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| Chemical Cycles
Balance is essential to the earth. Chemical cycles keep the amount of elements on the earth in a perfect balance. Oxygen Cycle The amount of oxygen in and around the earth is fixed. But this oxygen is fed again and again through the world's living systems in a never-ending circle called the oxygen cycle. Our needs are just part of this cycle. The cycle involves a continual exchange of gases between the air and animals and plants. In a process called respiration, animals and plants take oxygen from air and give back carbon dioxide. In a process called photosynthesis, plants take carbon dioxide from air and water and give back oxygen. Respiration and photosynthesis are effectively opposite processes. Respiration is an oxidation reaction, which takes oxygen from the air or from water. Photosynthesis is a reduction reaction. It adds oxygen gas to the air. Enormous quantities of oxygen are taken in by plants and animals every day, and huge quantities of oxygen are returned to the air by plants. These amount exactly balance so that overall the amount of oxygen in the air stays the same. Carbon Cycle Carbon is essential for the chemical processes that support life. It plays such an important role in life that sometimes we say that life is "carbon-based". But there is only a limited amount of carbon on the earth. So carbon is constantly cycling around the earth, turning up in a lot of different forms and places. The reactions that move carbon around make up a giant web called the carbon cycle. Plants get carbon by taking carbon dioxide from the air. They use the carbon dioxide and the energy from sunlight to make food. Animals get their carbon by eating those plants or by eating animals that have eaten those plants. When organisms breathe, they take oxygen from the air. During respiration, the oxygen reacts with food to provide energy. Respiration produces carbon dioxide which is released to the air. Volcanic eruptions are a source of carbon. When a volcano erupts, it releases huge amounts of carbon dioxide. But remember--the earth needs it elements to stay in balance. So the effect of volcanoes is balanced by weathering which is a chemical reaction between rainwater and rocks that absorbs carbon dioxide from the air to create rock carbonate minerals. Left to themselves, these natural processes are in perfect balance. But human activities can disturb the cycle and increase the amount of carbon dioxide in the atmosphere. This could cause problems in the future because carbon dioxide is vital for controlling the world's climate. Nitrogen Cycle Nitrogen atoms are constantly moving in a giant circle from the air, through the soil, into the bodies of plants and animals, and eventually back to the air. This whole process is called the nitrogen cycle. All living things need nitrogen to develop and grow. Even though the earth's atmosphere is made up of 78% nitrogen, plants and animals cannot use it in this form because the nitrogen atoms are too firmly bound together in molecules. So plants must draw their nitrogen from nitrogen compounds dissolved in the soil, and animals get their nitrogen by eating plants or by eating other animals that eat plants. The nitrogen gets into the soil in a couple of different ways. A small quantity of the nitrogen found the soil by way of lightning. Lightning changes atmospheric nitrogen into nitrogen dioxide which is soluble in water. The nitrogen oxides dissolve in rainwater to form nitric acid which is absorbed by soil. The rest of the nitrogen in soil comes from bacteria. Bacteria are the only living things capable of getting nitrogen directly from the air. This is called "fixing". The process is started by certain kinds of bacteria in the soil that can extract nitrogen from the air. Then other bacteria convert the nitrogen into nitrogen compounds called nitrates, This process is called nitrification. Plants absorb the nitrates and turn them into more complex nitrogen compounds. Bacteria also help return nitrogen to the air. Bacteria in the soil decompose animal waste and the remains of dead animals and plants and produce ammonia. Nitrifying bacteria turn the ammonia into nitrates. Other bacteria, called denitrifying bacteria, convert some of the nitrates back into nitrogen gas, which is released into the air. All these different steps form a massive cycle. The effect is that, over time, bacteria in the soil return almost the same amount of nitrogen to the air as other bacteria take from the air. This keeps the nitrogen content of the earth and its atmosphere in a perfect balance. Unfortunately, humans are interfering with the natural balance when they overuse artificially produced nitrates as agricultural fertilizers. Before these nitrates can be converted into atmospheric nitrogen, they are often carried off from the soil by rain or irrigation. These dissolved nitrates are carried to streams and rivers and even seep down to groundwater. In some parts of the world, water for humans and animals contains such high concentrations of nitrates that it is unsafe for consumption. These excessive amount of nitrates, when they reach rivers and lakes, cause too much algae to grow. This over-abundance of algae uses up too much of the oxygen in the water. When oxygen levels fall, other forms of life in the water die off. Sample some of the following activities to learn more about chemical cycles. Places To Go | People To See | Things To Do | Teacher Resources | Bibliography The following are places to go (some real and some virtual) to find out about chemical cycles.
The Periodic Table on the WWW Visit the periodic table on the web.
Iowa
Farmer: Corn Cam Volcano
World Introduction
to the Atmosphere 
Trip
Around the Carbon Cycle Ask
a Scientist Science
is Fun Understanding
the Global Carbon Cycle
Chem4Kids
- Carbon
Chem4Kids
- Nitrogen United
Nations Environment Program EPA
Global Warming Kids Site
Nitrogen
Cycle Nitrogen
Fact Sheet Photosynthesis:
How Life Keeps Going Exploring
Environment: Carbon Cycle Nutrient
Cycles Nitrogen
Cycle Nitrogen
Cycle What
Is the Carbon Cycle ChemiCool Periodic
Table The
Microbial World: The Nitrogen Cycle and Nitrogen Fixation Chemicals,
the Environment, and You Experiencing
Chemistry Chemistry Teacher Resources Lesson Plan/Webquests/
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