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