# 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 H 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 t^o

• The H bonds between H₂O molecules prevent them flying apart at normal t^o. Between 0^oC and 100^oC, 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 t^o (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 H₂O evaporates, 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 t^o by 1^oC. 
• The higher the kinetic energy the higher the t^o: a lot of energy is needed to raise t^o (to ↑ speed of H₂O molecules + break H bonds).

==> Bodies of H₂O (oceans, lake) do not change t^o as easily as air does.
       Bodies of organisms (with large amounts of H₂O) do not change t^o easily.

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 (t^o↓) 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)