General ---------Biology ------------------Online------------------------ Ch. 3 Water and the fitness of the environment

Read Heat and Temperature, Water's High Specific Heat, and Evaporative Cooling on page 43-44 of your text.  Critical terms: heat, kinetic energy, temperature, specific heat and evaporative cooling.  The following assumes you understand these terms.

Evaporative Cooling.  If you wish a liquid water molecule to evaporate (go from the liquid state to the gaseous state) you must give the molecule enough kinetic energy (heat it up) so that it can break it hydrogen bonds to adjacent water molecules and avoid forming any new hydrogen bonds on its way out of the liquid.  Once the "hot" water molecule leaves, it takes its energy with it, leaving the liquid slightly cooler.  If the liquid is sweat on your forehead, and lots of molecules are leaving, this has a cooling effect, especially if you stand in a breeze.

Condensation

The opposite of evaporation is condensation.  As two fast moving gaseous water molecules cool (loose kinetic energy and slow down) they may form hydrogen bonds as they collide or pass close to each other.  At this moment of condensation the two molecules give off some of their kinetic energy as heat to the surroundings.  This is why on a cool winter morning the temperature in my backyard goes up about 2 degrees F. when the dew point is reached and a fog occurs.  You can watch this phenomena if you have a thermometer attached to the outside of your kitchen window.  This heat of condensation is also why the tops of rain clouds show up on Doppler radar as hotter than clouds that have not started to rain; the more intense the rain, the hotter the cloud tops.  The energy liberated by condensing moisture heats the air causing it to rise (hot air rises).  This rising air causes the swirling updrafts of tornados, thunderstorms and hurricanes. 

Caution:  Covalent bonds work in the opposite fashion to hydrogen bonds.  When a covalent bond is broken, the atoms fly apart.  Some of the bond energy is converted into the kinetic energy of the two atoms, and the rest is given off in a minor explosion of heat, light or both (image).  This is what happens when you strike a match; the friction of the strike generates enough heat to break covalent bonds between the C, H and O's of the cellulose.  The energy liberated is enough to break nearby bonds, causing a chain reaction or a domino effect of breaking bonds, i.e. a burning match.

This is also what happens when your cells use enzymes to "burn" food for energy.  The energy from the breaking bonds is used to contract muscle fibers, and some is given off as heat, making you sweat. Both the flaming match stick and your cells need oxygen to break covalent bonds. 

Hydrogen bonds give off energy when they form, not when they break.  To see an illustration of this phenomena, click on the "heat of condensation" link above.

Summary:  Students should write summary statements below as bullet points. 
 
 
 
 
 

Test your knowledge:  Practice Quiz #   3,12, 16, 18, 28, 37, 42. Cohesive water Quiz.