With the help of suitable chemical equations, list the two main differences between roasting and calcination. How is metal reduced from the product obtained after roasting/calcination of the ore? Write the chemical equation for the reaction involved.

Chemical Reactions and Equations

Roasting and Calcination

Official Answer

The two main differences between roasting and calcination are:

Roasting:

It is the process of heating sulphide ores strongly in the presence of excess air to convert them into metal oxides.
Chemical Equation:

$2ZnS(s) + 3O_2(g) \xrightarrow{\text{Heat}} 2ZnO(s) + 2SO_2(g)$

Calcination:

It is the process of heating carbonate ores strongly in limited air or in the absence of air to convert them into metal oxides.
Chemical Equation:

$ZnCO_3(s) \xrightarrow{\text{Heat}} ZnO(s) + CO_2(g)$

Reduction of Metal Oxide:

The metal oxide obtained after roasting or calcination is reduced to the free metal by heating it with a suitable reducing agent such as carbon (coke).

Chemical Equation:

$ZnO(s) + C(s) \rightarrow Zn(s) + CO(g)$

RoastingCalcinationSulphide oreCarbonate oreExcess airLimited airReductionCarbonReducing agent

Marking Scheme

11 mark for stating the differences between roasting and calcination with correct definitions.
21 mark for writing the balanced chemical equations for both roasting and calcination.
31 mark for explaining the reduction process of the metal oxide using carbon along with the balanced chemical equation.

Hint

Think about the type of ore (sulphide vs carbonate) and the presence or absence of air during heating. For reduction, use carbon as a reducing agent.

Quick Oral Answer

Roasting is heating sulphide ores in excess air, while calcination is heating carbonate ores in limited air. The resulting metal oxides are reduced to metals using carbon.

Analysis & Explanation

Roasting is specifically used for sulphide ores, which require oxygen to oxidize the sulphur into gaseous sulphur dioxide ( $SO_2$ ). Calcination is used for carbonate or hydrated ores, which decompose thermally to release carbon dioxide ( $CO_2$ ) or water vapour without requiring external oxygen. Once the metal oxide is obtained, carbon acts as a reducing agent, removing oxygen from the metal oxide to yield the pure metal.

Common Mistakes

1Confusing roasting with calcination (e.g., writing that calcination is done in excess air).
2Forgetting to write the physical states or balancing the chemical equations.
3Using carbon dioxide instead of carbon as the reducing agent.

Interesting Facts

Sulphur dioxide gas produced during roasting is often captured and used to manufacture sulphuric acid, preventing it from polluting the atmosphere.

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Frequently Asked Questions

How many marks does this question carry in CBSE Class 10 Science 2023?

This question carries 3 marks in the CBSE Class 10 Science 2023 examination.

Which chapter does this question come from in Science?

This question is from the chapter "Chemical Reactions and Equations" in the CBSE Class 10 Science syllabus.

What topic does this question cover in Science?

This question covers the topic "Roasting and Calcination" from CBSE Class 10 Science.

What type of question is this in the CBSE Class 10 Science 2023 paper?

This is a Short Answer question from Section C in the CBSE Class 10 Science 2023 paper.