1. The Chemistry of Life
2.2 Properties of water

2. Bellwork question
Why is it important that biologists have a good understanding of chemistry?
Like all matter, living things are made up of elements and chemical compounds
Survival of living things depends on chemical reactions
Take place both within and outside their body
To understand living things, biologists need to have a good understanding of chemistry

3. Water is essential to life on Earth
It makes up 60% of the human body
Is known as the universal solvent
Presence of liquid water means that life might be present somewhere
Why is water so special?

4. The Water molecule
Found in a liquid state over most of the Earth’s surface
Neutral charge – 10 Protons and 10 electrons
Polarity of water molecule
A polar molecule is one where the charges are unevenly distributed

5. Hydrogen bonding Polar molecules can attract to each other
Attraction between partial +ve charge of H atom and partial –ve charge of another molecule is a hydrogen bond
Oxygen, nitrogen and fluorine are common partial negative charged particles
Hydrogen bonds are not strong
But they give water a number of unique characteristics
Explain why water expands upon freezing
Explains why water can dissolve so many substances

6. Cohesion
Cohesion is an attraction between molecules of the same substance
Water is very cohesive
One water molecule can have 4 hydrogen bonds at the same time
Results in water being molecules being drawn together
Explains why drops of water form beads on a smooth surface
Provides surface tension – allowing for some insects to walk on the a ponds surface

7. Adhesion The attraction between molecules of different substances
Water will stick to the side of glass tubes, or graduated cylinders
Results in a meniscus
Adhesion between water and glass is stronger than cohesion between water molecules
Water can also rise in narrow tubes against the force of gravity – capillary action.
One of the forces that draws water out of roots and into stems and leaves in plants
But, cohesion holds the column of water itself together as it rises

8. Heat Capacity
Heat Capacity – the amount of heat energy required to rise the temperature
The multiple hydrogen bonds means that it takes a large amount of heat energy to cause the molecules to move faster, raising the temperature of the water
Water has a relatively high heat capacity
Large bodies of water can absorb a lot of heat with only small changes in temperature
Protects organisms in the water
Water can also aid in the regulation of cell temperature

9. What is a mixture?
Mixture – material composed of 2 or more elements or compounds
Physically mixed together
Not chemically combined
Water can form 2 types of mixtures
Solutions and Suspensions

10. How does water’s polarity influence it’s properties of a solvent?
In an ionically bonded compound, ions are attracted to polar water molecule
Ions break away
Become dispersed
Form a solution
Solutions are evenly distributed throughout the solution
In Salt water solution
Table salt – solute
Water - solvent

11. The universal solvent Water is a very versatile solvent
It can break dissolve:
Salts
Sugars
Minerals
Gasses
Other solvents…
All because of it’s polarity
When no more solute can be dissolved the solution is saturated.

12. Besides tasting the water, how could you test to show that the salt is there, even though you can’t see it?
Weigh it
Measure changes in volume….

13. Suspension
When something in water separates into pieces so small that they do not settle out
Movement of water molecules keeps them suspended
Mixture known as a suspension
Blood is a great example of a solution and suspension
Blood – mostly water
Many dissolved compounds
Cells can’t dissolve, and are held in suspension

14. Complete the table Substance Definition Examples
Physical combination of to or more substances
Cinnamon sugar
Solute
Water in saltwater
Mixture of water and non dissolved substance
blood
Solution

15. Complete the table Substance Definition Examples Mixture
Physical combination of to or more substances
Cinnamon sugar
Solute
Substance that is dissolved
Salt in saltwater
Solvent
Substance in which Solute is dissolved
Water in saltwater
Suspension
Mixture of water and non dissolved substance
blood
Solution
Mixture in which all substances are evenly distributed
Saltwater

16. Acids, bases and pH
Water molecules can split into H+ ions and OH- ions
In pure water only 1 in 550 million molecules split
Number of Positive and Negative ions are equal
pH is neutral - 7

18. The pH Scale Indicates the concentration of H+ ions in solution
Ranges from 0 to 14
Below 7 is acidic
Above 7 is basic
The lower pH the greater the acidity
pH scale is logarithmic
Each step on the pH scale represents a factor of 10 increase from the previous step

19. Acids and bases
Both can be very corrosive

20. Acid Any compound that forms H+ ions in solution
Have higher H+ concentrations than pure water
Therefore have a pH of less than 7
1 – 3 is a strong acid
Example is hydrochloric acid in your stomach

21. Base A base is a compound that produces OH- ions (hydroxide)
Lower concentrations of H+ ions than pure water
pH > 7
Strong bases such as NaOH (used in soap making) have pH’s ranging from 11 – 14

22. Buffer
A weak acid and/or a base that can react with a strong acid or base to prevent a sharp, sudden change
Important in the human body
Where pH must be kept between 6.5 and 7.5
If pH is lower or higher than can affect chemical reactions that take place within cells
Controlling pH is one form of homeostasis
Blood must be kept at pH 7.4
Changes are controlled by bicarbonate and phosphate ions

23. Buffers cont… Buffers do not keep a solution neutral
There are many different buffers in the world – each is designed for a fixed pH range
One may work from 2 – 6
Another from 10 – 12
All contain a weak acid and a weak base component
Acid component will release H+ ions if base is added
Base component will absorb H+ ions if acid is added
The solution does not become as acidic or basic as it would have without the buffer

24. Writing exercise
Suppose you are a writer for a natural history magazine for children. This month’s issue will feature insects. Write a paragraph explaining why some bugs, such as the water strider, can walk on water