Water Chemistry & Properties of Water 1

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 is a Polar Molecule -has oppositely charged ends 2

Water molecules form Hydrogen bonds slightly positive charge hydrogen bond between (+) and (-) areas of different water molecules slightly negative charge 3 3

High Heat of Vaporization Solid water (ice) is less dense than liquid Water’s Properties Cohesion Adhesion Capillarity High Specific Heat High Heat of Vaporization Solid water (ice) is less dense than liquid Solvent Transparent 4

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 5

Adhesion refers to attraction to other substances. Water is adhesive to any substance with which it can form hydrogen bonds. 6

High Specific Heat High specific heat Amount of heat that must be absorbed or expended to change the temperature of 1g of a substance 1o C. 7

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! 8

High Heat 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 9

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. 10

“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. 11

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 12

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 13