Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 2 - 7.3 Anomalous properties of water-Study Notes - New Syllabus

7.3 (i) High Melting and Boiling Temperature of Water

Water (\( \mathrm{H_2O} \)) has unusually high melting and boiling points compared to other molecules of similar size due to hydrogen bonding.

Key Observation  

• \( \mathrm{H_2O} \): boiling point = 100°C
• Similar molecules (e.g. \( \mathrm{H_2S} \)) have much lower boiling points.

Reason: Hydrogen Bonding

• Each water molecule forms up to 4 hydrogen bonds.
• Creates a strong 3D intermolecular network.
• Large amount of energy required to break these interactions.

Comparison with Similar Molecules

• \( \mathrm{H_2S} \), \( \mathrm{H_2Se} \):
  • Only have London forces + weak dipole interactions
  • No hydrogen bonding

Therefore, much less energy is needed to separate molecules.

 Trend (Group 16 Hydrides)

• Boiling point generally increases down the group.
• Water is an anomaly (much higher than expected).

Conclusion

Water has a much higher melting and boiling point than similar molecules because it forms an extensive network of strong hydrogen bonds, which require significant energy to overcome.

7.3 (ii) Density of Ice Compared to Water

Water shows an unusual property: ice is less dense than liquid water, which is why ice floats. This anomaly is due to hydrogen bonding.

Key Observation

• Most substances: solid is more dense than liquid.
• Water: solid (ice) is less dense than liquid.

Structure of Ice

• Each \( \mathrm{H_2O} \) molecule forms 4 hydrogen bonds.
• Forms a rigid, open hexagonal lattice.
• Molecules are held further apart.

Structure of Liquid Water

• Hydrogen bonds are constantly breaking and reforming.
• Molecules can move closer together.
• Structure is more compact.

Resulting Effect

• Ice: molecules further apart → lower density.
• Liquid water: molecules closer → higher density.

Conclusion

Ice is less dense than water because hydrogen bonding forms a rigid, open lattice structure in ice, keeping molecules further apart than in liquid water, where hydrogen bonds are constantly broken and reformed allowing closer packing.