Friday, March 7, 2014

Trying to Teach Science at the Dinner Table


Our exchange student from Kazakhstan is learning about gas laws.
Short version:
Temperature can make the pressure of a gas in a confined volume increase.
Temperature can make the volume of a gas in a non-confined container increase
At a constant temperature, if the pressure on a gas increases, the volume will decrease.
At a constant temperature, if the pressure on a gas decreases, the volume will increase.

I decided to show her how it works.

I put a floating candle in a bowl of water.
I lit it.
I put a glass over the flame.
After a while the flame went out.
The air over the candle cooled.
The water rose about 10% of the way up the glass, above the level of the water in the bowl.

I tried to talk her through what she had just seen.
The sticking point was that the flame had warmed the air and when the flame went out, the air cooled.

She wasn’t believing me. She wanted an explanation based on pressure, that did not include temperature.

Then my husband, the alien, walked in.  He remembered that fire converts oxygen to carbon dioxide and that carbon dioxide dissolves in water.

Our student liked this explanation because it didn’t require her to consider the effect of temperature.

But since oxygen is 20% of the volume of air, and the water only rose 10% of the way up the glass, something else is going on here.

Possibilities include:
Only some of the carbon dioxide got dissolved in the water.

What we saw was a combination of effects from the cooling of the air and the dissolving of the carbon dioxide.

I checked on the web.  Air must contain 16% oxygen to sustain fire.  So even if 4% of the air was converted to carbon dioxide and all of it dissolved in the water, that would not account for a 10% rise in the water.

Plus according to this  http://www.pwtag.org/researchdocs/Used%20Ref%20docs/52%20Carbondioxide%20in%20water%20equilibrium.pdf

warming the water would decrease the amount of carbon dioxide that can be dissolved in the water.

So, if anything, less carbon dioxide was dissolved in the water after the fire, than before.

And our student’s propensity to believe men over women has led her to the wrong conclusion.



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