What is an isotonic solution? (Remember, before the solutions became isotonic, there were 5 solute molecules on each side.) An isotonic solution is one in which equilibrium has been reached -- there's the same concentration of solute molecules in it compared to the concentration of solute molecules in an adjacent solution. An isotonic solution is one in which equilibrium has been reached -- there are the same number of solute molecules in it as there are in a nearby solution.