Osmotic pressure is driven by the difference in *solute concentration*, not just the substance itself.
* Osmosis is the movement of solvent (usually water) across a semipermeable membrane from an area of high solvent concentration to an area of low solvent concentration. This movement is driven by the desire to equalize the concentration on both sides of the membrane.
* Osmotic pressure is the pressure that needs to be applied to the solution with the higher solute concentration to prevent the flow of solvent across the membrane.
Here's a breakdown of how different solutions of the same substance can have different osmotic pressures:
1. Concentration Matters: Even if the substance is the same, the concentration of the solute in each solution can vary. A higher concentration of solute means a lower concentration of solvent, creating a larger osmotic pressure difference.
2. Dissociation: Some substances, like salts, dissociate into ions when dissolved in water. This means a single molecule of salt can contribute to multiple particles in solution, leading to a higher effective concentration. For example, NaCl dissociates into Na+ and Cl- ions, effectively doubling the particle concentration compared to a non-dissociating molecule.
Example:
* Two solutions of sucrose (sugar) at the same temperature, but one is 1M (1 mole per liter) and the other is 0.5M will have different osmotic pressures. The 1M solution will have a higher osmotic pressure because it has a higher concentration of sucrose molecules.
In summary:
While the temperature is constant, the key factor determining osmotic pressure is the concentration of the solute, taking into account any dissociation effects.
