Air facts for kids
Air refers to the Earth's atmosphere. Air is a mixture of many gases and dust particles. It is the clear gas in which living things live and breathe. It has an indefinite shape and volume. It has mass and weight, because it is matter. The weight of air creates atmospheric pressure. There is no air in outer space.
Animals live and need to breathe the oxygen in the air. In breathing, the lungs put oxygen into the blood, and send back carbon dioxide to the air. Plants need the carbon dioxide in the air to live. They give off the oxygen that we breathe. Without it we die of asphyxia.
Air can be polluted by some gases (such as carbon monoxide, hydrocarbons, and nitrogen oxides), smoke, and ash. This air pollution causes various problems including smog, acid rain and global warming. It can damage people's health and the environment.
Since early times, air has been used to create technology. Ships moved with sails and windmills used the mechanical motion of air. Aircraft use propellers to move air over a wing, which allows them to fly. Pneumatics use air pressure to move things. Since the late 1900s, air power is also used to generate electricity.
Air is invisible: it cannot be seen by the eye, though a shimmering in hot air can be seen.
Air is one of the four classical elements in the ancient Greek concept of four basic elements (earth, water, air, and fire). It was considered the driving force for the birth of the cosmos.
The evolution of the Earth's atmosphere
The history of the Earth's atmosphere prior to one billion years ago is poorly understood, this remains an active area of research.
The modern atmosphere is sometimes referred to as Earth's "third atmosphere", in order to distinguish the current chemical composition from two notably different previous compositions. The original atmosphere was primarily helium and hydrogen.
About 3.5 billion years ago, the surface had cooled enough to form a crust, still heavily populated with volcanoes which released steam, carbon dioxide, and ammonia. This led to the "second atmosphere", which was primarily carbon dioxide and water vapor, with some nitrogen but virtually no oxygen. This second atmosphere had approximately 100 times as much gas as the current atmosphere. It is generally believed that the greenhouse effect, caused by high levels of carbon dioxide, kept the Earth from freezing.
During the next few billion years, water vapor condensed to form rain and oceans, which began to dissolve carbon dioxide. Approximately 50% of the carbon dioxide would be absorbed into the oceans. One of the earliest types of bacteria were the cyanobacteria.
Fossil evidence indicates that these bacteria existed approximately 3.3 billion years ago and were the first oxygen-producing evolving organisms. They were responsible for the initial conversion of the earth's atmosphere from a state without oxygen to a state with oxygen. Being the first to carry out oxygenic photosynthesis, they were able to convert carbon dioxide into oxygen, playing a major role in oxygenating the atmosphere.
Photosynthesizing plants would later evolve and convert more carbon dioxide into oxygen. Over time, excess carbon became locked in fossil fuels, sedimentary rocks (notably limestone), and animal shells.
As oxygen was released, it reacted with ammonia to create nitrogen; in addition, bacteria would also convert ammonia into nitrogen.
As more plants appeared, the levels of oxygen increased significantly, while carbon dioxide levels dropped. At first the oxygen combined with various elements (such as iron), but eventually oxygen accumulated in the atmosphere, resulting in mass extinctions and further evolution.
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Air Facts for Kids. Kiddle Encyclopedia.