• anonymous
How does temperature change affect waters cohesion?
  • Stacey Warren - Expert
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  • anonymous
The molecule H2O (water) has a funny shape, the two hydrogen atoms form a "V" shape, rather than a straight line The electrons spend more time orbiting the oxygen nucleus than orbiting the two hydrogen nuclei, leaving the oxygen more negatively charged and the hydrogen more positively charge. Therefore, the entire molecule is "Polar", that is with one end slightly positive and one end slightly negative. The two ends of these polar molecules can attract one another (positive to negative). Water and its Properties Water is one of the most remarkable compounds in the universe. It has several unique physical and chemical properties, all based on its "bent", "polar", structure. Water has an unusually high freezing point (OoC) and a high boiling point (100oC) in comparison to other small molecules, such as CO2. Since the temperatures on the earth's surface commonly fall between waters melting and boiling point (O C and 100 C), water exists as a liquid most of the time. The thermal properties of water have important biological implications. Two particularly significant characteristics are a high "heat of vaporization", the amount of heat needed to turn a given amount of liquid water into water vapour (gas). Water also has a high "specific heat", the amount of heat needed to raise the temperature of 1 gm of water by 1 C. (Waters heat of vaporization is 580 calories per gram, and its specific heat is 1 calorie per gram per degree centigrade). A high heat vaporization means that in order for water to reach the gaseous state, it must absorb a great deal of heat from the surroundings. For many plants and animals, this property is the basis of a natural cooling system. Water evaporating from leaves, skin or lungs uses up heat from the organisms in the process of changing from liquid to gas. That is the reason mammals have evolved sweat glands, when the body is overheated, the glands pour watery "sweat" onto the skin, as the water evaporated, large amounts of body heat are used up and the body is cooled.Due to it high specific heat, water is slow to undergo changes in temperature. Much heat must be added or removed before the temperature of water changes much. The temperature of the water within living organisms tends to change more slowly than does that of the surrounding air or soil, so that the living cells are buffered somewhat against temperature fluctuations. This kind of protection is important because many biochemical reactions will take place only within a narrow range of temperatures. Water has several other properties that make it important to living things. Water molecules exhibit strong "cohesion" ie. the tendency of "like" molecules to stick together (water to water). This is the result of the polar nature of water molecules and the resulting hydrogen bonding. In the photos below water is transported up inside trees and other vascular plants inside tiny tubes call "xylem vessels". As water transpires off the leaves the cohesion pulls more water up through the vessels. Thus, water can dissociate into "hydrogen ions" (H+) (actually H3O+ "Hydronium ions") and hydroxyl ions (OH-). The number of hydrogen and hydroxyl ions in a sample of H2O is equal and constant. In one litre of water, there id 1.0 X 10-7 mole of H+ and an equal amount 1.0 x 10-7 mole of HO-. The number 1.0x10-7 mole H is a difficult number to write so we often express this hydrogen concentration as -log10 [H+] -log[1.0x10-7] = 7. Adding a solute to water may change the concentration of the hydrogen ions. Any substance that gives up or "donates" H+ ions to a solution and thereby increases the H+ content of the solution is called an "acid" compounds that decrease the amount of H= ions in the solution is called a "base". A base actually "sucks" up H+ ions so there are fewer in the solution. The concentration of H+ in "acidic" and "basic" solution is expressed in terms of the "pH" scale/ Recall that the concentration if H+ ions in water was 1.0x10-7 mole H+ ions/LH2O and that if we took the -Log10[H+] the value was 7. The "pH scale" expresses the [H+] in solution as the -Log[H+]. The scale ranges from 0 to 14. pH Scale -Log[H+] [H+] 1.0X10-14 14 Very basic 1.0x10-8 8 Slightly basic 1.0x10-7 7 Neutral 1.0x10-6 6 Slightly acidic 1.0x10-2 2 Very acidic Since the pH scale is a "logarithmic" scale each unit represents a tenfold change in the concentration of H+ ions. A solution with a pH of 6, for example, is ten times more acidic than pure water at a pH of 7. **further knowledge Oxidation-Reduction Reactions Oxidation: is defined as a chemical process in which an atom, ion or molecule loses electrons. In rusting, iron (Fe) is changed from its metallic state to its iron(III) (Fe3+) state; it has lost electrons therefore it has become oxidized. 4Fe ------> 4Fe3+ + 12e- Simultaneously oxygen is changed from its molecular state to its charged state 3O2 + 12e- ------> 6O2- When oxygen accepts the electrons removed from the iron, it is being reduced. Reduction: is a chemical process in which an atom, ion or molecule gains electrons. Oxidation and reduction reactions occur simultaneously because on substance must accept the electrons, and some substance must donate the electrons. Oxidation-reduction reactions are sometimes referred to as redox reactions.

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