Unit: Chemical InteractionsChapter 8: Solutions

When substances dissolve to form solutions, the properties of the mixture change.

8.1: A solution is a type of mixture8.2: The amount of solute that dissolves

can vary8.3: Solutions can be acidic, basic, or

neutral8.4: 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 (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)

8.2: The amount of solute that dissolves can vary

Link – 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

Solute

Inc. Temp Dec. Temp

Solid Inc. in solubility Dec. in solubility

Gas Dec. in solubility

Inc. 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

Solute

Inc. Pressure Dec. Pressure

Solid No effect on solubility

No effect on solubility

Gas Inc. in solubility Dec. 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”