Why are the melting and boiling temperatures of water high for its size?

Water molecules form hydrogen bonds, which are much stronger than London forces, so more energy is needed to overcome them.

What is overcome when water boils?

The intermolecular forces (hydrogen bonds). The covalent O–H bonds stay intact.

Boiling temperatures: H₂S 213 K, H₂Se 232 K, H₂Te 271 K. Why do they rise?

The number of electrons increases, so London forces get stronger.

What would the boiling temperature of water be if it followed the trend of the other Group 16 hydrides?

Lower than 213 K, because water has the fewest electrons.

Why do H₂S, H₂Se and H₂Te not hydrogen-bond?

S, Se and Te are not electronegative enough.

Why does methane have a much lower boiling temperature than water?

It is non-polar with only London forces, with no hydrogen bonds.

Describe the structure of ice.

A regular, open lattice in which each molecule forms four hydrogen bonds to its neighbours, leaving empty space.

Why is ice less dense than liquid water?

The molecules in ice are held further apart in an open hydrogen-bonded lattice than in the liquid.

What happens to the hydrogen bonds when ice melts?

Some break and the lattice collapses, so the molecules pack more closely. Hydrogen bonds still form and break in the liquid.

What are the approximate densities of ice and water at 273 K?

Ice about 0.92 g cm⁻³ and liquid water about 1.00 g cm⁻³.

What is a consequence of ice being less dense than water?

Ice floats on water, and frozen water can burst pipes.

Hydrogen bonding and properties of waterEdexcel International A Level Chemistry: Flashcards

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How many hydrogen bonds can one water molecule form?

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How many hydrogen bonds can one water molecule form?: Up to four: two through its δ+ hydrogens and two through its lone pairs on oxygen.
Why are the melting and boiling temperatures of water high for its size?: Water molecules form hydrogen bonds, which are much stronger than London forces, so more energy is needed to overcome them.
What is overcome when water boils?: The intermolecular forces (hydrogen bonds). The covalent O–H bonds stay intact.
Boiling temperatures: H₂S 213 K, H₂Se 232 K, H₂Te 271 K. Why do they rise?: The number of electrons increases, so London forces get stronger.
What would the boiling temperature of water be if it followed the trend of the other Group 16 hydrides?: Lower than 213 K, because water has the fewest electrons.
Why do H₂S, H₂Se and H₂Te not hydrogen-bond?: S, Se and Te are not electronegative enough.
Why does methane have a much lower boiling temperature than water?: It is non-polar with only London forces, with no hydrogen bonds.
Describe the structure of ice.: A regular, open lattice in which each molecule forms four hydrogen bonds to its neighbours, leaving empty space.
Why is ice less dense than liquid water?: The molecules in ice are held further apart in an open hydrogen-bonded lattice than in the liquid.
What happens to the hydrogen bonds when ice melts?: Some break and the lattice collapses, so the molecules pack more closely. Hydrogen bonds still form and break in the liquid.
What are the approximate densities of ice and water at 273 K?: Ice about 0.92 g cm⁻³ and liquid water about 1.00 g cm⁻³.
By roughly how much does the volume of water increase when it freezes?: About 9%.
What is a consequence of ice being less dense than water?: Ice floats on water, and frozen water can burst pipes.

Exam questions on Hydrogen bonding and properties of water

Water is an unusual substance. Ice floats on the surface of liquid water, whereas the solid form of most substances sinks in its own liquid. Each water molecule in ice is held in position by hydrogen bonds.
Explain why ice floats on liquid water.2 marks
The boiling temperatures of the hydrides of the Group 16 elements are: hydrogen sulfide, H₂S, 213 K; hydrogen selenide, H₂Se, 232 K; hydrogen telluride, H₂Te, 271 K; water, H₂O, 373 K.
Explain why the actual boiling temperature of water is much higher than the value predicted from the trend in the other hydrides.2 marks
Methane, CH₄, and water have similar relative molecular masses (16 and 18). Methane melts at 91 K and boils at 112 K. Water melts at 273 K and boils at 373 K. At 273 K the density of ice is 0.92 g cm⁻³ and the density of liquid water is 1.00 g cm⁻³.
Explain why water has much higher melting and boiling temperatures than methane.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).