LYCOS RETRIEVER 
Nitric Acid: Water
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Nitric acid is one of the components of acid rain. Clean air has very little nitric acid in it, but some types of pollution generate this acid. For example, the engines of motor vehicles produce nitrogen oxide compounds when they burn their fuel. Nitrogen dioxide (NO2) from car exhaust turns into nitric acid when it mixes with water vapor. This mixture falls from the sky as acid rain.
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Nitric acid has a characteristic choking odor that is acrid and suffocating. It is a strong, monobasic acid and an oxidizing agent. It is a caustic and corrosive liquid which will attack some forms of plastics, rubber, and coatings. It is not combustible, but may give off poisonous oxides of nitrogen and acid fumes when heated in fires. It will react with water or steam to produce heat and toxic, corrosive, and flammable vapors. It reacts violently with alcohol, turpentine, charcoal, sodium and potassium analogs, and organic refuse.
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Nitric acid is subject to a certain degree of self-decomposition, depending on the concentration and temperature. The general rule is: the higher the concentration or the temperature, the faster the decomposition rate. During decomposition nitrogen dioxide and dinitrogen tetroxide are formed in addition to water and oxygen and these are absorbed in the nitric acid. This leads to an orange-brown coloration and an increase in the dinitrogen tetroxide concentration. Nitric acid must therefore be transported from the manufacturer to the user within the shortest possible time, particularly in the summer months.
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The salts of nitric acid, known as nitrates, are mostly readily soluble in water and crystallize well. They are all decomposed when heated to a sufficiently high temperature, with evolution for the most part of oxygen and nitrogen peroxide, leaving a residue of oxide of the metal. They may be recognized by the fact that on the addition of a solution of ferrous sulphate, followed by that of concentrated sulphuric acid (the mixture being kept quite cold), the ferrous sulphate solution becomes of a deep brown colour, owing to the reducing action of the ferrous sulphate on the nitric acid which is liberated by the action of the sulphuric acid on the nitrate. As an alternative method the nitrate may be warmed with some fragments of copper and sulphuric acid which has been diluted with its own volume of water, when characteristic brown vapours will be seen.
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Dilute nitric acid may be concentrated by distillation up to 68% acid, which is an azeotropic mixture with 32% water. Further concentration involves distillation with sulfuric acid which acts as a dehydrating agent. In the laboratory, such distillations must be done with all-glass apparatus at reduced pressure, to prevent decomposition of the acid.
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Concentrated nitric acid (69 to 71%) is a transparent, colorless or yellowish fuming, suffocating, hygroscopic, corrosive liquid. Attacks almost all metals. Miscible with water and decomposes in alcohol. Dangerous fire risk in contact with organic materials. Highly toxic by inhalation, corrosive to skin and mucous membranes, strong oxidizer.
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