Unit 2 Review

Editor's Notes

• #10 Usually, some energy is "lost" or transferred into friction (heat and movement loss), so efficiency is never 100%. Still the overall amount of energy remains the same.
• #15 All because of the atoms that form them - different atoms have different properties that react depending on their number of protons, electrons & neutrons
• #17 Review 5 states of matter: solid, liquid, gas, plasma, BE condensate Explore ptable.com to see state changes at different temperatures
• #19 The letter “H” stands for hydrogen while the letter “O” stands for oxygen. The subscript of “2” after the “H” tells us that there are 2 hydrogen atoms in a water molecule. The fact that there is no subscript after the “O” tells us that there is one oxygen atom in a water molecule. Thus, the chemical symbol “H 2 O” means “two hydrogen atoms and one oxygen atom.” Chemical symbols like this are called chemical formulas , because they provide a formula by which you can understand the chemical makeup of any substance. For example, natural gas stoves, water heaters, and furnaces burn a gas called methane, whose chemical formula is CH 4 . As we already know, the chemical symbol for hydrogen is “H. The chemical symbol for carbon is “C.” Thus, this chemical formula tells us that a molecule of methane contains one carbon atom (there is no subscript after the carbon's symbol) and four hydrogen atoms (there is a subscript of “4” after hydrogen's symbol). These atoms, when linked together in those numbers, make a molecule of methane. Notice what I needed to know to interpret a chemical formula. First, I needed to know the chemical symbols for each atom. Second, I needed to realize that if there is no subscript after a chemical symbol, that means there is only one of those atoms in the molecule. If there is a subscript, then the subscript tells me how many of those atoms exists in the molecule. So, in order to really be able to use chemical formulas, we will need to memorize all of the symbols for all of the atoms out there, right? Of course not. In this course, I will tell you the chemical symbol for any atom that you need to know. Eventually, you will become used to associating the most popular atoms with their symbols. You need to remember, however, that not all atomic symbols are composed of just one letter. Some atoms have two letters in their symbol. The chemical symbol for neon, an atom that comprises the gas used in neon signs, is “Ne.” Notice that even though there are two letters in this symbol, only one of them is capitalized. That is a general rule. All atomic symbols have only one capital letter. If there is a second letter in the symbol, it is always the lower case version of that letter. Also, you must realize that chemical symbols are not always as easily recognized as “C” for carbon and “Ne” for neon. The symbol for an iron atom, for example, is “Fe.” Where does that come from? Well, the Latin name for iron begins with the letters “f” and “e.” So sometimes we use two letters in an atomic symbol and sometimes we use one. Also, sometimes we base the symbol for an atom on its English name and sometimes we base it on its Latin name. Given all of that, make sure you understand the concept of chemical formulas by solving the “on your own” problems that follow.
• #20 What comes to mind when you think of "poles" (geographic N/S pole, battery terminals). Ionic bonds = electrons stripped/stolen/donated to form ions (remember NaCl?). Covalent bonds = shared electrons. In the case of water, there is actually a tug-of-war going on between the electrons that are supposedly being “shared” by the atoms. Much like two little children who continually fight over a toy that they are supposedly “sharing,” the oxygen atom and each hydrogen atom fight over the electrons that they are supposedly sharing. Continuing our analogy, suppose one of the children is stronger than the other. In the absence of proper adult supervision, the stronger child will end up with the toy more often than the weaker child, right? Well, it turns out that oxygen is stronger at pulling on electrons than is hydrogen so the oxygen atom will end up with the electrons more often than will the hydrogen. What's the big deal? Sure, oxygen isn't “playing fair,” but why worry about it? Remember that electrons are negatively charged. Since the oxygen gets the electrons more often than the hydrogens, it possesses more than its “fair share” of electrons. Since it possesses more than its “fair share” of electrons, it develops a very slight negative electrical charge. In the same way, since the hydrogen atoms get less than their “fair share” of electrons, they have less negative charge than they should. With less negative charge than they should have, they end up having a slight positive charge. The symbol δ is the lower-case Greek letter delta, and it is used to signify the fact that the electrical charges in the water molecule are very small. When you have positive and negative charges within the same structure, the phenomenon is called polarity (pol' air uh tee). As a result, we call water a polar molecule . Polar molecule - A molecule that has slight positive and negative charges due to an imbalance in the way electrons are shared
• #21 Notice what hydrogen bonds do. They link molecules together. Chemical bonds link atoms together to form molecules. Hydrogen bonds, on the other hand, bring individual molecules close together, linking them. Thus, while a chemical bond forms between atoms, a hydrogen bond forms between molecules. Also, since hydrogen bonds are weak, they can be easily broken. For example, if you boil water, you are adding enough energy to the water to pull the water molecules far apart, breaking the hydrogen bonds. Despite the fact that the hydrogen bonds break, the chemical bonds which hold the two hydrogen atoms to the oxygen atom do not break. Water vapor is still H2O, so the chemical bonds still hold. The molecules are so far apart, however, that the hydrogen bonds are eliminated.
• #22 At sea level, pure water boils at 100°C & freezes at 0°C Water's polarity forces molecules, as they slow down, to arrange themselves in a fixed crystal lattice formation. This formation causes ice to be less dense than liquid water. In its liquid form, water molecules are constantly moving around, sliding past each other and frequently forming and breaking hydrogen bonds. Because of this molecular movement, the molecules overall are closer together than in its solid state. Why is this important?
• #23 How do plants and trees hundreds of feet tall transport water up from the roots to the leaves without a pumping circulatory system? How do water bugs (or a paper clip) "float" on the top of ponds? Polarity of water results in cohesion (stickiness of molecules) that also creates surface tension Force of gravity on paper clip (or water bug) is less than the cohesive force of water (molecules clinging to each other at the surface more than they do in the solution because they are attracted to each other much more than they are attracted to the air).
• #24 Water has a very high specific heat - lots of energy to raise or lower the temperature of water. stickiness makes water molecules cling together & resistant to vaporizing slipperiness makes water molecules repellant and keep sliding past each other (resistant to freezing) Water can absorb or release relatively large amounts of heat with only a slight change in its own temperature. Water stabilizes air temperatures by absorbing heat from warmer air and releasing heat to cooler air.
• #26 Perhaps the largest aquifer in the world is the Ogallala aquifer located in the Midwestern part of the United States. This aquifer is named after a Sioux Indian tribe. It is estimated to be more than two million years old and to hold about 650 trillion gallons (2,500 trillion liters)! It underlies parts of 8 states, stretching about 800 miles (1,288 km) from South Dakota to Texas. The Ogallala aquifer supplies vast amounts of water to irrigate the crops grown in this vitally important agricultural area.
• #27 When scientists speak of “salt,” they are actually talking about a large class of substances, of which sodium chloride is a member. There are many other substances that fall within the classification of “salt,” however. Have you ever heard of someone soaking their feet in a mixture of Epsom salt and water? Epsom salt is another member of the “salt” class; its specific name is magnesium sulfate. To a scientist, then, many of the substances I lumped into the “other” category are considered “salts” as well. Thus, you might hear someone say that ocean water is about 96.5% water and about 3.5% salt. That's true, but the person saying that is obviously using the term “salt” to mean a class of substances, not just table salt.
• #28 After a while, the whole mass of ice and snow gets so heavy that it starts to slowly slide down the mountain. The speed at which glaciers slide is rather slow, usually about 1 meter (a little more than 3 feet) per day. Glaciers have been recorded moving more rapidly, however. In 1937, the Black Rapids Glacier in Alaska was observed moving more than 30 meters (more than 100 feet) per day. As a glacier moves, it tends to sculpt the earth over which it is traveling, plowing earth and rocks out of the way, often making valleys where there were none before. As the glacier travels down the mountain, it will often reach an altitude where the snow and ice begin to melt faster than new snowfall can replenish it. At that point, the glacier starts to melt, feeding many freshwater sources in the hydrosphere. Glaciers in the polar regions of the earth, however, never reach that point, and they continue to flow into the sea, forming huge sheets of ice (ice shelves that extend over the ocean). The continent of Greenland, for example, is almost entirely covered in a huge sheet of ice that comes from two glaciers! The entire Antarctic continent is covered by a glacier which occupies an area of more than 13 million square kilometers (5 million square miles)! photos: top - piedmont glacier (room to spread out creates a wide "bowl") bottom - satellite photo of Greenland's continental ice sheet
• #29 Glaciers move by internal deformation and/or by sliding at the base. Internal deformation occurs when the weight and mass of a glacier causes it to spread out due to gravity. Sliding occurs when the glacier slides on a thin layer of water at the bottom of the glacier. This water may come from glacial melting due to the pressure of the overlying ice, or from water that has worked its way through cracks in the glacier. Glaciers can also readily slide on a soft sediment bed that has some water in it. When a glacier moves rapidly, internal stresses build up in the ice which cannot be relieved by deformation alone, and cracks (called crevasses) form at the surface of the glacier. Glaciers erode the rock underneath them. A glacier can "carve" a valley, wearing away rocks and soil through abrasion and plucking up and moving large pieces of rock and debris. The glacier pushes this earth and rock forward as it advances, almost like a conveyor belt, and dumps it to the side along the way or at the end of the glacier (deposition). Depositional features include moraines, drumlins, and eskers.
• #31 At some point, such glaciers move so far out into the sea that they end up in water deeper than they are thick. At that point, since ice floats in water, the glacier begins to float. Typically, large chunks of the end of the glacier break away, forming icebergs . As you probably know, the vast majority (about 90%) of an iceberg exists below the surface of the water, so what we can see of an iceberg is really only about 10% of the total. This is easy to understand if you look at an ice cube floating in a glass of water. We know that ice floats in water, but not very well. Since the mass of ice is only slightly smaller than the mass of an equal volume of water, only a small portion of the ice cube can actually float above the surface of the water. The same is true for an iceberg. This is what makes them dangerous to ships. As a ship travels, it might see the tip of an iceberg and steer clear of it. However, since 90% of the iceberg is under water, it is very possible that even though the ship steers clear of the portion of the iceberg that can be seen, the bottom of the boat can still collide with the other 90% of the iceberg that is under water! Sometimes the glaciers do not break apart as they float, and they end up forming vast ice shelves that extend out into the ocean. One such ice shelf is called the Ross Ice Shelf , and it covers a portion of the Antarctic ocean that is about the size of Texas. In the end, then, an iceberg is really a portion of a glacier that has broken off and floats in the ocean. Ice shelves are simply the ends of glaciers that float in the ocean. A glacier is the result of heavy snowfall in the mountains that does not melt away during the summer. So the largest sources of freshwater on the planet (icebergs and glaciers) are really the result of precipitation. After all, without snowfall in the mountains, none of this would ever happen!
• #32 RELATIVE HUMIDITY:  The relative humidity tells how much water the air is holding compared to how much it could hold at a certain temperature. Lets imagine we have a blob of air with moisture in it. The temperature of our blob of air is 80 degrees. So, how do we tell how much moisture is in it ???? If our blob of air has a relative humidity of 50% then that means it is holding half of the amount of water a blob of air 80 degrees could hold. The relative humidity can change if the moisture changes or if the temperature changes. At 37 degrees C, air is capable of holding onto 44 mg of water vapor in each liter - so an absolute humidity reading of 22 would be 50% relative humidity.
• #33 Although not considered forms of precipitation, dew and frost are two other means by which water can leave the atmosphere and make it back to the earth. Dew forms when air near the surface of the earth gets cool. As this happens, the water vapor in the air will tend to condense into liquid. The problem, however, is that water vapor will not condense unless it can do so onto something. That's why clouds need cloud condensation nuclei in order to form. Near the surface of the earth, water will condense onto plants and soil, forming water droplets. These water droplets are called dew. The temperature at which dew forms, called the dew point, depends on the pressure and humidity of the air. The higher the pressure and humidity, the higher the dew point.
• #34 During autumn or winter, the air near the surface of the earth may get colder than the freezing point of water. When this happens, water vapor skips the liquid stage and immediately freezes on any surface with which it can come into contact, covering the surface with frost. Like dew, however, frost can only form on a surface. The temperature at which this occurs is called the frost point. The low temperatures which cause frost can be deadly for certain plants, so gardeners often cover their plants during cold nights to try and keep the cold air away from them.