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Water
The search for life is
really a search for
water.
We’ve met and discovered
many organisms that have
surprised us in their
abilities to survive and even
thrive in many harsh and
inhospitable environments.
Mars video
Why?
The water
molecule
1. Water is clingy
Polarity
Due to the unequal sharing of electrons in a covalent bond
Hydrogen bonding…
Leading to the
property of cohesion
(the attraction among
like molecules)
Surface Tension
How difficult it is to stretch or
break the surface of a liquid.
Water is not the only substance with charged regions, so the
charged areas of the water molecule can be attracted to oppositely
charged areas on other types of molecules resulting in
adhesion…the attraction of unlike molecules.
Capillarity (capillary action)
The combined effects of
cohesion and adhesion
Water conducting cells
Figure 3.3
100 µm
2. Water absorbs and
releases heat s-l-o-w-l-y.
Heat
Is a measure of the total amount of
kinetic energy of the molecules
Temperature
Measures the average speed of one
molecule of the substance.
Think of a cool swimming pool vs a hot cup of tea.
Which has the higher temperature?
Which has more heat energy?
The specific heat of a substance:
The amount of heat that must be absorbed
or lost for 1 gram of that substance to
change its temperature by 1’C.
Water has an extremely high specific heat, which allows it to
minimize temperature fluctuations on the Earth and in your body.
The specific heat of water is 1 cal/g ‘C
So…
A calorie is the amount of heat required to raise
1g of water by 1’C.
A kilocalorie (kcal, or Calorie) is the amount of
heat required to raise 1kg of water by 1’C.
Have you ever gone in the ocean on a
nice, hot day in early June?
What about a mildly cool day in midSeptember?
Most deserts are extremely hot during
the day, but what happens when the sun
goes down?
Water moderates air temperature by absorbing heat
from air that is warmer and releasing the stored
heat to air that is cooler.
The Gulf Stream
redistributes the heat
energy from the equator
to the higher latitudes of
Western Europe.
Evaporative Cooling
A similar thing happens when you perspire…each droplet of sweat contains a
large amount of heat energy that is sent to the surface of your body to be
released to the air around you as it turns to a gas. You warm up the air when you
sweat!
Heat of Vaporization
In order for one gram of liquid water to turn into a gas at room
temp, 580 calories of heat energy must be absorbed. Again,
water has a very high heat of vaporization compared to most
other substances.
Remember…breaking bonds releases energy (exergonic) and
forming bonds requires energy (endergonic).
3. Solid water FLOATS!!!
Ice is less dense than liquid water and therefore
floats on it. It serves to insulate the water below the
ice. (Ponds are not frozen to the bottom!...life
continues all winter).
To form the crystal
lattice, water
molecules “push
away” from each
other.
Figure 3.5
Hydrogen
bond
Ice
Liquid water
Hydrogen bonds are stable
Hydrogen bonds
constantly break and re-form
4. Universal Solvent
Because water is polar it can dissolve most
charged solutes and ionic compounds.
Hydration Shell
Negative
oxygen regions
of polar water
molecules are attracted
to sodium cations (Na+).
Positive
hydrogen regions
of water molecules
cling to chloride anions
(Cl–).
–
Na+
+
–
+
–
Na+
+
Cl–
Cl
–
+
+
–
Figure 3.6
+
–
–
–
+
–
+
–
–
–
This oxygen is
attracted to a slight
positive charge on the
lysozyme molecule.
+
This hydrogen is attracted to a
slight negative charge on the
lysozyme molecule.
(a) Lysozyme molecule
in a nonaqueous
environment
Figure 3.7
(b) Lysozyme molecule (purple)
in an aqueous environment
such as tears or saliva
(c) Ionic and polar regions on the protein’s
Surface attract water molecules.
Hydrophilic; “water-loving”
polar or ionic substances
carbohydrates, proteins, ionic compounds
Hydrophobic; “water-fearing”
non-polar substances
fats, oils, lipids
So what if I need to dissolve a
non-polar compound?
Use a non-polar solvent.
Remember the rule “Like
dissolves like”.
A mole
Represents an exact number of molecules of a
substance in a given mass
How many moles are there in 20 grams of hydrogen?
H’s atomic mass ~ 1
Moles = mass/relative mass = 20/1 = 20 moles.
Molarity
Is the number of moles of solute per liter of solution
Measures concentration
5. Acidity and Alkalinity
Water dissociates into hydronium and hydroxide ions
H
H
H
Figure on p. 34 of water
dissociating
H
–
+
H
H
H
Hydronium
ion (H3O+)
+
H
Hydroxide
ion (OH–)
Acids; increase hydrogen ion concentration
Bases; decrease hydrogen ion concentration (increase
hydroxide ion concentration)
Remember that
pH is based on the
negative log of
the concentration
of hydrogen ions,
so a higher pH
number is a lower
concentration of
ions.
Buffers in the blood…
The internal pH of most cells is 7
Human blood is about 7.4
Buffers are substances that minimize changes in the
concentrations of hydrogen and hydroxide ions in a solution
They consist of an acid-base pair that either donates or accepts
hydrogen ions in reversible reactions.
Living things make the buffers they need to maintain
homeostasis.