Water

Water (H2O)

• Most abundant substance in living systems .• Essential for all forms of life• Makes up 70 – 90% of the weight of most living organisms.• H and O atoms in a water molecule are linked by covalent bonds.• Each water molecule is linked to others by hydrogen bonds ,which is

responsible for many of the unique properties of water (high melting, , high heat of vaporization and cohesion adhesion properties.

• All the major components in cells (proteins, DNA, RNA

and polysaccharides) can dissolve in water .• Water and its ionization products OH- and H3O+ ions are important

determinants of the characteristic structures and properties of different biomolecules.

Properties of Water• Polar molecule• Cohesion and adhesion• High specific heat• High heat of

vaporization• Density – greatest at 4oC• Universal solvent of life

1-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.

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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 in liquid form

• Extraordinary Properties that are a result of hydrogen bonds.– Cohesive behavior– Resists changes in

temperature– High heat of vaporization,

high melting temperatures.– 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.

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Fig. 3.3

Cohesion …

Helps insects walk across water

Adhesion

Adhesion ; Attraction between two different substances.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.Example: transpiration process which plants and trees remove water from the soil, and paper towels soak up

water.

Adhesion Also Causes Water to …

Form spheres & hold onto

plant leaves

Attach to a silken spider

web

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Water Chemistry

adhesion

Capillary action-water molecules will “tow” each other along when in a thin glass tube. Capillary action is facillitated 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• The heat of vaporization is a direct measure of the

amount of energy required to overcome the attractive forces between adjacent molecules in a liquid so that individual molecules can escape from each other and enter the gaseous state.

• 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 .

Water has a high heat of vaporization.

- The evaporation of water from a surface causes cooling of that surface.

Evaporative Cooling• The cooling of a

surface occurs when the liquid evaporates

• This is responsible for:– Moderating earth’s

climate– Stabilizes

temperature in aquatic ecosystems

– Preventing organisms from overheating

Three-fourths of the earth is covered by water .The high Specific Heat of water is responsible for:

1. Prevention of temperature fluctuations that are outside the range suitable for life.

2. Coastal areas having a mild climate

3. A stable marine environment

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 1oC.

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 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.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 3.5

6-Solvent for Life• The polarity and hydrogen bonding properties of the water moleculemakes it a potent solvent for many ionic compounds and neutral molecules . • Water also disperses amphipathic molecules such as soaps to form micelles , which are clusters ofmolecules arranged such that the hydrophobic groups are hidden from exposure to

water and the hydrophilic (polar) groups are on the external surface exposed to water .

Hydrophobic and hydrophilic interactions

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Solvent properties Water is a good solvent for polar and ionic substances. The polarity and hydrogen-bonding capability of water makes it an excellent solvent for polar molecules. The reason is that water greatly weakens electrostaticforces and hydrogen bonding between ionic and polar molecules by competing for their attractions..

Solvent properties

The existence of life on Earth depends critically on

the capacity of water to dissolve

a remarkable array of polar

molecules that serve as fuels, building blocks,

catalysts, and information carriers.

High concentrations of these polar molecules can coexist in water, where they are free to diffuse and interact with one another.

• Occasionally, a hydrogen atom shared by two water molecules shifts from one molecule to the other.– 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

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Unnumbered Fig. 3.47

• A simpler way to view this process is that a water molecule dissociates into a hydrogen ion and a hydroxide ion:–H2O <=> H+ + OH-

• This reaction is reversible.• At equilibrium the concentration of water

molecules greatly exceeds that of H+ and OH-.

• In pure water only one water molecule in every 554 million is dissociated.–At equilibrium, the concentration of H+

or OH- is 10-7M (25°C) .