Question

When \[{(N{H_4})_2}C{r_2}{O_7}\] is heated, green coloured residue is:
(A) \[CrO\]
(B) \[C{r_2}{O_3}\]
(C) \[C{r_2}{O_5}\]
(D) \[C{r_2}{O_7}\]

Answer 1

Hint: In order to find what is the green-coloured residue obtained when heating \[{(N{H_4})_2}C{r_2}{O_7}\], we must know what is the reaction that is taking place. \[{(N{H_4})_2}C{r_2}{O_7}\] is an inorganic compound called ammonium dichromate.

Complete answer:
Let us first understand about \[{(N{H_4})_2}C{r_2}{O_7}\]. \[{(N{H_4})_2}C{r_2}{O_7}\] is an inorganic compound called ammonium dichromate. This is an orange red crystal which is obtained by a crystallisation process. Ammonium dichromate is very sensitive to light and also it is very toxic salt. The salt is odourless and it is soluble in alcohol and insoluble in acetone. Being a highly flammable compound, it should be stored under certain specific conditions.
When ammonium dichromate is heated, it will decompose into nitrogen gas, chromium oxide and water molecules. The reaction of decomposition of ammonium dichromate is given below:
\[{(N{H_4})_2}C{r_2}{O_7}(s) \to C{r_2}{O_3}(s) + {N_2}(g) + 4{H_2}O(g)\]
Hence the green coloured residue which is formed by heating of ammonium dichromate is chromium oxide, \[C{r_2}{O_3}\].

Therefore, the correct answer is option (B) .

Note:
We have to remember that the ammonium dichromate has various uses such as:
- Ammonium dichromate is used in magnetic recording material.
- It is used in the lithography to sensitize solutions.
- It is used in photoengraving.
- It is used in pyrotechnics.
- Ammonium is used as pesticide.
- It is used as a mordant for dyeing and also used as pigment.
- It is used for manufacture of alizarin and chrome alum.
- It is used in the purification of oil and leather tanning.