Have you ever seen a water strider bug walking on water and wondered how he was able to do that without sinking? According to the USGS, the water strider is an example in nature of surface tension. The bug does not weigh enough to penetrate the surface of the water.
What Is Surface Tension?
The Merriam-Webster dictionary defines surface tension as: "The force that causes the molecules on the surface of a liquid to be pushed together and form a layer."
Surface tension is basically an elastic membrane formed by water molecules. It can be disturbed by detergents, which lower the tension of the water, heavy objects or by the water being stirred. A good example is the shape of a raindrop. The raindrop holds together because of surface tension, but when it strikes a hard surface, the raindrop bursts. To see the concept of surface tension and how it can be broken, try the two experiments below.
Pepper My Water
One of the easiest ways to show surface tension is to sprinkle something on the surface of water that is lighter than the water, such as pepper or dried herbs. Pepper creates the most dramatic effect as it is dark against the water, but you could also use a colorful spice such as paprika for the same effect.
What This Experiment Teaches
This experiment teaches the concept of surface tension. The surface of the water in the bowl is bond together to create a sort of membrane. Only something heavier than the water can penetrate the surface. The pepper is lighter than the water, so it does not penetrate the membrane. However, if the surface tension is disrupted, which it is by the dishwashing soap, the membrane is broken and the lighter object will go beneath the surface of the water.
- Spoon (does not have to be metal)
- Ground black pepper or other spice
- Dishwashing soap (Dawn works best)
What to Do
- Fill the bowl with water to about one inch below the edge of the bowl.
- Pour about a dime-size of Dawn into the spoon and then use your fingers to completely coat the spoon with the dishwashing soap. It shouldn't be dripping from the spoon, but the spoon should be coated.
- Sprinkle pepper until it covers the surface of the water as pictured below.
- Watch carefully and place the spoon very gently in the water in the center of the bowl. Do not stir or move the spoon. You only want to break the surface tension, not disturb the water. The pepper should scatter to the sides of the bowl where the surface tension has not been disturbed.
Tie-Dye Surface Tension
This experiment uses food coloring to show your child what happens when the surface tension of water is disturbed.
What This Experiment Teaches
This is a good experiment to begin teaching the concept of chemical bonds. Milk is made up of different vitamins and thus reacts a little differently than water to the surface tension, or chemical bonds, being broken. Where the pepper experiment saw the pepper scatter from the center out to the edges, this experiment will create a swirl of colors.
The dishwashing soap is polar on one end and non-polar on the other (bi-polar) and therefore weakens the chemical bonds of the milk and breaks the surface tension.
- Paper plate or shallow bowl
- Milk (whole, 2%, etc.)
- Food coloring in green, red, blue and yellow
- Dishwashing soap (Dawn works well)
What to Do
- Pour enough milk into the plate or bowl to create a solid surface. You should not be able to see the surface of the plate or the bowl.
- Place three drops of each color in four circles in the center of the milk. It's okay if the drops of food coloring are not perfectly round. The experiment will still work.
- Coat a spoon with dish soap. The soap should not be dripping off the spoon but the spoon should be completely coated with the soap.
- Wash hands.
- Place the spoon in the center of the milk and hold it there.
- Watch as the colors swirl and mix together into a tie-dye design.
Polarity of Water Molecules
Water is made up of two hydrogen molecules and one oxygen molecule. Surface tension is caused because of the polarity of water molecules and bonding of the hydrogen molecules. Only by disturbing this polarity can surface tension be broken. Surface tension makes for an interesting study and some fun experiments. Isn't science fascinating?