IB Biology 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
- ib_null_biology_001_001_005
- Level
- All levels
Quick summary
Water is an effective solvent because its molecules are polar. The partially negative oxygen is attracted to positive ions, while the partially positive hydrogens are attracted to negative ions. Water molecules can surround separated ions to form hydration shells and can form hydrogen bonds with many polar molecules. Dissolved particles can then move through aqueous fluids such as blood plasma, cytoplasm and sweat, supporting transport and metabolic reactions. This does not mean that water dissolves every substance: non-polar molecules are generally hydrophobic and interact poorly with water. Exam questions commonly ask students to connect solvent behaviour to polarity, identify a biological transport example, or distinguish solvent properties from cohesion and thermal properties. Strong explanations state both the molecular interaction and its biological consequence rather than merely saying that water is a universal solvent.
Concept visual
One diagram for the relationship students need to remember.
Common mistakes
Calling water a universal solvent that dissolves all substances.
Reversing the orientation of water around positive and negative ions.
Using high specific heat capacity to explain solubility.
Naming transport in blood without explaining polarity or ion–dipole attraction.
Key takeaways
- 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.
