Investigate convection by using food coloring and water at different temperatures.
- Completely fill two bottles with hot water. Keep filling until a meniscus (an upward bulge) forms on the surface of the water.
- Completely fill two bottles with cold water. Keep filling until a meniscus (an upward bulge) forms on the surface of the water.
- Add food coloring to the bottles. Use one color for the two hot-water bottles, and another color for the two cold-water bottles. (In the photo below, we’ve used yellow for the hot-water bottles and blue for the cold.) Watch how the drops of food coloring mix in the water.
- Place the two plastic plates on a table. Put one of the cold-water bottles on one plate, and one of the hot-water bottles on the other plate.
- Cut a piece of index card so it’s slightly bigger than the opening of a bottle, and then place the card on the mouth of the second hot-water bottle. Gently tap the index card. This will help to make sure that the card is in contact with the entire rim of the bottle.
- Carefully and slowly invert the bottle without touching the paper, and place it directly on top of the cold-water bottle on the plate. Line up the mouths of the bottles, but leave the index card in place.
- Repeat Steps 5 and 6, topping the remaining cold-water bottle with a piece of index card, inverted and placed on top of the hot-water bottle on the plate. When you’re done, you should have two experiments set up: a hot-water bottle on top of a cold-water bottle, and a cold-water bottle on top of a hot-water bottle, each with dividing index cards in place.
Try to do this next step at the same time to both sets of bottles: Carefully slide the card out from between each set of bottles without spilling the water. (You might need a helping hand to do this.) Watch what happens to the fluid in each set of bottles.
When you removed the cards from one set of bottles, the hot water stayed on top and the cold water stayed on the bottom, with the colors staying pretty much the same. In the other set, however, something very different happened. The hot water rose, and the cold water sank. As this motion occurred, the colors mixed. This happened because of differences in density, which is defined by the amount of material in a given volume.
Everything is made of molecules. Hot molecules move more than cold molecules, and things that are hot typically take up more space than the same things when they are cold. This means it takes fewer hot-water molecules to fill a bottle than cold-water molecules. Hot water is therefore less dense than cold water.
Gravity can separate fluids by their density. Because the cold water has more mass per unit volume than hot water, the force of gravity on a given amount of cold water is larger than that on the same amount of hot water. This forces the cold water downward and causes the hot water to be pushed or lifted upward. This motion of fluids is called convection. In the set of bottles where the hot water was above the cold water, the cold water was already on the bottom, so there was no convection.
Have you ever climbed on a stepstool or ladder to change a lightbulb? If so, you might have noticed that the air higher up in the room is warmer. This is due to convection.
The next time you go swimming in a pool, try noticing the temperature difference between the surface water and the deeper water. Again, convection may have separated fluids by density, and the water below will be cooler.
Compare two bowls of hot soup. Leave one alone and blow across the surface of the other. Compare them, and you’ll find that the bowl you blow on will cool faster than the one you leave alone. When you blow on hot soup, you help drive the process of convection. The top surface cools and sinks, and the hot soup below rises and also gets cooled.
Convection affects fluid movement on small scales, as in this Snack, but it affects fluid movement on very large scales, too. As a result, this investigation can also be used to teach earth and space science phenomena. Convection is an important part of the weather cycle. It drives ocean currents, as well as the motion of semi-solid rock within the earth. Convection even moves material in stars.