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

2. Life depends on Hydrogen Bonds in water
Ice floats
Solid water is less dense than liquid water, allowing it to float on top
Think of glass of ice water
When water freezes, air pockets are trapped in between crystallized water molecules
Why is this property of water important for living things?
Important because it allows organisms to live even when bodies of water freeze
Image: AHAJOKES.COM
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3. Your body is made up of mostly water
Your body is made up of mostly water. Approximately 85% of your brain, 80% of your blood and 70% of your muscle is water. Every cell in your body needs water to live.
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4. Life depends on hydrogen bonds in water.
Water is a polar molecule.
Polar molecules have slightly
charged regions. O H _ +
Polar molecules—molecules that have regions with slight electrical charges due to uneven
pull on electrons
Nonpolar molecules—molecules without charged regions due to equal pull on electrons
1. hydrogen bonds
2. formed by attraction between slightly positive hydrogen atom and slightly negative
(oxygen) atom
Atom 1—oxygen; slightly negative
Atom 2—hydrogen; slightly positive
Nonpolar molecules do not have charged regions.
Hydrogen bond is a weak bond form between
slightly positive hydrogen atoms and slightly negative
atoms. 3

5. Cohesion - water molecules Adhesion - water molecules
Hydrogen bonds are responsible for three important properties of water.
large amount of energy needed to produce an increase in temperature
High specific heat-
Cohesion - water molecules
“stick” to each other
Adhesion - water molecules
“stick” to other substances
The water’s surface (left, dyed red) is curved down because the water has a greater adhesion than cohesion. The surface of the mercury (right) is curved up because mercury has greater cohesion than adhesion.
1. High specific heat— large amount of energy needed to produce an increase in temperature
2. Cohesion— water molecules “stick” to each other
3. Adhesion— water molecules “stick” to other substances 4

6. Cohesion
When the negative end of a water molecule attracts the positive end of another water molecule, and the molecules stick together, this is called cohesion.
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7. This figure shows water adhesion
Adhesion, or capillary action, is when water molecules are attracted to other substances with a negative or positive charges. This is why water "climbs" up a piece of paper or a straw above the surface level of the water.        
This figure shows water adhesion
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Demo straw

8. Because of the combination of adhesive and cohesive forces, trees such as the giant sequoia are able to transport water molecules from the soil to their leaves 300 ft. above.
How does water get to the top of the tree?
300 ft.
Water molecule released into the atmosphere
Water molecules pulled upward
As each water molecule evaporates, it pulls additional water up through the tree because of the “sticky-ness” of the hydrogen bonds that link the water molecules.
6-ft.-tall man
What is Life? A Guide to Biology
2010 W.H. Freeman and Co
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Water molecule pulled into root system

9. Many compounds dissolve in water.
A solution is formed when one substance dissolves in another.
A solution is a homogeneous
mixture.
Solvents dissolve other
substances.
Solutes dissolve in a solvent.
solution
Solutions—a mixture that is the same throughout (homogeneous)
Solvents—substance present in greatest concentration; dissolves other substances
Solutes—substance present in lower concentration; dissolves in solvent
The solvent is the substance that is present in the greatest amount, and is the substance that dissolves solutes.
The solute is the substance being dissolved. 5

10. “Like dissolves like.”
Polar solvents dissolve polar solutes.
Nonpolar solvents dissolve nonpolar solutes.
Polar substances and
nonpolar substances generally remain separate. 6

11. Some compounds form acids or bases.
An acid releases a hydrogen ion when it dissolves in water.
high H+ concentration
pH less than 7
more acidic
stomach acid pH between 1 and 3
Acids—release H+ ions in solution; high H+ concentration; low pH (<7)
Neutral—neither acidic nor basic; pH of 7
Bases—remove H+ ions from solution; low H+ concentration; high pH (>7)
A pH scale is a negative logarithm. A pH of 1 is equal to the log of a H+ ion concentration of , whereas a pH of 10 is equal to the log of a H+ ion concentration of There is a tenfold difference in H+ ion concentration from one pH value to the next. Ask, What is the difference in H+ ion concentration between lemon juice (pH 2) and tomato juice (pH 4)?
Lemon juice has a H+ ion concentration 100 times greater than tomato juice. 7

12. A base removes hydrogen ions from a solution.
low H+ concentration
pH greater than 7
bile pH between 8 and 9
more basic 8

13. A neutral solution has a pH of 7.
pure water pH 7 9

14. pH Scale