Solvation, Solubility, and Colligative Properties Solutions Solvation, Solubility, and Colligative Properties

Pure Substances vs. Mixtures Pure Substances- can’t be broken down without a chemical reaction Elements and Compounds Mixtures- can be separated Heterogeneous- not the same throughout Suspensions- particles settle Colloids- particles make “beam” (Tyndall effect) Homogeneous- the same throughout Solutions

Solutions Solutions made up of… Solute- the thing that is dissolved Solvent- the substance that the solute is dissolved in Ex: When making Kool-Aid, the powder is the SOLUTE and the water is the SOLVENT

What is happening when something dissolves? Polar Solvent & Polar (Ionic) Solute: Polar ends attract the ions in the solid and pull them away from the crystal They become SOLVATED – surrounded by solvent molecules If attractions between the ions in the solute are stronger than the attractions from the solvent, it is INSOLUBLE

Nonpolar Solvent & Nonpolar solute Covalent compounds do not break up in solutions like ionic ones do Nonpolar- no attractive OR repulsive forces Don’t really “dissolve”, they mix because of entropy

Solubility Curves Show the amount of a solute (g) that can be dissolved in a certain amount of solvent (usually 100 g of H2O) at a certain temperature Unsaturated solution- can dissolve more solute (under line) Saturated solution- cannot hold any more solute at that temperature (on line) Supersaturated- holds more solute than normal for that temperature (above line) Must be heated and cooled quickly. One seed crystal can cause crystallization (rock candy)

Practice Problem How many grams of KNO3 should be added to 250g of water at 40°C to make a saturated solution?

Colligative Properties Colligative properties are dependent on the number of particles in a solution Vapor Pressure Reduction/ Boiling Point Elevation Freezing Point Depression

Vapor Pressure Reduction and Boiling Point Elevation Vapor Pressure- pressure exerted by vapor of the liquid in a closed system If a solute is added, the vapor pressure is LOWERED Dissolved particles at the liquid surface, so less surface area for the solvent particles to escape Results in less solvent in air Boiling Point Elevation: Because adding a solute lowers vapor pressure, it takes more energy to vaporize the particles.

Boiling Point Elevation

Freezing Point Depression Freezing Point Depression: Adding solute causes the freezing point to be LOWER Freezing requires particles to form a crystal Solute molecules “get in the way” and prevent the molecules from freezing (and melting)

Freezing Point Depression

Changing Colligative Properties Colligative properties are changed by adding solute Change is proportional to the number of particles the solute makes NaCl would decrease by twice as much as glucose (C6H12O6) would glucose= covalent, NaCl= ionic Only ionic compounds break up in solution. MgCl2 (3 particles) would decrease it more than NaCl (2 particles)