Question

What happens when ${K_2}C{r_2}{O_7}$ is heated strongly?

Answer 1

Hint: Try to recall that ${K_2}C{r_2}{O_7}$ is an orange red crystalline solid and it decomposes on strong heating and a yellow coloured compound is obtained along with an oxide of chromium. Now, by using this you can easily answer the given question.

Complete step by step solution:
It is known to you that when potassium dichromate (${K_2}C{r_2}{O_7}$) is heated strongly it gets decomposed into a yellow coloured compound, an oxide of chromium which is green in colour and oxygen gas.
The yellow coloured compound is potassium chromate (${K_2}Cr{O_4}$) and the oxide of chromium is chromic oxide ($C{r_2}{O_3}$). The reaction is as follows:
$4{K_2}C{r_2}{O_7}\xrightarrow{\Delta }4{K_2}Cr{O_4} + 2C{r_2}{O_3} + {O_2}$

Additional information:
When potassium dichromate is heated with conc. Sulphuric acid and a soluble metal chloride, orange red vapors of chromyl chloride are evolved. When these vapors are passed through sodium hydroxide solution, yellow coloration is obtained. This reaction is used in the detection of $C{l^ - }$ ions in qualitative analysis and the test is known as chromyl chloride test.
On heating potassium dichromate with alkalies, the orange color of solution changes to yellow due to the formation of chromate ions.

Note: It should be remembered to you that sodium dichromate is more soluble than potassium dichromate. Therefore, potassium dichromate is prepared by mixing a hot concentrated solution of sodium dichromate and potassium chloride in equimolar proportions.
Also, you should remember that potassium dichromate is used for the preparation of a large number of chromium compounds such as chrome yellow, chrome red, chrome alum etc.