Last month, we conducted an experiment to determine how quickly molecules move in water. This month, we’ll explore how molecules naturally move toward equilibrium – a process that can take a long time if left undisturbed. Even when it seems like nothing is happening, the molecules are constantly in motion, just more slowly than we might expect. One demonstration I use in my classroom to illustrate this concept is the floating golf ball experiment.
Materials:
Vase (tall) or plastic tennis ball container
Golf ball
Salt
Water
Food coloring (optional)
Experiment:
Fill the vase or container about ¼ full with salt and drop the golf ball on top of it. Very slowly, pour water down the side of the container until it’s nearly full. If the water is added carefully, the golf ball should remain resting on the salt layer. Over the course of days, you’ll notice the ball gradually rising as the salt and water begin to mix. If you’d like to speed up the process, you can fill the container halfway with water (don’t worry about going slowly). Shake the container, making sure that water and salt don’t spill. Now, slowly pour in water colored with food coloring, again down the side of the container to avoid mixing the layers. You should see the ball floating in the middle of the water, suspended between the denser saltwater below and the lighter colored water above.
What’s happening?
The position of the golf ball in the solution is determined by the density of the materials involved. Water has a density of 1.0 g/mL, a saturated salt solution has a density of about 1.2 g/mL, and the golf ball has a density of about 1.1 g/mL. This means the less dense water stays at the top, the denser salt solution settles at the bottom, and the ball floats in between.
The container in the picture demonstrates where the golf balls have settled after about one year. As the salt at the bottom continues to slowly dissolve (there is some at the bottom of the container), the overall density of the solution increases, causing the balls to gradually rise.
This slow movement of the golf ball over time highlights molecular motion. The change occurs because salt continues to dissolve, releasing ions that move through the solution. Meanwhile, water molecules migrate downward to dissolve more salt. Over time, the system moves toward equilibrium, although true equilibrium won't be reached until the solution is fully saturated or all the solid salt has dissolved.
Extension:
Epsom salts are made out of MgSO4, which can make the solution even denser (1.3 g/mL). Using salt from CaCl2 (ice melt salt) will give a density of 1.4 g/mL. Will these cause the golf ball to rise faster? You can purchase golf balls that float on just water (they have a density of 0.98 g/mL).
References:
http://blog.teachersource.com/2016/07/15/floating-golf-ball-density-experiment/
To view past “ChemShorts for Kids” activities, go to:
https://chicagoacs.org/ChemShorts.
