Savvy Affiliate

Right now, this blog is in desparate need of some high PR incoming links.  So when I have a chance at a PR 4 or 5 link I have to take it.

 And when I can get it by answering science questions, so much the better.

 The question comes from Cool Tips and Tricks it is basically this.

You have a jar, a plate with a penny sitting on it, water in the plate which covers the penny, a candle and a match.  How do you get the penny out off the plate without getting wet or moving the plate (or the table).

Answer :

1) Light the candle

2) Place it on the plate in the water, but not too near the penny.

3) Invert the jar and place it over the candle

4) The candle will burn itself out

5) The jar will suck up the water on the plate, leaving the penny high and dry

But the real question, the one that can get me a decent link, is why does this happen.

Answer:

1) When you place the jar over the candle the air inside the jar heats up.

2) From the ideal gas law, PV = nRT.    The volume of the jar is constant.  The number of moles is constant (for now)  Therefore when the air heats up, the pressure must increase.

3) When the pressure increases, it pushes any water out of the jar, then air bubbles out of the jar decreasing the number of moles of gas.

4) Eventually the candle burns out, and the temperature of the gas decreases again.  Going back to the ideal gas law, the pressure inside the jar decreases.

5) Because some air escaped the jar the pressure in the jar is less now than when it was originally inverted.  Therefore the pressure in the jar is less than the air pressure.  As you can see with any barometer the exterior air pressure pushes the water into the jar.

6) As the height of the water increases, the volume of the air decreases, and the pressure of the air remaining in the jar increases.

7) The height of the water in the jar increases until ( P-atmosphere - P-jar ) = Gravity * Water Density * Height

Another thought might be that the candle burns up the oxygen in the air.  Since air is 20% oxygen there will be 20% less moles of gas, so the jar will have 20% less pressure and will suck up water until you get a pressure match.   However this is an incorrect assumption.   Combustion is C2 + 2O2 => 2C02    Combustion does not yield a net descrease in the number of moles of gas in and of itself.  Heating is the primary cause here.  You could achieve a similar effect by taking a blow torch to the jar, (assuming you didn’t evaporate all of the water) 

Moral :

Science is fun.

Try this trick and amaze your friends.

I need more pennies.