Transcript Covalent Bon - missballinger

Chapter 11:
Water: essential to life
Where there is no water, there is
no life.
Water:
• is a special chemical, both common and
unique
• Is the most abundant liquid on Earth, covering
over 70% of our planet.
• It moderates our weather, shapes our lands
and is essential for the existence of life.
The Water Cycle
The water on Earth exists in solid, liquid and gaseous states and readily
changes from one state into another.
The water cycle could be said to begin with evaporation from:
• Oceans
• Lakes
(solar energy being the major energy source)
• Rivers
• Human activities (combustion of fossil fuels) produce steam
which also contributes to the water cycle
Water vapour in the air is transported around the globe until it condenses to
form clouds
It returns to the ground as rain water or ice crystals in hail or snow
The Cycle begins again
Water and Living things
Water is involved in the reactions of life: photosynthesis and respiration
It is also important as:
Water provides a function in plants and animals where it
transports nutrients and soluble wastes. The fact that water is a
solvent and can dissolve many things explains why it is a very useful
system to transport materials to where they are needed.
Water can store large amounts of heat energy and transfer it away from
cells where it is produced, to the body’s surface where it is transferred
into the atmosphere. As living things contain large amounts of water this
makes for a very efficient heat transferring system.
Water on the skin of plants and animals can also act in cooling the
body. Heat energy is transferred up into the water on the skin which
then evaporates, taking away with it the heat energy that it absorbs.
Water provides a transport system.
• Transports nutrients and soluble wastes in the
body.
• Food is consumed in a liquid environment.
Water provides a heat transfer system.
• It transfers heat energy from the cells
(respiration) to the body’s surface, where it is
lost to the surroundings.
• It acts as a thermal regulator, reducing
temperature changes.
Water provides a cooling system.
• Humans lose 1 litre of water a day from
perspiration and the surface of the lungs.
• Evaporation of water from a surface is the
major cooling mechanism available to living
organisms.
Water provides an environment.
• For aquatic animals and plants.
• Is the main source of their nutrients.
• Aquatic organisms depend on the buoyancy of
water for physical support and their
environment.
Water shapes the environment
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Oceans, rivers, lakes etc.
Rainfall impacts on landscape
Water erosion can alter a region over time.
Water can freeze in the cracks of rocks causing
them to split.
• The distribution of water influence the type
and abundance of vegetation and animals.
• Coastal towns usually have a milder climate.
Water as a chemical reactant
• Water reacts in many naturally occurring
process. Eg photosynthesis.
• Water is produced in many process. Eg.
Combustion.
Water is important for reproductive
process.
• Some water plants achieve fertilization by the
release of spores into water.
• Developing embryo’s require an aqueous
solution.
The properties of water
The key to all the understanding is in the bonding between water molecules.
Covalent bonding
Strongly polar
Extensive hydrogen bonding
Hydrogen Bonding occurs when a hydrogen atom bonds with a very
highly electronegative atom.
A very obvious example is the Hydrogen bonding that occurs in
water between the Hydrogen atoms in one molecule and the Oxygen
atom in another neighboring molecule. This explains how water
molecules attract to each other and stay together.
(The Term HFON refers to the elements that can take part in
Hydrogen bonding, Hydrogen, Fluorine, Oxygen and Nitrogen.)
The properties of water
The key to all the understanding is in the bonding between water molecules.
Covalent bonding
Strongly polar
Extensive hydrogen bonding
Hydrogen Bonding occurs when a hydrogen atom bonds with a very
highly electronegative atom.
A very obvious example is the Hydrogen bonding that occurs in
water between the Hydrogen atoms in one molecule and the Oxygen
atom in another neighboring molecule. This explains how water
molecules attract to each other and stay together.
(The Term HFON refers to the elements that can take part in
Hydrogen bonding, Hydrogen, Fluorine, Oxygen and Nitrogen.)
Melting and boiling points
• Water consists of very small molecules.
• Other substances of similar size are often gas at
room temp, yet water is a liquid.
• Water freezes at 0.C and boils at 100.C
• These values are higher than those for other
substances consisting of similar sized molecules.
• The reason for these high values is the strong
hydrogen bonding that occurs between molecules in
water.
Thermal Properties
• Water is good at storing heat energy.
• The specific heat capacity is 4.2 J per gram per
degree. This is the energy required to increase the
temperature by one degree.
• The Latent heat value measures the energy required
to change the state of a substance eg. From ice to
liquid, or liquid to water vapour.
• These two values of water are quite high compared
to other substances and make water ideal in heating
and cooling systems.
Density
• Water expands on freezing.
• Ice (lower density) floats on liquid water.
• Water is the only natural substance that
expands on freezing.
• The density of liquid water at 3 degrees is 1
g/mL, whilst the density of ice is 0.917 g/mL.
Ice
• In liquid water, the hydrogen bonds
continually break and form as the molecules
move around.
• However, in a solid state, the hydrogen bonds
are permanent.
• In ice, each water molecule becomes
hydrogen bonded to four others in a
tetrahedral arrangement.
Ice continued
• This causes the water molecules to remain
slightly further apart than they are in liquid
water.
• Hence, ice is less dense than water and it
floats.
• N.B. Snap freezing occurs too quickly for the
hydrogen bonds to form, thus allowing no cell
damage to occur.
A good solvent
• Water will dissolve a wide range of substances
due to its polar nature.
• It will dissolve polar molecules and many ionic
substances.
• However, it is a poor solvent for non-polar
substances such at oil, fats and petrol.
Water
Specific Heat Calculations
Chapter 10: Question 4, 5, 13, 15
Handout : Wonderful Water