L2.4: Colligative Properties

• Colligative properties are properties that depend on the number of solute particles, not their chemical identity. The number of particles in solution can be predicted from the formula and type of the solute.
• An electrolyte separates into ions when it dissolves in water. Strong electrolytes dissociate completely while weak electrolytes dissociate very little. A nonelectrolyte does not dissociate to form ions. Each mole of dissolved compound yields 1 mole of particles in solution.
• A volatile solute readily escapes into the vapor phase. A nonvolatile solute does not readily escape, so it has a negligible vapor pressure at a given temperature.
• The vapor pressure above a solution of a nonvolatile solute is lower than the vapor pressure of the pure solvent.
• A solution always boils at a higher temperature than the pure solvent. This colligative property is a result of vapor pressure lowering. The boiling point elevation is proportional to the molality of the solution.
• A solution always freezes at a lower temperature than the pure solvent. The freezing point depression is proportional to the molality of the solution.
• Osmosis is the movement of solvent particles from a region of higher to a region of lower concentration through a semipermeable membrane. Solvent will always flow from a more dilute solution to a more concentrated one.
• Osmotic pressure is the pressure that must be applied to prevent the net flow of solvent.
• Osmotic pressure is the pressure that prevents the flow of additional solvent into a solution on one side of a semipermeable membrane.
• Two solutions with the same osmotic pressure are said to be isotonic.
• A hypotonic solution has a lower osmotic pressure than body fluids.

  •The concentration of particles outside the cell is lower than the concentration of particles inside the cell.

  •Water diffuses into the cell, so the cell swells and eventually bursts.

  •For red blood cells, this swelling and rupture is called hemolysis.
• A hypertonic solution has a higher osmotic pressure than body fluids.

  •The concentration of particles outside the cell is higher than the concentration of particles inside the cell.

  •Water diffuses out of cell, so the cell shrinks.

  •This process is called crenation.
• •In the human body, blood is filtered through the kidneys by the process of dialysis.

  •In dialysis, water, small molecules, and ions can pass through the semipermeable (dialyzing) membrane; only large biological molecules like proteins and starch molecules cannot.

  •This allows waste products like urea to be removed from the bloodstream and eliminated in the urine.