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 occurs when polar water molecules surround and interact with dissolved ions or polar solute molecules.
Water's partially negative oxygen is attracted to positive ions, while its partially positive hydrogens are attracted to negative ions. Polar solutes can also form hydrogen bonds with water.
These attractions form hydration shells, separate solute particles and keep them dispersed. Water molecules engaged around solutes have less freedom of movement than in pure water.
When sodium chloride dissolves, oxygen ends of water face Na⁺ and hydrogen ends face Cl⁻, producing oriented hydration shells.
Water does not form hydrogen bonds with every solute: ion–dipole attraction hydrates ions, while hydrogen bonding requires suitable polar groups.
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.