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The Magic of Water - Acids and Bases.

An acid is commonly defined as a substance that "donates" (releases) one or more hydrogen (H+) ions, while a base is defined as a substance that "accepts" (combines with) one or more H+ ions. Interestingly enough, water can behave as an acid or a base, depending on prevailing conditions.

In a sample of pure water, a small proportion of H2O molecules undergoes dissociation to produce H+ ions and OH- (hydroxide) ions. Given that the concentrations of these ions are extremely small, scientists use the pH scale, which ranges in value from 0 to 14. [For the mathematically inclined, pH is defined as the negative logarithm of the concentration of H+ ions.] When the concentrations of H+ and OH- ions are equal to each other, as in pure water, the pH value is 7.00. In acidic solutions, pH values fall below 7, while in basic (or alkaline) solutions, they go above 7.

When hydrochloric acid (HCl) is added to water, it dissociates into H+ and Cl- ions. The water molecules quickly combine with the H+ ions to form H3O+ (hydronium) ions, thus behaving as a base. Given that the concentration of H+ (or H3O+) ions has increased, the pH value falls below 7.

On the other hand, when the base sodium hydroxide (NaOH) is added to water, it dissociates into Na+ and OH- ions. Each OH- ion rapidly combines with one H+ ion that is pulled out of a nearby H2O molecule, forming new H2O and OH- entities, and the process continues. In this case, water behaves as an acid that donates H+ ions. As the concentration of H+ ions decreases, the pH rises above 7.

If both HCl and NaOH are dissolved in water, the H+ ions from HCl combine with the OH- ions from NaOH, to produce H2O. Thus the acid and base neutralize each other. While water is the solvent for this reaction, it can also participate in the interactions of H+ and OH- ions.


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Author:Dadachanji, Dinshaw K.
Publication:World and I
Article Type:Brief Article
Date:Jan 1, 2003
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