Monday, September 27, 2010

Water's Cohesion

Let's start with the definition of cohesion.  Cohesion is "the phenomenon that occurs when individual molecules are so strongly attracted to each other that they tend to stay together, even when exposed to tension."  Per Apologia's Exploring Creation with Physical Science, "water molecules are attracted to one another by hydrogen bonds as well as the electrical charges that are the result of polarity.  Because they are attracted to one another, they do not like to be pulled apart.

To visually see this, we performed two (2) experiments. 

1)  Supplies needed: small metal paper clips, bowl of water, small piece of toilet paper, and dish soap. 

 First, we filled a bowl with water.  Then, we cut a piece a toilet paper about 2 inches longer and wider than our small metal paper clip.  Set the paper clip on the paper and place it gently on the surface of the water.  After the paper drops to the bottom, the paper clip should be floating.

The cohesion of the water molecules allows the water strider to "walk" on water!  Since the water molecules want to stay close together, it forms a 'skin' on the water, called surface tension.

Now, let's see what happens when you break the hydrogen bonds.  Add 1-2 drops of liquid dish soap to the bowl of water.  What happens?  The bonds are broken and the paper clip drops to the bottom of the bowl.

2)  Supplies needed: a clean penny, water dropper, and water.

This is a simple experiment but also shows how strong the bonds in water molecules are.  Rinse the penny in water and place on a flat surface.  Fill a small glass with water and fill the eye dropper with water.  Now, drop the water onto the penny one drop at a time.  Count how many drops your penny can hold.  We were able to get to 33-40 drops before the water spilled over the side.

If you look closely, you can see the 'bubble' over the penny.  How many drops does your penny hold?

1 comment:

  1. we did a few of those experiments last year when we studied water. very fun!


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