Water Chemistry & Properties of Water Presentation courtesy of www.cascade.k12.mt.us
Water is a Polar Molecule -has oppositely charged ends • Water consists of an oxygen atom bound to two hydrogen atoms by two single covalent bonds. • Oxygen has unpaired & paired electrons which gives it a slightly negative charge while Hydrogen has no unpaired electrons and shares all others with Oxygen • Leaves molecule with positively and negative charged ends
Water molecules form Hydrogen bonds slightly positive charge hydrogen bond between (+) and (-) areas of different water molecules slightly negative charge
Water’s Properties • Cohesion • Adhesion • Capillarity • High Specific Heat • High Heat of Vaporization • Solid water (ice) is less dense than liquid • Solvent • Transparent
Cohesion • Water clings to polar molecules through hydrogen bonding • Cohesion refers to attraction to other water molecules. • responsible for surface tension • a measure of the force necessary to stretch or break the surface of a liquid
Adhesion • Adhesion refers to attraction to other substances. • Water is adhesive to any substance with which it can form hydrogen bonds.
water evaporates from leaves = transpiration All thanks to hydrogen bonding! Capillary action adhesion, cohesion and capillary action water taken up by roots
trees have specialized structures to transport water: xylem and phloem “plumbing” • water molecules are “dragged” from the roots to the top of the tree by capillary action and cohesion: hydrogen bonds help water molecules to each other
High Specific Heat • Highspecific heat • Amount of heat that must be absorbed or expended to change the temperature of 1g of a substance 1o C.
Impact of water’s high specific heat ranges from the level of the whole environment of Earth to that of individual organisms. • A large body of water can absorb a large amount of heat from the sun in daytime and during the summer, while warming only a few degrees. • At night and during the winter, the warm water will warm cooler air. • Therefore, ocean temperatures and coastal land areas have more stable temperatures than inland areas. • The water that dominates the composition of biological organisms moderates changes in temperature better than if composed of a liquid with a lower specific heat. The Earth is over 75% water!
High Heat of Vaporization • Highheat of vaporization • Amount of energy required to change 1g of liquid water into a gas (586 calories). • large number of hydrogen bonds broken when heat energy is applied
As a liquid evaporates, the surface of the liquid that remains behind cools - Evaporative cooling. • Evaporative cooling moderates temperature in lakes and ponds and prevents terrestrial organisms from overheating. • Evaporation of water from the leaves of plants or the skin of animals removes excess heat.
“Universal” Solvent • A liquid that is a completely homogeneous mixture of two or more substances is called a solution. • A sugar cube in a glass of water will eventually dissolve to form a uniform mixture of sugar and water. • The dissolving agent is the solvent and the substance that is dissolved is the solute. • In our example, water is the solvent and sugar the solute. • In an aqueous solution, water is the solvent. • Water is not really a universal solvent, but it is very versatile because of the polarity of water molecules.
Water is an effective solvent as it can form hydrogen bonds. • Water clings to polar molecules causing them to be soluble in water. • Hydrophilic - attracted to water • Water tends to exclude nonpolar molecules. • Hydrophobic - repelled by water
Water transports molecules dissolved in it • Blood, a water-based solution, transports molecules of nutrients and wastes organisms • Nutrients dissolved in water get transported through plants • Unicellular organisms that live in water absorb needed dissolved substances
Solid water (ice) is less dense than liquid • Ice is less dense than water: the molecules are spread out to their maximum distance Density = mass/volume same mass but a larger volume
Oceans and lakes don’t freeze solid because ice floats • water expands as it solidifies • water reaches maximum density at 4-degrees C • water freezes from the top down • organisms can still live in the water underneath the ice during winter
Water is Transparent • The fact that water is clear allows light to pass through it • Aquatic plants can receive sunlight • Light can pass through the eyeball to receptor cells in the back
pH • Water ionizes into H+ and OH- • H2O H+ and OH- • pH scale expresses hydrogen ion (H+) concentration in a solution. • logarithmic scale ranging from 0-14 • neutral = 7 • Below 7 = acid • Above 7 = base • Water at 25oC contains 1/10,000,000 mole of H+ ions = 10 -7 moles/liter • pH = -log [H+]
Acids • Acids dissociate in water to increase the concentration of H+. • Have many H+ ions • Sour taste • HCl is hydrochloric acid or stomach acid
Bases • Bases combine with H+ ions when dissolved in water, thus decreasing H+ concentration. • Have many OH- (hydroxide) ions • Bitter taste • NaOH = sodium hydroxide or baking soda
Buffers • Buffers • act as a reservoir for hydrogen ions, donating or removing them from solution as necessary • Offer protection from extreme pH levels • Produced naturally by organisms: • Organisms can’t tolerate much pH change • Cells function best within a narrow pH range