Floating Water

Floating Water

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.

Materials:
Mason jar (pint size) with twist-on lid, circular plastic screen insert, index cards

Place the plastic screen material over the opening of the jar and screw on the lid (sealing ban). Remove the lid and use scissors to cut around the indentation "ring." What you're left with is the 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 the 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. 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 it works:-->

How Does It Work:

Air Pressure: The atmosphere exerts about 15 pounds of pressure per square inch of surface at sea level. Because it's a gas, it not only pushes down, but also upwards and sideways. The card remains in place because the air pressure is pushing upward harder than the water is pushing downward.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 like 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 to form a thin membrane between each opening in the screen. Be careful not to giggle the jar or touch the screen because you'll break the surface tension and surprise everyone with a gush of water!

Additional Information:

Use the demonstration to explore the properties of air, air pressure and surface tension. For the older students, you can have them calculate the amount of force being applied to the circular opening of the jar by the air pressure as compared to the downward force resulting from the weight of the water. For example, if the diameter of the jar's opening is 3 inches, then the surface area is found by multiplying _ (pi) times the square of the radius, or 3.14 x 1.5" x 1.5" = 6.75 square inches. Multiply that times 15 pounds per square inch to determine the total force exerted by the air pressure (6.75 square inches x 15 pounds per square inch = 101.25 pounds). Your jar contains about a pint (16 ounces) of water. The weight of the water is about 448 grams or slightly less than 1 pound, so it's easy to see why the card stays in place when the jar is turned upside down. Another interesting statistic to ponder is that the density (mass/volume) of air at sea level is about 1/800th that 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, etcetera. 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.Let's recap. The jar was filled with water, a card covered the mouth of the jar, and the whole thing was turned upside down. Voila! The card held the water in place. Here comes the science magic. You do the unthinkable. You remove the card! Your volunteer dives for cover! Fear not, the water magically "floats" in the upside down jar. Kids scream, "How did you do that?" Take your well-deserved bow as you turn the jar right side up and empty the water back into the pitcher. Now that's science magic!