$1\text{ g}$ of copper powder was taken in a China dish and heated. What change takes place on heating? When hydrogen gas is passed over this heated substance, a visible change is seen in it. Give the chemical equations of reactions, the name and the color of the products formed in each case.

Chemical Reactions and Equations

Oxidation and Reduction Reactions

Official Answer

When $1\text{ g}$ of copper powder is heated in a China dish, it reacts with oxygen present in the air to form a black-colored substance, copper(II) oxide. This is an oxidation reaction.

1. Reaction on Heating Copper Powder:

Chemical Equation:

$2\text{Cu(s)} + \text{O}_2\text{(g)} \xrightarrow{\text{Heat}} 2\text{CuO(s)}$

Product Name: Copper(II) oxide (or Cupric oxide)
Color of Product: Black

When hydrogen gas is passed over this heated black copper(II) oxide, the black coating turns reddish-brown because copper metal is recovered. This is a reduction reaction.

2. Reaction on Passing Hydrogen Gas:

Chemical Equation:

$\text{CuO(s)} + \text{H}_2\text{(g)} \xrightarrow{\text{Heat}} \text{Cu(s)} + \text{H}_2\text{O(g)}$

Product Name: Copper and Water (as steam)
Color of Product (Copper): Reddish-brown

copper(II) oxideblackreddish-brownoxidationreduction2Cu + O2 -> 2CuOCuO + H2 -> Cu + H2O

Marking Scheme

1Observation and balanced chemical equation for the heating of copper powder (including product name and color): 1.5 Marks
2Observation and balanced chemical equation for passing hydrogen gas over the heated substance (including product name and color): 1.5 Marks

Hint

Recall that heating copper in air causes it to combine with oxygen to form a black oxide. Passing hydrogen gas over this hot black oxide reverses the process, reducing it back to brown copper metal.

Quick Oral Answer

When copper is heated in air, it gains oxygen to form black copper(II) oxide. When hydrogen gas is passed over this hot black oxide, it loses oxygen to form reddish-brown copper metal and water.

Analysis & Explanation

This two-step process demonstrates a classic redox (reduction-oxidation) reaction. In the first step, copper is oxidized to copper(II) oxide by gaining oxygen. In the second step, when hydrogen gas is passed over the hot copper(II) oxide, hydrogen acts as a reducing agent. It removes oxygen from copper(II) oxide, reducing it back to elemental copper, while hydrogen itself gets oxidized to water. The visual transition from reddish-brown copper to black copper oxide, and back to reddish-brown copper, serves as a clear indicator of these chemical changes.

Common Mistakes

1Writing the formula of copper(II) oxide incorrectly as $\text{Cu}_2\text{O}$ instead of $\text{CuO}$ .
2Forgetting to mention the color changes (reddish-brown to black, and black back to reddish-brown).
3Not balancing the chemical equations or omitting the state symbols and reaction conditions like 'Heat'.

Interesting Facts

Copper does not burn with a flame when heated; instead, it slowly reacts with oxygen to form a thin, protective black layer of copper(II) oxide on its surface.

This reaction is widely used in metallurgy to purify copper and study gas-solid reduction processes.

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Frequently Asked Questions

How many marks does this question carry in CBSE Class 10 Science 2020?

This question carries 3 marks in the CBSE Class 10 Science 2020 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 "Oxidation and Reduction Reactions" from CBSE Class 10 Science.

What type of question is this in the CBSE Class 10 Science 2020 paper?

This is a Short Answer question from Section B in the CBSE Class 10 Science 2020 paper.