WATER

What is water?• Chemical substance with the

chemical formula H2O.• It contains one oxygen and two

hydrogen atoms which are connected by covalent bond.

• Life on earth is believed originated from water

• Most abundant liquid on earth and covers 71% of the earth surface

• Exists in the form of solid(ice), liquid(water) or gaseous state(water vapour)

• Makes up between 65-95% by mass of plant and animals

• Essential for life• Unique chemical and physical

properties

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Properties of water

1. Polarity of water

• Water is formed when 2 hydrogen atoms combine with an oxygen atom by sharing electrons and covalent bonds are formed

• The shape of the water molecule is triangular and the angle between the nuclei of the atoms is about 104.50

• For water molecule, there is partial negative charge on oxygen atom and partial positive charge on hydrogen atom

• This unequal distribution of electrical charge makes the water molecule to become polar molecule (water known as dipolar molecule)

• This property makes water a good solvent and important for our daily life (polar and ionic substances can dissolve on it)

• Water molecule forms weak bonds (hydrogen bond) with other water molecules nearby

• One water molecule can connect to other 4 water molecules by hydrogen bonding

• Hydrogen bonding is about 20 times weaker than covalent bond

• Hydrogen bond is an important force to hold together water molecules and makes it stable substance

2. Water and temperature

• Water exists as liquid at room temperature due to hydrogen bonding

• Water molecules can exists in three forms that are solid, liquid or gaseous

• Water has high specify heat of capacity which is 4.2 kJ/kg/K, that means 4.2 kJ is needed to raise the temperature of 1 kg water by 1 K.

• The amount of heat is used to break the hydrogen bonding which restrict the movement of molecules

• This makes water as a good temperature buffer because the temperature of water does not increase as fast as other substances but remain relatively constant

• Water has high latent heat of vaporisation which is about 2260 kJ /kg that means 2260 kJ of heat energy is required to vaporise the water to break the hydrogen bonding so that they can escape as gas

• As water evaporates from a surface, it draws heat energy underneath and creating cooling effect (sweating)

Heat energy is drawn

so that cooling effect occurs

• Water has high latent heat of fusion which is 340 kJ/ kg that means 340 kJ of heat energy is required to melt ice OR loss of heat of liquid water to freeze

• When water at 0 0C, it must lose a lot of heat energy before it forms ice crystals (freeze).

3. Water density• Water is relatively high density

compared to with air and most dense at 40C

• The temperature of bottom of ocean or big lake remain relatively constant at 40C so that the aquatic animals can survive in deep ocean and lake when the environmental temperature falls

• High density of water also allows buoyancy in aquatic animals

• When water freezes, it forms 4 hydrogen bonding with 4 other water molecules

• This makes rigid lattice which hold water molecules further apart

• That’s why water expands when it freezes

• So, ice less dense that liquid water and floats on water

Lattice = geometrical arrangement

4. Water viscosity• Viscosity is the property of a fluid

that resists the force that cause the fluid to flow

• Water has low viscosity due to hydrogen bonds between water molecules continually broken and reformed

• This allow water molecules to slide easily and flow less friction through narrow vessels

• Acts as lubricant to allow food moves down alimentary canal

Alimentary canal

5. Cohesive and adhesive forces and surface tension• Cohesion = attraction between

molecules of similar substances• Adhesion = attraction between

molecules of different substances• Thus water molecules can stick

together and stick to other surfaces• These forces enable water to move

along stem of plants (xylem)

• Capillarity = a process against gravity where attraction between molecules enables water to enter and move along very narrow spaces

• At air-water interface, the inwardly acting cohesive forces produce high surface tension at the surface of water

• This surface tension is strong enough to support the weight of small animal so that they can skipping on it

• Water striders can walk on surface on water because fine hairs at the end of their legs spread their weight over a large area

Other properties of water• Transparent to sunlight• Difficult to compress• Conducts electricity• A neutral medium

Summary of some biologically important

functions of water

Functions Examples1. Cell structure - Important component of protoplasm

(60-95%)2. Solvent and hydrolysis -Solvent for solutes (biochemical

reaction)- hydrolyses polysaccharide into monosaccharides, protein into amino acids, fats into fatty acids and glycerol

3. Metabolites - Water is needed for photosynthesis process

4. Medium for diffusion - Enables diffusion of substances across plasma membrane

5. Transport medium -Blood plasma transport food, gases, hormones, salt…- helps in osmoregulation of body fluids

6. Support -Hydroskeleton for earthworm- amniotic fluid supports and protects foetus

7. Lubricants and secretions -Mucus in alimentary canal-Synovial fluid in joints

8. Habitat - Provide aquatic environment for aquatic organisms like whale

9. Medium for movement, dispersal and migration

-Medium for swimming gametes, dispersal of fruits and seeds- helps to break pericarp of some fruits and their testa for germination

10. Temperature control - Evaporation of sweat cools our body

11. transpiration - Creates tension to pull water and dissolved minerals from root to the aerial parts

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Transpiration

Exercise 1. A water molecule has ______

charge distribution within the molecule. The oxygen atom has a slight _____ charge and the hydrogen atom has a slight _____ charge. It is said to be ______ molecule. One water molecule can form bonds with _______ other water molecules.

2. Water is called the universal _____ because polar and ionic substances can dissolve readily in it.

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