?

ACT® Science

Free Version

Upgrade subject to access all content

Difficult

New Information

ACTSCI-KBVTZO

Water may be one of the most abundant compounds on Earth, but it is also the most mysterious. Like most liquids, it becomes denser as it cools. But unlike them, it reaches a state of maximum density at 4 degrees Celsius, then decreases in density as it freezes.

In solid form, it is even less dense, the major reason why standard ice floats on water. This is one reason life on Earth flourished. If ice were denser than water, lakes and oceans would freeze solid from the bottom up, preventing life from happening in the way we know it.

A current unsolved mystery about water is known as the Mpemba Effect. The phenomenon is named after a Tanzanian student who in 1963 observed that hot ice cream mix freezes faster than cold. These same observation about the rate at which water freezes were also made by classical scientists such as Aristotle, Bacon, and Descartes. Mpemba published a short article announcing his ideas, but he had no experimental evidence to back up the thoughts.

A student who works closely with the researcher hears a talk at a conference about water molecules. He records the following notes as he listens to the talk.

“Water is composed of one oxygen atom bonded covalently to two hydrogen atoms. Separate water molecules are also bound by weaker forces generated by hydrogen bonds. A hydrogen bond forms when the hydrogen atom from one water molecule sits close to an oxygen atom of a second water molecule.

The combination of these two bonds gives water many of its unique properties. As water molecules get close together, the covalent bonds “stretch” and store heat energy. When the water heats up, hydrogen bonds shrink, increasing the density of the water, and in turn causing the hydrogen bonds to gain energy as well.”

The student believes this information would be useful in helping to explain the Mpemba effect.

Why would the student believe this to be true?

A

The stretching of hydrogen bonds and shrinking of covalent bonds causes warmer water to lose heat twice as fast as cold water.

B

Covalent bonds stretch and hydrogen bonds shrink, so warm water will lose energy three times faster than cooler water.

C

Hydrogen bonds sit next to covalent bonds and rob them of their energy, causing cool water to lose energy faster.

D

Water gains energy from covalent bonds and loses it at twice the rate of cool water.