Effect of Temperature and Pressure of Density and Viscosity



TEMP. - As temp increases as density decreases (Inversely)

For example at low temp. the bitumen is thick, more dense. As temp. increases, the density of bitumen is lower.

PRESSURE - As pressure increases as density increases (Directly)

At the core of earth when the pressure is the high, the core liquids is thick, more dense.


TEMP. - As temp increases the density decreases (Indirectly)

The gas at the surface of heath gets hotter and its moves up, less dense. Cold gases come down on the earth.

PRESSURE - As pressure increases the density increases (Directly)

For example at low pressures the gas are moving at lower speeds, low inter-molecular forces. As temp. increases, the gas molecules more around faster, more dense.



While liquids get runnier as they get hotter, gases get thicker. (If one can imagine a “thick” gas.)


TEMP. - As temp increases the viscosity decreases (Inversely)

Honey and syrups can be made to flow more readily when heated. Engine oil and hydraulic fluids thicken appreciably on cold days and significantly affect the performance of cars and other machinery during the winter months. the viscosity of a simple liquid decreases with increasing temperature (and vice versa).

As temperature increases, the average speed of the molecules in a liquid increases and the amount of time they spend “in contact” with their nearest neighbors decreases. Thus, as temperature increases, the average intermolecular forces decrease.

PRESSURE - At very high pressures, as pressure increases the viscosity increases (Directly)

Viscosity is normally independent of pressure, but liquids under extreme pressure often experience an increase in viscosity. Since liquids are normally incompressible, an increase in pressure doesn’t really bring the molecules significantly closer together.


TEMP. - As temp increases the viscosity increases (Directly)

The viscosity of gases increases as temperature increases and is approximately proportional to the square root of temperature. This is due to the increase in the frequency of intermolecular collisions at higher temperatures.

Since most of the time the molecules in a gas are flying freely through the void, anything that increases the number of times one molecule is in contact with another will decrease the ability of the molecules as a whole to engage in the coordinated movement.

The more these molecules collide with one another, the more disorganized their motion becomes.

PRESSURE - Independent

The viscosity of an ideal gas is independent of pressure, and this is almost true for real gases. In gases, Viscosity arises mainly because of the transfer and exchange of molecular momentum. How come pressure doesn’t affect the viscosity then?

Double the pressure, and you double the number of molecules arriving at a surface, but on average they will only have come from half as far away, and the effects cancel out

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