Question

What is an isotonic solution? Explain with an example.

Hint:

Same osmotic pressure at the same temperature means the same molar concentration, so no osmosis occurs. Think of 0.9% NaCl and blood cells.

Answer 1

Step 1: Definition. Two solutions that have the same osmotic pressure at a given temperature are called isotonic (or iso-osmotic) solutions.
Step 2: Consequence. Since osmotic pressure depends on molar concentration \((\pi = CRT)\), isotonic solutions have the same molar concentration of solute particles. When they are separated by a semipermeable membrane there is no net osmosis (no flow of solvent) between them.
Step 3: Example. A 0.9% (mass/volume) solution of NaCl, called normal saline, is isotonic with the fluid inside human red blood cells. That is why 0.9% NaCl is used for intravenous injections: the blood cells neither swell nor shrink in it.
Step 4: Comparison. If a cell is placed in a hypertonic solution (higher osmotic pressure) it shrinks, and in a hypotonic solution (lower osmotic pressure) it swells and may burst. Only in an isotonic solution does it stay unchanged.