15 August 2014

# 15. Water

About 80% of the organism's body is H2O.
Its molecule has a small negative charge (β-) on the O atom                                        a small positive charge (β+) on each atom. 
                         This is called a dipole.

This makes H2O an excellent solvent.

Hydrogen bond = attraction between (β-) and (β+) parts of neighbouring H2O molecules.

Solvent properties of water
  • The dipoles on H2O molecules make water an excellent solvent. 
  • If you stir NaOH into H2O, the Na+ and Cl- separate and spread between the H2O molecules   --> They dissolve in the water. 
  • The Cl is attracted to the small (+) charge on the H of H2O molecules.
  • The Na+ is attracted to the small (-) charge on the O of H2O molecules.     

Any substance that has fairly small molecules with charges on them, or that can
separate into ions, can dissolve in water.

Being a good solvent, H2O helps:
- To transport substances around the bodies of organisms. The blood plasma of mammals is mostly water, and carries many substances in solution: glucose, oxygen, ions (Na...).
- To dissolve reactants ---> enable metabolic reactions.

Thermal properties of water

1. H2O is liquid at normal Earth to

  • The H bonds between H2molecules prevent them flying apart at normal to. Between 0oand 100oC, water is in the liquid state. The H2O molecules move randomly, forming transitory H bonds with each other.
  • Other substances with similar molecule structure, such as hydrogen sulfide (H2S), are gases at these to (no H bonds to attract their molecules to each other). 

H2O and H2S molecules 
have similar structure.

2. H2O has a high latent heat of evaporation 
  • When a liquid is heated, its molecules gain kinetic energy, moving faster + a lot of heat energy is needed to break H bonds between water molecules. Those molecules with the most energy are able to fly off into the air.
  • When H2evaporates, it absorbs a lot of heat from its surroundings ---> The evaporation of H2O from the skin of mammals when they sweat and the transpiration from plant leaves has a cooling effect
3. H2O has a high specific heat capacity
  • Specific heat capacity is the amount of heat energy that has to be added to a given mass of a substance to raise its tby 1oC
  • The higher the kinetic energy the higher the to: a lot of energy is needed to raise to (to ↑ speed of H2O molecules + break H bonds).
==> Bodies of H2O (oceans, lake) do not change to as easily as air does.
       Bodies of organisms (with large amounts of H2O) do not change teasily.

4. H2O freezes from the top down

Most substances are more dense in solid form than liquid form and will sink if submerged in their liquid state. But H2O is LESS dense in its solid state, and will float. This has to do with the crystal structure of ice. 

When water cools (to↓) the density of water ↑ (molecules lose kinetic energy, getting closer).
Below 4°C this trend is reversed: When H2O approaches freezing point, molecules form a lattice and stretches its very elastic H bonds --> density↓ (lower than density at 4°C) --> Ice floats on water.

The layer of ice acts as an insulator, slowing down the loss of heat from H2O beneath it, which tends to remain at 4°C.

The H2O under the ice remains liquid, allowing organisms to continue to live in it even when air temperatures are below the freezing point of H2O.

Inorganic ions

 Syllabus 2015

(i) describe and explain the roles of water in living organisms and as an environment for organisms;

Syllabus 2016 - 2018

Water  is a special  molecule with extraordinary properties that  make  life possible on this planet  150 million kilometres from the Sun.

d)   explain how hydrogen bonding  occurs between water molecules and relate  the properties of water to its roles in living organisms (limited to solvent action,  specific  heat capacity  and latent  heat  of vapourisation)

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