1. KEY CONCEPT Water’s unique properties allow life to exist on Earth.

2. Life depends on hydrogen bonds in water.
Water is a polar molecule.
Polar molecules have slightly charged regions, or 2 “poles,” O H _ +
These charged regions result in an unequal sharing of electrons leading to a positive end and a negative end, kind of like the ends of a magnet.
This gives water a “Mickey Mouse” appearance

3. Hydrogen bonding between Water molecules
Hydrogen bonds form between slightly positive hydrogen atoms and slightly negative atoms.
The bonds between Oxygen and Hydrogen are “covalent” meaning the electrons are “shared” between the atoms.

4. PHASES AND PHASE CHANGES (of water)

5. PHASE CHANGES (cont’d)
Liquid water becomes more dense at is cools to 4*C (almost freezing)
But ice is less dense than liquid water!
This is why ice floats
Fish in certain habitats can survive in winter because ice floats—they continue to live and function in the water beneath the ice because the water below holds a certain amount of heat in (specific heat, we will discuss in a few minutes)
See your textbook page 162 Figure 20 after presentation for more details

6. Hydrogen bonds are responsible for 4 important properties of water.
high specific heat- water can hold large amounts of heat before the temperature begins to rise
Cohesion – water is attracted to itself
Adhesion - water is attracted to other substances
Capillary Action – is the result of adhesion.
adhesion
cohesion

7. Some other ways Cohesion and Adhesion are important in Biology
Capillary action is the result of adhesion. Water travels up the stem of a plan, and seeds swell and germinate by capillary action.

8. SPECIFIC HEAT CAPACITY
Water has the highest specific heat capacity, which means the earth’s oceans and water systems can hold heat and maintain the planet’s temperatures within a moderate range capable of supporting life.
Specific heat capacity is the amount of heat required to raise the temperature of a certain amount of substance.

9. MIXTURES WITH WATER: Solutions
A solution is formed when one substance dissolves in another.
Solvents dissolve other substances.
Solutes dissolve in a solvent. Water is the universal solvent!
A solution is a homogeneous mixture meaning they are mixtures of 2 or more substances that cannot be separated.
solution

10. More mixtures: Heterogeneous mixtures: Suspensions
Suspensions are mixtures with undissolved particles.
Since they are not evenly mixed, they are called “heterogeneous” or “different phases”

11. Some solutions form acids or bases
Some solutions form acids or bases. pH is a measure of Hydrogen ions in a solution
An acid releases a hydrogen ion when it dissolves in water.
high H+ concentration
pH less than 7
A base removes hydrogen ions from a solution
Low H+ concentration
pH greater than 7
Pure Water is neutral at pH of 7

13. pH and Buffers
The majority of biological processes in the cells in your body work best at a pH between 6.5 and 7.5.
To maintain homeostasis, it is important to control H+ levels.
Buffers can do this. For example, antacids help neutralize stomach acid when you have an upset stomach. The antacids have a pH higher (more basic) than the stomach acid so it “balances out” the pH
Another example is the pH of blood which is around 7.4
The blood has buffers in it that help maintain that pH.

14. Review
How do polar molecules form hydrogen bonds?

15. How do acids and bases differ?

16. Describe an example of cohesion and adhesion that you might observe during your daily life.

17. Give an example of a solution.
Give an example of a suspension (heterogeneous mixture)