Acidified $K_2Cr_2O_7$ solution turns green when $Na_2SO_3$ is added to it. This is due to the formation of

Assign $A$,$B$,$C$,$D$ from the given type of reaction.
$CaSO_3(s) + H_2SO_4(aq) \longrightarrow CaSO_4(s) + SO_2(g) + H_2O(l)$

$A$ solution of colourless salt $H$ on boiling with excess $NaOH$ produces a non-flammable gas. The gas evolution ceases after sometime. Upon addition of $Zn$ dust to the same solution,the gas evolution restarts. The colourless salt$(s)$ $H$ is (are):

Assign $A$,$B$,$C$,$D$ from the given type of reaction.
$BaCO_3 \downarrow + 2CH_3COOH \longrightarrow Ba(CH_3COO)_2 + CO_2 \uparrow + H_2O$

Assign $A$,$B$,$C$,$D$ from the given type of reaction.
$2BaCrO_4 \downarrow + 4HCl \longrightarrow 2BaCl_2 + H_2Cr_2O_7 + H_2O$

$Fe^{3+}$ is reduced to $Fe^{2+}$ by using:
$A$. $H_2O_2$ in presence of $NaOH$
$B$. $Na_2O_2$ in water
$C$. $H_2O_2$ in presence of $H_2SO_4$
$D$. $Na_2O_2$ in presence of $H_2SO_4$