Name the gaseous products from the following $A$ and $B$ reactions,respectively.
$A.$ Hydrochloric acid is added to sodium sulphide.
$B.$ Concentrated sulphuric acid is added to a mixture of sodium chloride and manganese dioxide.

Which of the following elements has a half-life in milliseconds?

The gas evolved on heating $CaF_2$ and $SiO_2$ with concentrated $H_2SO_4$,on hydrolysis gives a white gelatinous precipitate. The precipitate is

The correct order of acidic strength of oxy-acids of chlorine is

Consider the following reactions:
$(A) (\text{Ionic halide}) + (B) (\text{Good oxidising agent}) + \text{conc. } H_2SO_4 \to (C) (\text{Red-Orange gas is evolved with few by-products})$
$(D) (\text{Other ionic halide}) + (B) + \text{conc. } H_2SO_4 \to (E) (\text{Has same colour as } (C))$
Which of the following properties are different for $(C)$ and $(E)$?

Explain why fluorine forms only one oxoacid,$HOF$.