why pressure is a scalar quantity?
As answered previously...
Pressure has no direction, they are compared in terms of their magnitude only. Consider mercury column used for measuring atmospheric pressure. The weight of mercury column acts downward, still the pressure exerted by it is equivalent to that of atmospheric pressure. In case of liquid, if you exert pressure at any point, it is transmitted equally in all directions, and not just in the direction of pressure exerted. It forms the basis of Pascal's law and many hydraulic machines. Thus, you can conclude that pressure is scalar and has no direction.