Unit 8: SolutionsUnit 8: Solutions

Mathematicians have Mathematicians have ProblemsProblems,,

But Chemists have But Chemists have SolutionsSolutions

WaterWater

Water is a Water is a polar polar molecule. molecule. This means the This means the oxygenoxygen side is side is negativelynegatively

charged and the charged and the hydrogenhydrogen side is side is positivelypositively charged.charged.

This charge allows water molecules to be This charge allows water molecules to be attracted to other water molecules through attracted to other water molecules through intermolecularintermolecular forces. forces.

HH

O

The forces that hold water molecules together The forces that hold water molecules together are called are called hydrogenhydrogen bonding. bonding.

Because of water’s strong hydrogen bonding, it Because of water’s strong hydrogen bonding, it is a liquid at normal temperatures even though is is a liquid at normal temperatures even though is has such a low has such a low molecular weightmolecular weight..

It is also the reason it has such an unusually It is also the reason it has such an unusually highhigh boiling point (100 boiling point (100ooC) and high C) and high specific specific heat capacityheat capacity. .

(this means it takes a lot of energy (4.18 J/g (this means it takes a lot of energy (4.18 J/g ooC )to raise C )to raise the temperature of liquid water!)the temperature of liquid water!)

Physical Properties of WaterSurface Tension—The resistance of a liquid to increase

in surface area The Result : Water beads as a drop instead of spreading

out --Cohesive forces are those holding the liquid

together to present the minimum of surface area.

Water has a very high surface tension due to strong hydrogen bonding!

Capillary Action—The spontaneous rising of liquid in a narrow tube due to two forces

Cohesive and Adhesive forces. Adhesive forces are those connecting a liquid

and its container. For Water- Adhesive forces are stronger than

the cohesive forces, the liquid is higher nearer the container and lower in the middle (the meniscus is concave.)

Solutions are mixturesSolutions are mixtures…… SOLUTIONS areSOLUTIONS are HOMOGENOUSHOMOGENOUS MIXTURES. MIXTURES.

They cannot be separated by filtration or settlingThey cannot be separated by filtration or settling..

Solutions have two components-Solutions have two components- 1. 1. SOLUTESOLUTE-- the solid that gets dissolved, or the the solid that gets dissolved, or the

liquid that is present in smaller amount.liquid that is present in smaller amount.

2. 2. SOLVENTSOLVENT-- the liquid that does the dissolving. the liquid that does the dissolving. It is always present in a larger amount.It is always present in a larger amount.

SolvationSolvation – The dissolving process where – The dissolving process where solutesolute particles are surrounded by particles are surrounded by solventsolvent molecules. molecules.

Example: Salt waterExample: Salt water What is the solute?___________What is the solute?___________

What is the solvent?___________What is the solvent?___________

Example – 70% ethanol solution.Example – 70% ethanol solution. Solute _______________Solute _______________

Solvent________________Solvent________________

Salt

water

Water

ethanol

Solubility Principle

Solutes will dissolve in solvents with similar polarity.

“Like Dissolves Like”

Polar substances will dissolve in Polar substances will dissolve in polarpolar solvents and solvents and nonpolar substances will only dissolve in nonpolar substances will only dissolve in nonpolar nonpolar substances.substances.

Water is considered a universal solvent because it can Water is considered a universal solvent because it can dissolve both dissolve both ionicionic and and polarpolar covalentcovalent compoundscompounds

Example – Alcohol will dissolve in water because both are polar molecules.

Like Dissolves LikeLike Dissolves Like

Q: Why does salad dressing separate into Q: Why does salad dressing separate into its oil and vinegar components? its oil and vinegar components?

The polarity of vinegar and oil are not the same.

Oil is non-polar and vinegar is polar.

Therefore, they do not mix.

Ionic CompoundsIonic Compounds

In an In an aqueous solutionaqueous solution, water is the solvent., water is the solvent.

HCl (aq), NaCl(aq), CuSOHCl (aq), NaCl(aq), CuSO44 (aq) (aq)

Ionic compounds are considered polar Ionic compounds will separate into its ionic

parts when dissolved in water.

NaCl + water

K2SO4 + water

Na+ + Cl-

2K+ + SO42-

SolvationSolvation Water dissolves ionic solutes through a process called Water dissolves ionic solutes through a process called

solvationsolvation..

Steps to solvationSteps to solvation:: 1. An ionic solute is added to water.1. An ionic solute is added to water.

2. Water molecules bombard the ionic compound until it 2. Water molecules bombard the ionic compound until it breaks into its ions.breaks into its ions.

3. The positive “H’s” in water molecules surround the 3. The positive “H’s” in water molecules surround the negativenegative solute ion. solute ion.

4. The negative “O” in water molecules surround the 4. The negative “O” in water molecules surround the positivepositive solute ion. solute ion.

Ions that are completely surrounded by water molecules Ions that are completely surrounded by water molecules form form hydration sphereshydration spheres and are said to be “hydrated”. and are said to be “hydrated”.

Solvation AnimationSolvation Animation

http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/molvie1.swf

Hydration Spheres

OHH

OH

H

OH

H

OH

H

OHH

OHH

Na+

Cl-

+

+

+

+ +

+