1. Basic Chemistry for Biology Water and Solutions

2. Water’s Life Supporting Properties Important to all living things 1.Moderation of temperature 2.Lower density of ice 3.The solvent of life 4.Sensitivity to acidic and basic conditions

3. 1. Moderation of Temperature Water resists changes in 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

4. 2. Lower Density of Ice Water is less dense as a solid than a liquid 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 bonds form and break Ice: Stable hydrogen bonds

5. 3. The Solvent of Life Water is a versatile solvent due to its polarity 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 in solution Salt crystal

6. 4. Sensitivity to Acidic & Basic Conditions In aqueous solutions, a small percentage of water molecules break apart into ions 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

7. Acidic & Basic Solutions Figure 2.14_3 Acidic solution Neutral solution Basic solution

8. pH Scale The pH scale ranges from 1 to 14 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 Accept H + when in excess acid Donate H + when in excess base Donate H + when in excess base