When acidified $K_2Cr_2O_7$ reacts with $H_2O_2$,a blue-coloured solution is obtained. This blue colour is due to the formation of:

Paris green is

Generally,metal ions and their salts are coloured due to the presence of unpaired electrons in the metal ions. Which of the following compounds is coloured?

Among the following oxides of $3d$ elements, the number of mixed oxides are . . . . . .
$Ti_{2}O_{3}, V_{2}O_{4}, Cr_{2}O_{3}, Mn_{3}O_{4}, Fe_{3}O_{4}, Fe_{2}O_{3}, Co_{3}O_{4}$

The first transition series metal $M$ has the highest enthalpy of atomisation in its series. One of its aquated ion $(M^{n+})$ exists in green colour. The nature of the oxide formed by the above $M^{n+}$ ion is $:$

Which of the following is used for making railway tracks?