(a) how hydrogen bonding occurs between water molecules, and relate this,…

The idea

Water is a polar molecule: the oxygen atom carries a partial negative charge (δ⁻) and each hydrogen a partial positive charge (δ⁺). The δ⁺ hydrogen of one molecule is attracted to the δ⁻ oxygen of a neighbouring molecule, forming a hydrogen bond. Each water molecule can hydrogen bond with up to four others, giving a network that explains water's properties. High specific heat capacity and high latent heat of vaporisation buffer temperature changes and allow cooling by sweating. Cohesion and surface tension support columns of water in xylem and allow insects to move on water. Being a solvent for ions and polar substances lets water act as a transport medium in blood plasma and as a medium for metabolic reactions. Ice is less dense than liquid water, so it floats and insulates water below.

What earns the marks

• Hydrogen bonding arises because water is polar: oxygen is δ⁻ and hydrogen is δ⁺, so the δ⁺ hydrogen of one molecule is attracted to the δ⁻ oxygen of another.
• Each water molecule can form hydrogen bonds with up to four neighbouring molecules, creating an extensive network.
• High specific heat capacity means large energy changes cause only small temperature changes, so water buffers temperature in organisms and habitats.
• High latent heat of vaporisation means much energy is needed to evaporate water, so sweating or transpiration cools organisms effectively.
• Cohesion between water molecules and surface tension allow continuous columns in xylem and support organisms on the water surface.
• Water is a good solvent for ions and polar molecules, so it acts as a transport medium (for example in blood plasma) and as a medium for metabolic reactions.
• Ice is less dense than liquid water, so it floats and insulates the water beneath, helping organisms survive in cold conditions.

Common mistakes and how to put them right

• Thinking hydrogen bonds are covalent bonds within a water molecule.

  Put it right by stating they are attractions between molecules, not shared-electron bonds.

• Saying water is non-polar or that oxygen is δ⁺.

  Put it right by stating oxygen is δ⁻ and hydrogen is δ⁺.

• Confusing high specific heat capacity with high latent heat of vaporisation.

  Put it right by linking specific heat capacity to resisting temperature change and latent heat to energy needed for a change of state.

• Claiming ice is denser than water.

  Put it right by stating ice is less dense and floats, insulating the water below.

Exam tips

• Read the question stem to see whether it asks about the bonding itself or about a property linked to a role, and answer that part specifically.
• When linking a property to a role, name both the property and the living process it supports, for example cohesion supporting the transpiration stream.
• Eliminate options that describe covalent bonding or non-polar water before choosing between the remaining plausible properties.