$HBr$ and $HI$ can reduce sulfuric acid; $HCl$ can reduce $KMnO_4$. Which of the following can $HF$ reduce?

$NaCl$ (solid) $+ K_2Cr_2O_7$ (solid) $+$ conc. $H_2SO_4 \xrightarrow{\text{warm}} X$. Reddish brown fumes of $X$. The oxidation state of central atom in compound $X$ is

All the compounds listed in Column $I$ react with water. Match the result of the respective reactions with the appropriate options listed in Column $II$.

Column $I$	Column $II$
$(A). (CH_3)_2 SiCl_2$	$(p).$ Hydrogen halide formation
$(B). XeF_4$	$(q).$ Redox reaction
$(C). Cl_2$	$(r).$ Reacts with glass
$(D). VCl_5$	$(s).$ Polymerization
	$(t). O_2$ formation

When iodine reacts with $NaF,$ $NaBr$ and $NaCl,$ what happens?

$SF_{6}$ is known but $SCl_{6}$ is not. Why?

The acid used for making permanent markings on the glass surface is: