Water and the Fitness of the Environment Chapter 3

Unique Properties of Water Powerful, versatile solvent – due to polarity Cohesive & adhesive behavior High Surface Tension High Specific Heat High Boiling Point – High Heat of Vaporization Good evaporative coolant High freezing point and lower density as a solid than a liquid (density anomaly)

Solutions Solution: a liquid that is a completely homogeneous mixture of two or more substances. consists of solvent (dissolving agent) and solute (substance that is dissolved) If water is the solvent, the solution is an aqueous solution

Polar Molecules Easily Surround & Separate! Polar water molecules surrounding and separating salt…

Hydration Shell http://www. sumanasinc Refers to the sphere of water molecules around each dissolved ion. Water will work inward from the surface of the solute until it dissolves all of it (provided that the solute is soluble in water)

Hydrophilic v/s Hydrophobic Hydrophilic – substances that have an affinity for water Does not mean that the substance has to dissolve – cotton fibers have a high affinity for water, so make great towels!!! But cotton absorbs water without dissolving! Hydrophobic – substances that fear or repel water “water fearing” - form clusters called hydrophobic interactions. Major components of cell membranes! non-ionic and nonpolar substances Ex. Vegetable oil

Cohesion Defined as the attraction of water to other water molecules. Results from hydrogen bonding

Adhesion Adhesion is the attraction of different substances to one another. Ex. Water to side of straw

Capillarity Cohesion and adhesion work together to give capillarity – the ability of water to spread through fine pores or to move upward through narrow tubes against the force of gravity.

Capillarity in Plants - Xylem

Surface Tension Surface Tension is a measure of how difficult it is to stretch or break the surface of a liquid. The interface between water and air is an ordered arrangement of water molecules that are hydrogen bonded to one another and to other water molecules below; So…water behaves as if coated with an invisible film. Many organisms us this property of water to stand, walk, or run on water without breaking the surface Ex. water strider

Surface Tension

Resistance to Temperature Changes A greater input of energy is required to raise the temperature of water than the temperature of air! That’s why see big differences in water temperature when we are swimming…

Water Temperature

Specific Heat Specific Heat is the amount of heat that must be absorbed or lost for 1 gram of that substance to change by 1 degree Celsius. calorie – the amount of heat energy it takes to raise the temperature of 1 gram of water 1 degree Celsius. kilocalorie – quantity of heat required to raise the temperature of 1 kg of water by 1 degree Celsius. (“calories” on food packages are actually kilocalories) High specific heat of water is due to hydrogen bonding – H-bonds tend to restrict molecular movement, so when we add heat energy to water, it must break bonds first rather than increase molecular motion.

Homeostasis & High Specific Heat World is covered mostly by water. Large bodies of water can absorb and store a huge amount of heat from the sun in the daytime and during summer while warming up only a few degrees. Gradually cooling water warms air at night. Organisms are made primarily of water, so these properties help to maintain body temperatures. This is vital because many biologically important chemical reactions take place in a very narrow temperature range.

Heat of Vaporization Quantity of heat a liquid must absorb for 1 gram of it to be converted from the liquid to the gaseous state. To evaporate one g of water at 25 degrees Celsius, about 580 cal of heat is needed – double that needed for 1 g of alcohol or ammonia. Again, caused by hydrogen bonds… On earth, this helps moderate Earth’s climate – much of the solar heat absorbed by tropical seas is consumed during evaporation of surface water. Then, as moist tropical air circulates poleward, it releases heat as it condenses to form rain.

Ice, Water, and Steam

Evaporative Cooling As a liquid evaporates, heat is taken away and the surface of the remaining liquid cools down. Contributes to the stability of temperature in lakes and ponds. Provides mechanism for cooling organisms – sweating.

Evaporative Cooling

Freezing Normally, density increases as temperature decreases. This occurs because molecules are moving more slowly and so, spaces between them decrease, leading to more molecules in same volume. This occurs with water until it nears 4 degrees Celsius – here, the water molecules come so close together that every one of them can form H-bonds simultaneously with 4 other molecules.

Density Anomaly Due to geometry of water molecule, they must move slightly apart to maintain the max number of H bonds in a stable structure. So at Zero degrees Celsius, an open latticework is formed, allowing air in – thus ice becomes less dense than liquid water floats on top of the water.

Density Anomaly

Ice Floats! frozen benzene ice

Density of Ice is Crucial to Life If water continued to contract as it froze, it would be heavier than liquid water. Bodies of water would freeze from the bottom up, not melt, and all life would be destroyed. Instead, layer of ice effectively insulates water below it, keeping temperatures at or above the freezing point of water. When ice freezes, it releases heat to surroundings, so ice and snow act as temperature stabilizers, particularly in transition periods, and allows organisms time to make seasonal adjustments essential to survival.

Floating Ice and the Fitness of the Environment