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Properties of Water• Polar molecule• Forms hydrogen bonds• High Cohesion and

surface tension • Density – greatest at 4oC• Universal solvent • Forms hydrates

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Polarity of Water

• 2 H2 + O2 2H2O–O is more electronegative– The two H atoms have a partial

positive charge.

• A water molecule is a polar molecule – negative at the oxygen end and positive at the hydrogen ends

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Water has unusual properties because of its polarity which allows it to form a hydrogen bond.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 3.1

Polarity of Water

Hydrogen bonding is the attraction of a slightly positive hydrogen in one water molecule to a slightly negative oxygen in another water molecule

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HYDROGEN BONDS• Hold water molecules

together• Each water molecule can

form a maximum of 4 hydrogen bonds

• The hydrogen bonds joining water molecules are weak, about 1/20th as strong as covalent bonds.

• They form, break, and reform with great frequency

• Extraordinary Properties that are a result of hydrogen bonds.– Cohesive behavior– Resists changes in temperature– High heat of vaporization– Expands when it freezes– Versatile solvent

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• Surface tension, a measure of the force necessary to stretch or break the surface of a liquid, is related to cohesion.– Water - greater surface tension than most other

liquids because of hydrogen bonds– Water behaves as if

covered by an invisible film.

– Some animals can stand, walk, or run on water without breaking the surface.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 3.36

Surface Tension

• Surfactant – a substance that interferes with the hydrogen bonding between water molecules. “It makes water wetter.”

• ex. soaps and detergents 

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Solid water . . . Ice, Ice, Baby!• Due to hydrogen

bonding, when water solidifies it expands into a hexagonal lattice work and becomes less dense. It forms a “honeycomb”. This is why snowflakes have six sides.

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Density of Water• Ice (solid) has a lower

density than water (liquid). That’s why ice cubes float.

The density of water:

1. Prevents water from freezing from the bottom up.

2. Ice forms on the surface first—the freezing of the water releases heat to the water below creating insulation.

3. Makes transition between season less abrupt.9

• Aqueous solution - water that contains dissolves substances

• Solvent – in a solution, the dissolving medium

• Solute – in a solution, the dissolved particles

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“LIKE DISSOLVES LIKE”

• Substances that dissolve easily into water are ionic compounds and polar covalent molecules. – think salt water

• Nonpolar compounds (oil, grease and gasoline) do not dissolve in water.

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HYDRATES

• A compound that contains water of hydration (crystalline form).

Copper (II) sulfate pentahydrateCuSO4 · 5 H2O

Cobalt (II) chloride hexahydrateCoCl2 · 6 H2O

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HYDRATES

When heated, they lose their water of hydration and become anhydrous.

CuSO4 · 5 H2O → CuSO4 (s) + 5 H2O (g)

CoCl2 · 6 H2O → CoCl2 (s) + 6 H2O (g)

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% Water in a Hydrate

Sodium carbonate decahydrateNa2CO3 · 10 H2O

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Try these:

CuSO4 · 5 H2O

CoCl2 · 6 H2O

36.1%

45.4%

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