Water

1
Chapter 12

• Water

2
Water Molecules

• Waters 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

3
Surface Tension

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

4
Surface Tension
d-

• One water molecule H bonds to another.
• Can H bond to molecules all around.

d
d
d-
d
5
Surface Tension

• A water molecule in the middle of solution is
  pulled in all directions.

6
Surface Tension

• Not true at the surface.
• Only pulled down and to each side.
• Holds the molecules together.
• Causes surface tension.

7
Surface Tension

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

8
Surface Tension

• Glass has polar molecules.
• Glass can hydrogen bond.
• Attracts the water molecules.
• Some of the pull is up.

9
Meniscus

• Water curves up along the side.
• This makes the meniscus.

10
Meniscus
In Plastic
In Glass
11
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 DT
• Calculate the heat need to raise the temperature
  of both iron and water by 75ºC.

12
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.

13
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?

14
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.

15
Liquid
Solid
16
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.

17
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.

18
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 dont mix.
• Salt is ionic- salt water.

19
How Ionic solids dissolve

• Called solvation.
• Water breaks the and - charged pieces apart and
  surround them.

20
How Ionic solids dissolve
21

• 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 doesnt dissolve nonpolar molecules because
  it the water molecules cant hold onto them.
• The water molecule hold onto each other and
  separate from the polar molecules.

22
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.

23
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.

24

• Non electrolytes do not conduct electricity when
  dissolved in water.
• Polar covalent molecules such as methanol CH3OH
  dont fall apart into ions when they dissolve.
• Weak electrolytes dont fall completely apart
  into ions.
• Strong electrolytes do ionizes completely.

25
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.