Plaster of Paris,when mixed with the correct amount of water,sets into a solid mass due to the formation of:

The equation of the sphere passing through the points $(1,0,0)$, $(0,1,0)$, and $(1,1,1)$ and having the smallest radius is:

$A$ coordination complex contains $Co^{3+}$,$Cl^{-}$,and $NH_3$. When dissolved in water,one mole of this complex gives a total of $3$ moles of ions. The complex is

Half-life of a radioactive substance $A$ is two times the half-life of another radioactive substance $B$. Initially,the number of nuclei of $A$ and $B$ are $N_A$ and $N_B$ respectively. After three half-lives of $A$,the number of nuclei of both are equal. Then the ratio $\frac{N_A}{N_B}$ is

Light of wavelength $2475 \, Å$ is incident on barium. Photoelectrons emitted describe a circle of radius $100 \, cm$ in a magnetic field of flux density $\frac{1}{\sqrt{17}} \times 10^{-5} \, T$. The work function of the barium is ............. $eV$ (given $\frac{e}{m} = 1.7 \times 10^{11} \, C/kg$).

When did $R.H. Whittaker$ propose the five-kingdom classification system?