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Q & A: Creating a Vacuum

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Most recent answer: 03/31/2011
Q:
How can you create a vacuum at home? I know you can stick a match in the bottle, but how could you seal it off before air gets back inside?
- Courthead (age 15)
A:
Contrary to popular belief, lighting a match in a bottle does not create a vacuum by using up oxygen. Although the flame burns up oxygen, it produces an equal quantity of carbon dioxide, another gas. Instead, the fire just heats up the air in the bottle. The air stays at atmospheric pressure, which means that the density must go down when the temperature goes up, since the pressure is approximately proportional to the product of density and absolute temperature. You've got plenty of time to seal the bottle before it cools down. When it does cool down, the pressure will then drop to less than atmospheric pressure, so you'll have a partial vacuum.

Probably the easiest way to create a vacuum at home is with a suction cup. If you press a suction cup flat against a wall and pull back, the inside of the cup will contain a vacuum. (This is why the cup sticks to the wall.) You can also create a vacuum inside of a syringe. If you seal the syringe up and pull back the plunger, there will be a vacuum inside.

If you want to create a vacuum in something larger than these, try hooking a container up to the hose attachment of your vacuum cleaner. A household vacuum cleaner should be able to produce a partial vacuum.

Of course none of these 'vacuums' are close to being completely empty of molecules, but they can have pressures much less than the atmosphere.

-Tamara

(published on 10/22/2007)

Follow-Up #1: making vacuum at home

Q:
if we need to creat complete vacuum at home, what are the options.
- zahid (age 23)
islamabad/pakistan
A:
If you really mean a complete vacuum, there are no options even in the lab. I'm guessing, based on the question you're following up, that you mean a good vacuum maintained for a long time. (The syringe vacuum can be pretty good, but it doesn't last. A vacuum made with flowing water only reaches the vapor pressure of water, not very low. etc.)

This isn't very creative, but I think that a commercial vacuum pump is about the most practical solution. I don't know how low a vacuum you need or how much gas-handling capacity so I can't say how much the pump would cost. There are lots of basic pumps available for about $120 (US).

Mike W.

(published on 03/31/2011)

Follow-up on this answer.