Topic 7 · page 7 of 14

Why water melts and boils so high

Down a group, the hydrides boil higher as London forces grow. The first member of groups 15, 16 and 17 breaks the pattern because its molecules form hydrogen bonds. Water is the most extreme, because it forms the most hydrogen bonds per molecule.

14 min read7.37.3(i)

Read first: Hydrogen bonds

The pattern in the hydrides 7.37.3(i)

The hydrides of a group are compounds of hydrogen with each element in the group, such as CH₄, SiH₄, GeH₄ and SnH₄ for group 14. They are all small, simple molecules, so they are similar molecules to water, and they make a fair comparison.

Graph of boiling temperature against period for the hydrides of groups 14, 15, 16 and 17. Group 14 (CH4, SiH4, GeH4, SnH4) rises smoothly from minus 162 to minus 52 degrees Celsius. In groups 15, 16 and 17 the period 2 hydrides NH3 at minus 33, H2O at 100 and HF at 20 are far above the next member, which then rises with period. Water is the highest point on the graph. — Boiling temperatures of the group 14 to 17 hydrides. NH₃, H₂O and HF sit far above the trend.

Group 14 rises smoothly from CH₄ (−162 °C) to SnH₄ (−52 °C). The molecules are non-polar, so they have London forces only, and these grow as the number of electrons increases down the group.
Groups 15, 16 and 17 follow the same rise from the third period downwards: more electrons, stronger London forces, higher boiling temperature.
NH₃, H₂O and HF are far above the line you would expect. Their molecules have hydrogen bonds, which act on top of London forces, so much more energy is needed to separate them.
Water is the highest point of all. It boils at 100 °C, much higher than NH₃ (−33 °C) and HF (20 °C).

Methane, ammonia, water and hydrogen fluoride all have 10 electrons. Their London forces are of similar size. Yet CH₄ boils at −162 °C, NH₃ at −33 °C, HF at 20 °C and H₂O at 100 °C. The large differences come from hydrogen bonds: none in CH₄, and the most per molecule in H₂O.

Water against the rest of group 16 7.37.3(i)

Compare water with the other group 16 hydrides.

Hydride	$M_\text{r}$	Melting temperature / °C	Boiling temperature / °C
H₂O	18	0	100
H₂S	34	−86	−60
H₂Se	81	−66	−41
H₂Te	130	−49	−2

Down the group the heavier molecules have more electrons, so London forces and boiling temperatures rise: −60, −41, −2 °C. Water is the smallest and lightest and should be the lowest. If it followed the trend, you would expect it to boil at about −80 °C.

Graph of boiling temperature for the group 16 hydrides. H2S at minus 60, H2Se at minus 41 and H2Te at minus 2 degrees Celsius rise steadily. A dashed straight line through H2S and H2Se, continued back to period 2, predicts that H2O would boil at about minus 80 degrees Celsius. The real value for H2O is 100 degrees Celsius, about 180 degrees higher. — Following the trend, water would boil at about −80 °C. It boils at 100 °C, about 180 °C higher.

Worked exampleHow far above the trend is water?

From H₂S to H₂Se the boiling temperature rises by −41 − (−60) = 19 °C.
Step back the same amount from H₂S: −60 − 19 = −79 °C, which is about −80 °C.
Real boiling temperature of water: 100 °C. Difference: 100 − (−79) = 179, so about 180 °C higher than the trend suggests.
The same is true of the melting temperature: water melts at 0 °C, which is 86 °C higher than H₂S at −86 °C.

The line is a teaching device. The real trend is not perfectly straight (the rise from H₂Se to H₂Te is bigger), so the number is only a guide. The point stands: water is much higher than any trend predicts.

The explanation. Water molecules are held to each other by hydrogen bonds as well as London forces. H₂S has only London forces and weak permanent dipole–dipole interactions, because S is not N, O or F and cannot form hydrogen bonds. More energy is needed to overcome the hydrogen bonds in water, so water has both a high melting temperature and a high boiling temperature compared with similar molecules.

Check yourself

Explain why water (100 °C) boils at a much higher temperature than hydrogen sulfide (−60 °C).
Explain why the boiling temperature rises from H₂S to H₂Te.
How much higher is the melting temperature of water than that of H₂S?
H₂S boils at −60 °C and H₂Se at −41 °C. Estimate the boiling temperature of H₂O if it followed the same pattern, and say how far above this the real value is.
Name two other hydrides whose boiling temperatures are much higher than their group trend, and explain why.
A student writes: "Water has a high boiling temperature because the O–H covalent bonds are very strong." Give the mistake and a correct explanation.

Answers cover these until you have tried the questions

Water has hydrogen bonds between its molecules, as well as London forces. H₂S has only London forces and weak permanent dipole–dipole interactions, because S is not N, O or F. Hydrogen bonds are stronger, so more energy is needed to separate water molecules.
From H₂S to H₂Te the molecules have more electrons, so the London forces are stronger. More energy is needed to separate the molecules, so the boiling temperature rises (−60, −41, −2 °C).
0 − (−86) = 86 °C higher.
The rise from H₂S to H₂Se is −41 − (−60) = 19 °C. Going back one period from H₂S: −60 − 19 = −79 °C, about −80 °C. The real value, 100 °C, is 179 °C (about 180 °C) above it.
NH₃ (−33 °C, far above PH₃ at −88 °C) and HF (20 °C, far above HCl at −85 °C). Both have an H bonded to N or F and a lone pair, so they have hydrogen bonds as well as London forces.
The mistake is to use the covalent bonds within molecules. Boiling does not break them. Water boils high because of the hydrogen bonds between molecules, which need more energy to overcome.