Questions: Properties of Water and Their Importance for Life

Question 1

Cities on a coast usually have milder winters and cooler summers than inland cities at the same latitude. Which property of water explains this best?

• A. High specific heat
• B. Low density of ice
• C. Water's ability to dissolve salt
• D. Surface tension

Answer: (A)

Water's specific heat is about 4.18 joules per gram per degree Celsius, much higher than that of rock or soil. A large body of water absorbs heat in summer and releases it in winter while changing temperature only a little, which moderates nearby air temperatures.

Question 2

Ice floats on liquid water because it is less dense.

• T. True
• F. False

Answer: True

In ice each molecule forms stable hydrogen bonds with four neighbors in an open crystal lattice. In liquid water the bonds keep breaking and re-forming, so molecules pack closer together. Floating ice insulates the water below it, which helps aquatic life survive winter.

Question 3

Why does sweating cool the body?

Model answer: To evaporate, water molecules must break their hydrogen bonds with neighbors, which takes a large amount of heat (water has a high heat of vaporization). As sweat evaporates, it carries that heat away from the skin.

The molecules that escape are the ones with the most energy, so the remaining liquid and the skin beneath it cool down. The same effect cools leaves during transpiration.

Question 4

Phospholipids placed in water spontaneously form a bilayer with their tails pointing inward. Which property of water drives this?

• A. Water's high specific heat
• B. The hydrophobic effect: water molecules hydrogen-bond with each other and with polar heads, excluding the nonpolar tails
• C. Ice's lower density
• D. Water's neutral pH

Answer: (B)

The nonpolar fatty acid tails cannot hydrogen-bond with water. The arrangement that disturbs water's hydrogen-bond network least is one where the tails cluster together away from water and the polar heads face outward. The same effect helps fold proteins with hydrophobic side chains buried inside.