Most of the value of hand-washing is the physical rubbing and rinsing action that dislodges bacteria from the skin’s surface.
A study at Rutgers University in the US found that hot water was no better than cold at removing bacteria. In fact, water that is uncomfortably hot actually increases the bacterial load, because it damages the natural protective barrier of your skin.
Warm water does a better job at washing than cold water for the same reason salt and sugar dissolve more easily in warm water than cold water.