Which alloy is used for making coins?

Which transition metal compound is used in a dry battery cell?

The bonds present in the structure of dichromate ion are:

Which among the following statements is incorrect for interstitial compounds?
$1$. They are very hard and rigid
$2$. They have higher melting point than pure metal
$3$. They do not show conductivity
$4$. They are chemically inert

$CrO _4^{2-} \underset{ pH = y }{\stackrel{ pH = x }{\longleftrightarrow}} Cr _2 O _7^{2-}$
$pH$ values of $x$ and $y$ are respectively

Which oxide of $Mn$ is acidic in nature?