Viscosity and surface tension are two physical characteristics of a liquid. Viscosity is the measure of how resistant to flow a liquid is, while surface tension is defined as how resistant the surface of a liquid is to penetration. Both viscosity and surface tension are affected by changes in temperature.
What is Viscosity?
Viscosity is determined by the time it takes a given amount of liquid to flow through an instrument called a viscometer tube; essentially a narrow pipe. A good example of viscosity is liquid flowing through a straw: water, which has a low viscosity, will flow more freely than honey, which has a high viscosity. Liquids like honey have a higher viscosity because they contain more complex molecular structures; while water consists of simple hydrogen and oxygen bonds, honey also contains sugars.
Viscosity and Temperature
When a liquid heats up, its molecules become excited and begin to move. The energy of this movement is enough to overcome the forces that bind the molecules together, allowing the liquid to become more fluid and decreasing its viscosity. For example, when syrup is cold it has a high viscosity and can be difficult to pour. When heated in a microwave, the viscosity decreases and the syrup flows more freely.
What is Surface Tension?
Surface tension is what makes it possible to float a needle in a cup of water, or for water-skimming insects to glide across the surface of a lake. The molecules on the surface of a liquid are bound to the molecules beside and below them, but have nothing above them to balance out these attractive forces. Because of this imbalance, the molecules on the surface of the liquid will be drawn more strongly to those around it, creating a sheet of tightly bound molecules on the surface of the liquid.
Surface Tension and Temperature
As the temperature of a liquid increases, its surface tension decreases. When water heats up, the movement of its molecules disrupts the imbalanced forces on the surface of the water and weakens its sheet-like barrier of tightly bound molecules, thereby lowering the surface tension. This is why hot water is more effective when cleaning; its low surface tension allows it to more easily penetrate the fibers of a material like fabric and wash away stains.