When any type of fuel is burnt, lots of different chemicals are produced in the air which can result in air pollution. The smoke that comes from a fire or the fumes of a car exhaust contain lots of invisible gases that are harmful to the environment.
In addition, power stations and factories all burn fuels that produce gases that pollute the air. Some of the gases, especially nitrogen oxides and sulphur dioxide, react with tiny droplets of water in clouds to form sulphuric and nitric acids. When the rain from these clouds fall, it falls as a very weak acid known as acid rain.
A scale called a pH scale can be used to measure the acidity of rain, and ranges from 0 to 14 with 0 as the most acidic and 14 as the most alkaline, or the opposite of acidic. Something with a pH value of 7 would be neutral; it is neither acidic or alkaline.
The very strong acids would burn if they contacted the skin and can also destroy metals, but acid rain is much weaker and could never burn the skin. However, rain is almost always slightly acidic because it mixes with naturally occurring acids in the air. Unpolluted rain would have a pH value of about 5 or 6, but when the air is more polluted with nitrogen oxides or sulphuric dioxide the pH value could increase to a value of 4, which is more acidic. There have been instances of rain measuring as being a pH of 2.
Vinegar has a pH value of 2.2 and lemon juice a 2.3, and though two substances do not cause harm, acid rain can negatively affect the environment. It can be carried long distances in the atmosphere from continent to continent, and the acid can also take the form of snow and other precipitation. The acid rain can fall many miles from the source of the pollution and can have a serious effect on soil, trees, buildings, and water.
It can cause trees to grow much more slowly in forests or may cause them to die prematurely. In addition, as acid rain falls on a forest to trickles through the leaves of the trees and into the soil below. Some of it then finds its way into streams, rivers, and lakes.
Acid rain can dissolve and wash away nutrients and minerals in the soil, cause the release of harmful substances such as aluminum into the soil, and wear away the waxy protective coating of leaves. The process of photosynthesis may also be affected. Trees and other plants can become weakened and become more susceptible and attacked by diseases, insects, and bad weather.
In lakes, rivers, and other bodies of water, the effects of acid rain are much more obvious. When the acidity of a lake increases, the water becomes clearer and the number of fish and other animals in the water decrease. Though there are some species of plant and animals that can survive the acid rain, freshwater shrimp, snails, and mussels are the most quickly affected, and the young of many fish are the worst affected. The acid rain can cause deformity in young fish and can prevent eggs from hatching properly.
All building materials usually become eroded too, but acid rain causes the natural process of erosion to increase. Statues, buildings, vehicles, cables, and pipes can all be negatively affected by acid rain. There are some things made from limestone or sandstone that are particularly susceptible to the effects of acid rain.
At one time, acid rain was thought to only affect the area where the pollution was occurring. Later, though, after some study, pollution in one area was found to be causing acid rain far away from the source of the pollution. The wind carries the pollution many hundreds of miles away where it eventually falls as acid rain.
Preventing acid rain is not impossible, and one thing that can be done is to reduce emissions from the burning of fossil fuels, find alternative sources for energy, conserve resources, and then restore damage done by acid rain. A process called liming may be used to neutralize the water in lakes and rivers by adding powdered limestone to the water. However, it is very expensive.
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