How do you know what is alkali and what is acid? Watch

annathecat
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Hi.

I would really appreciate some kind soul helping me figure out this simple question: For a substance, how do you know if it's acid or alkali? When the formula is given. For instance, sodium carbonate is an alkali.

Is there a general rule that determines whether something is acid or alkali-for instance something like 'all carbonates are alkali' ??
If there are such general rules, please post me those 'cause at the moment, I have no clue!

Thanks
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ElMoro
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(Original post by annathecat)
Hi.

I would really appreciate some kind soul helping me figure out this simple question: For a substance, how do you know if it's acid or alkali? When the formula is given. For instance, sodium carbonate is an alkali.

Is there a general rule that determines whether something is acid or alkali-for instance something like 'all carbonates are alkali' ??
If there are such general rules, please post me those 'cause at the moment, I have no clue!

Thanks
If there are OH^{-} ions present then it is an alkali.

If there are H^{+} ions present then it is an acid.

I'm only at GCSE level so I don't know if there's more to it
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Blob2491
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I'm interested to know too, I just remember that an alkali is a base that can be dissolved in water?
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Alakamind
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The only acids you need to be familiar with are Hydrochloric acid and Sulfuric acid. Alkalis often have hydroxides at the end of their name e.g Sodium Hydroxide. The hydroxide stands for the OH- hroup attached to them. Acids only have H+ and no OH- group. For example, sulfuric acids molecular formula is H2SO4, it contains a H+ group but no OH-. Although it contains both oxygen and hydrogen they are seperate and do not form a hydroxyl group.
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confuchaz
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the bonsted lowry defination of an acid is something that can donate a proton (H+ ion) and a base/alkali can accept a H+.

so look for something that can donate a H or accept one. i don't know what level you're at but if your a level then you should be able to notice whether it can accept it or not.

its not just as simple as H+ and OH- 's because water arguably has both, and yet can act as a very very weak acid.
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limetang
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Bronsted Lowry theory states that an acid is a proton donator and a base/alkali is a proton aceptor.
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limetang
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(Original post by Alakamind)
The only acids you need to be familiar with are Hydrochloric acid and Sulfuric acid. Alkalis often have hydroxides at the end of their name e.g Sodium Hydroxide. The hydroxide stands for the OH- hroup attached to them. Acids only have H+ and no OH- group. For example, sulfuric acids molecular formula is H2SO4, it contains a H+ group but no OH-. Although it contains both oxygen and hydrogen they are seperate and do not form a hydroxyl group.
Erm sorry to knitpick but that's wrong. You see you have can have things called Carboxyllic acids. These have the general formula R-COOH. You can also have things called phenols these are benzene rings with OH groups directly attatched to the ring. What happens in these molecules is they donate H+ ions directly from their OH groups.
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shadow12345
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Acids are proton donors and alkalis are proton acceptors....

Remember that a H+ is a proton...
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ShayCamp
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Alkali blue, acid red? LOOL I haven't done Chem in about 2 years.
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illusionz
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There are three definitions of acids/bases:

Arrhenius: As defined by Arrhenius, acid–base reactions are characterized by Arrhenius acids, which dissociate in aqueous solution to form hydrogen ions (H+) and Arrhenius bases, which form hydroxide (OH-) ions

Bronsted-Lowry: Acid is an H+ donor, base is an H+ acceptor. eg an alcohol can lose it's H from the OH and an amine can pick up an H.

Lewis: Acid - lone pair acceptor, base - lone pair donor. For example you may have heard of AlCl3 being described as a lewis acid

Each of these definitions includes substances defined by the ones above, but is a more broad definition.
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annathecat
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Thanks everybody-keep posting!!
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Clumsy_Chemist
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(Original post by annathecat)
Thanks everybody-keep posting!!
I could give you an ideal answer/mini essay on acids/alkalis if I knew what level you're studying at. GCSE? AS? A2?
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NiallHoran
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Use something called the PH scale and then add universal indicator to the substance and it will turn anywhere from red to purple. Red being very acidic and purple being very alkali and green being neutral
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Goppyfish
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Basically acids are H+ whereas; alkali are OH-. Also if it ends in hydroxide it probs is a alkali.
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samina_ay
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alkali is the one with OH- ions
acid is the one with H+ ions
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Fish40
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(Original post by samina_ay)
alkali is the one with OH- ions
acid is the one with H+ ions
Samina_ay you are gorgeous

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samina_ay
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(Original post by Fish40)
Samina_ay you are gorgeous

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Thank you
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alow
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(Original post by ElMoro)
If there are \text{OH}^{-} ions present then it is an alkali.

If there are \text{H}^{+} ions present then it is an acid.

I'm only at GCSE level so I don't know if there's more to it
pls no italic chemicals

Also things can be basic without hydroxide ions.
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Pigster
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I too have an irresistible urge to post in this most ancient of threads.

I'd like to point out that acid solutions contain the OH- ion.

And I'd be correct too.

That'll confuzzle a lot of you.
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Pigster
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Oh and whilst I'm in the confuzzling mood, a solution with a pH of more than 7 can be acidic.
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