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 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.

5 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

6 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

7 Colloids Colloid are formed of clusters of atoms or molecules. The particles are larger than the solvent molecules in a true solution. Ex. mayonnaise, egg whites and gelatin which never separate upon standing and do not qualify as true solutions.

8 Suspensions Suspensions are mixtures that settle upon standing. Suspensions can be separated by filtering. Ex. Dirt and debris in lake water. All the mud is at the bottom of the bucket.

9 Tyndall Effect If you shine a flashlight through a jar of a translucent colloid, the particles scatter the light, making the beam visible. Ex. Fog is an example of a colloid. This is why you can see headlights on a foggy day.

10  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

11 Calculating Concentration of solutions One way of calculating the concentration of a solution is percent by mass. Example: 3% hydrogen peroxide solution 6 g of hydrogen peroxide 200 g of water + 6 g of hydrogen peroxide x 100 Often the mass of the solute is disregarded because it is insignificant to the mass of the solvent

12 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

Moles of solute divided by Kilograms of solvent.  1000 grams (density of water = 1.00 g / mL and 1000 mL of water in a liter) g is 1.00 kg, so: Molality 13

Dilution M 1 V 1 = M 2 V 2 Diluting a solution  Transfer a measured amount of original solution to a flask containing some water.  Add water to the flask to the mark (with swirling) and mix by inverting the flask.

15 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.

16 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.

17 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

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

19 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

20 How Much? Solubility Solubility- The maximum amount of substance that will dissolve at that temperature (usually g/L). 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