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A1.1.5—Solvent properties of water

Water’s partial charges surround ions and interact with polar molecules, allowing many biologically important solutes to dissolve and move in aqueous fluids.

Syllabus
First assessment 2025
Objective
A1.1.5
Level
SL

Exam analysis

Chance of appearing3%of analysed past papers
Latest appearanceMay 2018
Most common paper—
Typical marks1–3

Common command terms

  • Identify
  • Explain

Scoring notes

Common mistake
Calling water a universal solvent that dissolves all substances.

Recent exam appearances

May 2018Paper1 SL · TZ17[ 1 ]A1.1.5—Solvent properties of water
November 2017Paper2 SL · TZ03(b)[ 3 ]A1.1.5—Solvent properties of water
November 2016Paper1 SL · TZ08[ 1 ]A1.1.5—Solvent properties of water
November 2014Paper1 SL · TZ07[ 1 ]A1.1.5—Solvent properties of water

Coverage 2014–2018 · Updated 14 Jul 2026

Life runs in a shared liquid environment

The earliest cells arose in water, where the surrounding ocean reduced exposure to ultraviolet radiation. Modern cells still contain a high proportion of water, and most cellular chemistry occurs in an aqueous phase.

As a biological medium, water allows:

  • dissolved reactants to diffuse and encounter one another
  • enzymes and substrates to interact in the same liquid phase
  • solutes to move through cytoplasm, blood, tissue fluid and plant transport systems

A medium provides the environment for a process. Water is a reactant only in reactions that chemically consume it. Its value as a medium depends on the molecular interactions developed next.

Partial charges predict what water can carry

Around Na+, the δ− oxygen ends of water face inward; around Cl−, the δ+ hydrogen ends face inward. Water–ion attractions separate the ions and stabilize a mobile hydration shell around each one.

Hydration shells around sodium and chloride ions, with oxygen ends facing Na+ and hydrogen ends facing Cl−.
Substance Interaction with water Biological result
ions and polar molecules charges or polar groups attract water; polar groups may hydrogen-bond dissolution enables diffusion, reactions and transport
non-polar molecules or regions no equivalent favourable attraction poor solubility helps form water-excluding boundaries such as membrane interiors

O₂ and N₂ are non-polar and only slightly soluble. Respiratory pigments such as haemoglobin therefore raise the oxygen-carrying capacity of blood far above that of water alone.

Recover the chain from molecule to biological consequence

Unequal electron sharing + bent geometry → a polar molecule → hydrogen bonding and attraction to charged or polar matter. Begin every explanation from the interaction that produces the property.

Interaction or physical property Biological opportunity Constraint or boundary
water–water attraction xylem continuity; surface tension cohesion does not create transpiration pull
water–surface attraction capillary movement and wet cell walls adhesion is not the main force resisting xylem tension
attraction to ions and polar solutes reactions, diffusion and transport non-polar O₂ and N₂ remain poorly soluble
heat capacity, conductivity, buoyancy and viscosity stable habitat, support and propulsion heat loss, diving cost and drag require adaptations

Use solvent behaviour to explain transport

[ 1 ]

Which can be explained by the solvent properties of water?

Concept essentials

  • Polarity allows water to interact with ions and polar molecules.
  • Water molecules form oriented hydration shells around ions.
  • Many dissolved solutes can be transported in aqueous body fluids.
  • Non-polar substances generally dissolve poorly in water.