Question

Water is considered to be unusual when compared to other substances with a similar molar mass (for example, carbon dioxide). Which of these properties would correctly illustrate this fact?

1. Water has a very low melting point
   compared to other substances.
2. Water has a very low specific heat capacity
   compared to other substances
3. Water is a liquid at room temperature
   while many similar substances are solids.
4. Water has a very high heat of vaporization
   compared to other substances.

Answer 1

The answer is 4. Water has a very high heat of vaporization compared to other substances.

Explanation:

Carbon dioxide is not a good comparison to water. Its molar mass is `"44.009 g/mol"`, which is `2.4` times the molar mass of water. Also, at normal atmospheric pressure, carbon dioxide sublimes from the solid state directly to the gaseous state, so it has a heat of sublimation, not a heat of vaporization.

`Delta"H"_"vap"` is the symbol for heat of vaporization (also called enthalpy of vaporization).

Ammonia and methane are better comparisons, because they do have molar masses similar to that of water.

Water: `"H"_2"O":`

Molar mass: `"18.015 g/mol"`

`Delta"H"_"vap"="40660 J/mol"` or `"2257 J/g"`

Ammonia: `"NH"_3":`

Molar mass: `"17.013 g/mol"`

`Delta"H"_"vap"="23,350 J/mol"` or `"1371 J/g"`

Methane: `("CH"_4")`

Molar mass: `"16.043 g/mol"`

`Delta"H"_"vap"="8170 J/mol"` or `"480.6 J/g"`