Chemistry of Life

Properties of Water Chapter 3

Pre Assessment

1. What causes water to travel up the roots of a plant?

2. What allows bugs to walk on top of water?3. Why does ice float in water if they are both

the same compounds?4. Draw a pH scale and label-acids, bases, and

neutral substances. 5. Why is water known as the universal solvent?

Pre Assessment Answers

WaterWhy are we studying water?Why are we studying water?

All life occurs in water inside & outside the cell

All life occurs in water inside & outside the cell

Makes up 70-90% of all living things.75% of the earth’s surface is covered with water

Makes up 70-90% of all living things.75% of the earth’s surface is covered with water

Figure 3.x1 Water

Chemistry of water• H2O molecules form H-bonds

with each other– +H attracted to –O– creates a

sticky molecule

Water Molecules

• Hydrogen bonds~ relatively weak bonds last only 1/billionth of a second in liquid water, but are constantly broken and reformed.

• Polar —shared electrons of the covalent bonds spend more time around the oxygen atom than they do around the hydrogen nuclei.

• water animation

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!

1. Cohesion & Adhesion

Walking on water

• Cohesion– H bonding between H2O molecules– water is “sticky”

• surface tension• drinking straw

• Adhesion– H bonding between H2O & other substances

• capillary action• meniscus• water climbs up

paper towel or cloth

Water molecules are cohesive and adhesive.

Cohesion— water molecules cling to other water molecules because of hydrogen bonding.

Adhesion— water molecules adhere to polar surfaces because they have positive and negative poles (polarity).

How does H2O get to top of trees?Transpiration is built on cohesion & adhesion

That’s cool!!

2. Water is the solvent of life• Polarity makes H2O a good solvent

– polar H2O molecules surround + & – ions– Solvents dissolve solutes creating solutions

What dissolves in water?• Hydrophilic

– substances have attraction to H2O– Philic=affinity – polar or non-polar?

Figure 3.7 A crystal of table salt dissolving in water

Figure 3.8 A water-soluble protein

What doesn’t dissolve in water?• Hydrophobic

– substances that have an attraction to H2O

– Phobic= fear– polar or non-polar?

fat (triglycerol)

Oh, lookhydrocarbons!

3. The special case of ice• Most (all?) substances are more dense when

they are solid, butnot water…

• Ice floats!– H bonds form a crystal

And this hasmade all the difference!

Ice floats

Figure 3.5x1 Ice, water, and steam

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

4. Specific heat• H2O resists changes in temperature

– high specific heat – takes a lot to heat it up– takes a lot to cool it down

• H2O moderates temperatures on Earth

Water has a high heat capacity.

specific heat—the amount of heat a given amount of a substance requires for an increase in temperature.Examples: water = 1 ethyl alcohol = 0.6

sucrose = 0.3 liquid ammonia = 1.23calorie —the amount of heat that will raise the temperature of 1 g (1 mL or 1 cc) of water 1o Celsius.

5. Heat of vaporizationEvaporative cooling

Organisms rely on heat of vaporization to remove body heat

Water has a high heat of vaporization.

• It requires 540 calories of heat energy to convert 1 g of the hottest water to a gas.

– This is 60 X as much as for ether and 2X as much as for ammonia.

Why does water have this high heat of vaporization?Hydrogen bonds must be broken before water boils and water molecules vaporize.

Why does water have a high heat capacity?The many hydrogen bonds that link water molecules help it to absorb heat without a great change in temperature.

What are the consequences of this property to living things?Because the temperature of water rises and falls slowly, organisms are better able to maintain their normal internal temperatures and are protected from rapid temperature changes (Homeostasis)

Evaporative cooling

I feel the evaporative cooling

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–H2O H+ + OH–

pH Scale10–1

H+ IonConcentration

Examples of Solutions

Stomach acid, Lemon juice1

pH100 Hydrochloric acid0

10–2 2

10–3 Vinegar, cola, beer3

10–4 Tomatoes4

10–5 Black coffee, Rainwater5

10–6 Urine, Saliva6

10–7 Pure water, Blood7

10–8 Seawater8

10–9 Baking soda9

10–10 Great Salt Lake10

10–11 Household ammonia11

10–12 Household bleach12

10–13 Oven cleaner13

10–14 Sodium hydroxide14

tenfold changein H+ ions

pH1 pH210-1 10-2

10 times less H+

pH8 pH710-8 10-7

10 times more H+

pH10 pH810-10 10-8

100 times more H+

Check Point

Acid –Base-or Neutral???1. pH of 8.42. A solution with more H+ ions the OH- ions3. A solution with an equal amount of H+ ions

and OH- ions4. A pH of 75. A base and an acid combined

Check Point

Acid –Base-or Neutral???

1001

2

3

45

6

7

89

3Amount of base added

Bufferingrange

4 52

pH

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

Solutions

• Solvent— the substance that does the dissolving

• Solute— the substance that is dissolved

• Solution— a homogeneous mixture that will not separate upon standing

Aqueous Solutions• Mole— the number of grams of a substance that equals

its molecular weight in daltons.Example: sucrose, C12H22O11

12 carbons = 14422 hydrogens =2211 oxygens = 176

One mole is equal to 342 grams. To make a 0.5 M solution, add water to 171 grams to make 1 Liter of solution.

He’s gonnaearn a Darin Award!

AnyQuestions?

Post Assesment

1. Name a benefit of cohesion and adhesion to animals and plants.

2. Name a solution-identify the solute and the solvent.

3. Name an example of a neutral substance in our bodies.

4. Name an a solution in our bodies that is acidic? How does it get neutralized?

5. How many times more acidic is a solution with a pH of 1 than a solution with a pH of 2?

Post Assessment Answers