Properties of Water

• Water is H2O

• One oxygen with 2 hydrogens
• But notice that the two hydrogen atoms are not on opposite sides of the
• oxygen. They are bunched together on the same side.
• This means that one end of the molecule is different than the other end.

• Polarity

• The oxygen end of the molecule has a negative charge
• the hydrogen end has a positive charge.
• This makes water a POLAR molecule.

• One pole is positive
• the other is negative.

• Cohesion

• Because opposites attract, the positive end of one water molecule is attracted to the negative end of any nearby water molecule.
• That’s why water droplets stick to one another.
• Water is cohesive. It sticks to itself.
• Water sticks to itself, forming droplets.
• Water may stick to other polar molecules.

• Nonpolar molecules

• Nonpolar molecules won’t mix with water.
• That’s why oil and water will separate even if you try to mix them together.

• Density

• a measure of how close molecules are in a given space

• determines whether things float or sink
• lower density materials float over higher density
• is NOT equal to weight or mass or heaviness

• Hot air is less dense than cold air
• Air is less dense than water
• Oil separates from water because of polarity, but oil floats on top of water because of density (oil: 0.86 g/mL; water: 1.00 g/mL)

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• Water does a very strange thing

• It expands when it freezes at 0° C.
• (most substances condense)
• Because it expands it gets less dense.
• Because it gets less dense, ice floats.
• Because ice floats, there are icebergs.

• Why does water expand when it freezes?

• As it cools, like all materials, the molecules come closer together.
• But because of water's...

• polarity
• size
• angle between hydrogen
• atoms hydrogen bonding

• ...the water molecules realign and push each other away.
• This makes water less dense than expected.

• Why do snowflakes have six sides?

• The angle between hydrogen atoms in water is 104.5o.
• The interior angle of a regular hexagon is 120o.
• The relative size of oxygen atoms prevents the shape from being smaller.

• After all, the interior angle of a pentagon is 108o. 
• But the molecules just can't squeeze into that shape.

• The next lowest energy shape is the hexagon.

• What would happen to lakes in the winter if water didn’t expand when it freezes? How would life on Earth be different?

• In winter the air gets cold.
• The water at the surface, in contact with this air, freezes.
• Because the ice is less dense than the water below, it floats on the surface.
• Since ice is a good insulator, it protects the lake water below from 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 the surface would freeze and sink until......
• the lake would freeze solid
• And everything in the lake would die.

• Universal Solvent

• Water has been called the UNIVERSAL SOLVENT.
• A solvent is a material, usually a liquid, in which other materials dissolve.
• If you want to clean a dirty paint brush you buy a paint solvent that dissolves the old paint.
• Water dissolves so many different substances that it is called the universal solvent.

• YOU are a big, walking solution of trillions of molecules all dissolved in water!

• That's why astronomers who are searching the
• universe for alien life forms always say FOLLOW THE WATER
• Where there's liquid water, there's life

• Why are the oceans salty?

• Water dissolves substances as it flows over and through the land on its way to the sea.
• Any soluble substance, such as salt, will be dissolved and will end up in the ocean.

• Does that mean that the oceans used to be less salty than they are today?

• Actually, no.

• When oceans formed, chloride ions were also outgassed, creating salts in ocean

• The concentration of salt remains almost constant.
• Salt is removed (precipitates) when water evaporates

• Salt is left behind, forming new sedimentary rocks
• Lava reacts with ions, clay absorbs some
• Seawater sprays salt water onto land, evaporates, and leaves salt on land

• Salt is replenished in part by rivers, water-caused erosion

• Why does the salt accumulate in the oceans?

• Water can evaporate leaving the salt behind or
• Water can freeze leaving the salt behind
• The only way salt can leave the ocean is if the entire ocean evaporates.

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• What kind of salt is in the ocean?

• Mostly sodium chloride (NaCl), which is the same as table salt
• The ocean has a salt concentration of 3.5%

• Why does salt dissolve in water?

• Sodium chloride is formed from ionic bonds.
• Sodium (metal) gives its valence electron to chlorine (nonmetal).

• Sodium becomes positively charged.
• Chlorine becomes negatively charged.

Na+ (aq)                 Cl-  (aq)

• (aq) means "aqueous" or "dissolved in water"

• Remember, water is polar.

• One side of the molecule (oxygen side) is more negative.
• One side of the molecule (hydrogen side) is more positive.

• Opposites attract

• Chlorine ions (-) are drawn to the hydrogen side (+).
• Sodium ions (+) are drawn to the oxygen side (-).

• What else is dissolved in sea water besides salt?

• Lots of things like gypsum which is used to make wallboard and plaster,
• and calcium carbonate which forms limestone, chalk, and marble.

• How do limestone and chalk form?

• Clams, oysters, and other organisms extract the calcium carbonate from the sea water to make their shells. When they die, their shells pile up and form limestone which may become marble.

• Why is this important?

• Limestone and marble are mined for commercial use, such as in the making of cement.
• But more important is the billions of tons of carbon that remain in the oceans and don't get into the air where it would cause global warming on a MASSIVE scale.

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• Buoyancy

• ability or tendency to float in water, air, or other substance
• if weight of object is equal to or less than buoyant force, it floats
• if weight of object is more than buoyant force, it sinks
• greater density usually means more buoyant force
• saltwater > freshwater

• What does salt (etc.) do to the water itself?

• It lowers the melting (freezing) point (temperature)
• It raises the boiling (condensation) point
• What does this mean?

• It melts/freezes at a lower temperature and boils/condenses at a higher temperature
• Water remains a liquid in a bigger range of temperatures
• That's why we put salt on ice in the winter…
• It lowers the freezing point, so the ice turns to water because it's no longer below the freezing point.

• Specific heat

• Water has the highest specific heat of any common substance.
• Specific heat refers to the amount of energy water needs to absorb before it changes temperature.
• It takes a LOT of heat energy to heat water so water heats up and cools down VERY SLOWLY.

• What difference does the specific heat of water make in my life?

• Locations near the oceans have cooler summers and warmer winters than inland locations.
• Large bodies of water eliminate extremes of temperature

• This affects the growing season and what crops can be grown in a region.

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• Water has Surface Tension

• Surface tension is the tendency of the surface of a liquid that allows it to resist an external fo
• Remember that water molecules are cohesive..... they stick together.
• The water molecules IN the liquid are attracted equally in every direction so all the forces acting on them cancel each out.
• But the forces acting on the molecules on the surface are unequal.

• Surface molecules are pulled inward towards the center.
• This creates a force, or tension.

• This tension pulls the water into a spherical shape or droplet.
• Water striders are insects that can walk on water.

• They can do this because their mass is distributed over a wide area and because their legs are nonpolar. They repel water.
• The force of gravity is less than the force needed to break the surface tension.

• Water shows Capillarity (aka capillary action)

• Capillarity is the movement of water molecules due to the attraction of those molecules to the surface they are on.
• Remember, water molecules are polar. One end of a molecule is positive, the other negative.
• These charges are attracted to charges on a surface such as a tube which pulls the water up.
• The attraction between water molecules and paper fibers pulls the water up and into the paper.
• Capillarity in the xylem tubes of plants helps to explain how plants can “pull” water up to their leaves against the force of gravity.

• If water didn’t have this property the tallest plant on Earth would be only a few inches tall and giant sequoias (as well as all other trees) wouldn't exist.

• Some gases dissolve in water

• This includes oxygen [O2] (not to be confused with the oxygen that is part of the water molecule).
• Dissolved oxygen can be easily extracted.
• Fish do it all day long. So could you if you had gills.

• Carbon dioxide (CO2) also dissolves in water.

• When carbon dioxide dissolves in water it forms Carbonic Acid, H2CO3 
• You are probably more familiar with carbonic acid as seltzer or club soda.

• It is a very weak acid but it does react with.......
• Calcium carbonate [CaCO₃] 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 limestone it converts the limestone to carbon dioxide (bad for global warming) and calcium and creates caves.

• So what is so special about water?

• It is polar.
• It has cohesive properties.
• It dissolves other substances such as salt, and calcium carbonate.
• It has surface tension, which results in capillarity.
• It is known as the universal solvent.
• Gases, including oxygen, dissolve in water.
• It dissolves CO2 forming carbonic acid.
• It has a high specific heat which means it changes temperature, up or down, slowly.
• It expands when it freezes so that ice is less dense than liquid water.
• When it evaporates or freezes the substances that may be dissolved in it, salt for example, remain behind.
• In its solid form, ice, it is a good insulator.

Project

Water Activist Project

Find a water crisis around the world and create a brochure to education people about it. Check The Water Project. Your brochure must include two paragraphs that explain the situation and have five additional bullets with other related facts. Create an informative event to bring people in and teach them about the situation and try to enlish their help. Create a name and logo for your foundation. Also be sure to include at least five related photos. To be printed and handed in.