Water-and-its-Properties-1.pdf

Water and its Properties  Polarity of Water In a water molecule two hydrogen atoms form single polar covalent bonds with an oxygen atom. Gives water more structure than other liquids – Because oxygen is more electronegative, the region around oxygen has a partial...

Water and its Properties  Polarity of Water In a water molecule two hydrogen atoms form single polar covalent bonds with an oxygen atom. Gives water more structure than other liquids – Because oxygen is more electronegative, the region around oxygen has a partial negative charge. – The region near the two hydrogen atoms has a partial positive charge. A water molecule is a polar molecule with opposite ends of the molecule with opposite charges.  Water has a variety of unusual properties because of attractions between these polar molecules. – The slightly negative regions of one molecule are attracted to the slightly positive regions of nearby molecules, forming a hydrogen bond. – Each water molecule can form hydrogen bonds with up to four neighbors. Fig. 3.1 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings  HYDROGEN BONDS Hold water molecules Extraordinary Properties together that are a result of hydrogen Each water molecule can bonds. – Cohesive behavior form a maximum of 4 – Resists changes in hydrogen bonds temperature They form, break, and – High heat of vaporization reform with great – Expands when it freezes frequency. – Versatile solvent  Organisms Depend on Cohesion Hydrogen bonds hold the substance together, a phenomenon called cohesion Cohesion is responsible for the transport of the water column in plants Cohesion among water molecules plays a key role in the transport of water against gravity in plants Adhesion, clinging of one substance to another, contributes too, as water adheres to the wall of the vessels.  Surface tension, a measure of the force necessary to stretch or break the surface of a liquid, is related to cohesion. – Water has a greater surface tension than most other liquids because hydrogen bonds among surface water molecules resist stretching or breaking the surface. – Water behaves as if covered by an invisible film. – Some animals can stand, walk, or run on water without breaking the Fig. 3.3 Copyright ©surface. 2002 Pearson Education, Inc., publishing as Benjamin Cummings Moderates Temperatures on Earth Water stabilizes air temperatures by absorbing heat from warmer air and releasing heat to cooler air. Water can absorb or release relatively large amounts of heat with only a slight change in its own temperature. Celsius Scale at Sea Level What is kinetic energy? Heat? 100oC Water boils Temperature? 37oC Human body Calorie? temperature What is the difference 23oC Room temperature in cal and Cal? 0oC Water freezes What is specific heat?  Specific Heat is the amount of heat that must be absorbed or lost for one gram of a substance to change its temperature by 1oC. Three-fourths of the earth is covered by water. The water serves as a large heat sink responsible for: 1. Prevention of temperature fluctuations that are outside the range for suitable for life. 2. Coastal areas having a mild climate 3. A stable marine environment Evaporative Cooling The cooling of a surface occurs when the liquid evaporates This is responsible for: – Moderating earth’s climate – Stabilizes temperature in aquatic ecosystems – Preventing organisms from overheating  Density of Water Most dense at 4oC Contracts until 4oC Expands from 4oC to 0oC The density of water: 1. Prevents water from freezing from the bottom up. 2. Ice forms on the surface first—the freezing of the water releases heat to the water below creating insulation. 3. Makes transition between season less abrupt.  Universal Solvent Solution – Solute – solvent Most biological reactions take place because of water Hydrophilic – Ionic compounds dissolve in water – Polar molecules (generally) are water soluble Hydrophobic – Nonpolar compounds