Two identical barrels with identical valves, first barrel is filled with hot water & second one with cold water...

Question

Two identical barrels with identical valves at the bottom are placed on a table so that the water can freely flow to the ground. The first barrel is filled with hot water, the second one is filled with cold water. If we open the valves at the same time, which one of the barrels will be empty first?

Answer 1

Viscosity of water decreases markedly with increasing temperature. When the viscosity decreases, the flow resistance decreases. So for the same driving force, that is the pressure drop per unit length, the water flow rate will be higher. So the barrel with hot water will be empty first.