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Properties of Water �

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More about Water

Why are we studying water?

All life occurs in water

    • inside & outside the cell

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Chemistry of water

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The structure of water is the key to its special properties. Water is made up of one atom of oxygen and two atoms of hydrogen, bonded to form a molecule (H2O)

Water molecules are polar meaning one region of the molecule has a more positively charged side and a more negatively charged side.

​

H2O molecules form H-bonds �with each other

    • +H attracted to –O
    • creates a �sticky molecule
    • Each water molecule can form a maximum of

four hydrogen bonds at a time.

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Elixir of Life

  • Special properties of water

1. cohesion & adhesion

      • surface tension, capillary action

2. good solvent

      • many molecules dissolve in H2O
      • hydrophilic vs. hydrophobic

3. lower density as a solid

      • ice floats!

4. high specific heat

      • water stores heat

5. high heat of vaporization

      • heats & cools slowly

Ice! �I could use �more ice!

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1. Cohesion & Adhesion

  • Cohesion (Linking of Like)
    • H bonding between H2O molecules
    • water is “sticky”
      • surface tension
      • drinking straw
  • Adhesion (Linking unlike)
    • H bonding between H2O & other substances
      • capillary action
      • meniscus
      • water climbs up�paper towel or cloth

Try that � with flour…�or sugar…

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How does H2O get to top of trees?

  • The movement of water molecules up the very tiny xylem tubes and their evaporation from the stomata in plants. The water molecules cling to each other by cohesion and to the walls of the xylem tubes by adhesion.

Let’s go to the �videotape!

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2. Water is the solvent of life

  • Solvent: The substance something is dissolved in
  • Solute: The substance being dissolved
  • Solution: The solvent mixed with the solute

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What dissolves in water?

  • Hydrophilic: water-soluble. Examples: solutions which dissolve in water. Example: many salts, sugars, polar molecules, some proteins.
  • Hydrophobic: water repelling. Example: lipids, oils which are nonpolar.

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What doesn’t dissolve in water?

  • Hydrophobic
    • substances that don’t have �an attraction to H2O
    • polar or non-polar?

fat (triglycerol)

Oh, look�hydrocarbons!

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3. The special case of ice

  • Most (all?) substances are more dense when they are solid, but

not water…

  • Ice floats!
    • H bonds form a crystal

And this has�made all the �difference!

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Ice floats

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Why is “ice floats” important?

  • Oceans & lakes don’t freeze solid
    • surface ice insulates water below
      • allowing life to survive the winter
    • if ice sank…
      • ponds, lakes & even oceans would freeze solid
      • in summer, only upper few inches would thaw
    • seasonal turnover of lakes
      • sinking cold H2O cycles nutrients in autumn

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4. Specific heat

  • H2O resists changes in temperature
    • high specific heat
    • takes a lot to heat it up (break H-bonds)
    • takes a lot to cool it down (form H-bonds)
  • H2O moderates temperatures on Earth

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Specific heat�& climate

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5. Heat of vaporization

Organisms rely on heat of vaporization to remove body heat

Evaporative cooling

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Ionization of water & pH

  • Water ionizes
    • H+ splits off from H2O, leaving OH–
      • if [H+] = [-OH], water is neutral
      • if [H+] > [-OH], water is acidic
      • if [H+] < [-OH], water is basic
  • pH scale
    • how acid or basic solution is
    • 1 → 7 → 14

H2O → H+ + OH–

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pH Scale

10–1

H+ Ion

Concentration

Examples of Solutions

Stomach acid, Lemon juice

1

pH

100

Hydrochloric acid

0

10–2

2

10–3

Vinegar, cola, beer

3

10–4

Tomatoes

4

10–5

Black coffee, Rainwater

5

10–6

Urine, Saliva

6

10–7

Pure water, Blood

7

10–8

Seawater

8

10–9

Baking soda

9

10–10

Great Salt Lake

10

10–11

Household ammonia

11

10–12

Household bleach

12

10–13

Oven cleaner

13

10–14

Sodium hydroxide

14

tenfold change�in H+ ions

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pH1 → pH2

10-1 → 10-2

10 times less H+

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pH8 → pH7

10-8 → 10-7

10 times more H+

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pH10 → pH8

10-10 → 10-8

100 times more H+

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Buffers & cellular regulation

  • pH of cells must be kept ~7
    • pH affects shape of molecules
    • shape of molecules affect function
    • pH affects cellular function
  • Control pH by buffers
    • reservoir of H+
      • donate H+ when �[H+] falls
      • absorb H+ when �[H+] rises

1

0

0

1

2

3

4

5

6

7

8

9

3

Amount of base added

Buffering

range

4

5

2

pH

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He’s gonna�earn a �Darwin Award!

Any�Questions?

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