Kids' Chemistry: Acids and Bases

"This will be a basic science class," the teacher said acidly.

At any rate, you can't talk about chemistry with out talking about acids and bases.

An acid is a chemical with extra hydronium (H+) ions.  It tastes sour (but you don't want to taste many of them), feels astringent, reacts with metals (remember rusting steel wool with vinegar?) and bases (baking soda and vinegar anyone?), and it turns indicator strips red. It also can burn you if the acid is strong enough.

 A base, on the other hand, is a chemical with extra hydroxide (OH-) ions.  It tastes bitter (do you want to taste one?), feels slippery or soapy, reacts with acids, and it turns indicator strips blue.  It also can burn you if the base is strong enough.

When an acid reacts with a base, that H+ and OH- get together and form... H2O, water!  The rest of the acid and base form a salt.

Not necessarily salt, but a salt.  There are a lot of salts in chemistry, and not many of them are edible!  I asked the kids what color a salt was.  They said white, which is correct, but a salt can also be red, blue, green, yellow, orange, or purple.  Next I asked them how a salt tastes.  Salty, of course!  But a salt can also taste sweet, sour, bitter, or umami (MSG is a salt).

Next up, the pH scale.  This is a scale which tells you how  acidic an acid is and how basic a base is.  On a scale of 0 to 14, 7 is neutral, anything higher than a 7 is basic, and anything lower than 7 is acidic.  I didn't go into the fact that you can have acids lower than 0, and bases higher than 14, but you can.  If that sounds more dangerous than anything they are likely encounter before grad school, it is, particularly when you consider that the pH scale is logarithmic: each number you go down (or up) is 10 times as acidic (or basic) as the last.

At any rate, I had set out 13 random carefully selected substances for them to test, along with pH strips and a table to record their findings (Hey, look!  Something from the class that we can actually put in our portfolios!).

We had everything from laundry detergent to pickle juice, but my favorites were milk (base) and buttermilk (acid).


Then it was back to the kitchen floor for the demonstration reactions!

I had put baking soda (and a drop of soap) in a graduated cylinder and I poured in some vinegar.  It foamed and overflowed obligingly.  We poured in more vinegar.  It reacted again.  But, eventually, the reaction stopped, no matter how much vinegar we added.  We had "used up" the baking soda, of course, but in chemistry we say, "The baking soda was the limiting reagent."

So we added more baking soda, and it reacted again!  This time we kept adding baking soda until the reaction stopped, or, The vinegar was the limiting reagent."  I suspect the kids could have gone on and on all day adding one reagent then the other, but I had one more thing I wanted them to see.

Will an acid and a base always react with each other?

There is one circumstance where they won't.  I am holding a bath fizzy which we made around Christmas time.  Bath fizzies are basically powdered citric acid mixed with baking soda. The acid and base don't react with each other because the acid base reaction requires water!

Regular baking powder works the same way,which is why baking powder goes bad if exposed to enough humidity (double acting baking powder has a third component that activates when exposed to  heat).  Plain baking powder is baking soda with tartaric acid, which is why you use baking soda with cream of tartar if you ever run out of baking powder.  I've never been able to use that fact because I always run out of cream of tartar before I run out of baking powder, but your mileage may vary!

The kids had lots of  fun putting the bath fizzies in water and watching them fizz.  They also tried putting baking powder in the water in pinches and powdery handfuls.  Makes me glad I buy baking powder in one pound containers at the warehouse club.  It'll be the first time I used one up completely before it lost strength!

Tune in next week for our last chemistry class when we'll be looking at crystal formation!