Acid rain is a type of rain that is more acidic than usual. Although the extra acid can come from volcanic gases and rotting vegetation, man-made sources can add acid to rain when fossil fuels are burnt and release certain gases into the air. Acid rain and sulfuric acid are closely intertwined, as sulfuric acid makes up the majority of the acidic component of rainwater.
Sulfuric acid is a molecule that has two hydrogen atoms, one sulfur atom and four oxygen atoms. This gives the acid the chemical formula of H2SO4. This substance is present in acid rain, although not exactly in that form. Powerful acids like sulfuric acid tend to mix in readily with water molecules, and breaks up into two parts when in the water.
These segments are a hydrogen atom, and the rest of the molecule, which now is HSO4. The hydrogen atom is positively charged when it falls off the initial acid molecule, so it is a positive ion. As most chemicals are balanced in charge, the other portion of the sulfuric acid molecule is negatively charged. pH, which is a measure of acidity, assigns acid values to substances based on how many hydrogen ions it contains. Acid rain and sulfuric acid therefore become more acidic the more hydrogen ions are present.
Before the sulfur compound even gets into the rain, it has to get into the atmosphere. This happens because gases that contains sulfur can float up in the air. Natural sources of these gases, which are hydrogen sulfide and sulfur dioxide, are emissions from volcanoes, or gas produced by rotting plant matter, respectively.
Hydrogen sulfide has the chemical formula H2S, which means that it has two hydrogen atoms and one sulfur atom. This particular sulfur gas reacts with the oxygen already in the air, and turns into sulfur dioxide. Sulfur dioxide, from H2S reactions, or from volcanic emissions, is a less complex molecule than when it becomes an acid, as it only contains two oxygen atoms and one sulfur atom.
Chemically, sulfur dioxide is represented by SO2. Most of this gas in the earth's atmosphere is from human activity. Primarily, this comes from the heat breakdown of fossil fuels. Fossil fuels are a form of decayed plant matter, which has turned into coal, oil or gas over millions of years. Electrical power stations make the most, but industry, home heating and car emissions can also contribute.
This gas floats up in the air and mixes with the droplets of water in rain clouds. The SO2 turns to SO3, from interaction with atmospheric oxygen, before mixing with the water in the cloud. Water, which contains two hydrogen atoms and one oxygen, reacts with the SO3 to form H2SO4, which is sulfuric acid.
When acid rain and sulfuric acid fall to earth, the low pH can be damaging to organisms and inanimate objects. Watercourses that receive too much acid rain and sulfuric acid can become unhealthy places for fish and plants to live in. Construction materials like marble can become slowly eroded by the acid. Generally, even normal rainfall can have an acidic pH, which is about 5.6. Rain that has a pH of less than this, such as 3.0, is regarded as acid rain, and may be a sign of local pollution.