Cloud in a Bottle

Adult supervision is required!

1. Put on your safety glasses and start by pouring just enough warm water in the bottle to cover the bottom.
2. Swirl the water around and then put the rubber stopper in the bottle.
3. Start by pumping the foot pump five times. You will notice that as you start to pump, the rubber stopper will want to pop right out. Hold it in the bottle tightly, being very careful not to let it fly out of the bottle.
4. After five pumps, pull the stopper out of the bottle. You'll likely see a very faint "poof" of a cloud. There wasn't enough pressure in the bottle to make a good cloud, but now you are starting to get the feel of the foot pump.
5. Repeat the experiment again, but instead of five pumps, pump the foot pump ten times. You'll notice that the more you pump, the harder it is to keep the stopper in the bottle. Just remember to hold it in there tightly. When you are done pumping, pull out the stopper. You should see a slightly more visible cloud this time.
6. Now that you have a good feel for how the experiment works, fill the bottom of the bottle again and pump the foot pump 15-20 times. You want to put about 9 kg (20 lbs) of pressure in the bottle.
7. When you remove the rubber stopper, you should see a good cloud.

Okay, so you've mastered the technique and you're ready for an even better cloud? Make sure you are still wearing your safety glasses. Place just a few drops of rubbing alcohol in the bottom of the 1-liter bottle. Swirl the alcohol around in the bottle, making sure to coat the sides. Then put the rubber stopper in the bottle. Follow steps 3-7 above to make a more visible (and more impressive) cloud.

How does it work?

Even though we don't see them, water molecules are in the air all around us. These airborne water molecules are called water vapor. When the molecules are bouncing around in the atmosphere, they don't normally stick together.

Pumping the bottle forces the molecules to squeeze together or compress. Releasing the pressure allows the air to expand, and in doing so, the temperature of the air becomes cooler. This cooling process allows the molecules to stick together - or condense - more easily, forming tiny droplets. Clouds are nothing more than groups of tiny water droplets!

The reason the rubbing alcohol forms a more visible cloud is because alcohol evaporates more quickly than water. Alcohol molecules have weaker bonds than water molecules, so they let go of each other more easily. Since there are more evaporated alcohol molecules in the bottle, there are also more molecules able to condense. This is why you can see the alcohol cloud more clearly than the water cloud.

Clouds on Earth form when warm air rises and its pressure is reduced. The air expands and cools, and clouds form as the temperature drops below the dew point. Invisible particles in the air in the form of pollution, smoke, dust or even tiny particles of dirt help form a nucleus on which the water molecules can attach.

Additional Info

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Sources for this information included the Exploratorium website and the National Hands-on Science Institute.