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The Uniqueness of Water

Before closing this chapter, let us consider briefly the properties of water. Table 10.2 summarizes these properties. The same table also lists the properties of methane, a compound of similar molecular weight (therefore expected to be similar to water in many physical properties), and of hydrogen sulfide (the hydride of sulfur), an element in the same column of the periodic table as oxygen. Our study of the periodic table showed that the compounds water and hydrogen sulfide can be expected to be quite similar. Notice that both methane and hydrogen sulfide are gaseous under normal conditions. Water, on the other hand, is a liquid under normal conditions and a solid or gas at temperatures not far from normal.

Table 10.2 A comparison of water with methane (similar molecular weight) and hydrogen sulfide (a hydride of an element similarly located in the periodic table)
Methane, CH4 Water, H2O Hydrogen sulfide, H2S
Formula weight 16.0 18.0 34.1
Melting point -182°C 0°C -85°C
Boiling point -16°C 100°C -61°C
Density: solid (g/cm3)
0.999

: liquid (g/mL)
1.000

: gas (g/L)

 0.71 0.80 1.5
Specific heat: solid (J/g°C)
2.05

: liquid (J/g°C)
4.18

: gas (J/g°C)

 2.1 2.01
Heat of fusion (kJ/mol) 0.934 6.02 2.37
Heat of vaporization (kJ/mol) 8.90 40.7 18.7
Toxicity high none very high
Odor bad none bad
Usefulness as solvent none* excellent none*
* Both would be useful solvents below their boiling points but, as both melt well below room temperature and are highly toxic, they have limited use as solvents.



The physical state of water at normal temperature can by and large be attributed to the polarity of its molecules and the strength of its hydrogen bonds. Methane is neither polar nor hydrogen bonded. Hydrogen sulfide is polar, but much less so than water, and is not hydrogen bonded.

What are some of the special properties of water? It is an excellent solvent. It dissolves ionic compounds, polar compounds, and low-molecular-weight nonpolar compounds. The implications of this property, particularly in biological processes, are enormous.

The specific heat and heat of vaporization of water keep most of our planet temperate. Consider how much less the daily temperature fluctuates near large bodies of water than in deserts. Our bodies evaporate water (sweat) in order to maintain their constant temperature.

The small difference in density between ice and water is of far-reaching importance. Ice is slightly less dense and floats on water. Lakes freeze from the top down, not only allowing a haven for fish in freezing water but allowing the ice to melt in summer.


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