properties of water

Water

Properties of Water
Polar moleculeCohesion and adhesionHigh specific heatDensity – greatest at 4oCUniversal solvent of life

Polarity of Water

In a water molecule two hydrogen atoms form single polar covalent bonds with an oxygen atom. Gives water more structure than other liquids Because oxygen is more electronegative, the region around oxygen has a partial negative charge. The region near the two hydrogen atoms has a partial positive charge. A water molecule is a polar molecule with opposite ends of the molecule with opposite charges.


Water has a variety of unusual properties because of attractions between these polar molecules. The slightly negative regions of one molecule are attracted to the slightly positive regions of nearby molecules, forming a hydrogen bond. Each water molecule can form hydrogen bonds with up to four neighbors.

Fig. 3.1

HYDROGEN BONDS
Hold water molecules together Each water molecule can form a maximum of 4 hydrogen bonds The hydrogen bonds joining water molecules are weak, about 1/20th as strong as covalent bonds. They form, break, and reform with great frequency
Extraordinary Properties that are a result of hydrogen bonds. Cohesive behavior Resists changes in temperature High heat of vaporization Expands when it freezes Versatile solvent



Organisms Depend on Cohesion
Cohesion is responsible for the transport of the water column in plants Cohesion among water molecules plays a key role in the transport of water against gravity in plants Adhesion, clinging of one substance to another, contributes too, as water adheres to the wall of the vessels.
Hydrogen bonds hold the substance together, a phenomenon called cohesion


Surface tension, a measure of the force necessary to stretch or break the surface of a liquid, is related to cohesion. Water has a greater surface tension than most other liquids because hydrogen bonds among surface water molecules resist stretching or breaking the surface. Water behaves as if covered by an invisible film. Some animals can stand, walk, or run on water without breaking the surface.

Fig. 3.3

Evaporative Cooling
The cooling of a surface occurs when the liquid evaporatesThis is responsible for:Moderating earth’s climateStabilizes temperature -Preventing - organisms from overheating

Density of Water

Most dense at 4oC Contracts until 4oC Expands from 4oC to 0oC

The density of water:Prevents water from freezing from the bottom up.Ice forms on the surface first—the freezing of the water releases heat to the water below creating insulation.

When water reaches 0oC, water becomes locked into a crystalline lattice with each molecule bonded to to the maximum of four partners. As ice starts to melt, some of the hydrogen bonds break and some water molecules can slip closer together than they can while in the ice state. Ice is about 10% less dense than water at 4oC.

Fig. 3.5

Solvent for Life

Solution Solute solvent Aqueous solution Hydrophilic Ionic compounds dissolve in water Polar molecules (generally) are water soluble Hydrophobic Nonpolar compounds


The hydrogen atom leaves its electron behind and is transferred as a single proton - a hydrogen ion (H+). The water molecule that lost a proton is now a hydroxide ion (OH-). The water molecule with the extra proton is a hydronium ion (H3O+).
Dissociation of Water Molecules

Unnumbered Fig. 3.47