Cloud in a Bottle Materials
- 1-liter bottle
- warm water
- Pour just enough warm water in the bottle to cover the bottom of the container.
- Light the match and place the match head inside the bottle.
- Allow the bottle to fill with smoke.
- Cap the bottle.
- Squeeze the bottle really hard a few times. When you release the bottle, you should see the cloud form. It may disappear between 'squeezes'.
Ok, here's the ideal gas law:
PV = nRT, where P is pressure, V is volume, n is number of moles, R is a constant, and T is temperature.
If we're not changing the amount of gas (as in a closed container) then if you raise the pressure, the only way for the temperature of the gas to be unchanged is by decreasing the container volume proportionally. I wasn't sure I could squeeze the bottle hard enough to achieve this (or that it would bounce back) and I wanted a really dense cloud for the photograph so I did the not-as-child-friendly version of this demonstration (still pretty safe). I poured water from my coffeemaker into the bottom of the bottle. Instant cloud! (... and a slight melting of the plastic) I couldn't find any matches, so I lit a strip of cardboard on fire, inserted it into the bottle, and let the bottle get nice and smoky (and melted more plastic... you can see the deformation in the photo). Dense cloud, no squeezing required, though of course it still worked.
How Clouds Form
Molecules of water vapor will bounce around like molecules of other gases unless you give them a reason to stick together. Cooling the vapor slows the molecules down, so they have less kinetic energy and more time to interact with each other. How do you cool the vapor? When you squeeze the bottle, you compress the gas and increase its temperature. Releasing the container lets the gas expand, which causes its temperature to go down. Real clouds form as warm air rises. As air gets higher, its pressure is reduced. The air expands, which causes it to cool. As it cools below the dew point, water vapor forms the droplets we see as clouds. Smoke acts the same in the atmosphere as it does in the bottle. Other nucleation particles include dust, pollution, dirt, and even bacteria.