Question

A brown substance X on heating in air gives substance Y. When hydrogen gas is passed over heated Y, it again turns back into X. Name the substance X and Y.

Answer 1

We should be familiar with the term called a redox reaction. Redox reaction is the process in which reduction and oxidation is involved. Also, we should know how to balance the chemical equations. The knowledge about reactivity series can help us to solve this question easily.

Complete step-by-step answer: A brown substance mentioned in the question is copper$Cu$. It is usually reddish brown in color.
On heating $Cu$in air, it gets oxidized to form copper oxide$CuO$. It is a black color coating.
The reaction is: $2Cu + {O_2} \to 2CuO$
(X) (Y)
When hydrogen gas is passed over heated$CuO$, the product formed is a reddish brown substance$Cu$, along with water. Thus reduction occurs here.
The reaction is as follows: $2CuO + {H_2} \to Cu + {H_2}O$
(Y) (X)
So, answer to the question is:
X is copper.
Y is copper oxide.
Additional information:
We should know about reactivity series as well. It is a series of metals, arranged in the order of descending order of their reactivities, which helps in determining the products of single displacement reaction. The less reactive element will be replaced by the more reactive element according to the reactivity series .The reactivity chart is as follows (highly reactive at the top and least reactive at the bottom):

Potassium($K$)

Sodium($Na$)
Calcium$\left( {Ca} \right)$
magnesium$\left( {Mg} \right)$
Aluminium $\left( {Al} \right)$
Zinc$\left( {Zn} \right)$
Iron$\left( {Fe} \right)$
Hydrogen$\left( H \right)$
Copper$\left( {Cu} \right)$
Mercury $\left( {Hg} \right)$
silver$\left( {Ag} \right)$
Gold$\left( {Au} \right)$
platinum$\left( {Pt} \right)$

Note: Balancing the reactions is mandatory. Also, the concept of reduction and oxidation should be clear. Corrosion of copper is due to the oxidation that happens when $Cu$is exposed to the atmosphere.