Unit 9: Solutions

H2O

Good solvent

High Surface tension

Low vapor pressure

High boiling point

WHAT PROPERTIES OF

WATER MAKE IT ESSENTIAL

TO LIFE OF EARTH?

Water is a polar

molecule. It

experiences

hydrogen bonding

due to its polar

nature.

After many hydrogen bonds are formed, you have a weak force holding all the water molecules to each other.

Hydrogen bonding is

the reason water freezes

into ice crystals of a

certain repeating shape

(hexagons)

Volume increases.

Hydrogen bonding is what gives proteins and

nucleic acids their three dimensional shape.

Hydrogen bonding also causes water

to have….

High Surface Tension (cohesion)

Surfactant

Surface Tension can be

decreased by adding a

surfactant – this type of

substance interferes

with hydrogen bonding.

Surfactants are used in

soaps, detergents,

paints, adhesives,

inks…

Hydrogen bonds also cause water to

have…

A high boiling point and a low vapor

pressure.

Water is also

attracted to other

substances like

glass.

Water can be drawn up into a thin

glass tube with no effort (capillary

action) because of the attraction

between the water and glass

molecules.

Matter

Pure Substances

Mixtures

Elements CompoundsHeterogeneous

MixturesHomogeneous

Mixtures

Classification of Matter

anything composed of atoms

Two types of mixtures!

Heterogeneous Mixtures

Two or more pure substances mixed

unevenly (you can see the different

components).

Example: fruit salad, pizza, granite

Suspensions

A suspension is a heterogeneous

mixture that has large particles that will

stay suspended as long as the mixture

is in motion

Once the motion stops, the particles

will fall to the bottom or settle out

Can be separated through filtering

Colloids

Homogeneous mixtures where the

medium sized particles are

dispersed throughout but are not

heavy enough to settle out.

Appear cloudy and opaque

Cannot be separated by filtering

Can be separated by a centrifuge

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How can light be used to tell the difference

between colloids and solutions?

Tyndall effect

A beam of light passing through a solution, such as air, is not visible.

Light passing through a colloid, such as fog, will be scattered by the larger particles and the light beam will be visible.

Homogeneous Mixtures

Two or more pure substances mixed evenly.

When you look at it, you can’t see separate

parts.

– Also called solutions (can be liquid, gas or

solid solutions)

Examples: salt water, soda, coffee,

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Parts of a solution

Solute – the substance being dissolved.

Solvent – the substance that is doing the dissolving (usually present in a greater amount).

Solvent

Solute

The solute dissolves INTO

THE solvent to form a

solution.

Miscible – two

substances that will mix

and dissolve together in

any proportion

Immiscible – two

substances that are

insoluble in one another.

Water and oil are

_______________.

Factors that affect the rate of dissolution

Temperature

– Dissolve faster at higher temp

Agitation

– Dissolve faster when the mixture is shaken or stirred.

Surface Area

– Dissolve faster if you increase their surface area

(broken up into smaller pieces)

VS.

Solubility in Water :

The simple rule to remember is ‘like dissolves like’

+

-

+

Water is a polar solvent and will dissolve polar

molecules and substances that contain

charged particles.

Substances that dissolve in H2O are said to be soluble

• Ex: Sugar, ethanol (type of alcohol) which are polar,

most ionic compounds

sucrose

Site of polarity can

form H-bonding

When solid sodium

chloride (NaCl)

dissolves in water it

breaks up into its

individual ions.

Water is polar, the different ends are attracted to the charged ions.

The positive and negative ions become surrounded by solvent-

solvation.

-

+

+

Cl-

Na+

Electrolytic Solutions

Ionic Compounds dissociate in

aqueous solutions and create

positive and negative ions in

solution. Their ability to move

nearly independently through the

solution permits them to carry

positive or negative electrical

charges from one place to

another. Hence the solution

conducts an electrical current

and are called electrolytes.

Electrolytic Solvation

Strong

Electrolyte

Non-

Electrolyte

solute exists as

ions only

- +

salt

- +

sugar

solute exists as

molecules

only

- +

acetic acid

Weak

Electrolyte

solute exists as

ions and

molecules

Electrolytic Solvation

Strong

Electrolyte

Non-

Electrolyte

ionic

compounds/

strong acids

and bases

- +

salt

- +

sugar

covalent

compounds

- +

acetic acid

Weak

Electrolyte

weak acids and

bases

Solution 1 Solution 2 Solution 3 Solution 4 Solution 5

Which beakers contain an electrolytic solution:

KCl

Solution

HNO3Solution

Sugar

Solution

Acetic

Solution

Ammonia

Solution

Substances that don’t dissolve are called insoluble

E.g. Petroleum (crude oil), which are non-polar

So if you want to dissolve grease which is non-polar, you

need to use a non-polar solvent.

Petroleum in a non-polar organic molecule

How is solubility of a compound denoted in a

chemical equation?

, yields

How do we know if

the products of a

chemical reaction

are (aq), a solution

or (s), a

suspension or a

precipitate?

AB + CD AD + CBWhich of the products produced will be soluble?

Double Replacement

Reactions

• potassium iodide + lead (II) nitrate

• barium nitrate + calcium carbonate

Saturation

MAXIMUM Amount of solute dissolved

Saturation – The

point when no more

solute can dissolve

into a solvent at a

given temperature ( a

dynamic equilibrium

exists between the

solution and

undissolved solute).

Unsaturated Solution

An unsaturated

solution is like a

sponge that can hold

more water.

More solute can be

dissolved at that

temperature.

I’m still

thirsty!

Supersaturated Solution

Can be prepared by

changing the conditions

of a saturated solution

(temperature, volume,

or pressure).

A supersaturated solution is a solution that contains more of the solute than it can normally hold at a given temperature.

Solubility

SATURATED

SOLUTION

no more solute

dissolves

UNSATURATED

SOLUTION

more solute

dissolves

SUPERSATURATED

SOLUTION

becomes unstable,

crystals form

concentration

SOLVATION & SOLUBILITY

CURVES

Solubility

Solubility Curve

– shows the

dependence of

solubility on

temperature

Solubility Curve

Most solubility curves

show the solubility of

more than one solute.

These graphs show

comparisons.

Which solute is most

soluble at 100˚C?

KNO3 is most soluble at

100 degrees.

Practice:

What is the solubility of

KBr at 80ºC?

Which salt is the most

soluble at 50ºC?

At which temperature

can you dissolve 160 g

of KNO3 in 100 g of

water?

Unsaturated, Saturated, and Supersaturated

solution

Solutions that are described by the line are always

SATURATED SOLUTIONS.Solutions that are above the line are always

SUPERSATURATED SOLUTIONS.

Solutions that are below the line are always

UNSATURATED SOLUTIONS.

Practice:

How many grams of

sodium chloride can be

dissolved in 100 g of

water at 80ºC?

If 40 g of potassium

bromide is dissolved in

100 g of water at 80ºC, is

the solution saturated,

unsaturated, or

supersaturated?

Solubility Curve What if we change the amount of water?

In order to reach saturation, if we double the

amount of water that is being used to prepare

the solution, we would need to also double the

solute.

20 g / 100g of water at 25 °C

40 g / 200g of water at 25 °C

Example:

How many grams of

KBr would be needed to

saturate 50g of water at

90°C?

It takes 100g of KBr to

saturate 100g of water,

so it would take 50g of KBr

to saturate 50g of water.

What do you notice

about the

relationship between

temperature and

solubility of solids

and gases in water?

Generally, as temperature

increases, the solubility of solids

increase.

As the temperature

increases, the

solubility of a gas

decreases.

Why?

Solubility and Temperature

Increased temperature causes an increase in

kinetic energy.

The higher kinetic energy causes more motion in

molecules which break intermolecular bonds and

escape from solution.

The solubility of gases decreases as the

temperature increases.

• Ex: room temp soda tastes more “flat” than cold

soda

Solubility of gases and

temperature

Gas pressure and

solubility

Gases increase in

solubility as pressure

increases (Henry’s law)

The gas molecules are

"forced" into the

solution since this will

best relieve the pressure

http://www.youtube.com/watch?v=8yU5y-cFXoo

Problems

A solution is prepared

with 10g of NaCl at

120°C. What type of

solution is it?

A. Unsaturated

B. Saturated

C. Supersaturated

D. Heterogeneous

Problems

How many grams of

KBr are needed to

saturate 100g of water

at 90°C?

A. 80g

B. 90g

C. 100g

D. 110g

Problems

How many grams of

KBr are needed to

saturate 200g of water

at 90°C?

A. 160g

B. 180g

C. 200g

D. 220g

CONCETRATIONS &

DILUTIONS

Concentration

Measure of how much solute is dissolved in solution, or “strength”

of solution.

– Concentrated: A solution with a relatively high

concentration of solute

• strong coffee

– Dilute: very little solute in the solution

• weak coffee

These statements are vague so lets get more specific…..

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Concentration

Molarity(M) – is used by chemist to

describe concentrations. It’s a measure

of how many moles of solute are present

for each liter of solution.

A 9M solutions is more concentrated

than a 3M solutions.

solution of L

solute of moles(M)molarity

• Suppose you have 2.00 moles of

sugar and you mix it with enough

water to make 1.00 liter of solution-

what is the molarity of the solution?

• 2M or “two molar”

Practice

• Find the molarity of a solution containing 75 g

of MgCl2 in 250 mL of water.

L

molM Given:

Mass = 75g

Volume = 250 mL

Unknown:

M=?

Practice

• How many grams of NaCl are required to make

a 1.54M solution using 0.500 L of water?

L

molM Given:

Molarity= 1.54 M

Volume = 0.500L

Unknown:

mass=?

Other ways to express

concentration…• Molality (m)

– Concentration calculated by dividing the

moles of solute by the kilograms of solvent

(usually water) used to dissolve.

m=mols solute/kg solvent

Suppose you dissolve 18.2 grams of sucrose

(C12H22O11) into 200. grams of water. What is

the molality of the solution?

Other ways to express

concentration…• Mole fraction (X)

X= moles of component/total moles of all components

• You mix 1.7 moles of sodium chloride with 6.7 moles of

water. What is the mole fraction of the sodium chloride?

Other ways to express

concentration…• Percent (%) mass

% mass = grams of component/total mass of mixture

• 16 grams of NaBr are mixed with 32 grams of

water. What is the % mass of sodium bromide?

2211 VMVM

Dilution

Preparation of a desired solution by adding

water to a concentrate.

Moles of solute remain the same.

What volume of 15.8M HNO3 is required to

make 250 mL of a 6.0M solution?

Practice

How much 0.05 M

HCl solution can

be made by

diluting 250 mL of

10 M HCl?

How much water

would I need to

add to 500 mL of

a 2.4 M KCl

solution to make

a 1.0 M solution?