Floating Water - Mason Jar Mystery
Fill the glass jar with water and cover it with a card. As you turn the whole thing upside down, the audience can hardly contain themselves. The room quiets down as you precariously position the inverted jar and card a few feet above someone's head. Just as they thought, no water spills out because the card magically sticks to the mouth of the upside down jar. But wait. . . there's more.
- Mason jar (pint size) with twist-on lid
- Circular plastic screen insert
- Index cards
- Tub or sink to practice over
- Place the plastic screen material over the opening of the jar and screw on the lid (sealing band). Remove the lid and use scissors to cut around the indentation. What you're left with is a screen insert that fits perfectly into the top of the sealing band.
- Place the screen over the opening of the jar and twist on the lid. Make sure that you do not accidentally show your audience the secret screen.
- When you're ready to perform the trick, fill the jar with water by simply pouring water through the screen. Cover the opening with the index card. Hold the card in place as you turn the card and the jar upside down. Gently let go of the card. The water doesn't spill!
- Carefully remove the card from the opening and the water mysteriously stays in the jar! Replace the card, turn the whole thing over, remove the card and pour out the water. That's amazing!
How Does It Work?
The water is mysteriously suspended in the jar because of air pressure and surface tension.
Air Pressure: The atmosphere exerts about 15 pounds of pressure per square inch of surface at sea level. Because it's a gas, air not only pushes down, but also upwards and sideways. The card remains in place because the pressure of the air molecules pushing up on the card is greater than the weight of the water pushing down. But how does the water stay in the jar when the card is removed? The answer is surface tension.
Surface Tension: The surface of a liquid behaves as if it has a thin membrane stretched over it. A force called cohesion, which is the attraction of similar molecules to each other, causes this effect. The surface tension "membrane" is always trying to contract, which explains why falling droplets of water are spherical or ball shaped. The water stays in the jar even though the card is removed because the molecules of water are joined together (through cohesion) to form a thin membrane between each opening in the screen. Be careful not to jiggle the jar or touch the screen because you'll break the surface tension and surprise everyone with a gush of water!
The Mysterious Water Suspension provides the framework for a hands-on lab that's both fun and effective. Kids can work alone or in teams, applying the scientific method to formulate theories, do research, devise and conduct experiments, gather data, and present their conclusions.
Experiment with different screens, some with fine mesh and some with coarse mesh to observe how surface tension and air pressure work together to accomplish the feat. For different screens, try materials such as cloth, plastic mesh from produce bags, etc. See what happens when different sizes and shapes of bottles are used.
Have your students explain why the card sticks better to the inverted jar when it's completely filled as compared to when it's only partially filled. Discuss the elastic properties of the trapped air inside the jar.
- Great , fun and educational Review by Shauna Lopez
We have done this experiment several times and love it every time . The kids get a kick out of what seems like a magic trick being science.
(Posted on March 7, 2012)