## Demo Notes:

### Can Crush

• Performing the Demonstration
• Materials
• An empty soda can
• A burner or electric heater
• A small amount of water
• A container of ice water
• A pair of tongs
• Put about 3 - 5 mL of water in the empty can
• Heat the can for 2 or 3 minutes, at least until water vapor comes steadily out of the opening
• Once you are sure the can is full of very hot water vapor grasp it carefully in the tongs and quickly submerge the top few cm of the can in the ice water
• If you have sealed the can in the water and heated it sufficiently it should be crushed immediately and spectacularly by atmospheric pressure; it will also partially fill with water
• Notes About What's Going On
• The can fills with steam and pushes cold air out; the water vapor is the steam condensing as it exits the can. Eventually, the can is full of steam and has pushed out at least some of the air. Just how much of the gas in the can is strictly steam and how much is air is not clear.
• The steam condenses rapidly upon immersion of the can in the ice water; when gases become liquids they reduce greatly in volume and the gas pressure inside the can drops suddenly; also, gases contract upon cooling (although this contraction upon cooling is probably only a minor contributor to what happens).
• The sudden drop in gas pressure inside the can (due to steam condensation), and the fact that it is sealed off from the atmosphere by the water, causes the can to be crushed by atmospheric pressure
• Atmospheric pressure is about 14.7 pounds per square inch or 1.03 kilograms per square centimeter
• Note: 1 mol of H2O as a liquid has a volume of about 18 mL. One mole of water as a gas at 100°C and 1 atm has a volume of 30.6 L (30,600 mL). That’s a difference of a factor of 1,700!!
• If you are making a connection to ideal gas laws, specifically Boyle’s Law, this demonstration can be confusing. Think about it this way:
1. The volume of the steam in the can goes down to practically zero because it turns into a liquid. This is not described by Boyle’s Law.
2. This results in the pressure inside the can becoming instantaneously zero. This is where it can get confusing. If volume decreases, doesn’t that mean that pressure increased? It does not in this case because a phase change is involved and Boyle’s Law is only concerned with describing the behavior of gases.
3. Because the pressure inside is now about 1700x smaller than outside the can there is an imbalance in forces. This crushes the can.
4. Think about it this way. The pressure outside the can is very large and the inside of the can must come into equilibrium with the now greater pressure. To do so the volume decreases according to our qualitative expectation based on Boyle’s Law.
• Safety
• This demonstration is safe enough for anyone with a bit of common sense to do.
• The can and the steam you create in it are very hot, use common caution
• The heat source should be carefully monitored and not allowed to burn/stay hot after you are done
• Cleanup
• Recycle the aluminum can!
Last updated: Nov 06, 2011            Home