Neutralization and the pH scale

Ask students, “How does an acid neutralize a base or vice versa?”

By just looking at the active component of an acid (H⁺) and a base (OH^–) they should see the answer. The hydrogen and hydroxyl ions attract, form a covalent bond and become a water molecule, HOH. Of course, this assumes exactly equal quantities of each. Suppose you are neutralizing an acid by adding a base. For example:

        HCl         +            NaOH           —>        H₂O   +   NaCl        Hydrochloric acid     sodium hydroxide                 water         salt

                                       Base

In more detail

   H⁺   +  Cl^–    +.   Na⁺   +   OH^–       —>          HOH  +  Na⁺  +   Cl^–

Not enough base, and an amount of acid (H⁺) will remain in the solution and the solution will remain acidic; too much base and you will overshoot and the solution will be more and more basic. Thus, have kids visualize a continuum from a high concentration of H⁺, then less and less,  to water, to a greater and greater predominance of OH^–.

      H⁺ <————————  H⁺ OH^– ———————>    OH^–

High conc.                           Equal amounts.                        High conc.

                                            water

Of course the same applies to neutralizing a base with an acid.

The pH scale

As the final step, scientists put numbers from zero to fourteen on this continuum with seven designating the neutral point of pure water in the center and call it the pH scale. The numbers are far from arbitrary. The actual concentration of hydrogen ions (H⁺) in a solution has been measured. The pH number stands for the negative log of the hydrogen ion concentration. Understanding this obviously requires higher levels of math knowledge. Therefore, I omit it for the sake of simplicity, two points should be stressed

First, even in pure water some molecules are ionized meaning there is a slight concentration (10^-7) of hydrogen ions; (and an equal concentration of OH^– ions) in pure water. Second, each unit on the scale stands for a tenfold increase/decrease in the concentration of H⁺ alone. Thus, a solution of pH6 has 10 times greater concentration of H⁺ that in pure water pH7; pH5 has 10 fold more H⁺ than pH 6; pH 3 has 100 times more than 5 and so on. Going the other way, pH 8 has 10 times less H⁺ than in water, pH7; pH 9 has 100 times less hydrogen ion concentration than water. However, think pH8 and pH9 as a 10 and 100 fold increase in OH over that of water.

The scale is eloquently described in the following video. 

Measuring pH

pH of natural waters is a critically important environmental parameter and, happily, very easy for kids to measure with pH indicator paper strips that readily available at reasonable cost. (Shop: pH indicator paper strips) The following video shows how to use the paper strips

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There are also various electronic instruments for measuring pH. I do not recommend them. They are expensive, painstaking to maintain, and prone to giving spurious results if not well maintained.