Question

why liquid in a beaker needs to be stirred constantly

Answer 1

If we don't stire the liquid then the solute particles settel down at bottom and the solution does not form completely.

Answer 2

think the answer is, it depends. There are two opposing effects here. Suppose you have a liquid-filled pot over a burner. If a layer of hot liquid forms against the bottom of the pot, that will reduce the rate of heat transfer from the burner. Stirring might keep cooler liquid in contact with the bottom of the pot, and therefore increase the efficiency of the heat transfer. On the other hand, in an unstirred pot, a cooler layer of liquid may form on the top of the pot, decreasing the rate of heat transfer to the air. Stirring the pot will bring hotter liquid to the surface, resulting if more heat transfer to the air. Which of these two effects dominates depends on lots of things, like the precise dimensions of the pot, the temperature of the burner, and the temperature of the air. However, in general, the expansion of the liquid due to heating will create convection currents that will naturally stir the mixture, bringing hot liquid to the top and cool liquid down to the bottom. So unless the "liquid" is fairly viscous, stirring probably won't make much difference one way or the other.