The Chemical History of a Candle · Lecture V, part 2 · Part 5: Lecture V: The air and carbonic acid
14. Air has weight and pushes hard
Goal: Your child can show with simple experiments that air has weight and presses on everything, and that it can be squeezed and springs back.
You'll need
- A small glass or plastic cup of water and a flat piece of card (work over the sink)
- Two rubber suction cups, if you have them
- A hard-boiled egg (cooled) and two egg cups
- A raw potato slice about half an inch thick and a sturdy drinking straw
- Science notebook
What's in the book
Faraday's table of the air, how he weighs gases with a copper bottle and a balance, the air in a box (a pound) and in the theater (over a ton), an air-pump holding his hand, a bladder bursting, brass hemispheres, the boys' sucker, the card on a glass of water, a potato popgun and an egg blown from cup to cup.
The lesson, step by step
Hook question
SayDoes air weigh anything? How could you possibly weigh it?
Read together
Read pages 134 to 146, stopping at "We will now turn to another very important part of our subject."
I have calculated the weight of the air in this room, — you would hardly imagine it, but it is above a ton.
How Faraday weighed air
He balanced a light copper bottle, pumped in extra air, and it sank on the scale. Then he let the extra air out into a jar of water so everyone could see how much it was, and the bottle balanced again. Say: Same bottle, more air squeezed in, more weight. So air has weight.
The weight of air above us
Watch only: don't try this at home. Faraday used an air-pump: his hand was held down on it, and a skin stretched over a jar burst with a loud report. He said every square inch carries:
fifteen pounds by weight, or nearly so
If you want to see it, search online for a video of a chemistry teacher doing this demonstration and watch it together. (Search for "Magdeburg hemispheres" too.)
Philosophy into toys (try these)
We young ones have a perfect right to take toys, and make them into philosophy, inasmuch as now-a-days we are turning philosophy into toys.
Suction cups: press one on a smooth table and try to lift it straight up.
Card and glass, over the sink: fill the cup to the brim, slide the card on, hold it, turn it upside down, and let go of the card.
Take a wine-glass, either quite full or half-full of water, and put a flat card on the top, turn it upside-down, and then see what becomes of the card and of the water.
Potato popgun: push the straw through the potato slice at both ends to make a plug at each end, then push one plug with a pencil. Point it away from faces. The squeezed air pops the far plug out.
The egg trick
Faraday blew an egg from one egg cup into the next. His own safety advice:
If you want to make the experiment, you had better boil the egg quite hard first, and then you may very safely try to blow it from one cup to the other, with a little care.
Set the two egg cups touching, the hard-boiled egg in one, and blow sharply down the near edge of the egg. It may take a few tries.
Say it back and check
Narration prompts: How did Faraday weigh air? What held the card on the glass? Why did the potato plug pop out?
Check questions:
1. Why was Faraday's hand held fast on the air-pump?
2. What does "elasticity" of the air mean?
3. What does the stack of five cubes help explain?
What we know now
Air pressure at sea level is about 14.7 pounds on every square inch, so Faraday's "fifteen pounds, or nearly so" is right. We don't feel it because it pushes equally from every side, and the air inside our bodies pushes back.
Afterwards
Notice it this week: drinking through a straw, a suction-cup hook, a squeezed and released bottle. Each one is the push of the air at work.
Tip
Do the card trick over the sink the first time. A slightly damp card makes a better seal.
Answers to the book's problems
1. The air was pumped out from under his hand, so the weight of the air above pressed it down, with nothing beneath to push back. 2. That air can be squeezed into less room, and springs back out (expands) when the pressure is taken away. 3. That the air above rests on the air below, like stacked cubes; take away the air underneath and the air above presses down.
Worked out for this site from the scan. If a number in your copy differs, trust the book.