Properties of Water

Polarity Polarity is when a molecule has a slightly negative side at one end and a slightly positive side at the other.

Hydrogen bonding Hydrogen bond is a weak bond between polar molecules. Hydrogen bonds occur because polar molecules are attracted to other polar molecules.

Cohesion Attractive forces between particles/molecules of the same kind. Water molecule + water molecule Cohesive forces resulting from water’s hydrogen bonding are strong enough to cause water to act as if it has a thin “skin” on its surface. This is why water appears to bulge from the sides of a glass filled to the brim.

Surface tension Attraction of water molecules to each other at the water’s surface.

Adhesion Is the attractive force between unlike substances. Water molecule + needle

Water sticks to itself; water is attracted to water Water sticks to other substances; water is attracted to other substances

Water exists naturally on Earth in all 3 states!

Ice, water, vapor States of Matter: Solid, liquid and gas

Hydrogen bonding (electric attraction) Ice Liquid

Water-soluble protein Universal Solvent Water-soluble protein Water can “dissolve” other polar or ionic substances.

Universal Solvent Water can “dissolve” other polar or ionic substances. When you add salt to water this is what happens→

Capillary Action Water will make hydrogen bonds with other surfaces such as glass, soil, plant tissues, and cotton. Capillary action-water molecules will “tow” each other along when in a thin glass tube. Capillary Action is the ability of water to flow upward against the flow a gravity. 1

Adhesion and cohesion Causes Capillary Action Which gives water the ability to “climb” structures Example: transpiration process which plants and trees remove water from the soil, and paper towels soak up water.

High Specific Heat Amount of heat needed to raise or lower 1g of a substance 1° C. Water resists temperature change, both for heating and cooling. Water can absorb or release large amounts of heat energy with little change in actual temperature. 1

High Heat of Vaporization Amount of energy to convert 1g of a substance from a liquid to a gas In order for water to evaporate, hydrogen bonds must be broken. As water evaporates, it removes a lot of heat with it. 1

High Heat of Vaporization Water's heat of vaporization is 540 cal/g. In order for water to evaporate, each gram must GAIN 540 calories (temperature doesn’t change --- 100oC). As water evaporates, it removes a lot of heat with it (cooling effect). 1

Heat radiated from the sun warmed surface of the earth is Water vapor forms a kind of global ‘‘blanket” which helps to keep the Earth warm. Heat radiated from the sun warmed surface of the earth is absorbed and held by the vapor. 1

“Dissociation” of water Breaking apart of water molecules into 2 ions of opposite charge. In water, the force of attraction between molecules is so strong that the oxygen atom of one water molecules can actually remove the hydrogen atom from the other water molecule

pH [H+]= 10-1M pH = – log [H+] Log scale means 10X change per unit!

Buffers Buffers resist changes to the pH of a solution when H+ or OH- is added to the solution. Buffers accept hydrogen ions from the solution when they are in excess and donate hydrogen ions when they have been depleted.

Overall lessons: Many properties of water are emergent properties due to hydrogen bonding. The cohesion of water molecules to each other is exploited by plants and animals. Water resists temperature changes by absorbing lots of heat. Lower density of ice causes it to float & insulate the water below. The polarity of water allows it to dissolve other polar molecules. Non-polar compounds are hydrophobic and not easily dissolved in water. A mole of a compound has a constant # of molecules. Adding or removing hydrogen ions changes the pH of a solution. Buffers resist pH changes by accepting or donating H ions when [H+] changes.