Water

Water is the most abundant substance in living systems,making up 70% or more of the weight of most organisms. The first living organisms doubtless arose in an aqueous environment, and the course of evolution has been shaped by the properties of the aqueous medium in which life began.

Properties of water

1- Polarity of Water In a water molecule two hydrogen atoms form single polar covalent bonds with an oxygen atom. 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.

HYDROGEN BONDS 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

2-Cohesion and adhesion - 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 plant. Cohesion ; Attraction between particles of the same substance (why water is attracted to itself) Results in Surface tension (a measure of the strength of water’s surface) Produces a surface film on water that allows insects to walk on the surface of water

Surface tension, a measure of the force necessary to stretch or break the surface of a liquid. 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.

Cohesion Helps insects walk across water

Adhesion Also Causes Water to Form spheres & hold onto plant leaves Attach to a silken spider web

Water Chemistry 12 Capillary action-water molecules will “tow” each other along when in a thin glass tube. Capillary action is facilitated by both the adhesion and cohesion properties of water

3- High Heat of Vaporization Amount of energy to convert 1g or 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.

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 o C). Water has a high heat of vaporization. - The evaporation of water from a surface causes cooling of that surface.

4-Specific Heat is the amount of heat that must be absorbed or lost for one gram of a substance to change its temperature by 1 o C.

5- Density of water - Ice water is less dense than liquid water that is why it floats, thus Bodies of water freeze from the top down. - Liquid water has hydrogen bonds that are constantly being broken and reformed. - Frozen water forms a crystal-like lattice whereby molecules are set at fixed distances.

When water reaches 0 o C, water becomes locked into a crystalline lattice with each molecule bonded 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 4 o C.

6- Water as solvents 19 Water is a good solvent. - Water dissolves polar molecules and ions

Solution

Hydrophobic and hydrophilic interactions

Osmosis

Ionization of water

When there are exactly equal concentrations of H and OH, as in pure water, the solution is said to be at neutral pH. At this pH, the concentration of H and OH can be calculated from the ion product of water as follows:

The pH scale

Acids and bases

Weak Acids and Bases Have Characteristic Dissociation Constants Hydrochloric, sulfuric, and nitric acids, commonly called strong acids, are completely ionized in dilute aqueous solutions; the strong bases NaOH and KOH are also completely ionized. Of more interest to biochemists is the behaviour of weak acids and bases—those not completely ionized when dissolved in water. The behaviour of aqueous solutions of weak acids and bases is best understood if we first define some terms.

Acids may be defined as proton donors and bases as proton acceptors. A proton donor and its corresponding proton acceptor make up a conjugate acid-base pair Acetic acid (CH3COOH), a proton donor, and the acetate anion (CH3COO), the corresponding proton acceptor, constitute a conjugate acid base pair, related by the reversible reaction:

Dissociation constant Ka