What Is an Arrhenius Acid? Understanding the Concept with Examples
Acids and bases are chemical compounds showing distinct properties which make them usable in certain scenarios. For instance, automobile batteries use sulphuric acid, few fertilisers and detergents use acids for its composition. Likewise, bases like Calcium Hydroxide find its application in the manufacturing of bleaching powder. These acids and bases have numerous applications in chemical units, industries, and day to day life.
While these have numerous applications, exposure to strong acids and bases can be harmful to you. Therefore, it is important to adhere to the safety measures while handling these corrosive chemicals.
Now, an acid or base can be defined in multiple ways as there are three proposed theories –
Arrhenius acid and base theory.
Bronsted-Lowry definition.
Lewis theory.
Here, we will discuss Arrhenius theory of acids in details.
Arrhenius Acid and Base Definition
A Swedish scientist Svante Arrhenius, in the year 1884, proposed acid and base as the two classifications of compounds. According to him, an acid is a compound which can readily give up protons or Hydrogen ion in aqueous or water solution.
For instance, take this equation into consideration.
HCl(aq) → H+(aq) + Cl–(aq)
Here, the solution of Hydrochloric acid with water entirely dissociates them into Hydrogen and Chlorine ions. When this aqueous solution of HCl is mixed with water, it releases hydronium ions as shown by the equation below –
HCl(aq) + H2O(l) → H3O+(aq) + Cl–(aq)
In a similar manner, acids such as HBr, CH3COOH (acetic acid), HNO3 (Nitric acid), HI (Hydro-iodic acid), H2SO3 (sulphurous acid), H2SO4 (sulphuric acid), H3PO4 (Phosphoric acid), H2CO3 (Carbonic acid), HCIO4 (Perchloric acid), HF (Hydrofluoric acid) follow the Arrhenius concept.
According to the theory, a compound is known as Arrhenius base if it dissociates into OH– ions in its aqueous or water solution form.
For instance, consider this equation to understand this theory.
NaOH (aq) → Na+ (aq) + OH– (aq)
Here, the aqueous solution of NaOH produces Hydroxide ions exhibiting the property of base.
Basicity Property of Arrhenius Acids
The basicity of any acid is the measure of H+ ions it can release. Further, understand this concept with the help of the following reaction –
H3PO4 → H+ + H2PO4–
H2PO4– → H+ + HPO42–
HPO42– → H+ + PO43–
From the above equations, it is clear that H3PO4 releases three Hydrogen ions in its aqueous or water solution. Therefore, the basicity of H3PO4 acid will be equal to 3.
Further, look at the table drawn below that shows the basicity values of different acids.
Arrhenius Acid as Electrolytes
Arrhenius was trying to learn the reason behind the conduction of electricity in solutions. He found out that the reason for conductivity was ions, primarily. His observation led to the speculation that acids like HNO3, HCl, H2SO4, etc. behave as electrolytes when they are dissolved in water.
As per the Arrhenius definition of acid, for an ideal strong electrolyte solution, if 100 molecules of HCl are mixed with water, then it releases 100 H+ & 100 Cl– ions. Ideally, there is no molecule of HCl in the solution as this acts as strong acid and reacts with water to produce ions.
Strong Acid
A strong acid has the capability to get completely dissociated or ionised in the aqueous solution so that it increases the number of protons or H+ ions in the solution. Here, the acid dissociation constant is represented as Ka and is proportional to the strength of an acid. Therefore, a strong acid has a high magnitude of Ka.
Weak Acid
Few compounds fail to dissociate completely when added to the aqueous solution. These are known as weak acids which are a solution of un-dissociated weak acid along with partially dissociated ions. In the solution, the number of H+ ions are extremely low and hence their pH value is greater than that of strong acids. For weak acids, magnitude of Ka or the value of acid-dissociation constant is lesser than strong acids.
Arrhenius Acid Example
The list of acids which qualify Arrhenius theory for acids is included in the table mentioned below. Have a look.
In the solid or pure state, these exist as covalent compounds as the hydrogen ions are generated only when these are mixed with water as per Arrhenius acid base theory.
Multiple-Choice Questions
Choose the appropriate option which is true for Arrhenius acid.
It is a compound which accepts hydrogen ions.
When mixed with water, these release hydroxide ions.
When mixed with water, these accept hydroxide ions.
It is a compound which releases hydrogen ions in the solution.
Choose the appropriate option which states the property of strong acid.
The bond between hydrogen atoms and other elements is stronger in case of strong acids.
Strong acids can easily bond with water.
Strong acids are capable of dissociating entirely in the solution.
Strong acids barely dissociate when mixed in the form of solution.
If you pour an Arrhenius base into a beaker full of water, which ions will you find inside it?
Hydroxide ions.
Hydronium ions.
Hydrogen ions.
Water.
Choose the appropriate option which depicts the hydronium ion formation.
HCl (aq) ---> H+ (aq) + Cl- (aq)
HCl(aq) + NaOH(aq) ---> H20(l) + NaCl(aq)
H3O+(aq) ---> H+(aq) + H20(l)
HCl(aq) ---> H3O+(aq) + Cl-(aq)
Choose the appropriate option for application of pH scale.
It is used to neutralise bases and acids.
It is used for describing the chemical reaction between bases and acids.
It is a scientific theory which helps determine the nature of bases and acids at a molecular level.
It is utilised for measuring the concentration of hydrogen ions or hydronium ions in an aqueous solution.
Choose the right option for an aqueous solution whose pH range is in between 1 and 6.
A base.
An acid.
A hydrocarbon.
A neutral solution.
Choose the appropriate equation depicting acid base neutralisation reaction.
H3O+ (aq) ---> H+(aq) + H20 (l)
HCl (aq) ---> H+ (aq) + Cl
HCl (aq) + NaOH (aq) ---> H20 (l) + NaCl (aq)
HCl (aq) ---> H3O+ (aq) + Cl- (aq)
What is the effect of strong acid on litmus paper?
It turns the red litmus paper blue.
It turns the blue litmus paper blue.
It turns the blue litmus paper blue.
It turns the red litmus paper red.
Now, get familiar with the important concepts related to Arrhenius with these study notes. You will be able to perform better in your examination and improve your knowledge about the two segregations of chemical compounds. So, if you want to give Arrhenius definition of an acid and a base precisely, refer to these study notes and learn with examples. You can also download Vedantu’s app for a more comprehensive learning approach. Refer to the study notes prepared by professional tutors and progress in your academic venture.
FAQs on Arrhenius Acid: Simple Guide, Properties & Examples
1. What is an Arrhenius acid according to the 2025-26 CBSE syllabus?
An Arrhenius acid is defined as any substance that contains hydrogen and ionises in an aqueous solution (dissolves in water) to increase the concentration of hydrogen ions (H⁺). In practice, these H⁺ ions bond with water molecules to form hydronium ions (H₃O⁺). This concept is a fundamental part of the 'Equilibrium' chapter in the CBSE chemistry syllabus.
2. What are some common examples of strong and weak Arrhenius acids?
Arrhenius acids are classified based on their ability to dissociate in water. The main types are:
- Strong Arrhenius Acids: These ionise almost completely in water, releasing a large number of H⁺ ions. Examples include Hydrochloric acid (HCl), Sulfuric acid (H₂SO₄), and Nitric acid (HNO₃).
- Weak Arrhenius Acids: These ionise only partially in water, meaning an equilibrium is established between the undissociated acid and its ions. Examples include Acetic acid (CH₃COOH) found in vinegar, and Carbonic acid (H₂CO₃) in soft drinks.
3. What are the key properties of a substance classified as an Arrhenius acid?
According to the Arrhenius theory, a substance must exhibit several key properties to be called an acid:
- It must possess at least one hydrogen atom in its chemical structure.
- When dissolved in water, it must dissociate to release hydrogen ions (H⁺).
- The resulting aqueous solution typically has a sour taste.
- It changes the colour of blue litmus paper to red.
- It reacts with an Arrhenius base in a neutralization reaction to produce salt and water.
4. How does an Arrhenius acid actually increase the concentration of H+ ions in water?
An Arrhenius acid increases the H⁺ ion concentration through a process known as dissociation or ionisation. When an acid molecule (e.g., HCl) enters water, the polar water molecules surround it. The strong attraction between the oxygen atom of a water molecule and the hydrogen atom of the acid breaks the original bond within the acid. This liberates a hydrogen ion (H⁺), which is then stabilised by bonding with a water molecule to form a hydronium ion (H₃O⁺), thereby increasing the concentration of these positive ions in the solution. The general equation is: HA(aq) → H⁺(aq) + A⁻(aq).
5. What are the main limitations of the Arrhenius theory for defining acids?
While foundational, the Arrhenius theory has several important limitations:
- Solvent Dependency: The theory is restricted to substances in aqueous solutions. It cannot explain the acidic behaviour of substances in non-aqueous solvents like benzene or liquid ammonia.
- Composition Requirement: It fails to explain the acidic properties of substances that do not contain hydrogen, such as acidic oxides like CO₂ and SO₃, which produce acidic solutions when dissolved in water.
- Nature of the Proton: The theory's concept of a free-floating proton (H⁺) is a simplification. Protons are highly reactive and do not exist independently in water; they exist as the more stable hydronium ion (H₃O⁺).
6. How does the Arrhenius definition of an acid compare to the Brønsted-Lowry definition?
The primary difference is in their scope and definition. The Arrhenius theory is narrow, defining an acid as a substance that produces H⁺ ions in water. The Brønsted-Lowry theory is broader, defining an acid as any species that can donate a proton (H⁺), regardless of the solvent. This means that while all Arrhenius acids are also Brønsted-Lowry acids, the reverse is not true. The Brønsted-Lowry concept can explain a wider variety of acid-base reactions, including those that do not occur in water.
7. Why is a substance like ammonia (NH₃) considered a base but not by the Arrhenius definition?
This is a classic example of the limitations of Arrhenius theory. Ammonia (NH₃) does not contain a hydroxide (OH⁻) group in its formula, so it cannot be classified as an Arrhenius base. However, when dissolved in water, ammonia reacts with a water molecule, accepting a proton to form the ammonium ion (NH₄⁺) and releasing a hydroxide ion (OH⁻) into the solution (NH₃ + H₂O ⇌ NH₄⁺ + OH⁻). Because it increases the OH⁻ concentration, it behaves as a base. This behaviour is perfectly explained by the Brønsted-Lowry theory, which defines a base as a proton acceptor.
8. What is an Arrhenius base and how does it participate in a neutralization reaction?
An Arrhenius base is a substance that contains a hydroxide group and dissociates in an aqueous solution to produce hydroxide ions (OH⁻). A common example is sodium hydroxide (NaOH). In a neutralization reaction, an Arrhenius acid reacts with an Arrhenius base. The fundamental process involves the H⁺ ions from the acid combining with the OH⁻ ions from the base to form neutral water (H₂O). The remaining ions, the cation from the base and the anion from the acid, combine to form a salt. For instance: HCl (acid) + NaOH (base) → NaCl (salt) + H₂O (water).
