Chapter 3 Properties of Water
Key Terms: surface tension, specific heat, hydrophilic, hydrophobic, hydrogen ion, hydroxide ion, pH
scale, buffer
WATER : THE CRADLE OF LIFE
Unique Properties of Water Necessary for Living Organisms
Exists as liquid at temperature of earth's surface
Provides a medium in which other molecules can interact
Composes two-thirds of most organisms
Forms weak chemical associations (hydrogen bonds)
Simple atomic structure, H2O
Water Acts Like a Magnet (Fig. 3.1)
Electronegativity attracts electrons of water molecules
Has distinct ends, each with a partial charge
Polar molecule results from magnet like poles
Polarity is crux of chemistry of water and life
Charge separation results in polar nature
Most stable configuration is tetrahedron, bond angle 104.50
Partial (d+) charges at apexes opposite hydrogens
Partial (d-) charge at oxygen
Polar molecules interact with one another
Opposite charges attract, form hydrogen bonds
Bonds are transient, cumulative effects important
Hydrogen bonds affect physical properties of water
Water Clings to Polar Molecules
Cohesion is attraction of water to water
Results in surface tension of water
Causes things to get wet in water
Adhesion is attraction of water to another molecule
Attraction is electrostatic
Results in capillary action, water rises in thin tube
Height inversely proportional to tube diameter
Water Stores Heat
Exhibits high specific heat
Amount of heat to change temperature of a substance
Associated with and proportional to polarity
Thermal energy must first disrupt hydrogen bonds
Heats up slowly
Retains heat longer than surroundings
Forms ice with decrease in temperature
Crystal-like lattice of hydrogen bonds
Less dense than liquid water
High heat of vaporization
Amount of heat required to change water to vapor
Evaporation of water produces cooling effect
Water Is a Powerful Solvent
Water molecules gather around charged molecules
Example: table salt
Water forms hydrogen bonds with Na+ and Cl- groups of salt
Each salt ion is surrounded by cloud of water molecules
Cloud is called the hydration shell
Negative faces Na+ and positive H’s face the Cl- (Fig. 3.7)
Water Organizes Nonpolar Molecules
Water excludes nonpolar molecules
Preferentially forms hydrogen bonds with itself
Minimizes disruption of hydrogen bonding
Hydrophobic: not soluble in water, nonpolar
Hydrophilic: soluble in water, polar
Hydrophobic exclusion
Forces nonpolar molecules to associate together
Shapes molecules with nonpolar regions
Water Ionizes
Ionization is spontaneous formation of ions
Results from breaking of covalent bonds of water
Proton (H+) dissociates from molecule
Remainder of molecule is OHMole of a substance is its molecular mass
Corresponds to combined atomic mass of all molecules
Molar concentration of H+ ions in water is 10-7 mole/liter
pH scale quantifies H+ concentration
pH = negative log of H+ ion concentration = -log[H+]
Acid = low pH value, <7, high concentration of H+
Base = high pH value, >7, low concentration of H+
Scale is logarithmic, change of one on scale is really tenfold
Changes in environmental pH caused by acid precipitation
Serious impact on living organisms
Erodes even limestone and marble
Buffers
pH of body fluids is about 7.4
Minimize changes in H+ and OH- concentration
Act as reservoirs for H+
Donate H+ to solutions when concentration of H+ falls (↑pH)
Take H+ from solutions when concentration H+ increases (↓pH)
Example: carbonic acid ↔ bicarbonate ion + H+ in blood