Is litmus test a reliable criterion for classifying compounds as acids or bases?

[PainterGuy]

Hi

All the chemical compounds can be categorized as acids, bases, and neutral compounds.

There are different theories, namely Arrhenius Theory, Brønsted-Lowry Theory, Lewis Theory, which define the acids and bases. Each successive theory becoming more general; Lewis Theory is the most generalized one of the three.

Neutral compounds could be salts, hydrocarbons like ethanol, oxides, and many more. A salt is always a result of a reaction between an acid and base.

The difference between a base and an alkali is that an alkali is a base which are soluble in water. In other words, all alkalies are bases but not all bases are alkalies.

I'm sure like bases there would be many acidic compounds which are not soluble in water. Please correct me if I'm wrong.

Is litmus test really a defining criterion for classification of a compound as an acid or base? I believe that to perform litmus test, the compound should be soluble in water, and we have already agreed that not all bases and acids form aqueous solution. It means that litmus test is not a main criterion for the classification. It is something of secondary importance.

Likewise, general properties given in school chemistry books are also not primary criteria. Stated differently, an acid doesn't always need to taste sour, turn litmus red, and a base doesn't have to feel slippery or turn litmus blue. Please let me know if I'm wrong.

Boron trifluoride is classified an acid. Boron has three electrons in its outer shell and it forms three covalent bonds with three fluorine atoms which give it total sum of six electrons. It's still deficient of two electrons and therefore it can accept an electron pair so it's Lewis acid.

Thank you.

Regards
PG

References:
1: http://www.krysstal.com/acidbase.html
2: http://chemwiki.ucdavis.edu/Physical_Chemistry/Acids_and_Bases/Acid
3: http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch11/lewis.php
4: http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch11/acidbase.php
5: http://www.bbc.co.uk/bitesize/ks3/science/chemical_material_behaviour/acids_bases_metals/revision/3/
6: https://answers.yahoo.com/question/index?qid=20091121212137AATbIfd
7: https://lh4.ggpht.com/mWIstW4evdppM4rZPwpihQgXmQdSZAMCcBpGrdwslvwsPmlDKlEQ5H-Vc7Edhkulg-E=h900

 

[Bystander]

Presumably there is a question in here somewhere.

 

[PainterGuy]

Presumably there is a question in here somewhere.

Yes, there are if you look little carefully! :) What I'm saying there might be incorrect that's why I have written 'Please let me know if I'm wrong' and then there are some direct questions. Thanks.

 

[symbolipoint]

Something like soluble metal hydroxide might be an alkali. Soluble organic bases like triethanolamine or monoethanolamine would be alkaline in water but would not be through of as alkalies.

 

[Borek]

Generalizations of the acid/base theory at some point became way too sophisticated for being used for simple cases. They help us classify some reactions as "acid base reactions" - and to some extent it makes sense, but to some extent it is just a way of helping us putting reactions into boxes with known names. Nature doesn't care about boxes we created and names we applied to them. So asking about litmus test in the context of Lewis acids doesn't make much sense - they belong to slightly different worlds.

 

[fireflies]

School chemistry books don't always show the actual figure. Like what I learned in school, much changed in college(high school) level. And more changes in university level. Like atomic theories of Dalton.

Though the topics seemed interesting. Particularly this one, I knew all acids are soluble? (I am not considering Lewis, rather the acids of the other two theories.

 

[Borek]

all acids are soluble?

This is difficult, for the reasons I have mentioned earlier (nature doesn't care about our classification attempts). Take fluoroantimonic acid (one of the strongest, or just the strongest superacid). It protonates everything, so it is a strong proton donor - as such it is pretty close to being the Arrhenius acid. Trick is, it hydrolyzes in water, so it is not possible to produce its solution. Does it make it soluble, or insoluble?

 

[fireflies]

Hydrolizes means soluble, right? I don't know

 

[Borek]

No, it means it reacts with water - producing something else.

 

[PainterGuy]

Thank you, Borek, fireflies.

I understand that to provide someone with an exact answer in chemistry is somewhat difficult. But could you please still address the specific questions from my previous post so that I could have a little clearer picture?

All the chemical compounds can be categorized as acids, bases, and neutral compounds.

There are different theories, namely Arrhenius Theory, Brønsted-Lowry Theory, Lewis Theory, which define the acids and bases. Each successive theory becoming more general; Lewis Theory is the most generalized one of the three.

Neutral compounds could be salts, hydrocarbons like ethanol, oxides, and many more. A salt is always a result of a reaction between an acid and base.

The difference between a base and an alkali is that an alkali is a base which are soluble in water. In other words, all alkalies are bases but not all bases are alkalies.

Q #1:
I'm sure like bases there would be many acidic compounds which are not soluble in water. Do I have it correct?

Q #2:
Is litmus test really a defining criterion for classification of a compound as an acid or base? I believe that to perform litmus test, the compound should be soluble in water, and we have already agreed that not all bases and acids form aqueous solution. It means that litmus test is not a main criterion for the classification. It is something of secondary importance. Do you agree?

Q #3:
Likewise, general properties given in school chemistry books are also not primary criteria. Stated differently, an acid doesn't always need to taste sour, turn litmus red, and a base doesn't have to feel slippery or turn litmus blue. Do you agree?

Boron trifluoride is classified an acid. Boron has three electrons in its outer shell and it forms three covalent bonds with three fluorine atoms which give it total sum of six electrons. It's still deficient of two electrons and therefore it can accept an electron pair so it's Lewis acid.

Thanks a lot.

Regards
PG

 

[fireflies]

I don't think litmus would be something defining acid. A neutral solution can change the colour provided one weak+one strong. Well, I am not certain about it. Phenopthalein does that in strong base and weak acid solution, that is though it is neutral but is basic. pH range for litmus paper is 6-8. So, acid and base with different strengths can show different result in such solution.

 

[symbolipoint]

I don't think litmus would be something defining acid. A neutral solution can change the colour provided one weak+one strong. Well, I am not certain about it. Phenopthalein does that in strong base and weak acid solution, that is though it is neutral but is basic. pH range for litmus paper is 6-8. So, acid and base with different strengths can show different result in such solution.

Too many misunderstandings to discuss. An introductory chemistry course would help you with your questions very well. Litmus paper was prepared by letting the paper absorb a color-changing acid-base indicator solution and compounds. The balance between hydroxide and hydronium ions in a test solution will then control the color of the indicator. Blue color indicates very low concentration of hydronium ion but much greater concentration of hydroxide ions. If concentration of hydronium is greater, then the indicator would turn red or pink. Litmus works in water.