Biology – the study of life
Water and it’s life giving properties How many drops of water can you fit on a penny?
Observations What observations did you make about water? What do you already know?
The Chemical Properties of Water
Your Objective
Be able to state at least 2 properties of water that help
support life AND
Be able to explain how the chemical nature of water gives
rise to that property
Water and Life
Life on Earth began in water and evolved there for 3 billion years.
Modern life still remains tied to water
Cells are composed of 70%-95% water
•Water is found as a liquid over 71% of the earth•The abundance of water is a major reason Earth is habitable
Studied in isolation, the water molecule is deceptively simple Its two hydrogen atoms are joined to one
oxygen atom by single covalent bonds
The structure of water
H
O
H
But the electrons of the covalent bonds are not shared equally between oxygen and hydrogen
This unequal sharing makes water a polar moleculeOxygen is more electronegative than hydrogen, so it has a greater pull on the electrons
() ()
() ()
The polarity of water results in weak electrical attractions between neighboring water molecules
These interactions are called hydrogen bonds
(b)
()
Hydrogen bond()
()()
()
()
()()
Polar Structure
Electronegativity of H20
Quick Think Why is a molecule
of water said to have polar covalent bonds?
What kind of bonds hold individual water molecules together?
Water’s Life Supporting Properties
The polarity of water molecules and the hydrogen bonding that results explain most of water’s life-supporting properties Water’s cohesive nature Water’s ability to moderate temperature Floating ice Versatility of water as a solvent
Water molecules stick together as a result of hydrogen bonding
This is called cohesion
The Cohesion of Water
Cohesion is vital for water transport in plants
Microscopic tubes
Quick think Explain how the water drops on a penny
demo exemplifies the property of cohesion.
Surface tension is the measure of how difficult it is to stretch or break the surface of a liquid
Hydrogen bonds give water an unusually high surface tension Cohesion between water molecules form a skin-like surface Can support animals like “water
striders” in ponds
Surface Tension
Water drops are round because all the molecules on the edge are pulled to the middle.
Adhesion Water will also
adhere to other polar substances
This is called adhesion
It is due to the polar nature of the water molecule
Caused by adhesion the water runs along the glass and does not fall straight.
Capillary Action
Glass has polar molecules.
Glass can hydrogen bond.
Attracts the water molecules.
Some of the pull is up.
Meniscus
Water curves up along the side.
This makes the meniscus.
Quick Think How are adhesion and
cohesion similar and different?
Give an example of how these properties help support life.
Warm up Use the clay and toothpicks to make a
model of a water molecule Working with someone near you, use your
models to demonstrate cohesion.
Objective: Explain and diagram the transpiration-cohesion mechanism responsible for water transport in plants
Water moderates temperature Because of hydrogen bonding, water has
a strong resistance to temperature change
Water moderates temperature Heat and temperature are related, but
different Heat is a measure of the amount of kinetic
energy in the atoms and molecules in something
Temperature measures the intensity of the heat
Whenever 2 objects meet, the cooler object absorbs heat from the warmer object until they are the same temperature
Water moderates temperature Water has a high specific heat
Specific heat = the amount of heat that must be absorbed or lost to change the temperature of 1g of the substance 1°C
Water moderates temperature much of the absorbed heat is used to
break hydrogen bonds, not increase the kinetic energy of the molecules
Water moderates temperatureEarth’s giant water
supply causes temperatures to stay within limits that permit life
How water moderates temperature
Water also has:High heat of fusion• The temp at which liquid turns solid
High heat of vaporization• The temp at which liquid turns to gas
Quick Think
Why does it take so much energy to change the temperature of water?
When water molecules get cold, they move apart, forming ice
The Biological Significance of Ice Floating
A chunk of ice has fewer molecules than an equal volume of liquid water
IceLiquid water
The density of ice is lower than liquid water This is why ice floats
Figure 2.15
Hydrogen bond
Liquid waterHydrogen bonds
constantly break and re-form
IceStable hydrogen bonds
Change of State
Dipole Structure Ice floats in water because all ice
molecules are held in hexagons Center is open
space, making ice 8% less dense than water.
Since ice floats, ponds, lakes, and even the oceans do not freeze solidMarine life could not survive if bodies of water froze solid
Floating ice insulates water below, preventing freezing: critical for ocean animals
Maximum density: 3.98oC
Below this temp, form hexagonal polymers and decrease density
Above this, molecules are energetic, water behaves like other liquids - expanding when warm and contracting when cool
Quick Think
Of all the properties we’ve learned so far, which ones could you competently write about in your essay?
A solution is a liquid consisting of two or more substances evenly mixed
Water as the Solvent of Life
The dissolving agent is called the solventThe dissolved substance is called the solute
Ion in solutionSalt crystal
Water is a good solvent because it is polar
Ionic (salts) and polar (sugars) compounds dissolve readily in water
Solvent Properties Water dissolves salts by surrounding the atoms in the
salt molecule and neutralizing the ionic bond holding the molecule together
Acids, Bases &
Buffers
Acid
Acids, Bases, and pH
A chemical compound that donates H+ ions to solutionsBase
A compound that accepts H+ ions and removes them from solution or a compound that dissociates in water to form hydroxide ions
Basicsolution
Neutralsolution
Acidicsolution
Oven cleaner
Household bleach
Household ammonia
Milk of magnesia
Seawater
Human bloodPure water
Urine
Tomato juice
Grapefruit juice
Lemon juice;gastric juice
pH scale
To describe the acidity of a solution, we use the pH scale
pH scale (log scale)
Each number on the scale is 10x difference from the number next to it
• pH 1 is 10x more acidic than pH 2, 100x more acidic than pH 3, 1000x more acidic that pH 4, and so on
pH - Percent HydroniumA measure of the percent of hydronium ions
in the solutionThe greater the percent hydronium ions, the
more acidic the solution is
H2CO3 -------------> H+ + HCO3 -
CARBONIC ACID HYDRONIUM BICARBONATE ION ION
Measuring Acidity
Buffers are substances that resist pH change
They accept H+ ions when they are in excessThey donate H+ ions when they are depleted
Buffering is not foolproof
Example: acid precipitation
normal rain water (pH of 5-6) pH of acid rain is between 3-4
Buffers Dissolved CO2 in water acts as a buffer, a
substance that prevents large shifts in pH.
Buffers help keep pool and spa water clean
Buffer SystemsCO2 + H2O H2CO3 H+ + HCO3
- H+ + CO3-2
H2CO3 is carbonic acid,
H+ is the hydronium ion HCO3
- is the bicarbonate ion
CO3-2 is the carbonate ion
Buffer Systems
CO2 + H2O H2CO3 H+ + HCO3-H+ + CO3
-2
Adding CO2 shifts the reaction to the right and produces more H+ ions making the water more acid.
Buffer SystemsCO2 + H2O H2CO3 H+ + HCO3
- H+ + CO3-2
Removing CO2 shifts the reaction to the left, combining H+ ions with carbonate and bicarbonate ions reducing the acidity.
Ocean Buffers