Re: Why does water have a dome on top layer that doesn't spill over imediately?

Dear Logan:
Water in a glass has a dome on the top that doesnıt spill over 
immediately because of SURFACE TENSION.

Water, although a common liquid, has a rather uncommon 
structure that gives it some very uncommon properties. Water 
molecules, H2O or  H­O­H, have very polar bonds between the 
hydrogen and oxygen atoms. This means that water molecules act 
somewhat like magnetic marbles: they stick to one another 
strongly.  In fact, water molecules have exceptionally strong 
attractions to one another.  [In contrast, oil molecules act 
more like glass marbles: they have essentially no attraction 
for one another.  And other liquids, like alcohol, act like 
really weak magnetic marbles.]

If you are a water molecule in the middle of a glass of water, 
you are pulling on and being pulled on by water molecules all 
around you: above you, to your sides, and below you. But if 
youıre on the surface of the glass, life is different. You are 
only pulling on and being pulled on by molecules to your sides 
and below. This creates an imbalance, or tension that we call 
³surface tension².  The result pulls water molecules inward in 
a protective network, feels like a skin, and lets water do 
some amazing tricks.

* Youıve already discovered the ³dome². You can fill a glass 
to overflowing without a spill, because the water molecules 
are the surface are safely held by the water molecules 
underneath. [Compare it to a glass filled with soapy water or 
rubbing alcohol.]

* You can float a needle on your glass of water. Just make 
sure the needle is free of dirt or oil (which would prevent 
neighboring water molecules from grabbing on to one another) 
and use a clean tweezers to gently place the needle on the 
water.  What happens when you add soapy water to the water 
near the needle?

* A droplet of water squeezed from an eye-dropper will be 
bigger than a droplet of soapy water or rubbing alcohol. The 
best way to see this is to use two disposable plastic pipets, 
and either look at the droplets under a magnifying glass or 
find the volume of 100 drops.  The water droplets are bigger 
because the water molecules in the droplet are holding on to 
the water droplets still in the eye-dropper. So it takes a 
bigger droplet, with more weight, to break their hold and let 
the drop fall.

If you wish to learn more about waterıs structure and unusual 
properties, check out any basic high school chemistry 
textbook. It will have good diagrams of water molecules, and a 
discussion of its properties.

Enjoy your explorations!
Yours,
Sue Klemmer
chemistry teacher
Camden-Rockport High School
Maine, USA