Water and Hydrogen Bonding

In a water molecule, the O-H bonds are polar because oxygen attracts the bonding electrons more strongly than hydrogen. Each hydrogen atom in water is therefore electron-poor, and the oxygen atom has lone pairs that can attract electron-poor hydrogen atoms on neighbouring water molecules.

The key word is intermolecular. The O-H covalent bonds inside each H2O molecule are strong intramolecular bonds. Hydrogen bonds act between different H2O molecules and are relatively strong compared with many other intermolecular forces, such as induced dipole-dipole interactions.

Water's relatively high melting point and boiling point follow from this model. To melt ice or boil liquid water, energy is needed to overcome hydrogen bonds between H2O molecules. The O-H covalent bonds inside the water molecules are not broken during normal melting or boiling.

Worked reasoning: why water boils much higher than expected for a small molecule

Hydrogen sulfide, H2S, is also a small covalent molecule, but it does not form hydrogen bonds between its molecules. Its strongest intermolecular forces are weaker than the hydrogen bonds between H2O molecules.

So, for water:

1. H2O molecules form hydrogen bonds with neighbouring H2O molecules.
2. Hydrogen bonds are relatively strong intermolecular forces.
3. More energy is needed to overcome these forces.
4. Therefore water has a relatively high boiling point for a small covalent molecule.

The explanation is not that the O-H covalent bonds are unusually easy or difficult to break. The explanation is about attractions between molecules.

Why Ice Is Less Dense Than Water

Most substances become denser when they freeze because their particles are held in a more closely packed solid structure. Water is an important exception. Ice is less dense than liquid water, so ice floats.

In ice, hydrogen bonds hold H2O molecules in a regular open lattice. The molecules are held further apart than they are, on average, in liquid water. In liquid water, some hydrogen bonds are broken and reforming all the time, so the molecules can move closer together.

Density is mass per unit volume. If the same number of H2O molecules occupies a larger volume in ice than in liquid water, the density of ice is lower.

Worked reasoning: correcting the usual solid-state assumption

A student writes: "Ice should be denser than water because particles in a solid are always closer together."

The assumption is wrong for water:

1. In ice, H2O molecules are linked by hydrogen bonds into an open lattice.
2. This open lattice holds the H2O molecules further apart than in liquid water.
3. A fixed mass of ice therefore occupies a larger volume than the same mass of liquid water.
4. The density of ice is lower, so ice floats on water.

Do not say the gaps are full of air. The important point is the spacing of H2O molecules in the hydrogen-bonded lattice.

Simple Molecular Lattices

A simple molecular lattice is a regular solid structure made from covalently bonded molecules. The molecules themselves are held together internally by covalent bonds, but the molecules are attracted to neighbouring molecules by intermolecular forces.

Ice is a simple molecular lattice made from H2O molecules. The H2O molecules contain O-H covalent bonds, and neighbouring H2O molecules are attracted by hydrogen bonds.

Iodine is another useful example. Solid iodine contains I2 molecules. Each I2 molecule has a covalent bond between the two iodine atoms. The separate I2 molecules are attracted to each other by induced dipole-dipole interactions.

When a simple molecular substance melts or boils, the molecules are separated from each other. The intermolecular forces between molecules are overcome. The covalent bonds inside each molecule usually remain intact.

Worked reasoning: what changes when solid iodine is heated

Solid iodine can form iodine vapour. The particles in the vapour are still I2 molecules.

The explanation is:

1. Solid iodine has a simple molecular lattice of I2 molecules.
2. There are covalent bonds within each I2 molecule.
3. There are induced dipole-dipole interactions between I2 molecules.
4. Heating overcomes the intermolecular forces between I2 molecules.
5. The I-I covalent bonds inside the molecules are not broken in this change of state.

Properties From Structure and Bonding

For simple molecular covalent compounds, physical properties depend mainly on the intermolecular forces between molecules and on whether there are mobile charged particles.

Melting and boiling points

Simple molecular substances often have low melting and boiling points compared with ionic, metallic or giant covalent substances. This is because only intermolecular forces between molecules need to be overcome during melting or boiling. Covalent bonds inside the molecules are much stronger, but they are not normally broken in a change of state.

The exact melting or boiling point depends on the strength of the intermolecular forces. Hydrogen bonding raises water's melting and boiling points relative to many small molecules. Larger non-polar molecules, such as I2, can also have stronger induced dipole-dipole interactions than smaller non-polar molecules because their electron clouds are more easily distorted.

Solubility

A simple molecular substance dissolves well only if attractions formed between solute and solvent particles can compensate for attractions that must be disrupted. A molecular substance that can form hydrogen bonds with water may dissolve in water. A non-polar molecular substance, such as iodine, is only slightly soluble in water because it cannot form strong attractions with water molecules.

Many non-polar molecular substances dissolve better in non-polar organic solvents because the intermolecular forces disrupted are replaced by similar intermolecular forces between solute and solvent molecules.

Electrical conductivity

Electrical conduction requires mobile charged particles, such as mobile ions or delocalised electrons. Simple molecular covalent substances are made of neutral molecules. They therefore do not usually conduct electricity as solids or liquids because they lack mobile charged particles.

Worked explanation: predicting properties of a non-polar molecular solid

Substance X is a covalent compound made of small non-polar molecules in a simple molecular lattice.

• Melting point: likely low, because only weak induced dipole-dipole interactions between molecules need to be overcome.
• Boiling point: likely low for the same reason, although boiling needs enough energy to separate molecules throughout the liquid.
• Solubility in water: likely low, because X cannot form strong attractions such as hydrogen bonds with water.
• Electrical conductivity: poor, because X is made of neutral molecules and has no mobile ions or delocalised electrons.

Writing Strong Property Explanations

A strong structure-and-bonding explanation has a clear chain:

1. Name the structure or particles.
2. Name the bonds or intermolecular forces.
3. State what is overcome, separated or mobile.
4. Link this to the property.

For example, an ice-density explanation should not just say "hydrogen bonding". It should connect hydrogen bonding to an open lattice, the spacing between H2O molecules, and lower density.

For example, an iodine melting-point explanation should not just say "covalent". It should separate the covalent bond inside each I2 molecule from the intermolecular forces between I2 molecules.

Common OCR traps

• Writing "atoms" when the structure contains molecules.
• Saying covalent bonds are broken when a simple molecular substance melts or boils.
• Saying ice is less dense because there is air in the gaps.
• Saying "hydrogen bonds are strong" without making the comparison clear. Say "hydrogen bonds are relatively strong intermolecular forces."
• Forgetting that electrical conductivity depends on mobile charged particles, not just whether a substance is solid or liquid.

A final check is to look for the exact thing being explained. If the property is density, talk about spacing and volume. If the property is melting point, talk about energy needed to overcome intermolecular forces. If the property is conductivity, talk about mobile charged particles.