Identify the cyclic silicate ion given in the figure below:

What is produced by the reaction of $Ca(OH)_2$ with $SO_2$?

Assertion $(A)$: The maximum covalency of $Si$ and $Ge$ is $6$ but that of carbon is $4$.
Reason $(R)$: Due to the presence of $d$-orbitals in $Si$ and $Ge$,both these elements can undergo $sp^3d^2$ hybridisation and covalency of $6$ is possible. Carbon has no $d$-orbitals in it and hence its covalency is $4$.
The correct answer is:

Assertion : Stannous chloride gives grey precipitate with mercuric chloride,but stannic chloride does not do so.
Reason : Stannous chloride is a powerful oxidising agent which oxidises mercuric chloride to metallic mercury.

The $[SiO_4]^{4-}$ unit has a tetrahedral structure. Which silicate is formed by sharing three oxygen atoms per $[SiO_4]^{4-}$ unit?

Deep sea divers use a mixture of ....... for respiration.