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Weird Science: Macroscopic Changes in Liquid Water Volume

NGSS Crosscutting Concepts:

Three cups of the same size are filled with the same mass of fresh water. One cup is put in a bucket of ice water so the water becomes cold, but does not freeze. One cup is heated so the water becomes hot, but does not boil. One cup is left at room temperature. At the macroscopic level, what someone can see with just their eyes, there appears to be very little difference in the volume of water between the three cups. However, at the molecular level, there is a difference in volume.

 

<p><strong>SF Fig. 2.1.</strong>&nbsp;<span style="line-height: 1.538em;">Heating and cooling water at the macroscopic and microscopic level.</span></p>

In SF Fig 2.1 the blue circles represent molecules of water. The cup in SF Fig. 2.1 A was heated. An increase in temperature caused the water molecules to gain energy and move more rapidly, which resulted in water molecules that are farther apart and an increase in water volume. The cup in SF Fig. 2.1 B was kept at room temperature and remained at the same volume. The cup in SF Fig. 2.1 C was chilled over ice. A decrease in temperature caused the water molecules to lose energy and slow down, which results in water molecules that are closer together and a decrease in water volume.

 

When water is heated, it expands, or increases in volume. When water increases in volume, it becomes less dense. As water cools, it contracts and decreases in volume. When water decreases in volume, it becomes more dense. For samples of water that have the same mass, warmer water is less dense and colder water is more dense.

 

Test this out yourself! Fill up three identical cups of water to the same level. Put one cup in the refrigerator to cool, heat one cup in the microwave, and leave one cup at room temperature. Look carefully to see if you can observe a difference in the volume of water.

Special Feature Type:

Exploring Our Fluid Earth, a product of the Curriculum Research & Development Group (CRDG), College of Education. University of Hawaii, 2011. This document may be freely reproduced and distributed for non-profit educational purposes.