Among the following series of transition metal ions,the one where all metal ions have $3d^2$ electronic configuration is

Three very large plates of the same area are kept parallel and close to each other. They are considered as ideal black surfaces and have very high thermal conductivity. The first and third plates are maintained at temperatures $2\,T$ and $3\,T$ respectively. The temperature of the middle (i.e.,second) plate under steady-state conditions is

If the curves $\frac{x^2}{a^2}+\frac{y^2}{b^2}=1$ and $\frac{x^2}{25}+\frac{y^2}{16}=1$ cut each other orthogonally,then $a^2-b^2$ equals to

During the first meiotic division,the exchange of genetic material between the chromatids of homologous chromosomes occurs. What is this process called?

The yellow colour solution of $Na_2CrO_4$ changes to orange-red on passing $CO_2$ gas due to the formation of

Which of the following ions does not exist in an aqueous solution?