HOMEWORK 3 ANSWER KEY ATMOSPHERIC MOISTURE GEO 131 Spring 2007 True/False : 1. The special properties of water (universal solvent, high boiling point) are related to its low molecular weight. False- it is related to its asymmetrical shape which creates a charge imbalance (i.e., a polar charge distribution). The hydrogen end is positively charged and the oxygen end is negatively charged. This charge imbalance enables water molecules to pull apart other molecules, thereby making water a universal solvent. The charge imbalance also causes the water molecules to be strongly attracted to one another, thereby raising the boiling point of water. 2. Water can dissolve almost anything because of the shape of the water molecules and the charge distribution on them. True- The positive charge on one end and the negative charge on the other end of the V-shaped water molecule creates an electrical attraction for the molecules near to them, and can thereby pull molecules apart. 3. If ice sufficiently compressed, it will melt- even at temperatures below freezing. True- Because ice must expand in order to develop the ordered crystalline structure characteristic of the frozen state, compression destroys this structure and results in melting. 4. On a cold winter day, an unfrozen lake will lose heat energy faster than a lake with a frozen surface. True- Ice acts like a layer of insulation because it slows the rate at which heat energy can escape from the underlying water. 5. The charge distribution on water molecules is relevant to dissolving things, but not to the phase changes that water undergoes. False- the charge distribution results in an attraction between water molecules - the positive end of one water molecule attracts the negative end of another. As a result, this increases the amount of energy required for phase changes. If it weren’t for this fact, water would not serve as the “fuel for storms.” 6. Many substances on earth undergo phase changes within the temperature range that occurs naturally on earth. False- only water does this - which is one of the reasons why it is so special. It is also why the hydrologic cycle occurs. If water did not possess this property, life as we know it would be impossible on earth.
7. The reason temperature changes result in phase changes is because the change in temperature affects the speed of the molecules. True- by affecting the speed of the molecules, a change in temperature affects the ability of the chemical bonds to hold molecules together. For example, as the temperature increases, the speed of the molecules increases, thereby reducing the effectiveness of the bonds. This leads to a change from either a solid to a liquid, or a liquid to a gas, depending upon the original state of the matter and the amount of temperature increase. 8.
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