Solution Composition --Concentration of a Solution--

Concentration What are some examples from every day life where we use the term concentration? Concentration: – How much solute has been dissolved in the solvent. What do you think: – Is there a limit to how much will dissolve?

Terms to know: Saturated: A solution that contains as much solute as will dissolve at that temperature. Unsaturated: A solution that has NOT reached the limit of solute that will dissolve. Supersaturated: A solution that contains more dissolved solid than a saturated solution will hold at that temperature. – supersaturated solutions are very unstable – Only certain substances are able to make supersaturated solutions

More Terms… Concentrated: A solution that contains a relatively large amount of dissolved solute. – STRONG coffee is concentrated Dilute: A solution that contains a relatively small amount of dissolved solute. – weak coffee is dilute

Factors Affecting the Rate of Dissolution Solids can be dissolved to form solutions rapidly or slowly. Three factors affect how fast it happens: 1.Surface Area – Dissolving happens at the surface! The more surface exposed, the faster the solute will dissolve. Granulated sugar dissolves much faster than a sugar cube!

Factors Affecting the Rate of Dissolution 2.Stirring – Stirring speeds up dissolving by constantly exposing the surface to fresh solvent. Sugar dissolves must faster when stirred! 3.Temperature – Higher temperatures cause solvent molecules to move more quickly, speeding up the dissolving process. Sugar dissolves much faster in hot tea than it does in cold tea!

Solubility Curves Solvents are able to dissolve different amounts of solute at different temperatures. – (Usually, the higher the temp, the more solute can dissolve) Solubility Curves tell us what mass of solute will dissolve in 100g (or 100 mL) of water over a range of temperatures

Solubility Curves Can you read a solubility curve? What mass of KNO 3 will dissolve in water at 70˚C? 130 grams What mass of NaCl will dissolve in water at 100˚C? 40 grams Which is most soluble n water at 15˚C? NaCl

Supersaturated Solutions How can solutions exceed their saturation point? – Under certain conditions it is possible for a solution to contain more solute than it normally would at a given temperature – These are usually created by: Heating a solution Dissolving enough solute so the solution is saturated Cooling the solution slowly so the solvent continues to hold the solute in solution

Supersaturated Solutions Supersaturated Solutions are VERY UNSTABLE – The excess dissolved solute can easily crystallize/fall out of solution if: A single crystal of solute is added to the solution (called “seeding”) The solution is bumped or disturbed in any way The solution becomes too cool