Solutions

Terms to know Solution – homogeneous mixture of a solvent and solute Solvent – substance does the dissolving Solutes – substance that gets dissolved Aqueous solution – solution with water as the solvent

Solubility Maximum amount of substance that will dissolve in a certain amount of water at a given temperature Solubility of ionic substances Ionic substances breakup into individual ions Conducts electricity in water

Water is polar Oxygen end is slightly negative (due to bigger electronegativity value) Hydrogen end is slightly positive (due to smaller electronegativity value)

Polar substances like ionic substances

Polar dissolves polar Ethanol Ethanol is soluble in water because of the polar OH bond

Why is sugar soluble in water?

Substances insoluble with water Nonpolar substances No polar end for water to be attracted to and pull apart Ex: oil

Like dissolves like Polar dissolves polar Nonpolar dissolves nonpolar Does water dissolve polar or nonpolar? Why? What is similar about table salt and sugar that makes then dissolve in water?

How much can dissolve? The solubility of a solute is limited. Saturated solution – contains as much solute as will dissolve at that temperature Unsaturated solution – has not reached the limit of solute that will dissolve More can dissolve

Supersaturated solution – occurs when a solution is saturated at an elevated temperature and then allowed to cool but all of the solid remains dissolved Contains more dissolved solid than a saturated solution at that temperature Unstable – adding a crystal causes precipitation All of extra solute that made it supersaturated will fall to the bottom

Solubility Curve Used to determine how much solute will dissolve in 100 g of H2O at a given temperature ALWAYS use 100 g of water (same as 100 mL)

Another way to describe solutions Solutions are mixtures So amount of solute dissolved can vary concentrated – relatively large amount of solute dilute – relatively small amount of solute

Which solution is more concentrated? B has more solute → more concentrated

Which solution is more concentrated? They are the same! X has 2 in 1 L Y has 4 in 2 L (just doubled!)

Factors affecting the rate of dissolving Surface area Dissolving occurs at the surface of the solid Exposes more surface to be attracted to solvent Stirring Removes newly dissolved particles from solid surface Exposes the surface to fresh solvent Temperature (for solids) Solvent molecules move more rapidly Increases rate of dissolving

Review Questions What does, “like dissolves like” mean in chemistry? Why does water alone not dissolve a grease stain? Can a dilute solution also be saturated? Explain

Gases Solubility As temperature increases Particles move faster Have energy to escape from a liquid So solubility DECREASES when temperature goes up

Describing solution composition Mass percent Practice problem

Practice Problems A 135 g sample of seawater is evaporated leaving 4.73 g of solid residue. What was the mass percent of solute present in the original seawater? What mass of water must be added to 425 g of formaldehyde to prepare a 40.0% by mass solution?

Parts per million  

Another way to describe solutions...  

Finding number of moles Moles will allow us to convert to grams (which is what can be weighed on a balance)

Practice Problems Calculate the molarity of a solution that has 1 g of ethanol C2H5OH in 101mL  How many grams of formaldehyde (HCHO) must be used to prepare 2.5 L of 12.3 M solution?

How to make a solution Weigh out grams of solid (found from calculations) Dissolve in most of the water Fill to line with water once all is dissolved

Diluting a stock solution Stock solution – concentrated Dilute to make concentration that is needed Moles of solute will remain the same (just adding more solvent) M1 x V1 = M2 x V2 Molarity times volume before dilution = molarity times volume after dilution

Electrolytes Substance that dissolves in water and produces ions that conduct electricity Ex: NaCl Nonelectrolyte – dissolves in water but does not allow the flow of electricity Ex: sucrose

Colligative Properties Any physical effect of the solute on the solvent Solvent has its own set properties which are now changed due to dissolving something (solute) in it So water no longer melts at 0 °C or boils at 100 °C because there is something mixed with it that hinders its ability to do that

What properties can change? How well water mixes with other substances, freezing point, boiling point Example – salt added to icy sidewalks Salt lowers the freezing point of the water, so ice is able to melt at a lower temperature Example - glycol in radiator Increases the boiling point of the water in the radiator, which prevents overheating Also lowers the freezing point, which is good for the radiator

Solute’s contribution to the property changes Any solute (electrolyte or nonelectrolyte) contributes How much dissolves and the concentration of the dissolved particles decides how much it changes the properties of the solvent More dissolved particles  Greater concentration of those particles  bigger effect on boiling and freezing point

Dissolved particles NaCl (s)  Na+1 (aq) + Cl-1 (aq) [2 dissolved particles] C12H22O11 (s)  C12H22O11 (aq) [1 dissolved particle] CaCl2 (s)  Ca+2 (aq) + 2 Cl-1 (aq) [3 dissolved particles] So CaCl2 will have the biggest effect on colligative properties because it produces the most dissolved particles