7.2.4: Strong, Weak, Concentrated, and Dilute Acids and Bases

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It can be very confusing when words have a different meaning in the scientific context than they do in everyday life. The words we use to describe solutions of acids and bases fall into this category of easily mixed-up definitions. We use the term strong to refer to acids that ionize completely in water, and weak for those acids that are only partially ionized (see Chapter 8 for more information on why). Strong and weak are used to describe an intrinsic property of the acid or base. The terms dilute and concentrated are used to describe the concentration of the acid in water. We could have a dilute solution (say 0.1 M) of the strong acid hydrochloric acid, or a concentrated solution (say 10 M) of the weak acid acetic acid. By contrast, when we refer to strong versus weak liquids in the everyday sense, we are referring to the concentration of the solution. For example, if you say, “This tea is very weak” or “I like my coffee strong” what you are really saying that you like a lot of tea or coffee dissolved in the solution you are drinking. It is important to remember this difference and understand that the scientific context can change the meaning of familiar words.

Questions to Answer

Draw out molecular-level pictures of a dilute solution of a strong acid and a weak acid.
Draw out molecular-level pictures of a concentrated solution of a strong acid and a weak acid.
What are the similarities and differences between all the representations you have drawn?
Consider what you have learned about the energy changes associated with the reaction of a strong acid with water. From a safety point of view, which of the following actions makes more sense when diluting a concentrated solution of a strong acid with water? Why?
A. Add water slowly (dropwise) to the concentrated strong acid or
B. Add the concentrated strong acid dropwise to water