Cohesion

Hi everyone! Today my subject is  cohesion. 

Cohesion: The phenomenon that occurs when individual molecules are so strongly attached to each other that the tend to stay together, even when exposed to tension.

Watch the video, then I will talk about it.

So in the video I had a shallow bowl of water, and two threads, I put a needle across the threads then carefully lifted it onto the water. When I laid them across the water, the threads sank, but the needle floated on the water! We have all done the 'will it float?' test, and we all know that a needle is heavy enough to sink. So why did it float? Because of hydrogen boding and water molecules polarity, which we have talked about before, the water wants to stick together. Even though the needle was putting tension or pressure on the water, the molecules has enough tension of their own to fight back.

I also put a drop of dish soap in the water, and when I did, the needle sank! What happened? The active ingredient in dish soap is called sodium stearate, it has long molecules that are attracted to the water molecules. Those sodium stearate molecules were able to get between the water molecules enough, that the waters cohesion was weakened, and the needle could sink.

What happened in this experiment is called surface tension. It is a part of cohesion, but as the name suggests, it only refers to the tension on the surface of liquids. Have you ever wondered how plants could suck up water? They have little tube called xylem that, thanks to cohesion, can carry the water up the plant!

I don't know about you, but I think God might have known what he was doing when He made water.

Until next time!

For more information on Cohesion and Surface Tension, see Dr. Jay L. Wiles book Exploring Creation with Physical Science module 4, pages 97-99.