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Water•Water is the most important inorganic molecule
•All living organisms need water to survive
•65% of human body weight is water
Water and it’s importance
• Most important compound in living organisms.
• Life processes rely on free movement of molecules and ions.– This occurs when substances are dissolved in
water.• Water has unique properties that make it
vital to life.– Many of water’s biological functions stem from
it’s chemical structure!
Polarity of Water• Atoms of water molecules are bonded together by
covalent bonds.• Sharing of electrons in the covalent bonds is not equal
– oxygen attracts electrons more strongly than hydrogen
• This gives water an assymetrical distribution of charge. – assymetrical = uneven– Uneven opposite ends or poles are created – POLAR covalent bonds exist within water molecules
This causes an unequal sharing of electrons
Hydrogen bonds form between water molecules
Oxygen is slightly negative(more electronegative)
Hydrogen is slightly positive(less electronegative)
Neighboringmolecules held by a hydrogen bonds
H-bonds are not true bonds, but an attractive force.
Water is a versatile solvent• When water and another solution are mixed,
you have an aqueous solution.• Solute = what is being dissolved• Solvent = what is doing the dissolving• Polarity of water allows a variety of substances
to dissolve easily – ionic compounds– polar molecules– Molecules with charged parts attract to water.
• Substances that repel water include non-polar and non-ionic substances
• Salt dissolving (animation)
Water as a solvent: Examples
• Polarity of water attracts other polar molecules – ex: sugar
• These molecules are considered to be “hydrophilic” or – water loving
• Molecules that are non-polar are repelled– Ex: oil– Non-polar molecules are considered
“hydrophobic” or – water fearing
• oil and water animation
• More about dissolving
Importance to Living Things• The chemistry of living things involves the
study of solutions ~~AQUEOUS solutions• Water is vital to life because the chemical
reactions (metabolism) of living things must take place in aqueous solutions.
• This occurs when substances are dissolved in water.
• Life processes rely on free movement of molecules and ions. (ex: electrical signals in nervous systems)
Examples of aqueous solutions:
• Blood plasma – Liquid portion only– Ions, larger molecules, gases all dissolved in water
• Cytosol– The gelatin-like aqueous solution inside cells.
• Interstitial fluid– Intercellular fluid of multicellular organisms~11 L in average adult human body
• Aquarium water….
Temperature Stabilizer• Water can absorb great amount of heat before it’s
temperature changes considerably– Due to hydrogen bonds– Energy added to water disrupts hydrogen bonds – Hydrogen bonds are constantly breaking and reforming
• Cells release heat as a result of metabolism– Water helps to minimize temperature changes in cells
• This also allows large bodies of water and large organisms to regulate temperature– If bonds stay broken, water molecules at the surface escape into the
atmosphere• Evaporation• This takes away some energy and cools surface left behind
– This is why sweating cools you down!
High Specific HeatSubstance C (J/g oC)
Air 1.01
Aluminum 0.902
Copper 0.385
Gold 0.129 Iron 0.450
Mercury 0.140
NaCl 0.864 Ice 2.03
Water 4.18
• Water (liquid) has a high specific heat compared to other substances.
• It requires more heat to increase it’s temperature
• 0-100 degrees Celsius range!
Specific Heat Definiton:
• The quantity of heat required to raise the temperature of a substance by one degree Celsius is called the specific heat capacity of the substance.
Freezing and Expansion• Water expands below 4o C.• Molecules are spaced further apart • It is less dense, therefore ice floats in
water.• Ice at surface of lakes and oceans provides a thermal insulator for life below the surface.
Water is Cohesive• Cohesion- ability of similar molecules to stick together
• Cohesion = stick TOGETHER – Like molecules– Like sticks to like– WATER sticks to WATER
• Water attracts other water molecules– Hydrogen bonds = attractions (water-water) – Results in surface tension
Surface Tension• Cohesion allows water to form an interface
with a surface.• Surface tension is the measure of how
difficult it is to break this interface.– Due to H-bonding between water molecules
• Ex: materials are able to rest on water (float), as long as the surface tension is not broken.
Adhesion• Sticking together of one substance to
another, different substance.
• Water is very adhesive; sticks to OTHER substances like glue.
• Capillary action is adhesion and cohesion in action.– Capillary action is the process of pulling water
up against gravity in plant vessels
Imbibition
• Process of soaking into a hydrophilic substance.
• Water soaking into seeds, paper towels, or sponge.
• Important for plants (seed germination usually requires imbibition)
pH• pH refers to the dissociation of water
molecules.• H2O H+ + OH-
• In pure water, concentrations of H+ and OH- are equal. (pH = neutral)
• Organisms are sensitive to changes in pH
Acids and Bases
• When acids dissolve in water they donate H+ to the solution
• An acid is a substance that increases the H+ concentration
• A substance that reduces the H+ concentration is a base– By accepting H+
– Or by dissociating to form hydroxide ions that combine with H+ to form water
pH• In any solution, the product of H+ and OH-
concentrations is constant at 10 -14
[H+][OH-] = 10 -14
Neutral solution (pH 7) = [H+] = 10-7 [OH-] = 10-7
[10-7] [10-7 ] = 10-14
– If [H+] increased to 10-5 M, then [OH-] would be 10-9 M
[10-5] [10-9] = 10-14
PH Scale: pH scale is used to determine concentration of these ions in a solution
Higher concentrationof Hydrogen ionsH+
Higher concentration ofHydroxide ions OH-
Each step is ten fold
Ex: How many more H+ at pH of 1 than pH of 3?
10
x
10
= 100
pH Changes• H+ and OH- are very reactive• pH changes greatly affect a cell’s proteins
and other molecules by altering their structure (shape).
• If the structure of a molecule changes, the result is a change in function.
In our pH lab, what changes were observed?
Buffers
• The control of pH is very important for living systems
• Most chemicals of life can only operate properly within a narrow pH range
• Buffers are substances that minimize changes in pH
Buffers• As shown on the pH scales above, the pH of
your internal body systems varies greatly.– Stomach fluid = acidic– Urine = acidic– Intestinal fluid = alkaline– Blood = alkaline
• Complex buffering systems maintain the proper pH values of your body’s many fluids
