1 Chemistry Chapter 15 Solutions and Colligative properties Zumdahl-World of Chemistry Last revision Fall 2009

2 What is a Solution? A solution is a mixture of two or more substances that is homogenous at the molecular level. Ex. of solutions include: potable water, mineral water, tap water, carbonated water and distilled water solutions.

3 What are the parts of a solution? Solute – This is the part of the solution that is dissolved. Solvent – This is the part of the solution that dissolves the solute. It is the part that is present in the greatest amount.

4 How Much? Solubility Solubility- The ability of something to dissolve. It will change with temperature. Saturated Solution- Saturated Solution- Contains the maximum amount of solid that can be dissolved. Unsaturated Solution- Unsaturated Solution- Can dissolve more solute. Supersaturated- Supersaturated- A solution that is temporarily holding more than it can, a seed crystal will make it come out

5 Solubility This is the ability of a substance to dissolve in a solvent (like water) Things that effect solubility are: 1.The volume of the solvent; more solvent will dissolve more solute. 2.The temperature of the solvent; the hotter the solvent the more solute can be dissolved. 3.The surface area of the solute.

6 Solubility of gas in water decreases with higher temperature. Higher temperatures cause gases to expand and become less dense which causes them to float to the top of the solution where they can escape the solution.Temperature

7 Why does the amount of solvent, surface area and temperature effect solubility? Solvents break apart the crystalline structure of the solute and allow the particles to slip in between the molecules of the solvent. More solvent= more spaces for solute Faster moving particles allow for more particles to be broken from the crystal and bumped in between the spaces The more surface area for the solute the more the solvent can interact with it and cause the particles to break away from the crystal. Agitation will also speed up the rate of dissolving.

8  You can use the Tyndall effect to see if your mixture is heterogeneous or homogeneous  If you can see the beam its heterogeneous, if you can’t it is a true solution or homogeneous

9 Another way of calculating concentration is molarity Example: 7 moles of NaCl = 1 mol/L or M 5 Liters of water Calculating Concentration of solutions

10 Colligative Properties Upon adding a solute to a solvent, the properties of the pure solvent are modified. Vapor pressure decreases Vapor pressure decreases Melting point decreases Melting point decreases Boiling point increases Boiling point increases These changes are called COLLIGATIVE PROPERTIES. They depend only on the NUMBER of solute particles relative to solvent particles, not on the KIND of solute particles.

11 Vapor Pressure  As solute molecules are added to a solution, the solvent become less volatile (able to evaporate) this causes a decrease in vapor pressure.  Solute-solvent intermolecular forces contribute to this effect.  Therefore, the vapor pressure of a solution is lower than that of the pure solvent.

12 Change in Freezing Point The freezing point of a solution is LOWER than that of the pure solvent because the solute particles get in the way of the crystal formation in freezing Pure water Ethylene glycol/water solution

13 Change in Freezing Point Common Applications of Freezing Point Depression Propylene glycol Ethylene glycol – deadly to animals and must be rinsed away if spilled

14 Boiling Point Elevation The temperature at which a solvent boils is increased because the particles of the solute get in the way of allowing the solvent to be bumped out of the liquid phase The temperature at which a solvent boils is increased because the particles of the solute get in the way of allowing the solvent to be bumped out of the liquid phase