Chapter 12

Water

Water Molecules Water’s bent shape and ability to

hydrogen bond gives water many special properties.

Water molecules are attracted to one another.

This gives water high surface tension, low vapor pressure, high specific heat, high heat of vaporization, and high boiling point

Surface Tension liquid water acts like it has a skin. Water forms round drops. All because water hydrogen bonds.

Surface Tension One water

molecule H bonds to another.

Can H bond to molecules all around.

H HO

+

+

-

H HO

+

-

Surface Tension A water molecule in

the middle of solution is pulled in all directions.

Surface Tension Not true at the

surface. Only pulled down and

to each side. Holds the molecules

together. Causes surface

tension.

Surface Tension Water drops are

round because all the molecules on the edge are pulled to the middle.

Surface Tension Glass has polar

molecules. Glass can

hydrogen bond. Attracts the water

molecules. Some of the pull

is up.

Meniscus Water curves up

along the side. This makes the

meniscus.

MeniscusIn Glass

In Plastic

Heat Capacity Water has a high heat capacity (same

as specific heat). It absorbs 1 cal/gºC while iron absorbs

only 0.107 cal/gºC. Remember SH = heat

Mass x T Calculate the heat need to raise the

temperature of both iron and water by 75ºC.

Heat of vaporization Because of the strong hydrogen bonds

it takes a large amount of energy to change water from a liquid to a solid.

540 cal/g is the heat of vaporization. It takes this much energy to boil water. You get this much energy back when it

condenses. Steam burns, but heats things well.

Ice Most liquids contract (get smaller) as

they are cooled. They get more dense. When they change to solid they are

more dense than the liquid. Solid metals sink in liquid metal. Ice floats in water. Why?

Ice Water becomes more dense as it cools

until it reaches 4ºC. Then it becomes less dense. As the molecules slow down they

arrange themselves into honeycomb shaped crystals.

These are held together by H-bonds.

HHO

H

HO

H

H

O

HH

O

H

H O HH

O

H

HO

HHOH

H

O

H

HO H H

O

HH

O

Liquid

Solid

Ice 10% less dense than water. Water freezes from the top down. It takes a great deal of energy to turn

solid water to liquid water. Heat of fusion is 80 cal/g.

Aqueous Solutions Solution - a homogenous mixture mixed

molecule by molecule. Solvent - the stuff that does the

dissolving. Solute -the stuff that is dissolved. Aqueous solution- a solution with water

as the solvent.

Aqueous Solutions Water dissolves ionic solids and polar

covalent solids best. The rule is “like dissolves like” Polar dissolves polar. Nonpolar dissolves nonpolar. Oil is non polar. Oil and water don’t mix. Salt is ionic- salt water.

How Ionic solids dissolve Called solvation. Water breaks the + and - charged

pieces apart and surround them.

How Ionic solids dissolve

H HOH

H OH

HO

H HO

HHO

HH

O

HH

OH

H

O

HH

O

Solids will dissolve if the attractive force of the water molecules is stronger than the attractive force of the crystal.

If not the solids are insoluble. Water doesn’t dissolve nonpolar

molecules because it the water molecules can’t hold onto them.

The water molecule hold onto each other and separate from the polar molecules.

Hydration Water molecules chemically bonded to

solid salt molecules. These compounds have fixed amounts

of water. The water can be driven off by heating.

CuSO4.5H2O CuSO4 + 5H2O

Called copper(II)sulfate pentahydrate.- heat

+ heat

Electrolytes Substances that conduct electricity

when dissolved in water. Must have charged particles that can

move. Ionic compounds break into charged

ions. NaCl Na+ + Cl- These ions can conduct electricity.

Non electrolytes do not conduct electricity when dissolved in water.

Polar covalent molecules such as methanol CH3OH don’t fall apart into

ions when they dissolve. Weak electrolytes don’t fall completely

apart into ions. Strong electrolytes do ionizes

completely.

Mixtures that are NOT Solutions

Suspensions are mixtures that slowly settle upon standing.

Particles of a suspension are more than 100 times bigger.

Can be separated by filtering. Colloids particles are between the size of

a solid and that of a liquid. Emulsions are colloids of liquids in liquids.