The Meniscus Effect

I saw a bowl of water and I realized something that I cannot see documented anywhere. It is a property of water that we usually do not stop to appreciate. If we fill a graduated cylinder or test tube with water, it displays a phenomenon know as a "meniscus".

The surface of the water does not form a straight line but is actually curved downward so that the water's surface is actually concave rather than flat. This is because the water molecules attach themselves to the glass walls of the cylinder more strongly than they attach themselves to each other.

Not all liquids display such a concave meniscus. Mercury behaves in the opposite way in that it will form a meniscus that curves upward, in other words a convex meniscus. This is because atoms of mercury attach themselves to each other more strongly than they will to the walls of a glass cylinder.

You do not need a graduated cylinder or a test tube to see this principle at work. If you pull a plate or cup from a bowl of water, the plate or cup will be wet and will need to be dried. This is because, once again, the water molecules attach themselves to the surface of the plate or cup than they do to each other. This causes some water to remain on it when it is pulled out of the water. If this were not so, the plate or cup would be dry when it was pulled out of the water.

I find that this property of water is as important to living things as the hydrogen bonding that takes place between water molecules. Water molecules are polar, that is one end is more positively-charged while the other end is more negatively-charged, this causes an attraction between water molecules that brings them together without which water would not be liquid at room temperature and life as we know it would be very difficult.

But I find the meniscus effect to be just as important to living things. If water molecules did not attach themselves to other surfaces more strongly than they do to each other, water would drain from high ground to lower ground during rain without the high ground even getting wet. This would cause most of the earth's surface to be barren desert even if it received abundant rain.

If water behaved more like mercury, plants would have a much more difficult time absorbing water than they do now. When animals drank liquid water, it would be much more difficult for their stomachs and intestines to absorb the water. In fact, I would go as far as to say that without the meniscus effect, life on land anything like we know it probably could not exist.

Just stop for a moment to appreciate how special water is. Not only do water molecules undergo the hydrogen bonding that makes it possible for water to exist in the liquid form that is necessary to life but it fortunately attaches itself to outside surfaces more strongly than it does to other water molecules so that life on land can exist.