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: Green coloured residue after heating ammonium dichromate is chromium oxide with two chromium atoms and three oxygen atoms.

Complete step by step solution:
The compound mentioned in the question is ammonium dichromate. It is an inorganic compound. Above reaction is an exothermic reaction and was generally used in schools for demonstrating volcanic explosions. This produces a green residue which is chromium oxide. The reaction is as follows:

${(N{H_4})_2}C{r_2}{O_7}(s) \to C{r_2}{O_3}(s) + {N_2}(g) + {H_2}O(g)$

As we can see that other products of this reaction are gases (g) and escape. So option B is the correct one. Since chromium is a transition metal in periodic table, it has multiple oxidation states so it also has many oxides. But since it was mentioned that residue was green in colour we said oxide in option B.

Note: The reaction produces dark green residue. It is also interesting to note that product chromium oxide is used as a colouring agent for paints, inks and glasses for green colour.