$SiCl_4$ on hydrolysis forms '$X$' and $HCl$. Compound '$X$' loses water at $1000^\circ C$ and gives '$Y$'. Compounds '$X$' and '$Y$' respectively are:

If the circle $x^2+y^2+8x-4y+c=0$ touches the circle $x^2+y^2+2x+4y-11=0$ externally and cuts the circle $x^2+y^2-6x+8y+k=0$ orthogonally,then $k=$

$X$-rays of wavelength $0.140 \ nm$ are scattered from a block of carbon. What will be the wavelength of $X$-rays scattered at $90^{\circ}$ (in $nm$)?

The oxide of nitrogen formed by thermal decomposition of $NH_4NO_3$ is

The formation of the oxide ion $O^{2-}_{(g)}$ requires first an exothermic and then an endothermic step as shown below:
$O_{(g)} + e^- \to O^-_{(g)}; \Delta H = -142 \ kJ/mol$
$O^-_{(g)} + e^- \to O^{2-}_{(g)}; \Delta H = 844 \ kJ/mol$
This is because:

Which of the following fluorides does not exist?