Unique properties of water

65% or more of your body is composed of water -all life occurs in water, both inside & outside of the cell

The Water Molecule -A water molecule is held together by covalent bonds. This type of bond forms between atoms that share electrons.

How does water become polar? -The atoms in a covalent bond do not always share electrons equally. In water, the oxygen atoms pull on the electrons more strongly than the hydrogen atoms do. This unequal sharing of electrons forms a polar covalent bond.

Where are the electrons spending more time?

Polarity between oxygen and hydrogen

Polarity and charges -Because electrons spend more time near the oxygen atom, they give this part of the water molecule a partial negative charge. The hydrogen atoms, in contrast, gain partial positive charges.

Does a water molecule have a charge? -Although the entire molecule is neutral, different parts of it carry charges. For this reason, water is described as a polar molecule.

Polarity leads to hydrogen bonding -Because water is a polar molecule, it is able to form multiple hydrogen bonds with other water molecules. - In a hydrogen bond, the positive region (hydrogen atom) of one water molecule attracts the negative region (oxygen atom) of a neighboring water molecule.

What is wrong with this?

Properties of Water -Hydrogen bonds are responsible for many of the unique and important properties of water.

1. Cohesion Cohesion - the sticking together of like molecules -Because a single water molecule may be involved in as many as four hydrogen bonds at the same time, water is extremely cohesive.

Cohesion and hydrogen bonds

Cohesion -Cohesion causes water molecules to be drawn together, which is why drops of water form beads on a smooth surface.

Surface tension -Cohesion causes a high surface tension, meaning that more force is required to break the surface of a liquid.

Water forms beads -Surface tension resists forces like gravity that would normally break the surface and allows water to form beads; it allows the surface of water to remain intact and maintains the least possible surface area.

The molecules at the surface don’t have to attract molecules above them…

Surface tension -Water allows materials to rest upon it if the surface tension is not broken.

2. Adhesion Adhesion - the sticking together of unlike molecules -Water is a good adhesive; It will cling to many objects and act like a glue.

Adhesion -Adhesion occurs when water’s hydrogen bonds have a greater attraction for other substances, such as glass, than the attraction they have to other water molecules (cohesion).

Capillary action Capillary Action - an example of cohesion & adhesion working together to move water up a thin tube

How does H2O get to top of trees? -Plants can move water through interconnected tube like cells from the roots to where it is needed. -As water evaporates from the surface of leaves, called transpiration, the water below it is pulled up to take its place all the way down to the roots. APBio/TOPICS/04Biochemistry/MoviesAP/03_03WaterTransport_A.swf

How does H2O get to top of trees? -The movement is due to cohesion between water molecules and adhesion of water to plant cell walls. APBio/TOPICS/04Biochemistry/MoviesAP/03_03WaterTransport_A.swf

3. Heat Capacity -Because of the multiple hydrogen bonds between water molecules, it takes a large amount of heat energy to cause those molecules to move faster and raise the temperature of the water. -Water’s specific heat, the amount of heat energy required to increase its temperature, is relatively high.

Changing water temperature -Compared to other substances, water requires the addition of a huge amount of energy to increase its temperature and an equal amount of energy to be removed to decrease its temperature.

Heat Capacity -Large bodies of water, such as oceans and lakes, can absorb or release heat slowly with only small changes in temperature. This protects organisms living within from drastic changes in temperature. -At the cellular level, water absorbs the heat produced by cell processes, regulating the temperature of the cell.

Freezing and boiling -The attraction created by hydrogen bonds keeps water as a liquid over a wider range of temperatures than is found for any other molecule its size. Ex> water has a great difference in its freezing (0ºC) and boiling(100ºC) point

Breaking Hydrogen Bonds -The evaporation of sweat cools the body due to the large amount of heat carried from the body as hydrogen bonds between water molecules break and allow sweat to evaporate, carrying heat with them.

4. “Universal solvent” -Almost all the chemical reactions in life processes occur in solutions of water.  Water is necessary for dissolving essential nutrients that plants and animals need to live, as well as wastes.

Water and polar substances -Polar substances, as well as ionic compounds, have an attraction to H2O What dissolves in water easily? polar or non-polar molecules? How about Oxygen? Does that dissolve in H2O?

Water and nonpolar substances -Nonpolar substances, such as oils, fats, and waxes, don’t have an attraction to H2O -Your cell membrane is insoluble….so you don’t dissolve in water! What dissolves in water easily? polar or non-polar molecules? How about Oxygen? Does that dissolve in H2O? fat (triglycerol)

The universal solvent -Because of its ability to dissolve so many substances, it is called the universal solvent.

Solution - a mixture in which all the components are evenly spread out -Salt is an ionic compound composed of two ions of opposite charge.

Formation of Solutions -Water pulls ions away from the salt crystal and the ions gradually become dispersed in the water, forming the solution.

Solution components Solute + Solvent = Solution -In a saltwater solution, table salt is the solute—the substance that is dissolved. -Water is the solvent—the substance in which the solute dissolves. Solute + Solvent = Solution

solubility Solubility- ability of one substance, the solute, to dissolve in another substance, the solvent “Like dissolves like” ex> polar solvents dissolve polar solutes

5. Density -Ice is less dense than liquid water. -Other substances are more dense as solids. When water freezes and becomes a solid, it expands and becomes less dense because of an increase in volume.  One visible result of this is that solid ice floats in liquid water.

Ice floats because it is less dense than liquid water

Lots of “space” between water molecules in the solid state

Why ice floating is important -Oceans & lakes don’t freeze solid; they freeze from the top down; surface ice insulates water below, allowing life to survive the winter. if ice sank… -ponds, lakes & even oceans would freeze solid -in summer, only the upper few inches would thaw