CHAPTER 17

WATER AND AQUEOUS SOLUTIONS

Objectives:

• 1. Describe the hydrogen bonding that occurs in water.

• 2. Explain the high surface tension and low vapor pressure of water in terms of

hydrogen bonding.

Water and its properties

Water molecule

1. triatomic H2O

2. O-H polar bond

3. polar molecule

Surface properties

surface tension

a. an inward force that tends to minimize the surface area of a liquid

b. Gives the spherical shape

c. Gravity helps flatten the shape

d. higher the surface tension the more spherical shape

Surfactant a. Wetting agent (soap or detergent)

b. Reduces surface tension (flattens shape)

c. Soap interferes with hydrogen bonds

• Vapor pressure

a. Caused by molecules that escape the surface of the water and enter the gas phase

b. Hydrogen bonds hold molecules together escape is low

c. Gives water unusually low vapor pressure

• Specific heat capacity

a. The quantity of heat, in joules or calories, required to raise the temperature

of 1 g of a substance 1 °C

b. Water’s = 4.184 J/g°C (helps moderate daily air temp.)

Evaporation and Condensation

1. heat of vaporization (evaporation)

a. amount of energy needed to convert 1 g of a substance from a liquid to gas

b. Water takes 2.26 kJ of energy to convert 1 g of liquid to 1 g of steam at 100 °C

c. Absorbs high amount of heat because of hydrogen

bonds

2. heat of condensation

a. Water needs 2.26 kJ of heat to turn from a gas to a liquid

b. Opposite of evaporation

c. You get a worse burn from steam

d. Water has a high boiling point due to hydrogen bonds

Ice

1. Expands as liquid turns to a solid

2. Density a. 4 °C water is most denses

b. Below 4 °C the density decreases

c. Lower density causes ice to float

d. Why does water act differently?

1. shape

2. Molecules have empty space 3.

4. Ice melts at 0oC (happens at the same time as freezing)

5. Heat absorbed when changing from a solid to a liquid is 334 J/g

6. pond

Objectives: By the end of class you will be able to:

• 4. Explain the significance of the statement “like dissolves like.”

• 5. Distinguish among strong electrolytes, weak electrolytes, and

nonelectrolytes.

Aqueous Solutions 1. Substances dissolved in water

2. Two parts

a. Solvent – the part that causes the dissolving

b. Solute – the part that dissolves

3. substances that dissolve in water readily are ionic and polar

4. The Process

Solvation

a. The process that occurs when a solute dissolves

b. Water molecules are in constant motion

c. The solvent (H2O) attract the solute

(Na+1 Cl-1)

d.

e. Some ionic compounds are insoluble

5. Electrolytes

a. Produce an electrical current in (aq) solutions

b. Ionic compounds

c. Some solids will when melted (molten state)

d. Dissociation: separating ions

examples: NaCl Na+ + Cl- (2 ions0

BaCl2 Ba+2 + 2Cl- (3 ions)

6.Nonelectrolytes Do not conduct an electric current

7. There are weak electrolytes and strong electrolytes

(see page 485) weak electrolytes dissociate slightly

8. Water of hydration

a. a crystal containing water

b. Called a hydrate

c. formula CuSO4 5H2O

d. reaction

CuSO4*5H2O(s) CuSO4(s) + 5H2O(l)

e. Effloresce 1. hydrate losing water

2. Has a higher vapor pressure than water

3. becomes coated with white powder

f. hygroscopic

1. lower vapor pressure than water

2. Remove water from the air

3. Solids will become slightly wet

4. Used as drying agents

g. desiccants

remove lots of water from the air and form a solution

h. % water calculations

percent H2O = mass of water X 100

mass of hydrate

Calculate the percent by mass of water in washing soda, or sodium carbonate decahydrate (Na2CO3*10H2O).

Objectives: By the end of class you will be able to:

• 6. Explain how colloids and suspensions differ from solutions.

• 7. Describe the Tyndall effect.

Heterogeneous Mixtures

1. Suspensions

a. Mixtures that settle out

b. Muddy water, Italian dressing

2. Colloids

a. Fall in between a suspension and solution

b. Dispersed phase

c. Gelatin, paint, smoke

d. appear cloudy or milky when

concentrated

e. look clear when dilute

f. Tyndall effect - scattering of visible light in all directions

g. colloid particles reflect light

h. colloid particles also absorb ions

i. Brownian motion: the chaotic movement of colloidal particles

3. emulsions

1. colloidal dispersions of liquids in liquids

2. example: oil + soap