Amphoterism Definition
Amphoteric meaning, in the simplest term, can be stated as any compound that can be mixed with other compounds both as a base and an acid. For example, water is amphoteric. It can be transformed into a compound that can be used as an alkali or an acid. Most amphoteric compounds are metal oxides or hydroxides.
Amphoterism is a popular term in the field of chemistry explaining the reactivity of a compound that shows alkaline nature when mixed with an acid and demonstrates acidic characteristics in an alkaline environment.
As water is amphoteric, it is used widely for such reactions. When acid reacts with water, it gives off H3O+ ion adding an H+ ion from the acid. On the other hand, when water is combined with a base, it donates an H+ ion to the base and releases OH- ion.
Besides water, many oxides and hydroxides are vastly utilized in amphoterism. Now that amphoteric meaning is clear enough, let us see how you can identify an amphoteric substance.
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How to Identify an Amphoteric Substance?
While experimenting in the laboratory, how would you recognize an amphoteric compound? How to match the properties to the amphoteric definition that you have just read? Here is how you can define the amphoteric characteristics in a compound.
When an amphoteric substance is mixed with an acid, it would take up hydrogen from the acid creating an H3O+ ion.
When the amphoteric compound reacts to a base, it supplies hydrogen to the alkaline compound and releases an OH- ion.
The Bronsted Lowry theory states that 'acids are proton donors and bases are proton acceptors.'. The substances that have both these characteristics are identified as amphoteric substances.
Amphoteric Hydroxides
Hydroxides are famous for using amphoterism. Not all metals can be used as amphoteric hydroxides. Hydroxides that are made of highly charged metal cations are used for amphoterism.
They can react in two conditions. They can either function as a Bronsted lowry base by receiving protons or work as Lewis acid by receiving a pair of electrons.
In acidic conditions, a simple neutralization reaction occurs. As mentioned before, here the hydroxide would react as a base and would either give off protons or accept electrons.
In an alkaline condition, the hydroxide will behave just like an acid by accepting protons from the other reactant.
Amphoteric Hydroxide Examples or Simply, Amphoteric Examples List is Given Below.
Aluminium hydroxide
Zinc hydroxide
Copper hydroxide
Chromium hydroxide
Beryllium hydroxide
Tin hydroxide
Amphoteric Amino Acid
The carboxyl group in amino acids is responsible for their amphoteric properties. They are widely used for amphoterism. An amphoteric amino acid contains both acidic and alkaline elements like COOH and NH2.
When present in an acidic condition, an amino acid would act as a base. It will carry away the positively charged ions and get attracted to the negative ions. It gives off the protons or accepts electrons.
On the other hand, while reacting with a base, it possesses acidic characteristics by carrying negative ions and accepting protons. Amino acids are excellent for amphoterism.
One example of amphoteric amino acids is glycine.
Amphoteric Oxides
Just like the hydroxides, some oxides are also amphoteric. The amphoteric characteristics depend on the oxidation state of the compound.
When a metal oxide reacts with an acid, you can witness a neutralization reaction just like an amphoteric hydroxide. It will accept protons from the acid and produce salt.
In contrast to this reaction, a metal oxide would produce salt and water when it reacts with an alkali. It would give off protons to the other reactant.
Amphoteric oxides are pretty similar to amphoteric hydroxides.
Some Common Oxide Amphoteric Examples are Given Below.
Aluminum oxide
Lead oxide
Zinc oxide
Tin oxide
Beryllium oxide
Chromium oxide
Copper oxide
Aluminum Amphoteric
Aluminum is well-known for its amphoteric properties. All compounds made with aluminum are amphoteric compounds. All of these compounds can be used as both acids or bases.
Aluminum amphoteric can be used in many forms to let them react as acids or bases. It reacts similarly to other amphoteric compounds. It accepts protons when mixed with an acid and releases protons when it reacts to an alkali.
Aluminum chloride, aluminum oxide, aluminum hydroxide are some examples of aluminum amphoteric.
As you can see in the amphoteric list above, you can imagine how important it is in the field of chemistry. Amphoteric reactions are used to study the properties of elements and analyze the periodic table.
The amphoteric examples given above have many compounds that can be applied both as an acid and an alkali. These compounds help in laboratory work as well. As one compound acts both as an acid and a base, you can observe various reactions with the same compound.
Fun Facts
Do you know that water is a neutral oxide as well as an amphoteric substance? Neutral substances hardly show any alkaline or acidic properties. But amphoteric compounds show both acidic and alkaline characteristics. Water, however, is the only compound that can be both a neutral substance and an amphoteric substance depending on the situation.
FAQs on Amphoterism
1. What is meant by amphoterism in chemistry?
Amphoterism is the property of a substance that allows it to act as both an acid and a base. An amphoteric substance will react with an acid to behave like a base, and it will react with a base to behave like an acid. This dual nature depends entirely on the chemical environment and the substance it is reacting with. Water is a classic example of an amphoteric substance.
2. What are some common examples of amphoteric substances?
Many substances exhibit amphoterism. The most common examples fall into these categories:
- Water (H₂O): It can donate a proton to act as an acid or accept a proton to act as a base.
- Metal Oxides: Oxides of certain metals like Aluminium oxide (Al₂O₃), Zinc oxide (ZnO), Lead(II) oxide (PbO), and Tin(II) oxide (SnO).
- Metal Hydroxides: Hydroxides such as Aluminium hydroxide (Al(OH)₃), Beryllium hydroxide (Be(OH)₂), and Zinc hydroxide (Zn(OH)₂).
- Amino Acids: As they contain both an acidic carboxyl group (-COOH) and a basic amino group (-NH₂), they are fundamentally amphoteric.
3. How can the amphoteric nature of zinc oxide (ZnO) be demonstrated with chemical equations?
The dual nature of zinc oxide can be shown by reacting it with a strong acid and a strong base:
- Acting as a base (reaction with acid): When it reacts with hydrochloric acid, it forms zinc chloride and water.
ZnO + 2HCl → ZnCl₂ + H₂O - Acting as an acid (reaction with base): When it reacts with a strong base like sodium hydroxide, it forms sodium zincate.
ZnO + 2NaOH → Na₂ZnO₂ + H₂O
These reactions clearly show ZnO's ability to neutralise both acids and bases.
4. How do amino acids exhibit amphoterism?
Amino acids demonstrate amphoterism due to their unique structure, which contains both a basic amino group (-NH₂) and an acidic carboxyl group (-COOH). In a neutral solution, these groups ionise to form a dipolar ion called a zwitterion (-NH₃⁺ and -COO⁻). This zwitterion can then react by either donating a proton from its -NH₃⁺ group (acting as an acid) or accepting a proton at its -COO⁻ group (acting as a base).
5. What is the key difference between an amphoteric and an amphiprotic substance?
This is a common point of confusion. The key difference lies in the specific acid-base theory being applied:
- An amphiprotic substance is one that can both donate and accept a proton (H⁺), according to the Brønsted-Lowry theory. Water (H₂O) is a perfect example.
- An amphoteric substance is a broader term for any substance that can act as an acid or a base, by any definition. This includes the Lewis acid-base theory (donating/accepting electron pairs).
Therefore, all amphiprotic substances are amphoteric, but not all amphoteric substances (like ZnO) are amphiprotic, as they may not be able to donate a proton.
6. Why are some metal oxides amphoteric while others are strictly acidic or basic?
The acidic, basic, or amphoteric character of a metal oxide is primarily determined by the electronegativity and oxidation state of the metal.
- Basic Oxides: Metals with very low electronegativity (like Sodium, Na) are highly electropositive and readily donate electrons, forming strongly basic oxides (e.g., Na₂O).
- Acidic Oxides: Non-metals with high electronegativity (like Sulphur, S) form acidic oxides (e.g., SO₃).
- Amphoteric Oxides: Metals with intermediate electronegativity (like Aluminium and Zinc) form oxides where the metal-oxygen bond has partial covalent character. This allows the bond to break in two different ways, enabling it to react with both strong acids and strong bases.
7. Which elements in the periodic table are known for forming amphoteric oxides and hydroxides?
Amphoteric behaviour is commonly found in elements located near the dividing line between metals and non-metals in the periodic table. Key elements that form well-known amphoteric oxides and hydroxides include:
- Beryllium (Be)
- Aluminium (Al)
- Zinc (Zn)
- Tin (Sn)
- Lead (Pb)
- Gallium (Ga)
- Chromium (in the +3 oxidation state, Cr₂O₃)
8. What is the importance of amphoterism in biological systems?
Amphoterism is vital for life, primarily through the action of amino acids and proteins. Because proteins are made of amphoteric amino acids, they can act as natural buffers in biological systems. They help maintain a stable pH in blood and cells by absorbing excess H⁺ (acid) or OH⁻ (base) ions, which is crucial for enzymes to function correctly and for overall metabolic stability.
