Clay or masking tape, or other another material to make a watertight seal
Clear glass or plastic straw (or a ballpoint pen with a clear exterior that you can disassemble to make a tube that's open at both ends)
Two larger containers, such as bowls or pots
Water (very hot tap water, very cold water—ice helps—and room-temperature water)
Permanent marker (not shown)
Optional: Food coloring
Fill the bottle completely with room-temperature water.
If you’re using food coloring, add a drop or two to the water in the bottle.
If you're using a ballpoint pen instead of a straw, carefully pull the pen cartridge and ball point out of one end and the end cap off of the other end. You may need pliers. Note: Some clear pens have a tiny hole in the side; if yours does, cover it with clear tape or find a different pen without a hole.
If you're using clay as a sealant, wrap a small piece near one end of the straw without letting the clay block the opening (click to enlarge the photo below).
If you're using masking tape, wrap the tape around and around the straw until it forms a "donut" of tape with a diameter large enough to fit into the top of the bottle and plug it tightly.
Place the straw and its attached ball of clay (or tape plug) into the top of the bottle, plug side first. Push the sealant into the top of the bottle to make a tight seal.
Gently press the plug into the bottle until you see water rising into the straw. If your seal is tight enough, the water level in the straw will remain constant. If the water level in the straw keeps dropping, you have a leak—try repeating Steps 4 and 5. Try to get the water level in the straw to stabilize at a level about 5 cm (2 inches) above the top of the bottle, as shown in the photo below.
With a permanent marker, mark the height of the water in the straw (see photo below).
To Do and Notice
Pour hot water into one of the large containers and very cold water into the other. The two containers should be full enough to cover most of your bottle, but not so full that they overflow when you place the bottle inside.
Place the bottle into the container of hot water and watch it for 5 to 10 minutes. What do you notice?
Can you guess what will happen when you place the bottle in the container of cold water? Try it and see.
What's Going On?
You probably noticed that the water level in your bottle rose when you placed it in the hot water and sank when you placed it in cold water. That's because the water in the bottle expanded with the heat of the hot water and shrank when it cooled back down. Most materials expand when heated—a property called thermal expansion—and liquid water is no exception.
As the water in the bottle gets warmer, molecular motion increases and causes the volume of water to increase as well. This raises the water level in the straw, even though no water was added to the system.
When the water is cooled, molecular motion decreases, and the volume of the water in the bottle contracts. The water level in the straw goes down.
The water in your bottle serves as a model for what's happening in our oceans. Ocean temperatures are getting warmer because our atmosphere is getting warmer. Increasing amounts of greenhouse gases in our atmosphere prevent heat from Earth’s surface from radiating back into space as freely as it used to. That makes the atmosphere warmer. The oceans absorb 90% of this excess heat, making them warmer, too.
Like the water in your bottle, ocean water expands when heated. This expansion and the melting of land ice, such as glaciers and ice sheets, are the two main factors contributing to rising sea levels around the globe. When land ice melts, the resulting water flows into the ocean, raising sea levels. Until the last decade or so, thermal expansion of water and melting land ice contributed roughly equally to rising sea levels. In the last decade, melting land ice has become the greater contributor to rising sea levels, responsible for twice the increase of that caused by thermal expansion (see diagram below).
You may notice the column of water in the straw temporarily goes down when the bottle is first placed in the container of hot water (and it may rise temporarily when the bottle is first placed in the cold water). This is due to the expansion (and contraction) of the bottle itself. The bottle expands when it's immersed in the hot water it, increasing its volume. As a result, some of the water in the straw flows back into the bottle and the water level in the straw decreases. After a few moments, the water in the bottle begins to heat up, expand, and push back up into the straw.
Getting a good seal around the straw and in the top of the bottle can take some time and patience. Be careful not to crush the straw. Younger individuals may need help with this step, as well as with handling hot water.
You may notice the column of water in the straw going down when the bottle is first placed in the container of hot water. If so, ask students why they think this might be happening.
A recent study shows the 2019 ocean temperature is about 0.075 degrees Celsius above the 1981-2010 average.
(Cheng, L., Abraham, J., Zhu, J. et al. Record-Setting Ocean Warmth Continued in 2019. Adv. Atmos. Sci. 37, 137–142 (2020). https://doi.org/10.1007/s00376-020-9283-7)