Basic Chemistry for Biology
Water and Solutions
Water’s Life Supporting Properties
Important to all living things1. Moderation of temperature2. Lower density of ice3. The solvent of life4. Sensitivity to acidic and basic
conditions
1. Moderation of Temperature
• Water resists changes in temperature– Hydrogen bond formation releases heat– Heat absorption is required to break hydrogen
bonds– The large numbers of hydrogen bonds in water
increase the amount of heat required to break them
2. Lower Density of Ice
• Water is less dense as a solid than a liquid– Due to hydrogen bonding– Stable hydrogen bonds form between
neighboring molecules as water freezes– Molecules are less densely packed ice floats– Forms an insulating layer protecting liquid water,
allowing life forms to survive cold temperatures
Liquid water:Hydrogen bondsform and break
Ice: Stablehydrogen bonds
3. The Solvent of Life
• Water is a versatile solvent due to its polarity– Aqueous solutions –
water is the solvent– Polar solutes dissolve
easily in water– Charged (ionic) solutes
dissolve and dissociate due to attractions to polar water molecules
Figure 2.13
Ion insolution
Salt crystal
4. Sensitivity to Acidic & Basic Conditions
• In aqueous solutions, a small percentage of water molecules break apart into ions– Some are hydrogen ions, others are hydroxide ions– Compounds that release H+ ions to a solution are
acidic– Compounds that release OH- ions to a solution are
basic– The pH scale describes how acidic or basic a
solution is
Acidic & Basic Solutions
Figure 2.14_3
Acidicsolution
Neutralsolution
Basicsolution
pH Scale
• The pH scale ranges from 1 to 14– 7 is neutral – pure water – equal numbers of H+ and
OH- ions– pH values under 7 are acidic – more H+ ions than OH-
ions– pH values over 7 are basic – more OH- ions than H+ ions– Each number on the scale represents 10X as many H+
ions as the next higher number– Buffers minimize changes in pH
• Accept H+ when in excess acid• Donate H+ when in excess base