______ which dissolves in ether producing a blue colouration.
A.\[{\text{Cr}}{{\text{O}}_3}\]
B.\[{\text{Cr}}{{\text{O}}_5}\]
C.\[{\text{C}}{{\text{r}}_2}{{\text{O}}_3}\]

Hint: Blue coloration is formed by that chromium compound with ether which has an oxidation state of 6. It has a butterfly-like structure.

Complete step by step answer:
The reaction between potassium dichromate and hydrogen peroxide produces chromium pentoxide, potassium sulphate and water the reaction can be written as:
\[{{\text{K}}_2}{\text{C}}{{\text{r}}_2}{{\text{O}}_7} + {{\text{H}}_2}{\text{S}}{{\text{O}}_4} + 4{{\text{H}}_2}{{\text{O}}_2} \to 2{\text{Cr}}{{\text{O}}_5} + {{\text{K}}_2}{\text{S}}{{\text{O}}_4} + 5{{\text{H}}_2}{\text{O}}\]
The chromium pentoxide form is relatively unstable and decomposes to chromium ion. Hence in order to store and keep the blue colour of chromium pentoxide it is dissolved in ether.

So, the correct option is chromium pentoxide that is option B.

Additional information:
Chromium is a d block element or transition metal element. Chromium belongs to group number 6 and period number 4 of modern periodic table. It belongs to 3 d series; it is grayish in colour and very less lustrous and hard. It is added in steel especially in stainless steel to increase the anti corrosive properties of the Steel.
Chromium oxides have toxic nature and that is why they are used in the preservation of wood. Chromated copper arsenate known as CCA is used to protect wood from the fungi from insects such as termites. Potassium dichromate is an inorganic compound. It is an oxidizing agent and is red- orange in colour. It is used in cleaning glassware and in construction.

Note:
\[{\text{Cr}}{{\text{O}}_5}\] has an oxidation state of 6, there are 4 oxygen atoms that are present with peroxide linkage. When oxygen is present as peroxide its oxidation state is \[ - 1\]. One oxygen is bonded with a double bond and hence has an oxidation state of \[ - 2\]. So the overall oxidation state becomes \[ + 6\] for chromium.