Chapter 3: Water & Life

Cohesion and Adhesion

*Cohesion* - water molecules stick to other water molecules by hydrogen bonding *Adhesion* - water molecules stick to other substance molecules by hydrogen bonding

Changes in Environmental pH Affect Ecosystems

*Acid precipitation*: rain, snow, or fog with a pH lower than 6 (corrosion of aquatic habitats, soil composition, etc.) *Ocean acidification*: decrease in ocean water pH - As atmospheric CO2 levels increase and dissolve in the ocean, levels of carbonic acid in the ocean rise - Leads to a decrease in available calcium for organisms to use in shells and other structures

More solutions

*Acidic solutions* have higher concentrations of H+ than OH- *Basic Solutions* have higher concentrations of OH- than H+ *Neutral solutions* have the same concentrations of H+ and OH- Whenever we know the concentration of either [H+] or [OH-] in an aqueous solution, we can calculate the concentration of the other ion!

Hydrogen Bond

*Hydrogen bond*: a hydrogen covalently bonded to one electronegative atom is attracted to another electronegative atom The electronegative partners are usually hydrogen bonds or polar covalent bonds

The pH scale

*pH*: the negative logarithm of the of the hydrogen ion concentration pH = -log[H+] For a neutral solution, [H+] is 10^-7 M: pH = -log [10^-7] = 7 A pH value < 7 denotes an acidic solution; A pH value > 7 denotes a basic solution (pure water is considered to be neutral)

Buffers resist pH change!

A buffer is a substance that resists changes in pH (overall effect of keeping [H+] relatively constant) Act by: - Releasing hydrogen ions when a base is added - Absorbing hydrogen ions when acid is added

Water is a Polar Molecule

A water molecule has 2 hydrogens joined to 1 oxygen by single covalent bonds Oxygen is *more electro-negative* than hydrogen Polar molecule: a molecule with unequal distribution of charge (2 ends of the molecule have opposite charges)

Water is the Biological Medium

All organisms are made mostly of water Most organisms are surrounded by water Cells are 70-95% water Living organisms require water more than any other substance Water exists in the natural environment in all three states of matter: solid, liquid, and gas

Buffers (cont.)

Most biological buffers consist of a pair of molecules, one an acid and one a base!

Aqueous solution

In an aqueous solution at 25 °C, the product of the H+ and OH- concentrations is constant at 10^-14: [H+][OH-] = 10^-14 In a neutral solution [H+] = 10^-7 & [OH-] = 10^-7

Example: pH Calculations

Measurements show that the pH of a particular lake is 4.0. What is the hydrogen ion concentration of the lake? What is the hydroxide ion concentration of the lake? Equations: pH = -log[H+] [H+][OH-]=10^-14

Hydrogen bonds (cont.)

The hydrogen bonds of liquid water are very fragile (1/20 as strong as a covalent bond) The hydrogen bonds of liquid water are constantly breaking and re-forming

Additional Properties of Water

Water has a *high specific heat* A large amount of energy is required to change the temperature of water Water has a high *evaporative cooling* The evaporation of water from a surface causes cooling of that surface Solid water is less dense than liquid water Bodies of water freeze from the top down

Ice water vs. Liquid water

Water is a good solvent Water dissolves polar molecules and ions Water organizes hydrogen molecules Hydrophilic "water-loving" Hydrophobic "water-fearing" Water causes hydrophobic molecules to aggregate Water can form ions: H2O → OH- (hydroxide ion) + H+ (hydrogen ion)

Water makes life possible!

We live in an environment dominated by water About 3/4 of Earth is covered by water

The single most outstanding chemical property of water is its ability to form *hydrogen bonds!*

Weak chemical associations that form between the partially negative O atoms and the partially positive H atoms of two *separate* water molecules.

*Base*

a substance that *decreases* the hydrogen ion concentration of a solution

*Acid*

a substance that *increases* the hydrogen ion concentration of a solution Ex. Strong acid: Hydrochloric acid Weak acid: Carbonic acid