Why Does Water Freeze?
Name: Mr. B.
I teach Kindergarten and was asked "why does water freeze?"
How about a kindergarten level answer?
All molecules, including water molecules (even those in a glassful of
still water) are constantly moving. Heat makes them move faster, cooling
slows them down. When water gets cool enough, molecular movement is slowed
enough that the molecules stick to each other and form ice crystals.
You can demonstrate the movement by drawing a glass of water, covering the
top with a piece of plastic wrap, and then placing the glass in a place
where it will not be disturbed. By the way, the plastic wrap is there to
exclude dust and retard evaporation during the duration of the demonstration.
In a day or two, the circulating currents resulting from filling the glass
will have died down. Carefully remove the plastic wrap without moving the
glass or disturbing the water. Add one small drop of red food coloring to
the center of the liquid surface, taking great care to not stir or mix it
into the water. Replace the plastic wrap, again taking care to not disturb
Observe the glass from day-to-day and you'll notice that the color is
spreading in the clear water. In time, the color will be uniform
throughout the liquid. This mixing effect is proof that the liquid
molecules are moving -- even when they might appear to be stationary.
Solids are slow,
liquids they flow...
liquids they freeze
when their molecules go slow.
Hi Mr. B.
Tell your kids that all things are made up of little parts called
molecules; you, me, plants, water, trees etc. All these particles are
always moving. As water gets cold, the particles slow down. When the
particles slow down, they line up in a nice crystal pattern and the water
become solid. We will not freeze because our bodies create heat energy
from the food we eat. It is like having a big furnace inside. Things
that do not eat might freeze because they have no way of keeping warm on
their own. Lizards and snakes
have to move where it is warm or cold to keep comfortable.
Is this good enough? It might also lead you into a discussion of warm
blooded and cold blooded animals. Have fun!
When things get cold, what is really happening is that the particles they
are made of move slower. Water particles (molecules) are a little "sticky":
they tend to hold together when they touch. At room temperature, the water
particles do not stick together for very long, because they are moving too
fast. When the temperature gets lower and the particles slow down, it is
easier for them to hold on to each other. Below 0 Celsius (32 Fahrenheit),
the water particles move slowly enough that they do not detach from each
other. So, they hold together in a solid block, which we call "ice".
Richard E. Barrans Jr., Ph.D.
PG Research Foundation, Darien, Illinois
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Update: June 2012