Problem 64 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

An element which can exist as a positive ion in acidic solution and also as a negative ion in basic solution is said to be ...............

Step-by-Step Solution

Answer

The term is 'amphoteric.'

Step 1: Understand the Problem

The exercise is asking for a term that describes an element which has the ability to become a positive ion (cation) in acidic conditions and a negative ion (anion) in basic conditions.

Step 2: Recall Relevant Concepts

We need to recall terms related to elements that can switch between positive and negative ionic states depending on the environment. An amphoteric element or compound fits this description, as it can react both as an acid and as a base.

Step 3: Apply Knowledge

Since the context involves an element changing ionic states based on the solution's pH, an appropriate term to describe such an element is 'amphoteric.' This term embodies the dual functionality of forming cations in acidic solutions and anions in basic ones.

Key Concepts

Cation Formation

Cation formation is an essential process where elements or compounds become positively charged ions by losing electrons. Typically, metals form cations.
In acidic solutions, a higher concentration of protons ( ext{H}^+) encourages this electron loss.
When an element loses one or more electrons, it generally attains a positive electrical charge, due to the balance tipping towards more protons than electrons.
To understand cation formation better, you should:

• Recognize that metals are predisposed to lose electrons due to their relatively low electronegativity.
• Note that in acidic conditions, the abundant ext{H}^+ ions can drive electron loss, assisting in cation formation.
• Remember that amphoteric elements like zinc and aluminum can become cations in acidic surroundings.

Anion Formation

Anion formation is the process by which atoms or molecules gain electrons, resulting in a negative charge. This commonly happens with non-metals which possess higher electronegativity, eager to accept electrons.
In basic solutions, ample ext{OH}^- ions exist which can facilitate electron gain by elements that are capable of becoming anions.
To clearly grasp anion formation:

• Understand that non-metals are inclined to gain electrons due to higher electronegativity.
• Recognize that in basic conditions, the availability of hydroxide ions can promote electron gain, leading to anion formation.
• Amphoteric elements, like lead and aluminum, can turn into anions when in basic environments.

Acidic and Basic Solutions

Acidic and basic solutions are defined by their ion concentrations, primarily affecting the behavior of elements and compounds within them.
An acidic solution is characterized by a higher concentration of hydronium ( ext{H}_3 ext{O}^+) or hydrogen ions ( ext{H}^+).
These conditions can lead to metal elements forming cations. On the flip side, a basic solution possesses a greater concentration of hydroxide ( ext{OH}^-). Such environments support non-metal elements or compounds forming anions.
Key features of acidic and basic solutions include:

• **Acidic Solutions:** Typically have a pH less than 7, high in ext{H}^+ ions, allowing metals to lose electrons and form cations.
• **Basic Solutions:** Possess a pH greater than 7, rich in ext{OH}^- ions, enabling non-metals to gain electrons and form anions.
• Understanding the pH scale is vital, with 7 neutral (pure water), less than 7 acidic, and greater than 7 basic.

The dual behavior of amphoteric elements gives them a unique ability to interact in both acidic and basic solutions.