Earth science

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Video

Materials

• A 1.5 or 2 L plastic bottle
• 1 cork stopper that fits the bottle (or is a little too big)
• 1 file (to shape the cork if needed)
• 1 awl
• 1 pump with a nozzle for inflatable toys
• Ethanol (for example in the form of ethanol fuel or rubbing alcohol)
• Safety equipment: 1 fire extinguisher, 1 pair of safety goggles

Short explanation

When you reduce the pressure in the bottle, it also cools down, which leads to vapor in the bottle condensing into small drops that are visible as a cloud.

Long explanation

When you pour ethanol into the bottle, some of the ethanol will change from a liquid state to a gaseous state (evaporates) and become invisible ethanol vapor in the bottle. A liquid always changes to a gaseous state to some extent, and vice versa, even if no change in temperature occurs. However, this so-called chemical equilibrium shifts in some direction when a temperature change occurs. There are now two things in the bottle's "atmosphere"; the gas mixture we know as air, plus ethanol vapor.

When you pump air into the bottle, the pressure inside increases. And then the temperature also increases. This is because the air molecules inside get closer to each other and collide more and more violently, which leads to their speed increasing - and temperature is a measure of how fast particles move. The air temperature may rise by a few degrees. This leads to the ethanol in the bottle also heating up and more liquid ethanol evaporating to ethanol vapor. When you then relieve the pressure, the temperature drops again, as the air molecules now come further apart. This causes a large amount of ethanol vapor to condense into liquid ethanol, and small ethanol droplets that are visible as a cloud are formed.

The cloud in the bottle has formed in the same way as clouds in the atmosphere. In nature, water constantly evaporates from Earth's surface by the Sun warming it. This water ends up as invisible water vapor in the air just above the ground. This air is also heated by Earth's surface, which causes it to rise and take the water vapor higher into the atmosphere. Up there it's cold and the water vapor turns into liquid water again (condenses). Now small water droplets are formed high up in the atmosphere, which form the clouds (some water droplets also freeze into small ice crystals).

In this demonstration, you have used ethanol instead of water, and the reason for this is that ethanol evaporates much faster. There will be a greater amount of vapor in the bottle, and a better cloud.

Experiment

You can turn this demonstration into an experiment. This will make it a better science project. To do that, try answering one of the following questions. The answer to the question will be your hypothesis. Then test the hypothesis by doing the experiment.

• What happens if you change the number of pumps?
• What happens if you change the amount of ethanol?
• What happens if you replace the ethanol with other liquids?
• What happens if you change the temperature of the ethanol?
• What happens if you change the size of the bottle?
• What happens if you add food coloring to the ethanol?
• What happens if you put the cork back on and leave the cloud in there?

Variation

You can replace the ethanol with the same amount of boiling hot water. The result is almost as good.

You can also make a cloud in a bottle using water and matches, and without the help of a pump. You can find this experiment here: Cloud in a bottle 1.

Gilla:		Dela: