The outermost shell of an atom is called Valence shell. why?

The number of electrons to be filled in each shell depends on Aufbau's principle and Hund's rule. Valency tells us only the total number of electrons present in the outermost shell of the atom and so it is different from the atomic number. Therefore, the outermost shell of an atom is known as valence shell.

a boiling water reactor does not boil the water. a pressurized water reactor boils the water faster. a pressurized water reactor is heated directly by the core. a pressurized water reactor does not use fission.

a  solution  is always transparent, light passes through with no scattering from solute particles which are molecule in size. the  solution  is homogeneous and does not settle out. a colloid is intermediate between asolution  and a  suspension. while asuspension  will separate out a colloid will not.