Which of the following carbonates undergoes decomposition upon heating?

The terminal velocity of a liquid drop of radius $r$ falling through air is $v$. If two such drops are combined to form a bigger drop, the terminal velocity with which the bigger drop falls through air is (ignore any buoyant force due to air).

Thermite is a mixture of $X$ parts of ferric oxide and $Y$ parts of aluminium powder. What are $X$ and $Y$ respectively?

At $27^{\circ} C$,a closed vessel contains a mixture of equal weights of helium (mol. wt. = $4$),methane (mol. wt. = $16$) and sulphur dioxide (mol. wt. = $64$). The pressure exerted by the mixture is $210 \ mm$. If the partial pressures of helium,methane and sulphur dioxide are $p_1, p_2$ and $p_3$ respectively,which one of the following is correct?

The orange coloured compound formed when $H_2O_2$ is added to $TiO_2$ solution acidified with conc $H_2SO_4$ is

Two equally charged metal spheres $A$ and $B$ repel each other with a force of $4 \times 10^{-5} ~N$. Another identical uncharged sphere $C$ is touched to $A$ and then placed at the midpoint of the line joining the spheres $A$ and $B$. The net electric force on the sphere $C$ is