Water, Acids, Bases and Salts Chapter 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

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

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)

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’

Capillary action Surface tension

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

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

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

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

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

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