1.1 Water

Properties of Water(a) describe chemical properties (solvent, bond angles & hydrogen bond) of water and relate its physiological roles in the organisms; (b) describe the physical properties (polarity, cohesiveness, density, surface tension, specific heat capacity, & latent heat of vaporisation) of water and relate its physiological roles in organisms.

Water MoleculeWater Molecule

• Polarity– Water molecule is polar – uneven distribution of e- between

O & H atoms.

(—)

– Water molecule is neutral

(+)

(+)

Water molecule has polar covalent bond.

Polar and Non-Polar Covalent Bonding

Some atoms have an unequal affinity for electrons.

Covalent bonds of H2O are highly polar.

The shared e- spend more time around one atom relative to another and the covalent bond is polar.

Oxygen draws e- to itself much more strongly than H.

Polar and Non-Polar Covalent Bonding

Polar and Non-Polar Covalent Bonding

The polar versus non-polar distinction determines which molecules will dissolve in a particular solute.

For example, sugar dissolves in water, but fat doesn’t.

Molecular Shape

Molecules have distinct shapes – and shape matters.

Summary

• Water is a polar molecule

• It forms weak hydrogen bonds

• It remains a liquid over a wide temperature range

• Water molecules stick to one another = cohesion (surface tension)

• Water molecules stick to other substances = adhesion (capillarity)

O

H

H

+

+-

Properties of Water

• Polar molecule• Cohesion -Surface Tension• Adhesion• High Heat of Vaporization• High specific heat

Cohesion

H-bond between adjacent water molecules

AdhesionAdhesion• Attraction between Attraction between

molecules of different molecules of different substancessubstances

• Responsible for capillary Responsible for capillary forces in plantsforces in plants

Adhesion

Water can also be attracted to other surfaces.

Capillary Action

Capillary action is a result of adhesion and cohesion.

1. High heat of vaporization [evaporation]

- it also involves the breaking of hydrogen bonds, water resists vaporizing (evaporating).

- Consequently, it takes a lot of heat to evaporate water.

- high heat of vaporization is utilized by organisms as a cooling process, e.g., sweat or panting.

High specific heat - a temperature increase is an increase in the motion of the molecules & atoms making up a substance.

- The cohesion of water molecules resist increasing their motion. Consequently, water resists heating; water has a very high specific heat.

- This tendency to not want to change temperature causes bodies of water (e.g., a lake) to strongly resist rapid changes in temperature.

- This temperature buffering capacity of water is taken advantage of to a great extent by organisms.

Surface Tension

•Species - Basiliscus basiliscus.

Cohesion•The attraction

between molecules of the same substance (e.g. water).

•Allows some insects and spiders to walk on water.

Surface tension - a measure of the strength of the water's surface film. - The attraction between water molecules creates a strong film, which among other common liquids is only surpassed by that of mercury.

• The surface tension permits water to hold up substances heavier & denser than itself.

• Some aquatic insects - water strider rely on surface tension to walk on water.

Water

• It has a high latent heat of vaporisation – so animals use water to cool themselves

• It has a high specific heat capacity – so water can maintain a reasonably constant temperature (homeostasis)

• It is less dense as a solid (ice)…• … and ice is a poor conductor• Water is a good solvent

Quiz

1. Which of the following is not an important

property of water

a) Its polar nature

b) Its low specific heat capacity

c) Its high latent heat of vaporisation

d) Its low density in solid form

Hydrogen BondsHydrogen Bonds• Polar water molecules act like

magnets and attract each other • Hydrogen Bonds

– The attraction of the Hydrogen end (+) of one molecule for the Oxygen end (-) of another water molecule.

– They are the strongest bonds that can form between molecules

Hydrogen BondsHydrogen Bonds

Solutions & Solutions & SuspensionsSuspensions

•Water is usually part of a mixture.

•There are two types of mixtures:–Solutions–Suspensions

SolutionSolution• Ionic compounds disperse as Ionic compounds disperse as

ions in waterions in water• Evenly distributedEvenly distributed• SOLUTESOLUTE

– Substance that is being Substance that is being dissolveddissolved

• SOLVENTSOLVENT– Substance into which the Substance into which the

solute dissolvessolute dissolves

SolutionSolution

SuspensionsSuspensions• Substances that Substances that

don’t dissolve don’t dissolve but separate but separate into tiny pieces.into tiny pieces.

• Water keeps Water keeps the pieces the pieces suspended so suspended so they don’t they don’t settle out.settle out.

Acids, Bases & pHAcids, Bases & pH• 1 water molecule in 550 million 1 water molecule in 550 million

naturally dissociates into a naturally dissociates into a Hydrogen IonHydrogen Ion and a and a Hydroxide IonHydroxide Ion

Hydrogen Ion Hydroxide IonHydrogen Ion Hydroxide Ion Acid Acid BaseBase

H2O H+ + OH-

The pH ScaleThe pH Scale• Indicates the concentration of H+

ions• Ranges from 0 – 14• pH of 7 is neutral• pH 0 – 6.99 is acid … H+ • pH 7.01 – 14 is basic… OH- • Each pH unit represents a factor

of 10 change in concentration

AcidsAcids•Strong

Acid = pH 1-3

H+ ions

BasesBases•Strong

Base = pH 11 – 14 OH-ions

Buffers• Weak acids or bases that react

with strong acids or bases to prevent sharp, sudden changes in pH.

Weak AcidWeak Acid Weak BaseWeak Base

• Some biologically important functions of water• All organisms• - Structure — high water content of protoplasm• - Solven and medium for diffusion• - Reagent in hydrolysis• - Support for aquatic organisms• - Dispersal of seeds, gametes & larval stages of

aquatic organisms, and seeds of some

terrestrial species e.g. coconut• Multipurpose solvent: medium for metabolism and transport.• Takes part in metabolic reactions

Plants-Osmosis and turgidity (important in many ways. such as cell enlargement, support, guard cell mechanism) -Reagent in photosynthesis-Transpiration and translocation of inorganic ions and organic compounds-Germination of seeds — swelling and breaking open the testa and further development

Animals-Osmoregulation-Cooling by evaporation, -- sweating, panting-Lubrication, as in joints-Support — hydrostatic skeleton-Protection, for example lachrymal fluid, mucus-Migration in ocean currents.-Fluid component: 90% of cytoplasm, 92% of blood plasma, 97% of tissue fluid and lymph.

Photosynthesis: water is a raw material in the light stage. - Respiration: aerobic respiration produces water. - Anabolism: produced when macromolecules are made (condensation). - Catabolism: water is used to break the bonds in macromolecules (hydrolysis). - Movement of materials through cell membranes: diffusion, osmosis and active transport.

Water:Importance of Water for OrganismsO Turgor plays an important role in the support of plants. Good absorber of heatO Water is a temperature-stable medium - which is important for homeostasis.O Vaporisation of water is an excellent cooling mechanism.