8.1: A solution is a type of mixture 8.2: The amount of solute that dissolves can vary

1. Unit: Chemical InteractionsChapter 8: SolutionsWhen 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

2. 8.2: The amount of solute that dissolves can vary 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

3. 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

4. 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

5. 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”

6. 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 (1st)( 2nd–how) • 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) Link – saturated, unsaturated, supersaturated, crystallization

7. 8.2: The amount of solute that dissolves can vary 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. 8.2: The amount of solute that dissolves can vary The solubility of a solute can be changed • …in two ways: • Change the temperature • Raise the temperature of the solute – most solids are more soluble at higher temperatures • Change the pressure • The solubility of gases in a liquid solvent increases at high pressure

9. 8.2: The amount of solute that dissolves can vary Temperature and Solubility • Increase temperature effects on most solid substances: • Solid solutes will dissolve more quickly • 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!

10. 8.2: The amount of solute that dissolves can vary Temperature and Solubility

11. 8.2: The amount of solute that dissolves can vary 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?

12. 8.2: The amount of solute that dissolves can vary Pressure and Solubility (Generic Graph:)

13. 8.2: The amount of solute that dissolves can vary 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) • Polar solvents dissolve polar solutes • Nonpolar solvents dissolve nonpolar solutes • “Like dissolves like”