Which substance is used to produce very low temperatures by magnetic effects?

Excess of $KI$ reacts with $CuSO_4$ solution and then $Na_2S_2O_3$ solution is added to it. Which of the statements is incorrect for this reaction?

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

Stainless steel is non-corrosive. This character is more prominent in

In the reaction $4K_2Cr_2O_7 \xrightarrow{\text{heat}} 4K_2CrO_4 + 3O_2 + X$,the species $X$ is:

Which of the following compounds of $Ti$ $(Z = 22)$ does not exist?