What makes hot water better cleaning agent than cold water?« Back to Questions List

Surface tension is created on the surface of liquids because of the force of attraction in all directions between the molecules of the liquid inside the liquid. This force is called as cohesive force. The molecules at the surface (the surface molecules) of the liquid experience a cohesive force only from the molecules inside the liquid whereas no force acts from above. This imbalance in force causes the liquid to behave like a stretched elastic membrane. Due to the imbalance in the force on the surface molecules, the surface of liquid acts as if it were under tension.

Water has a higher surface tension than most liquids. Due to the surface tension, small objects will "float" on the surface of a fluid, as long as the object cannot break through and separate the top layer of water molecules. You might have seen small insects walk on water because they cannot break the surface tension and reach the bottom.

When we rise the temperature of water, the water molecules are set in motion. This means the cohesive force is getting weak compared to the force of cohesion in cold water. That is why hot water has less surface tension compared to cold water. This makes hot water better cleaning agent than cold water. The lower surface tension in hot water makes it a better wetting agent to get into the pores and fissures rather than bridging them with surface tension. Soaps and detergents further lower the surface tension easing the process of washing and cleaning.