Using the boiling point of water to learn about hydrogen bonds.

The boiling point of water can be used to illustrate the importance of hydrogen bonds in living systems. The boiling point of a compound is the temperature at which a liquid becomes a gas. All other things being equal, the boiling point should be approximately proportional to molecular weight, since molecules are held together by weak forces that are roughly proportional to mass. When the temperature is high enough to separate the individual molecules from each other, they boil, or become a gas.

I give students the following chart:

Molecular Weight    Substance    Boiling Point

       2            [H.sub.2]
      18            [H.sub.2]O
      28            [N.sub.2]
      32            [O.sub.2]
      44            C[O.sub.2]

I then fill in the boiling points as follows. I round to the nearest whole number, but here I give the actual numbers for reference. Absolute zero is -273[degrees]C, or 0[degrees]Kelvin.

[H.sub.2] = -252.87[degrees]C

[N.sub.2] = -195.79[degrees]C

[O.sub.2] = -182.95[degrees]C

C[O.sub.2] = -78.5[degrees]C

The value for C[O.sub.2] is the sublimation point, when dry ice, a solid, becomes a gas.

I then ask students to predict the boiling point of [H.sub.2]O. They admit that it should be around -200[degrees]C. Of course, we all know that water boils at +100[degrees]C. The hydrogen bonds between the water molecules raise its boiling point by approximately 300[degrees]C, thereby causing water to be a liquid at room temperature and making oceans, rivers, lakes, and, indeed, life possible.

This year, my students commented that they knew everything I was telling them. They knew that [N.sub.2], [O.sub.2], and C[O.sub.2] are all gases at room temperature. But they had never quite thought about it this way.

DOI: 10.1525/abt.2012.74.5.12

Susan Offner is a biology teacher at Lexington High School, Lexington, MA 02421; e-mail: [e-mail].