1 the nature of molecules chapter 3. 2 water chemistry water consists of an oxygen atom bound to two...

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The Nature of Molecules

Chapter 3

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Water Chemistry

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

– stable atom with no electrical charge most outstanding property is its ability to

form weak chemical associations

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Hydrogen Bonding

• In a water molecule, both the oxygen and hydrogen atoms attract the shared electrons in the covalent bond

– Oxygen atom is more negative than the hydrogen atoms.

distinct ends with partial charge (polar)hydrogen bonds

4

Water Chemistry

• Water clings to polar molecules.– Cohesion refers to attraction

to other water molecules. responsible for surface

tension: water strider leaves can pull water up

the stem Seeds swell to germinate

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–Adhesion refers to attraction to other substances.

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

Capillary action on glass

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Water Chemistry

•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( sugar/salt)

–Water tends to exclude nonpolar molecules.

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WatermoleculesSalt

crystal

Na+

Cl–

Cl–

Na+

Na+

Na+

Cl–

Cl–

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Hydrophobic - repelled by water-form their own layer

Water and oil do not mix !

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pH

• pH scale expresses hydrogen ion (H+) concentration in a solution.

– logarithmic scale ranging from 0-14 neutral = 7

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pH

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

– pH values lower than 7

• Bases combine with H+ ions when dissolved in water, thus decreasing H+ concentration.

– pH values above 7

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pH

Neutral

MoreH ion

LessH ion OH

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How Acid and bases affect the homeostasis of our bodies

CO 2 is acidic: high levels in blood will send message back to brain to breathe faster: remove CO2 and return to normal pH

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