Unit: Chemical Interactions Chapter 8: Solutions When substances dissolve to form solutions, the properties of the mixture change. 8.1: A solution is a type of mixture 8.2: The amount of solute that dissolves can vary 8.3: Solutions can be acidic, basic, or neutral 8.4: Metal alloys are solid mixtures

8.2: The amount of solute that dissolves can vary  Before, you learned  Solutions are a type of mixture  A solution is made when a solute is dissolved in a solvent  Solutes change the properties of solvents  Now, you will learn  About the concentration of a solution  How a solute’s solubility can be changed  How solubility depends on molecular structure 8.2: The amount of solute that dissolves can vary

Review / warm-up  True or False?  Dissolving a solute in a solvent is a chemical change  False: physical change  Adding a solute to a solvent raises the solvent’s boiling point  True  The particles in a solution are larger than the particles in a suspension  False: smaller 8.2: The amount of solute that dissolves can vary

A solution with a high concentration contains a large amount of solute  Ocean water vs well water  Both contain salt (solute)  Ocean water contains more salt (solute) than the well water  The more solute, the higher concentration of the solute  Concentration: depends on the amount of solute dissolved in a solvent at a particular temperature  Adding lemonade mix to water  More powder you add, the higher concentration of the drink 8.2: The amount of solute that dissolves can vary

Degrees of Concentration  “Dilute solution”: a solution that has a low concentration of solute  Well water is a “dilute solution” - there is salt dissolved in the water but at a low concentration  Add more solvent, or pure water, you can dilute the solution more  A “concentrated solution” has a large amount of solute  A “saturated solution” – when no more solute can be dissolved into the solvent – it can’t “hold” any more at that temperature  If it contains less than this maximum amount, it is a “unsaturated solution” 8.2: The amount of solute that dissolves can vary

Supersaturated Solutions  “Supersaturated Solution”: a solution that contains more dissolved solute than is normally possible  Can occur if more solute is added while the temperatures is raised, then the solution is slowly cooled, the solute can remain dissolved  This solution is unstable  If the solution is disturbed, or more solute is added in the form of a crystal, the excess solute will quickly solidify and form a precipitate  Ex: chemical heat pack – sodium acetate and water (1 st )( 2 nd –how)sodium acetate and water2 nd  Bag contains a solution of sodium acetate (solute) and water (solvent)  When heated, the excess sodium acetate can dissolve into the water completely  supersaturated solution  Heat pack is activated by bending it – disturbing the solution, solidifying the sodium acetate and releasing a large amount of heat over a long period of time (liquid  solid releases heat) 8.2: The amount of solute that dissolves can vary LinkLink – saturated, unsaturated, supersaturated, crystallization

Solubility  The amount of a substance that will dissolve in a certain amount of solvent at a given temperature  Ex: household ammonia (solution) is ammonia (solute) and water (solvent)  A large amount of ammonia can dissolve in water  ammonia has a high solubility in water  Ex: carbon dioxide can dissolve in water, but only a small amount  Low solubility in water  Oils do not dissolve in water at all  Insoluble in water  The amount of solute needed to make a saturated solution depends on the solubility of a solute in a particular solvent  If the solute is highly soluble, a lot of solute will be needed for a saturated solution, and the saturated solution will be very concentrated  If the solute has a low solubility, little solute is needed for a saturated solution, and the saturated solution will be dilute 8.2: The amount of solute that dissolves can vary

The solubility of a solute can be changed  …in two ways: 1. Change the temperature  Raise the temperature of the solute – most solids are more soluble at higher temperatures 2. Change the pressure  The solubility of gases in a liquid solvent increases at high pressure 8.2: The amount of solute that dissolves can vary

Temperature and Solubility  Increase temperature effects on most solid substances: 1. Solid solutes will dissolve more quickly 2. A greater amount of the solid dissolves in a given amount of solvent  Opposite true for gases! Increase temperature:  Gases become less soluble in water  Warming tap water – as it approaches boiling point air bubbles come out of solution and rise to the top!  Recall: to supersaturate a solution, must raise the temperature first, then it cools slowly  The solid is less soluble in the cooler solution, but has not yet formed a precipitate  Consequences in real life:  a factory takes lake water for use as a coolant – returns warmer water to lake  Dissolved oxygen content drops  bad for some marine life that use oxygen to breathe! 8.2: The amount of solute that dissolves can vary

Temperature and Solubility 8.2: The amount of solute that dissolves can vary SoluteInc. TempDec. Temp SolidInc. in solubilityDec. in solubility GasDec. in solubilityInc. in solubility

Pressure and Solubility  Pressure change usually only affects gases  Solubility of any gas increases at higher pressures  Decreases at lower pressures  Ex: Soda – CO2 is added at higher pressure than typical air  When you open it – the CO2 bubbles fizz out  Ex: the bends – normal air we breathe: 78% N2, 21% O2  Under water pressure is higher, so N2 solubility into blood is higher  Rise to surface, the N2 comes out of solution, forming bubbles in diver’s blood vessels!  Avoidable? 8.2: The amount of solute that dissolves can vary

Pressure and Solubility 8.2: The amount of solute that dissolves can vary SoluteInc. PressureDec. Pressure SolidNo effect on solubility GasInc. in solubilityDec. in solubility (Generic Graph:)

Solubility depends on molecular structure  Ex: oil and water do not mix – they have different molecular structures  When a substance dissolves, its molecules or ions separate from one another and become evenly mixed with molecules of the solvent  Water contains polar covalent bonds (negative and positive regions)  Oil is non-polar  Therefore their molecules are not attracted to each other  Sugar is polar, and highly soluble in water  Ionic compounds are soluble in water, also (ions are charged) 8.2: The amount of solute that dissolves can vary Polar solvents dissolve polar solutes Nonpolar solvents dissolve nonpolar solutes “Like dissolves like”