| Acids need water to behave as acids |
| Arrhenius had suggested that an acid is a substance that produces hydrogen ions in water and that a base is one that produces hydroxide ions in water. This theory assumes a simple dissociation of these substances. |
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| A useful modification of Arrhenius' idea came from Bronstead and Lowry. They defined an acid as a substance that yields a hydrogen ion (commonly called a proton) and a base as that which accepts a hydrogen ion or a proton. |
| Thus an acid-base reaction involves donation of a proton by the acid to the base. A covalent compound like HCl behaves as an acid in water because of the reaction: |
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| Citric acid is a neutral solid. When dissolved in water, the solution has pH 5.0. i.e. It is acidic. Pure sulphuric acid does not react with metals. As soon as water is added, it reacts violently. |
| When a nonmetal is burned in the air, oxide is formed. This dissolves in water and the solution is acidic eg CO2, SO2. |
From car exhaust, NO2 is given out in the atmosphere. This dissolves in water vapour in the air, making HNO3 fumes. This gives sever irritation to the lung lining of people with bronchial problems.
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