1. Water, Acids, Bases and Salts
Chapter 2.2

2. Water About 70% of human body weight Important internal compound:
in photosynthesis
as a solvent for biological reactions
as a reactant or product in many chemical reactions
Also an important external environment for many organisms

3. Water molecules are polar
Oxygen is an electron ‘hog’
Causes an uneven covalent bond
Oxygen side has a slight – charge
Hydrogen sides have a slight + charge
A water molecule forms H bonds with up to four other water molecules

4. Because water is polar…
It is especially good at dissolving other polar compounds as well as ionic ones
Hydrophilic (‘water-loving’) substances dissolve readily in water
Examples: table sugar (polar) and salt (ionic)
Hydrophobic (‘water-fearing’) substances don’t dissolve easily in water and make good structural body parts and storage units
Examples: fats, oils, and waxes (all nonpolar)

5. Hydrogen bonding of water…
Makes water ‘sticky’
Cohesion – water molecules stick to each other
Adhesion – water molecules stick to other surfaces
These two properties explain:
Capillary action – the tendency of water to rise in a narrow tube even against the force of gravity – how water rises in plants
Surface tension – makes the top layer of water ‘stronger’

6. Capillary action
Surface tension

7. Hydrogen bonding also means…
Water has a high specific heat
The amount of energy required to raise the temperature is very large
This means that the temperature of water does not fluctuate as easily as air
Aquatic environments (both external and internal) do not change temperatures as quickly as the surroundings
Stable internal temperatures are necessary for the chemical reactions needed for life

8. Ice formation is related to hydrogen bonds
In water vapor, the hydrogen bonds between the water molecules are broken
In liquid water, the hydrogen bonds constantly break and re-form
In ice, the bonds are firmly fixed, resulting in a regular, evenly distributed crystalline lattice structure
This means ice is less dense than liquid water and floats
This has important biological consequences

10. Acids and Bases
Water molecules have a slight tendency to ionize into hydrogen ions (H+) and hydroxide ions (OH-)
In pure water the number of H+ = OH-
The solution is neutral
Acid – solution produces more H+ ions
It is a proton donor
Base – solution produces more OH- ions
It is a proton acceptor

11. pH scale The negative logarithm of the hydrogen ion concentration
Neutral = 7
Acidic < 7
Basic > 7
Most living things stay between 7.2 to 7.4

12. Buffers Resists changes in pH when an acid or base is added
Includes a weak acid and a weak base
Important in biological systems in order to maintain homeostasis

13. Salt formation Acid + base  water + salt HCl + NaOH  H2O + NaCl
Salt = metal and a nonmetal
A salt, acid or base dissolved in water will conduct an electric current: electrolytes