Colligative Properties

"Solubility" is not a yes/no answer, it's always a matter of degree.  Although we may describe something as "soluble", there obviously must be a limit to that solubility: NaCl is "soluble" in water, but you can't dissolve a bucket of NaCl in a glass of water.  Similarly, if we look back at the solubility rules we used in Gen Chem I, just because something is described as "insoluble" doesn't mean that none of it will dissolve, it just means that very little will dissolve.  It's an important distinction and we'll re-visit it in a few weeks.

When a solute is added to a solvent, the properties of that solvent are affected.  A colligative property is one that is dependent upon the number of solute particles present in the solution and not necessarily the identity of those solute particles.  If 0.1mol of sucrose and 0.1mol of fructose (0.2mols of solute particles) are dissolved in 10.0L of water, the colligative properties of the solution will change by the same amount as if 0.2mols of sucrose alone is dissolved in 10.0L of water.  We looked at vapor pressure depression as a colligative property; the presence of a solute decreases the vapor pressure of a solution due to solvent-solute interactions and surface blocking.

Next week we'll look at more colligative properties.  Have a good weekend.