What Is a Material That Can Easily Flow?

Substances such as water, acetone, motor oil and some acids have low viscosities and flow easily. Substances that resist flowing are the opposite and have high viscosity.

Viscosity varies with temperature, which means that most charts that compare densities have a temperature index, allowing an individual to find the viscosity of a liquid at a given temperature. Water has a very low viscosity, as do solutions primarily composed of it. However, sugar, cornstarch and many other chemicals tend to increase the density of water. Superfluid helium that has been cooled to within a few degrees of absolute zero has the lowest viscosity ever measured. Essentially, this bizarre liquid does not resist flowing at all, giving it a viscosity of zero.

Often, but not always, liquids with low viscosity have low specific gravity as well. However, this relationship is often coincidental, and the two terms should not be confused. Specific gravity is a measure of the density of a fluid, not its resistance to flow. Specific gravity is expressed as a comparison of a substance's density with that of water, and it uses no units. For example, water has a specific gravity of 1, while iron has a specific gravity of 7.85, meaning that iron is 7.85 times as dense as water, regardless of the units used.