Chapter 3: Chemistry of Life

Water

Properties of Water• There are 4 main properties of

water that make it an important molecule which allows life to exist.– 1. Cohesive behavior.– 2. Water’s ability to moderate

temperature.– 3. Expansion upon freezing.– 4. Water is a good solvent.

1. Cohesion• Cohesion is basically the

phenomenon of the hydrogen bonds working together.

• This leads to the idea of surface tension.

1. Cohesion--Surface Tension• Surface tension is the thing you

observe when you see a water bug walking on water or the beads of water forming on a freshly waxed car.

• The surface tension is created by the hydrogen bonds that form between the water molecules.

2. Moderation of Temperature• Water has a very high specific heat

capacity meaning that it takes a lot of energy to raise and lower its temperature.

• This high specific heat is due to hydrogen bonding.

• In order to break the bonds, heat must be absorbed and in order for them to form, heat must be released.

2. Moderation of Temperature• Thus, adding heat to the water does

little more than disrupt H-bonds, it is only when you add a lot of heat do you begin to see a change in temperature.

• When hydrogen bonds have been disrupted and the H2O molecules begin to move faster only then do we see a change in temperature.

2. Moderation of Temperature• These characteristics of water enable it

to moderate air temperatures in the regions near water.

• A large amount of heat can be absorbed with only a small change in temperature enabling life in the water to enjoy a relatively stable climate.

• Likewise, on cool nights “warm” water can give off a lot of energy without itself cooling much. This warms the nearby air again moderating the temperature.

3. Expansion Upon Freezing• One of the unique properties of water is

that it expands when it freezes. Most substance do not do this.

• It expands because as the water molecules lose energy, they align a certain way according to their hydrogen bonds.

• This creates a small pocket and makes the ice less dense than the liquid.

3. Expansion Upon Freezing• When the ice freezes at the top of the body of

water it acts to insulate the remaining water preventing it from freezing solid.

• Because it is less dense and floats, it prevents the body of water from freezing from the bottom up. If this were the case, only a small portion of the ice would ever thaw making life impossible.

4. Versatile Solvent• Water is a very versatile solvent.

Though not universal water has an amazing ability to dissolve many things that are important to our existence.

• Much of water’s ability to dissolve things is due to its polarity. The polar regions of the water molecule interact with the positively and negatively charged regions of the molecules with which they are interacting.

pH• pH of pure water has a

pH of 7. That is, H+ and OH- are present in equal amounts.

• In substances that are acids, H+ ion concentration is high and pH is said to be low. Likewise, OH- concentration is low.

• In basic solutions, OH- concentration is high, and H+ concentration is low, pH is said to be high.

Bases• Some substances are called bases

because they produce OH-. • Others are called bases because

they accept H+.• NaOH --> Na+ + OH- (produces

OH-)• NH3 --> NH4

+ (accepts H+)

Acids• Donate H+ ions to solution.• Some accept OH- ions from

solution.

Buffers• Buffers are important because

they minimize changes in pH by preventing wild swings in H+ and OH- concentration.