Viscosity is a scientific term describing the internal friction of a fluid or gas. Both have adjacent layers, and when pressure is applied, the friction between layers affects how much the substance will respond to external force. Viscosity, in its simplest form, can be evaluated by the thickness of a substance. A general rule is that gases are less viscous than liquids, and thicker liquids exhibit higher viscosity than thin liquids.

Viscosity may also be described as resistance of a liquid to penetration. Some refer to viscosity as the density of a liquid or gas. The term fluidity is opposite to viscosity, as it measures lack of resistance instead of amount of resistance.

Within each substance, the molecules reduce flow. They collide with each other, and also exhibit a degree of attraction. Molecular analysis can help determine specific viscosity measurements, thus helping to determine which substances will be of most use in a given application.

Temperature also effects viscosity. Raising the temperature of a fluid tends to make it less viscous. If one takes a thick liquid like molasses, and heats it up to boiling, the result will be a thin, easy to pour liquid. Cooking oils that are refrigerated, in most cases become more viscous, or almost solid due to colder temperatures, rendering them useless.

As crude oil is piped through climates of varying temperatures, the rate of flow in response to pressure changes. When oil is derived from Alaska, it is more viscous, than oil derived from the Persian Gulf, since the ground temperatures vary significantly. To address the issue of force needed to deliver oil through piping, sensors in some pipes measure the viscosity of the fluid and determine if greater or lesser pressure must be added to keep the flow of oil constant and steady.

Naturally, motor oil is also subject to changing viscosity when heated by an engine. Oil that becomes too thin from the engine’s heat will not work properly in the car engine. To address this, scientists developed additives, called polymers, which keep viscosity rates constant under higher temperatures.

Geologists use measurements of viscosity to evaluate magma under active or possibly soon to be active volcanoes. When magma exhibits a low level of viscosity, the volcano is more likely to erupt, because little pressure is needed to push the magma to the surface. Magma with greater viscosity causes volcanic eruptions less frequently. However if an eruption occurs with high viscosity magma, it results in huge explosions, since greater force is required to push magma outside of the volcano.