Question

Define the term "amphoteric"

Short answer

Amphoteric substances can act as both acids and bases.

Step-by-step solution

Understanding Amphoteric Substances

An amphoteric substance is one that can act as both an acid and a base according to the Brønsted-Lowry definition of acids and bases. This means they can donate protons (H+) like acids or accept them like bases, depending on the reaction environment.

Examples of Amphoteric Substances

A common example of an amphoteric substance is water (H2O). In acidic environments, water can act as a base by accepting a proton to form hydronium ion (H3O+), while in basic environments, it can donate a proton to form hydroxide ion (OH-). Another example is aluminum hydroxide (Al(OH)3), which can react with acids to form aluminum salts or with bases to form complex anions.

Key concepts

Brønsted-Lowry Definition

In the world of chemistry, the Brønsted-Lowry definition is a fundamental concept used to describe acids and bases. This definition, proposed by Johannes Nicolaus Brønsted and Thomas Martin Lowry in 1923, goes beyond the earlier definition by Arrhenius.
According to the Brønsted-Lowry theory, an acid is any substance that can donate a proton (H⁺ ion), while a base is any substance that can accept a proton.

• An acid is therefore a proton donor.
• A base is a proton acceptor.

This approach provides a broader spectrum for identifying acids and bases.
It is particularly useful in explaining acid-base reactions that occur in non-aqueous solvents, which the Arrhenius definition doesn't cover. The Brønsted-Lowry definition also lays the foundation for understanding amphoteric substances, which can act as either an acid or a base depending on the scenario.

Acid-Base Reactions

Acid-base reactions are a key component of many chemical processes. In the context of the Brønsted-Lowry theory, these reactions involve the transfer of protons from the acid to the base. This is also known as a proton transfer reaction.
When acid and base interact, the acid donates a proton, which the base accepts. This results in the formation of two new species: the conjugate base of the acid and the conjugate acid of the base.

• Acid donates a proton → forms conjugate base.
• Base accepts a proton → forms conjugate acid.

For example, when hydrochloric acid (HCl) reacts with water (H₂O):
The HCl donates a proton to water, forming hydronium ion (H₃O⁺) and chloride ion (Cl^-).
Reactions of this type illustrate how substances can act as both acids and bases under different conditions, which is the principle behind amphoteric behavior.

Amphoteric Examples

Amphoteric substances are unique because they can act as either an acid or a base, depending on the environment. Two common examples of such substances are water (H₂O) and aluminum hydroxide (Al(OH)₃).
In acidic environments, these substances will behave as bases, accepting protons:

• Water accepts a proton to become hydronium (H₃O⁺).
• Aluminum hydroxide reacts with acids to form aluminum salts.

In basic environments, they will donate protons, acting as acids:

• Water donates a proton to form hydroxide (OH^-).
• Aluminum hydroxide reacts with bases to form complex anions.

Understanding these examples helps clarify the concept of amphoteric substances, showcasing their ability to adapt by assuming different roles in chemical reactions.
This versatility is what makes them crucial players in many chemical processes, including solutions and reactions that do not conform to simple acid or base behavior.