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Name: Mr. B.
Status: educator
Age: 30s
Location: N/A
Country: N/A
Date: 3/7/2003

I teach Kindergarten and was asked "why does water freeze?" How about a kindergarten level answer?

Mr. B.:

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 the glass.

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.

ProfHoff 601

Solids are slow,
liquids they flow...
liquids they freeze
when their molecules go slow.

Peter Faletra

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!

Martha Croll

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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