Post on 29-Mar-2015
Properties of Water
What is water?
Water is H2O
One molecule ofoxygen and twomolecules of hydrogen.
But notice that the two hydrogenatoms are not on opposite sides of theoxygen. They are bunched together onthe same side.
This means that oneend of the moleculeis different than theother end.
The oxygen end of the molecule has anegative charge and.......................
the hydrogen endhas a positivecharge.
This makes watera POLAR molecule.One pole is positiveand the other isnegative.
Because opposites attract, the positive end of one water molecule is attracted to the negative end of any nearby water molecule.
That’s why waterdroplets stick toone another.
Water is cohesive. It sticks to itself.
Water sticks to itself formingdroplets!
Other molecules may also be polarso water molecules stick to them....
Some molecules are non-polar.
Non-polar molecules won’tmix with water.
That’s why oil and waterwill separate even if youtry to mix them together.
Water does a very strange thing.It expands when it freezes at 0° C.
Because it expands it gets less dense.
Because it gets less dense, ice floats!
Because ice floats there are ice burgs!
Chasing Ice
What would happen to lakes in the winter if water didn’t expend when it freezes? How would life on Earth be different?In winter the air gets cold.The water at the surface, incontact with this air freezes.Because the ice is less densethan the water below it floatson the surface.Since ice is a good insulator itprotects the lake water belowfrom the cold air.The lake never freezes solid. Life in the lake survives the winter.
If water didn’t expand when it froze, ice would be more dense and would sink.
More water at thesurface would freezeand sink until......
The lake would freeze solid
And everything inthe lake would die.
Water has been called theUNIVERSAL SOLVENT.
What is a solvent?A solvent is a material, usually a liquid, in whichother materials dissolve.
If you want to clean adirty paint brush youbuy a paint solventthat dissolves the oldpaint.
Water dissolves so many differentsubstances that it is called theuniversal solvent.
YOU are a big, walkingsolution of trillions ofmolecules all dissolvedin water!
That’s why............
Astronomers who are searching theuniverse for alien life forms always say
FOLLOW THE WATER!!!
Where there’s Liquid water......
There’s life!!!
Water dissolves substances asit flows over and through the land
on its way to the sea.Any soluble solublesubstance, such assalt, will be dissolvedand will end up inthe ocean.
This is why the seasare salty.
Does that mean that the oceansused to be less salty than they are
today?
YES!!!
Why does the salt accumulatein the oceans?
Water can evaporate leaving the salt behind orWater can freeze leaving the salt behind
The only way salt can leave the ocean is ifthe entire ocean evaporates..................
What else is dissolved in seawater besides salt?
Lots of things like gypsum which is used to makewallboard and plaster,and calcium carbonate which forms limestone,chalk, and marble.
How does limestone and chalk form?
Clams, oysters, and other organisms extract thecalcium carbonate from the seawater to maketheir shells. When they die their shells pile upand form limestone which may become marble.
Who cares?
Limestone and marble are mined for commercialuse but more important is the billions of tons ofcarbon that remains in the oceans and does notget into the air where it would cause global warming on a MASSIVE scale.
Specific Heat
Water has the highest specific heat of anycommon substance.
Specific heat refers to the amount of energywater needs to absorb before it changestemperature.
It takes a LOT of heat energy to heat water sowater heats up and cools down VERY SLOWLY
What difference does the specificheat of water make in my wonderful
and exciting life?
Locations nearthe oceans havecooler summersand warmerwinters thaninland locations.
Large bodies of water eliminate extremes of temperature!
This affectsthe growingseason and whatcrops can begrown in a region.
Water has Surface TensionSurface tension is the tendency of the surfaceof a liquid that allows it to resist an external force.Remember that water molecules are cohesive.....they stick together.
The water molecules IN theliquid are attracted equally inevery direction so all the forcesacting on them cancel eachout.
But the forces acting on themolecules on the surface areunequal.
Surface molecules are pulledinward towards the center.This creates a force, or tension
This tension pulls the waterinto a spherical shape ordroplet.
Water striders are insects that can walk on water.They can do this because their mass is distributedover a wide area and because their legs are non-polar. They repell water.
The force of gravity is less than the force needed to break thesurface tension.
Water shows Capillarity(aka capillary action)
Capillarity is the movement of water moleculesdue to the attraction of those molecules to thesurface they are on.Remember, water molecules arepolar. One end of a molecule is positive, the other negative.
These charges are attractedto charges on a surface such asa tube which pulls the water up.
The attraction betweenwater molecules andpaper fibers pulls thewater up and into thepaper.
Capillarity in the xylem tubesof plants helps to explain howplants can “pull” water upto their leaves against theforce of gravity.
If water didn’t have this propertythe tallest plant on Earth would be
only a few inches tall and
giant sequoias (aswell as all othertrees).... wouldn’t exist
Gases dissolve in water!
This includes oxygen (not to be confused with theoxygen that is part of the water molecule).Dissolved oxygen can be easily extracted!
Fish do it all day long.So could you if you hadgills.
Carbon dioxide (CO2 ) alsodissolves in water.
When carbon dioxide dissolves in water it formsCarbonic Acid, H2CO3 You are probably more familiar with carbonic acid as seltzer or club soda. It is a very weakacid but it does react with.......
Calcium carbonate which forms limestone, marble, and chalk.
Since all rainwater contains dissolved carbon dioxide, all rainwater is really a weak acid.
When the carbonic acid rain reacts with lime-stone it converts the limestone to carbondioxide (bad for global warming) and creates
CAVES
So what is so special about water?1) It is polar and because it’s polar it......
2) Dissolves other substances such as salt, andcalcium carbonate.
3) Has surface tension which results in capillarity.4) Gases, including oxygen, dissolve in water.
5) It dissolves CO2 forming carbonic acid.
6) It has a high specific heat which means itchanges temperature, up or down, very slowly.
7) It expands when it freezes so that ice is lessdense than liquid water.8) When it evaporates or freezes the substancesthat may be dissolved in it, salt for example, remain behind.
9) In its solid form, ice, it is a very good insulator.
How would YOUR life be differentif any of these properties changed?