How many ions liberate $H_2$ gas with dilute acids among the given species:
$Mn^{2+}, Cr^{3+}, Cr^{2+}, Zn^{2+}, Cu^{2+}, Sc^{3+}, Hg^{2+}$

Which of the following is not amphoteric?

Consider the following transformation:
$2CuX_2 \xrightarrow{\text{Room temperature}} 2CuX + X_2 \uparrow$
Then $X^-$ can be:

Which statement is incorrect?

Which of the following statements are correct?
$A$. The $M^{3+}/M^{2+}$ reduction potential for iron is greater than manganese.
$B$. The higher oxidation states of first row $d$-block elements get stabilized by oxide ion.
$C$. Aqueous solution of $Cr^{2+}$ can liberate hydrogen from dilute acid.
$D$. Magnetic moment of $V^{2+}$ is observed between $4.4-5.2 \ BM$.
Choose the correct answer from the options given below:

Which of the following characteristics of the transition metals is associated with their catalytic activity?