Properties of Water

What is water? Water is H2O One molecule of oxygen and two molecules of hydrogen.

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.

The oxygen end of the molecule has a negative charge and....................... the hydrogen end has a positive charge. This makes water a POLAR molecule. One pole is positive and the other is negative.

Water is cohesive. It sticks to itself. 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!

Other molecules may also be polar so water molecules stick to them....

Some molecules are non-polar. Non-polar molecules won’t mix with water. That’s why oil and water will separate even if you try 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, 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.

Water has been called the UNIVERSAL SOLVENT. What is a 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!!!

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

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

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..................

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 does limestone and chalk form? Clams, oysters, and other organisms extract the calcium carbonate from the seawater to make their shells. When they die their shells pile up and form limestone which may become marble.

Who cares? Limestone and marble are mined for commercial use but more important is the billions of tons of carbon that remains in the oceans and does not get into the air where it would cause global warming on a MASSIVE scale.

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 wonderful and exciting 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.

Water has Surface Tension Surface tension is the tendency of the surface of a liquid that allows it to resist an external force. 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 non- polar. They repell 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

Gases dissolve in water! This includes oxygen (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 which forms limestone, marble, and chalk.

CAVES 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 carbon dioxide (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, and calcium 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 it changes temperature, up or down, very slowly.

How would YOUR life be different if any of these properties changed? 7) It expands when it freezes so that ice is less dense than liquid water. 8) When it evaporates or freezes the substances that 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 different if any of these properties changed?