CBSE Class 12 Chemistry Chapter 6: General Principles and Processes of Isolation of Elements - NCERT Solutions

Reviewed by NCERT Help Editorial Team · Updated on 29 Jul 2026

NCERT Solutions PDF Class 12 PDF

This section provides comprehensive NCERT Solutions for Class 12 Chemistry, Chapter 6, focusing on the General Principles and Processes of Isolation of Elements. It covers key concepts like hydrometallurgy, froth flotation, reduction processes, zone refining, and chromatography. The solutions explain why certain metals can be extracted using specific methods while others cannot, detailing the role of reducing potentials and depressants. It also elaborates on the thermodynamic principles governing the extraction of metals from oxide and sulfide ores, and the purification techniques for obtaining high-purity elements. These solutions are designed to help students grasp the fundamental principles of metallurgy and prepare effectively for their board examinations.

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Board	CBSE
Class	Class 12
Subject	Chemiry
Session	2026
Language	English
Type	NCERT Solutions
Chapter	Chapter 6: General Principles and Processes of Isolation of Elements - NCERT Exercises Solutions

Chapter summary

Chapter 6 of the NCERT Class 12 Chemistry syllabus delves into the fundamental principles and processes involved in isolating elements from their ores. This solution set covers key metallurgical techniques, including hydrometallurgy, froth flotation, and reduction methods. It explains the thermodynamic basis for metal extraction, the role of reagents like depressants, and purification methods such as zone refining and chromatography. The exercises focus on understanding the applicability and limitations of these processes for different metals.

Learning outcomes

Understand the principles of hydrometallurgy and its limitations for metals like zinc.
Explain the function of depressants in the froth flotation process.
Compare the ease of extracting copper from oxide versus sulfide ores.
Describe the process and principle of zone refining for purifying metals.
Explain the working and application of column chromatography in element purification.

Topics covered

Paper topics

Hydrometallurgy
Froth flotation
Depressants
Reduction of metal oxides
Reduction of metal sulfides
Gibbs free energy in metallurgy
Zone refining
Column chromatography
Metal extraction principles
Purification of elements

Important topics

Principles of Hydrometallurgy vs. Pyrometallurgy
Role of Depressants in Froth Flotation
Thermodynamic Feasibility of Metal Extraction (Ellingham Diagrams implied)
Zone Refining Technique
Column Chromatography for Purification

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Questions and Solutions

Question 6.1

Copper can be extracted by hydrometallurgy but not zinc. Explain.

Solution: Hydrometallurgy involves using aqueous solutions to extract metals. This method is feasible if a more reactive metal (with a lower reduction potential) can displace the metal ion from its solution. Copper can be extracted this way because metals like iron (Fe) have lower reduction potentials and can displace Cu²⁺ ions from aqueous solutions, as shown:

$Fe_{(s)} + Cu^{2+}_{(aq)} \longrightarrow Fe^{2+}_{(aq)} + Cu_{(s)}$

However, zinc (Zn) has a significantly lower reduction potential than copper. To displace Zn²⁺ ions, metals like magnesium (Mg) or calcium (Ca) are needed. These metals are highly reactive and react vigorously with water, producing hydrogen gas, rather than displacing zinc ions from the solution:

$2K_{(s)} + 2H_2O_{(l)} \longrightarrow 2KOH_{(aq)} + H_{2(g)}$

Because these highly reactive metals cannot be used in aqueous solutions for displacement, zinc cannot be effectively extracted by hydrometallurgy.

Question 6.2

What is the role of depressant in the froth flotation process?

Solution: In the froth flotation process, a depressant is a chemical substance added to selectively inhibit the flotation of one or more desired minerals while allowing others to float. This is crucial for separating mixtures of sulphide ores that have similar properties. For instance, when separating zinc sulphide (ZnS) and lead sulphide (PbS), sodium cyanide (NaCN) can be used as a depressant. NaCN reacts with ZnS to form a soluble complex, sodium zinc cyanide ([Na₂[Zn(CN)₄]]), which does not form froth. PbS, however, does not react with NaCN under these conditions and can be selectively floated and collected.

The reaction showing the complex formation is:

$4\text{ NaCN} + \text{ZnS} \longrightarrow \text{Na}_2[\text{Zn(CN)}_4] + \text{Na}_2\text{S}$

Question 6.3

Explain: Why is the extraction of copper from its pyrites (like Cu₂S) more difficult than from its oxide ore (like Cu₂O) through reduction?

Solution: The ease of extracting a metal by reduction depends on the thermodynamic stability of its compound, often assessed using Gibbs free energy of formation (Δ<0xE2><0x82><0x99>G). For copper extraction, we compare the reduction of copper sulfide (Cu₂S) versus copper oxide (Cu₂O).

When considering reduction by carbon (or CO), we look at the Gibbs free energy changes for the formation of the metal compound and the oxide of the reducing agent (e.g., CO). The extraction is feasible if the Gibbs free energy change for the formation of the metal compound is more negative than that for the formation of the reducing agent's oxide.

For copper sulfide (Cu₂S), the Gibbs free energy of formation is relatively low (less negative). This means that the formation of CO from C is thermodynamically more favorable than the decomposition of Cu₂S into Cu and S. Therefore, carbon or CO cannot effectively reduce Cu₂S to copper.

Conversely, for copper oxide (Cu₂O), the Gibbs free energy of formation is more negative than that for CO formation. This indicates that the reduction of Cu₂O by carbon is thermodynamically favorable:

$C_{(s)} + Cu_2O_{(s)} \longrightarrow 2Cu_{(s)} + CO_{(g)}$

Hence, copper can be more easily extracted from its oxide ore by reduction compared to its sulfide ore.

Question 6.4 (i)

Explain: Zone refining.

Solution: Zone refining is a highly effective purification technique used for obtaining ultra-pure metals, particularly semiconductors like silicon and germanium. It is based on the principle that impurities are generally more soluble in the molten state of a metal than in its solid state.

The process involves heating a metal rod of impure metal with a moving circular heater. This creates a narrow molten zone that travels along the length of the rod. As the molten zone moves, the metal in contact with it melts, and as it cools behind the heater, pure metal crystallizes out. The impurities, being more soluble in the molten phase, tend to remain in the molten zone and are thus swept along with it towards one end of the rod.

This process is repeated multiple times. With each pass of the molten zone, the concentration of impurities at the moving end increases. Finally, the end of the rod containing the concentrated impurities is cut off and discarded, leaving behind a highly purified metal rod.

Question 6.4 (ii)

Explain: Column chromatography.

Solution: Column chromatography is a powerful separation and purification technique used to separate components of a mixture based on their differential adsorption onto a stationary phase. It is particularly useful for purifying elements or compounds that are present in small quantities or when the impurities have chemical properties very similar to the desired substance.

The technique involves packing a column with a suitable stationary phase, typically an adsorbent like alumina (Al₂O₃) or silica gel. The mixture to be separated is then introduced at the top of the column. A mobile phase (a solvent or a mixture of solvents) is allowed to flow through the column, carrying the mixture components downwards. Different components travel at different rates depending on their relative affinity for the stationary phase and the mobile phase. Components that are more strongly adsorbed by the stationary phase move slower, while those that are more soluble in the mobile phase move faster.

As the mobile phase flows continuously, the components separate into distinct bands within the column. These separated bands can then be collected individually as they elute from the bottom of the column, yielding purified substances.

Common mistakes

Confusing the relative reduction potentials of metals and their implications in displacement reactions.
Misunderstanding the selective action of depressants in separating different sulphide ores.
Not considering the Gibbs free energy changes when comparing the feasibility of reducing metal oxides versus sulfides.
Inaccurate description of the zone refining process or its underlying principle.

Revision tips

Focus on understanding the 'why' behind each extraction method's applicability or limitations.
Draw diagrams to visualize processes like zone refining and froth flotation.
Relate the concepts of reduction potential and Gibbs free energy to specific metal extraction examples.
Practice explaining the role of specific reagents (like NaCN) in separation techniques.

Practice MCQs

Q1. Why can copper be extracted by hydrometallurgy, but zinc cannot?

Q2. What is the primary function of a depressant in froth flotation?

Q3. Why is it more difficult to extract copper from Cu₂S than from Cu₂O by reduction?

Q4. Zone refining is based on which principle?

Q5. In column chromatography, what is the role of the stationary phase?

Frequently asked questions

What is hydrometallurgy and why is it not suitable for zinc extraction?

Hydrometallurgy involves extracting metals using aqueous solutions. Copper can be extracted because its ions can be displaced by metals like iron. Zinc extraction is difficult because it requires highly reactive metals (like Mg, Ca) which react with water, making the process impractical.

How does a depressant work in the froth flotation process?

A depressant selectively reacts with one of the sulphide minerals in a mixture, preventing it from forming froth. This allows the other mineral to be separated by flotation. For example, NaCN can depress ZnS while allowing PbS to float.

Why is copper extraction from its oxide ore easier than from its sulfide ore?

The extraction from oxide ore (Cu₂O) is easier because carbon can effectively reduce it, as the Gibbs free energy change for Cu₂O formation is more negative than that for CO. However, carbon cannot effectively reduce copper sulfide (Cu₂S) because the Gibbs free energy change for Cu₂S formation is less negative than for CO.

Explain the principle behind zone refining.

Zone refining relies on the difference in solubility of impurities between the solid and molten states of a metal. Impurities are more soluble in the molten state. By moving a molten zone along an impure metal rod, impurities are concentrated in the molten zone and moved to one end.

What is column chromatography used for in metallurgy?

Column chromatography is a purification technique used to separate components of a mixture based on their differential adsorption onto a stationary phase (like Al₂O₃). It's particularly useful for purifying elements present in small quantities or when impurities have similar chemical properties.

Which metals can be purified using zone refining?

Zone refining is typically used for purifying metals like silicon, boron, gallium, and indium, which are often required in very high purity for semiconductor applications.

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