D2.3.1—Solvation with water
Water solvates ions and polar solutes by forming hydration shells, reducing free water movement and lowering effective water availability in solutions.
- Syllabus
- First assessment 2025
- Objective
- D2.3.1
- Level
- HL
Water Surrounds Solutes during Solvation
Solvation is the attraction of water molecules to ions or polar molecules, forming a hydrated environment around the solute.
Water is polar: its partial charges orient toward charged or polar regions. These attractions separate particles and keep them dispersed, which helps cells transport dissolved substances.
Check: solute charge or polarity; water orientation; particle separation; resulting solution.
When sodium chloride dissolves, oxygen ends of water face Na+ while hydrogen ends face Cl−, forming hydration shells.
Non-polar solutes are not automatically soluble in water; polarity and the relevant intermolecular attractions matter.
Core Osmosis Effects
Water forms hydration shells around ions and polar solutes; hydrogen bonding and charge attraction reduce free water movement. Water moves by osmosis across partially permeable membranes from hypotonic/lower solute solutions toward hypertonic/higher solute solutions. Osmosis direction depends on internal and external solute concentration; isotonic conditions have dynamic water movement but no net osmosis. Plant tissue changes mass or length in sucrose solutions; percentage change graphs estimate isotonic or osmotic concentration. Animal cells can lyse in hypotonic solutions and crenate in hypertonic solutions; freshwater protists use contractile vacuoles to expel excess water. Plant cells become turgid in hypotonic solutions as vacuoles swell; hypertonic solutions cause flaccidity and plasmolysis from water loss. Isotonic saline prevents harmful water gain or loss in body cells; IV fluids and transplant organ baths must match tissue osmotic concentration.
Concept essentials
- Water forms hydration shells around ions and polar solutes.
- Hydrogen bonding and charge attraction associate water with solute particles.
- Solutes reduce the amount of freely moving water.
- Solvation helps explain why solutes lower water potential.
ConceptIB Biology HL