Why did anyone invent the concepts of acids and bases? [closed]








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I am home-schooling my son in first-year Chemistry, and I am struggling to teach Acids and Bases in a rewarding way.


I am not new to Chemistry. I went to grad school in Physics and I even taught High School Chemistry for four years. I admit, I am suspicious of the way Chemistry is taught in school today and I think it needs radical revamping. It's either too much like stamp collecting (memorizing a bunch of terms and rules) or esoteric (obsessing about significant figures in 8th grade seems misplaced) or else just a bunch of unit conversions (Stoichiometry) without focusing on bigger glorious ideas.


I have read a dozen textbook treatments of acids and bases. I can follow the expanding theories from Arrhenius through Lewis. I can do all the calculations with no problem.


What mystifies me is: Why do Chemists care so much about Acids and Bases?


Every treatment starts with Arrhenius studying dissolving ionic compounds in water and noticing that the resulting solution conducts electricity. I appreciate that needs an explanation, but seeing charged particles floating in water seems good enough. No need to invent "acids" and "bases" just for that.


Then, someone discovered that litmus paper and other indicator compounds will turn different colors in the presence of certain chemicals.


Maybe scientists realized that the same explanations for what makes water solutions conductors could also explain why these indicators change to different colors, and that was enough connection to warrant these new terms "acid" and "base"? And this is important enough to spend three chapters of a Chemistry textbook explaining?


Do professional chemists use the concept of Acids and Bases in their daily work? If so, how?


I know this is an unfocused question, but I hope I am conveying the type of answer that would be helpful to me. "Acids and Bases are not just a historical artifact, like phlogiston. They really are very important to understand in modern Chemistry because ...."




The words "acid" and "base" have both a historical origin.


The word "acid" is coming from the latin root "acies" which means "prickly". Its derivate "acidus" is an adjective used for describing a prickly, or hot, or sour liquid, because it pricks the tongue when drinking it. Another derivate is "acetum", latin name of vinegar, because vinegar is the first acidic liquid, the first known liquid pricking the tongue when drinking it.


The word "base" has another origin, which is more recent, but not so easy to explain. It needs a long explanation. In the Middle Age, it was known that acids can be destroyed (= neutralized) by adding soda, ashes or chalk. Soda ($\ce{Na2CO3}$) was obtained in Egyptian oasis. Ashes contains impure sodium and potassium carbonates, and chalk contains $\ce{CaCO3}$, but these data were unknown of course. Some alchemists called these strange substances "antacid". We will see that this name was never popular. The only fact that was accepted by all alchemists was that the reaction acid + antacid produces a new substance, a solid substance, which was called a "salt", because the sea salt was soon recognized to be also the result of an acid ($\ce{HCl}$) reacting on soda ($\ce{Na2CO3})$.


When heating a (solid) salt to a rather high temperature, it gets decomposed into two parts : nearly half of the stuff quits the solid phase as a gas (new word, coming from "ghost", created by van Helmont, b. 1577). Surprisingly enough, these vapors, when cooled, can be condensed and usually produce an acid !! And the remaining stuff, when cold, could have been described as "residue". But it was more informative to call it "base", because it stays in the lower part of the heated flask and refuses to pass into gas phase. Generally speaking, a "base" is the lower part of an object, or of a mountain. In these old times, a base is (or was) the part of the stuff that stay in the lower part of the hot container. Today, we know that this basic residue is made of oxides or carbonates. But even in the Middle Age, it was soon discovered that these bases were similar to the antacids that react with acids. So the word "antacid" was replaced by the word "base". The following "equations" could have been written .



```
        Acid + Base -> Solid salt (+ water + sometimes a gas)

        Salt (+ heat) -> Base(solid) + Acid(gas)

```

Although sea salt does not decompose this way, an important proportion of salts can be decomposed at high temperature according to this last equation, specially when the salt crystallizes with some crystallization water, like $\ce{ Na2SO3·7H2O}$, or $\ce{ Ca(NO3)2·4H2O}$, or $\ce{ Al2(SO4)3·12H2O}$. Examples : $$\ce{ Na2SO3·7H2O -> 2 NaOH(s) + SO2(g) + 6 H2O(g)}$$ followed at low temperature by : $\ce{SO2(g) + H2O(l) -> H2SO3(l)}$ $$\ce{ 2Ca(NO3)2·4H2O -> 2 CaO(s) + 4 NO2(g) + O2(g) + 4 H2O(g)}$$ followed at low temperature by : $\ce{4 NO2(g) + 2 H2O(l) + O2(g) -> 4 HNO3(l) }$ $$\ce{Al2(SO4)3·12H2O -> Al2O3(s) + 3 SO3(g) + 12 H2O(g)}$$followed at low temperature by : $\ce{SO3(g) + H2O(l) -> H2SO4(l)}$


The obtained residual base can be an oxide or a hydroxyde. It did not matter. And the acid may be partially decomposed. These facts were considered as secondary in these old times, where chemistry was not yet a science.


Today the words "acid" and "base" have a quite different meaning, of course. But it is interesting to know that these two names have a rather intuitive origin


 2

Thanks SO much for all the effort spent here!


@Maurice I loved learning the historical origins of "acid" and "base". It will certainly enrich my teaching.


If I understand correctly, it's not so much acids/bases that are crucial on their own, but acids/bases are a stepping stone to pH, and, for whatever reason, pH is a vital consideration in lots of biological processes.


Here's my attempt to coalesce the helpful comments into an ideal (to me, at least) intro textbook section on acids and bases:



> 
> Knowing whether a substance is a metal or a non-metal gives you lots of predictive power to guess how it will behave. Is this substance a good conductor of electricity and heat? Is it malleable? What's the melting point? All these questions can generally be answered simply by knowing whether it's a metal or non-metal.
> 
> 
> When discussing solutions, another useful classification is "acid" or "base" or neither. This lets you predict whether it conducts electricity, whether it tastes sour or bitter, and whether it feels slippery. This list of predictions is clearly less important than those concerning metallic nature, but there is one consequence of the acid/base distinction that's, literally, a matter of life or death:
> 
> 
> In Biology, there's a property called pH, which is intimately related to the acid/base character of a solution. Biological processes are extremely sensitive to the pH of the surrounding solutions, and even small differences in the pH lead to wildly different outcomes in the underlying chemical reactions.
> 
> 
> So, Chemists (and lots of other scientists) have learned to pay careful attention to pH, and to understand pH, you first need to understand acids and bases.
> 
> 
> 


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