your forefinger and immerse your finger in the water so that the paper clip remains flat.
Later, you can use a drop of detergent to break the surface tension on the clip will sink immediately, or the drop of water on the penny would collapse.
I didn't have a handout for this experience, I just had students record these 3 phenomena in their notebooks.
Water expands as it freezes, which is good for the life in a body of water (you wouldn't want the fish and plants to freeze!), but this expansion leads to burst water pipes and potholes in our roads.
Here is brief but very informative demonstration on how water pipes can burst:
Here is another worksheet that contains a little molecular geometry and intermolecular forces -- it could be a nice review!
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Each lab is presented with two possible report formats -- both with the same procedure -- one with 10 questions to be answered as a conclusion, the other with a full laboratory report required. This was to give the teacher the option of what type of report is desired!
Have a good week!