Common Student Misconceptions

1. "Substances containing H are acidic; substances containing OH are basic."
Many substances that contain H that are not acids and many substances that contain OH are not bases. Table sugar (sucrose), C6H12O6, contains H and OH, however, when dissolved in water it dissolves as intact molecules and does not ionize to produce any H⁺ or OH^- ions, as shown by the equation: C6H12O6(s) + H2O(l) Æ C6H12O6(aq). Since sugar molecules do not lose any H⁺ or OH^- ions, sugar is not considered an acid. Alcohols have a characteristic hydroxyl group, OH, covalently bonded to the rest of the molecule. When alcohols dissolve in water they also dissolve molecularly and do not ionize. Since no OH^- ions are released, alcohols are not bases.

2. "When a `proton donor' acid reacts, the nucleus of an atom loses a proton."
When we speak of acids as proton donors we are talking about the single proton in a hydrogen ion, H⁺, not a proton from the nucleus of some other atom. Although chemists commonly refer to acids and bases as proton donors and proton acceptors, it is important to realize that H⁺ is being moved from one chemical species to another, not a proton from one nucleus to another. Furthermore, if nuclear protons were transferred, then elements would change their identities in acid-base reactions, since the atomic numbers of the atoms involved would be altered.

3. "Strength and concentration mean the same thing."
These two terms are often confused by students. Concentration refers to the number of moles of solute (acid or base) per liter of solution. Strength refers to the percent of molecules that ionize and form ions in solution. Hydrochloric acid is considered a strong acid because in aqueous solutions nearly all the molecules ionize to produce H⁺ and Cl^- ions. Acetic acid is considered a weak acid because only about 1% of the molecules normally ionize. It is interesting to note that the percent of acid molecules that normally ionize in a weak acid actually increases as the acid concentration is decreased through dilution with water. Thus, acetic acid actually becomes slightly stronger as the solution is diluted.

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