what is the difference between Aqueous and liquid

Aqueous means it's in a solution, liquid is just a state.

are hydrogen atoms the cause of the acidic taste

Yes, though to be more specific, it is the H+ *ions* that indirectly cause an acidic food to taste sour. To put it simply, when acids found in the food combine with saliva in your mouth, H+ ions are produced. These H+ ions react with the protein molecules on your tongue and cause them to change shape. The change in shape causes the protein molecules to send an electrical signal to your brain that you experience as a sour taste. Source: Introductory Chemistry(Third Edition) by Nivaldo J. Tro

Are certain compounds acid only when in aqueous solutions? For example, I found something saying, "The compound, H2S, is called hydrogen sulfide when it is in pure form but it is called hydrosulfuric acid when its acidic properties in an aqueous solution is being discussed." I am a little confused

The thing is an acid is any substance that increases the hydronium ion concentration of the solution or yields hydronium ions on dissociation. Thus, in aqueous solution only can a substance dissociate and form hydronium ions and be classified as an acid. Not in any other form can it yield a H+ ion. I hope it helps...

What is meant by "dissociation?"

It means "to fall apart" or the opposite of parts coming together (opposite of association). Basically, when A-B are together then they can dissociate by forming A separated from B.

Are Arrhenius and Bronsted-Lowry just different ways to define an acid or a base or are they a different class of acids and bases all together??

It's a different way to define acids and bases. It's simply an extension to the theory of Arrhenius. Arrhenius defines acids and bases by the dissociation products that are formed when the acid or base is added to water, while Bronsted and Lowry define acids and bases by the reactions that occur when both are added together. Basically, acids in the Bronsted-Lowry theory donate protons (H+) to other substances while bases accept them to produce water (OH- + H+ = H2O).

i have read somewhere that water is a bit acidic but H is neutral in nature?so what can we say about it?

Water contains H⁺ ions, which are acidic, and an equal number of OH⁻ ions, which are basic. The two effects cancel each other, so water is neutral.

if we are able to separate cl- ion from hcl solution what do we get ? if we get hydromium ions , is it is acidic ?

HCl + H₂O → H₃O⁺ + Cl⁻ Yes, we get hydronium ions, and the solution is acidic because the HCl increases the concentration of hydronium ions in the water.

I don't understand that when hydronium exists instead of hydrogen ions in water, why doesn't hydronium react with water instantly to form something else since it's an ion.

Not all ions are reactive (think of dissolving salt in water to give Na+ and Cl- ions) - it depends on the circumstances. H+ ions are more reactive than H3O+ ions, so when an acid dissociates in the water, the protons immediately latch on to water molecules to give H3O+ ions which are more stable than H+ ions.

What other definitions are there?

Bronsted Lowry acids see basically the same thing as Arrhenius acids, but it also introduces the idea of conjugate bases and acids. Lewis acids accept a pair of electrons and Lewis bases donate a pair of electrons. Lots of metal chemistry is based on them being Lewis acids.

Main content

Arrhenius acids and bases

Google Classroom

Definition of Arrhenius acids and bases, and Arrhenius acid-base reactions

Key points

An Arrhenius acid is any species that increases the concentration of $H^{+}$ ‍ in aqueous solution.
An Arrhenius base is any species that increases the concentration of ${OH}^{-}$ ‍ in aqueous solution.
In aqueous solution, $H^{+}$ ‍ ions immediately react with water molecules to form hydronium ions, $H_{3} O^{+}$ ‍.
In an acid-base or neutralization reaction, an Arrhenius acid and base usually react to form water and a salt.

Introduction

From the vinegar in your kitchen cabinet to the soap in your shower, acids and bases are everywhere! But what does it mean to say that something is acidic or basic? In order to answer this question, we need to examine some of the theories describing acids and bases. In this article, we will focus on the Arrhenius theory.

Arrhenius acids

The Arrhenius theory of acids and bases was originally proposed by the Swedish chemist Svante Arrhenius in 1884. He suggested classifying certain compounds as acids or bases based on what kind of ions formed when the compound was added to water.

An Arrhenius acid is any species that increases the concentration of

H^{+}

ions—or protons—in aqueous solution. For example, let's consider the dissociation reaction for hydrochloric acid,

HCl

, in water:

H Cl (a q) \to H^{+} (a q) + {Cl}^{-} (a q)

When we make an aqueous solution of hydrochloric acid,

H Cl

dissociates into

H^{+}

ions and

{Cl}^{-}

ions. Since this results in an increase in the concentration of

H^{+}

ions in solution, hydrochloric acid is an Arrhenius acid.

Hydrogen or hydronium ions?

Let's say we made a

2 M

aqueous solution of hydrobromic acid,

HBr

, which is an Arrhenius acid. Does that mean we have

2 M

H^{+}

ions in our solution?

Actually, no. In practice, the positively charged protons react with the surrounding water molecules to form hydronium ions,

H_{3} O^{+}

. This reaction can be written as follows:

H^{+} (a q) + H_{2} O (l) \to H_{3} O^{+} (a q)

Even though we often write acid dissociation reactions showing the formation of

H^{+} (a q)

, there are no free

H^{+}

ions floating around in an aqueous solution. Instead, there are primarily

H_{3} O^{+}

ions, which form immediately when an acid dissociates in water. The following picture illustrates the formation of hydronium from water and hydrogen ions using molecular models:

In practice, most chemists talk about the concentration of

H^{+}

and the concentration of

H_{3} O^{+}

interchangeably. When we want to be more accurate—and less lazy!—we can write the dissociation of hydrobromic acid to explicitly show the formation of hydronium instead of protons:

\begin{aligned} HBr (a q) + H_{2} O (l) & \to H_{3} O^{+} (a q) + {Br}^{-} (a q) More accurate \\ vs. \\ HBr (a q) & \to H^{+} (a q) + {Br}^{-} (a q) Shorter and easier to write! \end{aligned}

In general, either description is acceptable for showing the dissociation of an Arrhenius acid.

Arrhenius bases

An Arrhenius base is defined as any species that increases the concentration of hydroxide ions,

{OH}^{-}

, in aqueous solution. An example of an Arrhenius base is the highly soluble sodium hydroxide,

NaOH

. Sodium hydroxide dissociates in water as follows:

Na OH (a q) \to {Na}^{+} (a q) + {OH}^{-} (a q)

In water, sodium hydroxide fully dissociates to form

{OH}^{-}

and

{Na}^{+}

ions, resulting in an increase in the concentration of hydroxide ions. Therefore,

NaOH

is an Arrhenius base. Common Arrhenius bases include other Group 1 and Group 2 hydroxides such as

LiOH

and

{Ba(OH)}_{2}

Even insoluble compounds such as

{Ca(OH)}_{2}

are often classified as Arrhenius bases because they have some tiny fraction that can dissolve in water, and the fraction that is in solution dissociates to form

{OH}^{-}

ions.

Another way to think about this is that complete solubility is not a requirement for something to be an Arrhenius base! As long as some of it gets into solution and increases the concentration of

{OH}^{-}

ions, we can classify it as an Arrhenius base.

Note that depending on your class—or textbook or teacher—non-hydroxide-containing bases may or may not be classified as Arrhenius bases. Some textbooks define an Arrhenius base more narrowly: a substance that increases the concentration of

{OH}^{-}

in aqueous solution and also contains at least one unit of

{OH}^{-}

in the chemical formula. While that doesn't change the classification of the Group 1 and 2 hydroxides, it can get a little confusing with compounds such as methylamine,

{CH}_{3} {NH}_{2}

When methylamine is added to water, the following reaction occurs:

{CH}_{3} {NH}_{2} (a q) + H_{2} O (l) ⇌ {CH}_{3} {NH}_{3}^{+} (a q) + {OH}^{-} (a q)

Based on our first definition, methylamine would be an Arrhenius base since the

{OH}^{-}

ion concentration increases in the solution. By the second definition, however, it would not count as an Arrhenius base since the chemical formula does not include hydroxide.

Acid-base reactions: Arrhenius acid + Arrhenius base = water + salt

When an Arrhenius acid reacts with an Arrhenius base, the products are usually water plus a salt. These reactions are also sometimes called neutralization reactions. For example, what happens when we combine aqueous solutions of hydrofluoric acid,

HF

, and lithium hydroxide,

LiOH

If we think about the acid solution and base solution separately, we know the following:

An Arrhenius acid increases the concentration of $H^{+} (a q)$ ‍:

H F (a q) ⇌ H^{+} (a q) + F^{-} (a q)

An Arrhenius base increases the concentration of ${OH}^{-} (a q)$ ‍:

Li OH (a q) \to {Li}^{+} (a q) + {OH}^{-} (a q)

When the acid and base combine in solution,

H_{2} O

is produced from the reaction between hydrogen ions and hydroxide ions, while the other ions form the salt

LiF (a q)

\begin{aligned} H^{+} (a q) + {OH}^{-} (a q) & \to H_{2} O (l) & Formation of water \\ {Li}^{+} (a q) + F^{-} (a q) & \to LiF (a q) & Formation of salt \end{aligned}

If we add the reactions for the formation of water and the formation of salt, we get our overall neutralization reaction between hydrofluoric acid and lithium hydroxide:

H F (a q) + Li OH (a q) \to H_{2} O (l) + LiF (a q)

Limitations of the Arrhenius definition

The Arrhenius theory is limited in that it can only describe acid-base chemistry in aqueous solutions. Similar reactions can also occur in non-aqueous solvents, however, as well as between molecules in the gas phase. As a result, modern chemists usually prefer the Brønsted-Lowry theory, which is useful in a broader range of chemical reactions. The Brønsted-Lowry theory of acids and bases will be discussed in a separate article!

Summary

An Arrhenius acid is any species that increases the concentration of $H^{+}$ ‍ in aqueous solution.
An Arrhenius base is any species that increases the concentration of ${OH}^{-}$ ‍ in aqueous solution.
In aqueous solution, $H^{+}$ ‍ ions immediately react with water molecules to form hydronium ions, $H_{3} O^{+}$ ‍.
In an acid-base or neutralization reaction, an Arrhenius acid and base usually react to form water and a salt.

Attributions

This article was adapted from the following article:

“Acid/Base Basics” from UC Davis ChemWiki, CC BY-NC-SA 3.0

The modified article is licensed under a CC-BY-NC-SA 4.0 license.

Additional References

Zumdahl, S. S., and S. A. Zumdahl. "Atomic Structure and Periodicity." In Chemistry, 290-94. 6th ed. Boston, MA: Hougton Mifflin Company, 2003.

Kotz, J. C., P. M. Treichel, J. R. Townsend, and D. A. Treichel. "Acids and Bases: The Arrhenius Definition." In Chemistry and Chemical Reactivity, Instructor's Edition, 116. 9th ed. Stamford, CT: Cengage Learning, 2015.

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ahmedhanif271
Posted 8 years ago. Direct link to ahmedhanif271's post “what is the difference be...”
what is the difference between Aqueous and liquid
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- Dina Tayyib
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  Aqueous means it's in a solution, liquid is just a state.
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are hydrogen atoms the cause of the acidic taste
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- Yellow Shiƒt»
  Posted 8 years ago. Direct link to Yellow Shiƒt»'s post “Yes, though to be more sp...”
  Yes, though to be more specific, it is the H+ ions that indirectly cause an acidic food to taste sour.
  
  To put it simply, when acids found in the food combine with saliva in your mouth, H+ ions are produced. These H+ ions react with the protein molecules on your tongue and cause them to change shape. The change in shape causes the protein molecules to send an electrical signal to your brain that you experience as a sour taste.
  
  Source: Introductory Chemistry(Third Edition) by Nivaldo J. Tro
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Caitlyn Limpioso
Posted 8 years ago. Direct link to Caitlyn Limpioso's post “Are certain compounds aci...”
Are certain compounds acid only when in aqueous solutions? For example, I found something saying, "The compound, H2S, is called hydrogen sulfide when it is in pure form but it is called hydrosulfuric acid when its acidic properties in an aqueous solution is being discussed." I am a little confused
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- gargi
  Posted 7 years ago. Direct link to gargi's post “The thing is an acid is a...”
  The thing is an acid is any substance that increases the hydronium ion concentration of the solution or yields hydronium ions on dissociation. Thus, in aqueous solution only can a substance dissociate and form hydronium ions and be classified as an acid. Not in any other form can it yield a H+ ion. I hope it helps...
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Jonathan Ziesmer
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What is meant by "dissociation?"
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Answer
- Matt B
  Posted 8 years ago. Direct link to Matt B's post “It means "to fall apart" ...”
  It means "to fall apart" or the opposite of parts coming together (opposite of association). Basically, when A-B are together then they can dissociate by forming A separated from B.
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Kartikeye
Posted 8 years ago. Direct link to Kartikeye's post “Are Arrhenius and Bronste...”
Are Arrhenius and Bronsted-Lowry just different ways to define an acid or a base or are they a different class of acids and bases all together??
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- Ian N. Friskus
  Posted 8 years ago. Direct link to Ian N. Friskus's post “It's a different way to d...”
  It's a different way to define acids and bases. It's simply an extension to the theory of Arrhenius. Arrhenius defines acids and bases by the dissociation products that are formed when the acid or base is added to water, while Bronsted and Lowry define acids and bases by the reactions that occur when both are added together. Basically, acids in the Bronsted-Lowry theory donate protons (H+) to other substances while bases accept them to produce water (OH- + H+ = H2O).
  Comment on Ian N. Friskus's post “It's a different way to d...”
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Pooja Goyal
Posted 8 years ago. Direct link to Pooja Goyal's post “if we are able to separat...”
if we are able to separate cl- ion from hcl solution what do we get ? if we get hydromium ions , is it is acidic ?
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Answer
- Ernest Zinck
  Posted 8 years ago. Direct link to Ernest Zinck's post “HCl + H₂O → H₃O⁺ + Cl⁻ Ye...”
  HCl + H₂O → H₃O⁺ + Cl⁻
  Yes, we get hydronium ions, and the solution is acidic because the HCl increases the concentration of hydronium ions in the water.
  Comment on Ernest Zinck's post “HCl + H₂O → H₃O⁺ + Cl⁻ Ye...”
  (9 votes)
vashist.ssy
Posted 8 years ago. Direct link to vashist.ssy's post “i have read somewhere tha...”
i have read somewhere that water is a bit acidic but H is neutral in nature?so what can we say about it?
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(6 votes)
Answer
- Ernest Zinck
  Posted 8 years ago. Direct link to Ernest Zinck's post “Water contains H⁺ ions, w...”
  Water contains H⁺ ions, which are acidic, and an equal number of OH⁻ ions, which are basic.
  The two effects cancel each other, so water is neutral.
  Comment on Ernest Zinck's post “Water contains H⁺ ions, w...”
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rezatahmid
Posted 7 years ago. Direct link to rezatahmid's post “I don't understand that w...”
I don't understand that when hydronium exists instead of hydrogen ions in water, why doesn't hydronium react with water instantly to form something else since it's an ion.
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(3 votes)
Answer
- RogerP
  Posted 7 years ago. Direct link to RogerP's post “Not all ions are reactive...”
  Not all ions are reactive (think of dissolving salt in water to give Na+ and Cl- ions) - it depends on the circumstances.
  
  H+ ions are more reactive than H3O+ ions, so when an acid dissociates in the water, the protons immediately latch on to water molecules to give H3O+ ions which are more stable than H+ ions.
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Brady
Posted 8 years ago. Direct link to Brady's post “What other definitions ar...”
What other definitions are there?
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- Ryan W
  Posted 8 years ago. Direct link to Ryan W's post “Bronsted Lowry acids see ...”
  Bronsted Lowry acids see basically the same thing as Arrhenius acids, but it also introduces the idea of conjugate bases and acids.
  
  Lewis acids accept a pair of electrons and Lewis bases donate a pair of electrons. Lots of metal chemistry is based on them being Lewis acids.
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  (5 votes)
༺𝓢𝓲𝓭𝓭𝓱𝓪𝓷𝓽༻
Posted 8 years ago. Direct link to ༺𝓢𝓲𝓭𝓭𝓱𝓪𝓷𝓽༻'s post “what is the difference be...”
what is the difference between an 'arrhenius acid' and a 'normal acid' ?
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- Toshaani Goel
  Posted 6 years ago. Direct link to Toshaani Goel's post “Arrhenius, Bronsted-Lowry...”
  Arrhenius, Bronsted-Lowry and Lewis concept are basically ways of characterising acids and bases. The substance proclaimed an acid by Arrhenius theory is referred to as Arrhenius acid. there is nothing like normal acid. All acids are normal explained by different theories
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