Which of the following compounds is amphoteric?

In making gun-barrel,the steel used is

What is obtained when gold chloride is heated?

On heating a mixture of common salt and $K_{2}Cr_{2}O_{7}$ with concentrated $H_{2}SO_{4}$, a gas is evolved. The formula of the gas evolved and the oxidation state of the central metal atom in the gas, respectively, are:

On heating $KMnO_4$,one among the following is not formed:

Which of the following ores is used for the production of $K_2Cr_2O_7$?