There is an adage that says, “Like attracts like.” It basically means that similar things tend to stay together. It is possible to witness this in action simply by dropping water onto a penny.
The Experiment
Supplies: A paper towel, a penny, a small glass, an eye dropper or pipette, tap water.
What to do: Wash the penny well with soap and water. Make sure to dry it thoroughly. Set the penny flat on the paper towel. Fill the glass with water. Use the pipette or eye dropper to remove some water from the glass. Carefully place drops of water onto the penny, one at a time. How many drops of water can you place onto the penny before the bulge of water runs off the coin?
What is happening: As you place the drops of water onto the coin, you should see the water forming a small, dome-shaped bulge or bubble. Eventually, the bubble will “burst”, causing the water to run off over the edge of the penny.
The water bubble is a result of two things – cohesion and surface tension. Cohesion is when particles of the same substance stick together – “like attracting like”. Water molecules behave sort of like a bar magnet. They have a positive end and a negative end. The positive end of one water molecule is drawn to the negative end of the next water molecule. This cohesion draws the molecules tightly together, forming the dome-shaped bubble.
The outside of the bubble has surface tension. The attractive force exerted upon the surface molecules of the water by the molecules beneath tends to draw the surface molecules into the bulk of the water and makes the water assume the shape having the least surface area. In other words, the cohesion of the molecules is trying really hard to make the surface of the water (that area of the bubble where the water molecule has no neighboring molecule to cling to) as small as possible. The molecules on the surface are more attracted to the other water molecules than they are the air molecules around them, so they form a bubble. As long as the surface tension is greater than the force of gravity, the bubble will remain intact on top of the penny.
Take It Further
If you repeat this test several times, you will discover that the number of drops you can put on the penny each time changes. This is because the water you are using is changing each time. When an element of an experiment can change, that element is called a variable, because it can vary from test to test.
Try repeating this experiment using different variables. Some options include changing the type of coin you are using. Try it on a nickle, a dime, and a quarter. What do your results look like when comparing water on coins with smooth edges vs. water on coins with ridged edges? Another variable could be the liquid. What happens if you use bottled water instead of tap water? What about using salt water? You could also try using other liquids, like soda, fruit juice, or vegetable oil. You can also try rubbing alcohol or hydrogen peroxide. Just remember, if you are going to change a variable, make sure you only change one thing at a time. That will allow you to identify how the change in variable alters your results.
Links
To see this experiment in action, watch this video from Sick Science!
To explore more about cohesion and adhesion of water, visit Khan Academy.