Data Entry - Physics

Which of the following can be added to remove the permanent hardness of water?

An electron collides with an atom in its ground state at rest. The atom then emits a photon of energy $E_{photon}$. In this process,the change $\Delta E_{elec}$ in the electron's energy is

The point at which the circles $x^2+y^2-4x-4y+7=0$ and $x^2+y^2-12x-10y+45=0$ touch each other is:

Which one of the following statements is correct with respect to basic character?

The $pH$ value of which one of the following is not equal to $1$?