There’s something awesome about answering questions for kids. Redditor HippySkippy noted that her 7-year-old son had recently developed a love for the wondrous world of chemistry. However, like many books written for children, the books she’d been able to find often said to mix baking soda (sodium bicarbonate) and vinegar (dilute acetic acid) to build an “at-home volcano.” Many of us know that an acid (like vinegar) and a base (like baking soda) react. But how many folks know why? So, Little Joey (that’s your name now), allow me to explain.
What is an acid? What is a base?
To determine if something is an acid or a base, the quickest method is to dissolve a little in water and then to test the pH with indicator.
Universal indicator is a mixture of substances which change color when exposed to acids or bases; tradition makes it such that acids are red and bases are blue. If you added universal indicator to pure, freshly distilled water, it would be a very pretty shade yellow. Add vinegar, it will turn orange-red. Add bleach, it will turn blue. What happens?
Normally, water breaks apart into H+ (hydronium) and OH– (hydroxide), but as fast as it breaks apart, other molecules come back together. This system is in balance. We describe the ratio of H+ and OH– by multiplying their concentrations – this number is referred to as the “dissociation constant of water” and usually written as KW. Water in perfect equilibrium at 25°C has a KW of 10-14. See the 14 there? It will be important in a minute.
When an acid – let’s use hydrochloric acid (HCl), for example – is added to water, it dissociates and dissolves. The water actually gets hot because of all the energy released when the hydrogen-chlorine bond breaks! When the HCl breaks apart, it makes H+ and Cl– and so the amount of H+ in the beaker increases. When a base is added to water, like sodium hydroxide (NaOH), it dissociates and forms Na+ and OH–. The OH– bind to the H+ in the water and decrease the amount of H+ in the beaker.
I said that 14 would be important. Turns out, we can use a scale from 0 to 14 to measure how much H+ is in something; we call this scale the pH scale. If you click this link, you’ll see one with all sorts of stuff from your house on it. When the H+ level gets really high, the number it gets on the pH scale gets lower and lower. Water is in the middle – a neutral solution – while acids are from 0-7 and bases are from 7-14.
How does an acid react with a base?
I really like HCl and NaOH for examples; HCl is a very strong acid and NaOH is a very strong base. Let’s look at the chemical equation:
As you can see, the acid (HCl) breaks apart in water just like the base (NaOH). The water wants to form, so if you add an equal amount of hydrochloric acid to sodium hydroxide, you’ll get a solution of nothing but hot salt water. What happens is that the hydronium (H+) really wants to get together with the hydroxide (OH–) and make water (H2O); they want to get together more than you’d like an ice cream cone on a hot summer day.
The solution will eventually even out and come to a point where there’s not much happening called “equilibrium” where the water is breaking up just as fast as it’s getting together again.
Why does an acid react with a base?
When you mix all this stuff in a beaker, you can’t help but wonder: why does it all react? Well, the answer here is simple: energy. The type of energy I’m talking about is called Gibb’s Free Energy which is just science-speak for the ability of the stuff in the beaker to do stuff. Let’s look at a picture (I’m a chemist, not an artist):
At the beginning, the reactants have a lot of Gibb’s Free Energy; they have the ability to do a lot of stuff! But at the end of the reaction, the products have almost no Gibb’s Free Energy and can’t do any work. Where did the energy go? Well, before I mentioned that the beaker would get hot if you mixed a strong acid and a strong base – that’s where the energy went. The reactants took all their stored up energy and released it as heat when they reacted.
So, why does an acid react with a base? Because the universe really likes it when molecules go from a high energy state to a lower energy state. Acids and bases have lots of energy to be spent when they get mixed together so they spend it all like a kid spends money at an arcade.
For those looking for a more advanced story of acid/base chemistry, here’s a great – free! – resource.
Also, David Kroll asked this same question. I hope I did a sufficient job.