Water is the most abundant inorganic compound in the human body – 66%! It is the transport/exchange medium in organisms (e.g. plasma, cytoplasm)

Properties of Water

1. Water is a polar molecule will dissolve other polar molecules (hydrophilic: water-loving) therefore, water is called the “universal” solvent solutes can be gases (e.g. O 2 and CO 2 ), liquids, or solids (e.g. proteins, sugars) But… water cannot dissolve non-polar molecules (these are called hydrophobic: water-hating)

2. High specific heat capacity absorbs/releases large amounts of heat helps maintain constant body temperature

3. High specific heat of vaporization good evaporative coolant sweating, panting cools the organism

4. Density Density of solid water (ice) allows it to float - anomaly! Water is most dense at 4⁰C Evolution - life on this planet began in the ocean because organisms could survive under the ice

5. Cohesion cohesive attraction to itself hydrogen bonds high surface tension e. g. water droplets, water striders

Surface tension…

6. Adhesion adhesive attraction to other polar molecules hydrogen bonds, dipole-dipole capillary action e. g. movement of water up the stem of a plant

Cohesion vs. Adhesion

7. Ionization its ability to separate (or dissociate)

A hydrogen atom moves from one water molecule to another water molecule Actually, a proton (H + ) leaves and the electron is left behind! H + then gets picked up by H 2 O  H 3 O + hydronium So, water can dissociate into a hydroxide ion (OH - ) and a hydronium ion (H 3 O + or H + or proton ) Pure water has an equal concentration of H + and OH - Adding substances, however, may change these concentrations

“Water is life's matrix, mother and medium. There is no life without water.” Albert Szent-Gyorgyi, Hungarian Biochemist 1937 Nobel Prize for Medicine