Everything you'll need for OCR A-Level Biology:
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-Key equations and their application
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1.1 : understand the importance of water as a solvent in transport, including its dipole nature
It is a covalent compound of hydrogen and oxygen.
Hydrogen atom is having a slight positive charge (δ+) and oxygen atom is having a slight negative
(δ-) charge.
It is dipolar in nature.
Water molecules form hydrogen bond between them.
Water has high melting and boiling point because of this hydrogen bonding between them.
POLARITY OF WATER MOLECULE
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 (hydrogen
atom with partial positive and oxygen with partial negative charge).
Properties of water
1) Unusual and excellent solvent
2) High surface tension
3) High latent heat of vaporization
4) High specific heat capacity
5) Density
1
,Universal solvent
Almost all the substances will dissolve in water due to its dipolar nature. All the polar or ionic substances
dissolve in water. These substances can form hydrogen bond with water.
It can form colloids (solute particles are larger than solvent) with non-polar substances.
It can form emulsion (liquid liquid) or suspension (liquid solid)
Dissolving sodium chloride in water
NaCl dissolve in water. The positive and negative ions of sodium chloride separate and become
surrounded by water molecules which keep them in solution.
Slightly negative Oxygen atom of the water molecules are attracted by the positively charged
sodium ions and slightly positive hydrogen atoms are attracted by the negatively charged chloride
ions.
How the dipolar nature of water is essential for living organisms.
Water can form hydrogen bonds
Water is a solvent and polar molecule can dissolve in water and can be transported.
Cohesion and adhesion lead to high surface tension
Hydrogen bonds holding water together as a liquid, so that it can move in mass flow systems
Has high specific heat capacity and high latent heat of vaporisation.
This helps to maintain body temperature without much fluctuation
The polarity of water allows it to dissolve other polar molecules.
High surface tension
The property of the liquid where it behaves like a stretched membrane.
High surface tension of water is due to the presence of hydrogen bonds
It is of great importance in plant transport systems and also affects life at the surface of ponds, lakes and
other water masses.
2
,Water behaves as if covered by an invisible film
Some animals can stand, walk, or run on water without breaking the surface.
This provides an excellent capillary action.
High latent heat of vaporization
It is the amount of heat energy needed to convert 1kg of water to its vapour state at a constant
temperature.
This property of water makes it a good coolant, as it absorbs heat from the body to evaporate.
Helps stabilize temperatures in organisms and bodies of water
Prevent organisms from Overheating.
3
, High latent heat of fusion
It is the amount of heat energy needed to convert 1kg of ice into water or water into ice at constant
temperature.
It ensures that water bodies do not freeze easily in cold climate.
Prevent cytoplasm from freezing at low temperatures.
Density and freezing properties
Most dense at 4oC
Contracts until 4oC
On further cooling it expands from 4oC to 0oC
Ice is less dense than water and tends to float on water
Prevent 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.
It reduces heat loss from the water body
It allows aquatic organisms to live underneath
High specific heat capacity
This is the amount of heat energy needed to increase the temperature of 1 kg of water by 1°C. (4200J/kg)
For a small change in temperature it needs to add or remove a lot of heat.
This prevents sudden fluctuations in temperature of aquatic environments.
This is extremely useful for organisms,
Helping them to avoid rapid changes in their internal temperature
Enabling them to maintain a steady temperature even when the temperature in their surroundings varies
considerably.
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