EHA: 10/01/10

Friday, October 1, 2010

Today's science terms and concepts:

There are 4 states of matter (Tutors, this will be in our Science grammar next semester): Solid, Liquid, Gas, and Plasma. Plasma is a really complex subject so I just told the children that stars (including our sun) is made of plasma and that is probably enough for them to know now.

Water at standard atmospheric pressure (normal pressure) which is "1 ATM" goes from solid (ice) to liquid (water) to gas (steam) as we increase temperature. However, if we increase or decrease pressure, it may behave differently. We can see how it behaves by seeing water's phase diagram. At 1 ATM, water goes from liquid to solid at 32 degrees F or 0 degrees C. It goes from liquid to gas at 212 degrees F or 100 degrees C (shouldn't we just convert to Celcius?! Much easier to remember!).

However, all materials have their own phase diagrams. Carbon Dioxide, at 1 ATM, or standard pressure, goes directly from solid (Dry Ice) to gas at around -110 degrees F. So any temperature above -110 degrees, it will become a gas. That is why dry ice is so good for putting in the fridge during power outages (should never be put in a working fridge) and mailing perishables. You won't be left with a wet mess after it melts.

At higher pressure, Carbon Dioxide will go from solid to liquid to gas. That is why soda manufacturers have to introduce high pressure in order to liquefy Carbon Dioxide to add to the rest of the junk that's in soda :).

The process for ice becoming water is melting.
The process for water becoming steam is boiling.

However, the process when you skip the liquid stage and go from solid directly to gas is sublimation. Even if they don't remember the word, they get the concept that different materials behave differently.

Dry ice precautions:

Dry ice isn't dangerous per se but it is so cold that it will kill your skin cells with very short contact, so should never be touched without thick gloves.

The gas is not dangerous but large amounts of it can asphyxiate.

Dry ice should not be put in a closed container. The gases will build up and if they can't escape, will explode the container it is in. This is just about true for any gas buildup.

Dry ice can be put in drinks to make an interesting show, but no one should drink the actual ice so it's safest to wait until all the ice is gone. And each time children should be reminded not to eat or drink science experiments without permission, no matter what they think.

Harris Teeter in Reston (near the Home Depot) has a dry ice fridge. Otherwise, it's a little difficult to find. And it usually sells out during power outages and Halloween.