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• Water consists of an oxygen atom bound to two hydrogen atoms by two single covalent bonds.

– Oxygen has unpaired & paired electrons which gives it a slightly negative charge while Hydrogen has no unpaired electrons and shares all others with Oxygen

– Leaves molecule with positively and negative charged ends

Water is a Polar Molecule-has oppositely charged

ends

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slightly positive charge

slightly negative charge

hydrogen bond between (+) and (-) areas of different water molecules

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Water’s Properties• Cohesion• Adhesion• Capillarity• High Specific Heat• High Heat of Vaporization• Solid water (ice) is less dense than liquid• Solvent• Transparent

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– Cohesion refers to attraction to other water molecules.

responsible for surface tension

a measure of the force necessary to stretch or break the surface of a liquid

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– Adhesion refers to attraction to other substances.

Water is adhesive to any substance with which it can form hydrogen bonds.

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water evaporates from leaves = transpiration

adhesion, cohesion and

capillary action

All thanks

to hydrogen

bonding!

water taken up by roots

Capillary action

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•Trees have specialized structures to transport water: xylem and phloem “plumbing”

• Water molecules are “dragged” from the roots to the top of the tree by capillary action and cohesion: hydrogen bonds help water molecules to each other

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High Specific Heat

– High specific heat Amount of heat that must be

absorbed or expended to change the temperature of 1g of a substance 1o C.

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• A large body of water can absorb a large amount of heat from the sun in daytime and during the summer, while warming only a few degrees.

• At night and during the winter, the warm water will warm cooler air.

• Ocean temperatures and coastal land areas have more stable temperatures than inland areas.

• The water that dominates the composition of biological organisms moderates changes in temperature better than if composed of a liquid with a lower specific heat.

Impact of water’s high specific heat

The Earth is over 75% water!

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– High heat of vaporization Amount of energy required to change

1g of liquid water into a gas (586 calories).

large number of hydrogen bonds broken when heat energy is applied

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• As a liquid evaporates, the surface of the liquid that remains behind cools - Evaporative cooling.

• Evaporative cooling moderates temperature in lakes and ponds and prevents terrestrial organisms from overheating.

• Evaporation of water from the leaves of plants or the skin of animals removes excess heat.

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“Universal” Solvent

• A liquid that is a completely homogeneous mixture of two or more substances is called a solution.

– A sugar cube in a glass of water will eventually dissolve to form a uniform mixture of sugar and water.

water is the solvent and sugar the solute.

• In an aqueous solution, water is the solvent.• Water is not really a universal solvent, but it is

very versatile because of the polarity of water molecules.

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• Water is an effective solvent as it can form hydrogen bonds.

– Water clings to polar molecules causing them to be soluble in water.

Hydrophilic - attracted to water

– Water tends to exclude nonpolar molecules.

Hydrophobic - repelled by water

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• Water transports molecules dissolved in it– Blood, a water-based solution, transports molecules

of nutrients and wastes organisms– Nutrients dissolved in water get transported through

plants– Unicellular organisms that live in water absorb

needed dissolved substances

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Solid water (ice) is less dense than liquid

same mass but a larger volume

• Ice is less dense than water: the molecules are spread out to their maximum distance

Density = mass/volume

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water expands as it solidifies

Water reaches maximum density at 4-degrees C

Water freezes from the top down

Oceans and lakes don’t freeze solid because ice floats

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Water is Transparent• The fact that water is clear allows light to pass

through it– Aquatic plants can receive sunlight– Light can pass through the eyeball to receptor

cells in the back

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pH• Water ionizes into H+ and OH-

• H2O H+ and OH-• pH scale expresses hydrogen ion (H+) concentration in

a solution.– logarithmic scale ranging from 0-14

neutral = 7 Below 7 = acid Above 7 = base Water at 25oC contains 1/10,000,000 mole of H+

ions = 10 -7 moles/liter pH = -log [H+]

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Acids

• Acids dissociate in water to increase the concentration of H+.

– Have many H+ ions– Sour taste– HCl is hydrochloric acid or stomach

acid

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Bases• Bases combine with H+ ions when

dissolved in water, thus decreasing H+ concentration.

– Have many OH- (hydroxide) ions– Bitter taste– NaOH = sodium hydroxide or baking

soda