The following experiments rely on a property of water called surface tension. In Championship Science Fair Projects, Sudipta Bardhan-Quallen explains: “Water is a polar molecule. That means the hydrogen atoms and oxygen atom of a water molecule are arranged so that one side of the molecule is more negatively charged (the oxygen side) and the other side is more positively charged (the hydrogen side)….”
Bardhan-Quallen notes that when lots of water molecules are in proximity, they tend to “stick” together because of connections called hydrogen bonds – the hydrogen atoms of one molecule of water connect with the oxygen atoms of other water molecules, forming something similar to a net.
This net creates the thin, invisible film known as surface tension.
How Full Is Full? – Science Experiment Demonstrating the Surface Tension of Water
- a clear drinking glass
- a pitcher or measuring cup
- several quarters
- a small plate or saucer
- Place the glass on the plate or saucer.
- Using the pitcher or measuring cup, fill the glass to the very top with water; the water’s surface should be slightly rounded upward, or convex, when you look at it from the side.
- Place a stack of about 20 quarters next to the glass.
- Ask a friend to guess how many quarters can be added to the water before the water spills over the top.
- Slowly and carefully drop quarters edge first one at a time into the glass until the water spills.
- How close was your friend’s guess?
Observation: The water’s surface becomes more and more convex with the addition of each quarter. Eventually, the water reaches peak convexity and spills over the edge of the glass.
Explanation: The invisible skin on the water’s surface allows the water to stretch into a high convex shape over the top of the glass. Usually as many as 12 to 16 quarters can be added to the glass before the water overflows.
Note: Don’t have enough quarters on hand? Use pennies, dimes or nickels instead – you’ll just have to add a few more to your glass of water.
Fun Science Project – Challenge Your Friends to a Boat Race in Your Kitchen
- a large oblong pan
- a bath towel
- a pitcher or measuring cup
- small pieces of aluminum foil (approximately 1 1/4 inches by 2 1/4 inches)
- liquid dish soap
- 2 to 4 participants
- Place the bath towel on a kitchen table or counter.
- Place the pan on top of the towel.
- Using the pitcher or measuring cup, fill the pan nearly to the top with water.
- Have each participant cut a simple boat shape out of a small piece of aluminum foil.
- Instruct each participant to cut a narrow slot at the back of the boat, opening into a small square or “window” about 1/2 inch from the back of the boat (see illustration in image following this article).
- Instruct each participant to smooth and flatten his boat.
- Place all boats at one end of the pan with the fronts pointing toward the other end.
- Instruct each participant to dip a toothpick into a little liquid dish soap.
- At exactly the same moment, each racer should gently touch the soapy toothpick to the water inside his boat’s window.
- What happens?
Observation: The boats shoot forward across the water.
Explanation: When the soap is placed in the boat’s window, it can spread in only one direction: out the slot at the back. The movement of the soap molecules creates a different surface tension at the back of the boat from that at the front and pushes the boat forward. The boats will stop moving when the soap has spread over the entire surface of the water in a layer that is one molecule thick, or a monolayer.
Note: If the participants’ boats have different shapes, observe which shape moves faster through the water. Does the design of a boat matter?
Tip: If you’d like to run the race again, pour out the soapy water and refill the pan with clean water.
Share Fun and Easy Science Projects With Your Friends
Surface tension is one of the many interesting scientific properties of water. Can you think of other ways to demonstrate surface tension in at-home experiments?