How does temperature affect viscosity of a liquid?

1. Nov 11, 2015

avito009

I know that

When there is increase in temperature the molecules in a viscious liquid move further apart and this decreases the viscosity of the fluid.

So does that mean the molecules are far apart in case of liquid and these molecules move further apart with increase in temperature. So a viscious liquid is like a solid because its molecules are closer and lesser viscious liquid has molecules far apart?

2. Nov 11, 2015

joknhial

The best application for your question that i can conjure is in hydrocarbons. hydrocarbon viscosity is determined by the length of a carbon molecule, the longer the chain, the more viscous, the shorter the chain, the less viscous. in the case of temperature being a factor, a molecule's kinetic energy will be raised, its atoms will move at a much higher rate, since viscosity is the relationship between one molecule's ability to interact with other molecules by encouraging homogeneous movement, atoms at a higher kinetic level would be more conducive to a less viscous substance as surplus energy is not required to facilitate movement in the corresponding molecules. ultimately what this means is that temperature affects viscosity by increasing the kinetic energy of each atom/molecule. a useful analogy is to imagine you are in a crowded room with a lot of people ( a party). but there is no music. when you move, you might nudge a few people and they move abit (high viscosity). however if the music was on (Music is temperature source) people will start to jump around (kinetic energy), and so do you. when you move, you will find your energy will jolt people more aggressively then if you were stationary, meaning viscosity has decreased.

3. Nov 11, 2015

Most liquids get less viscous with increasing temperature. The trend is reversed in gases.

4. Nov 12, 2015

Andy Resnick

This is not always true: we use solutions of methyl cellulose, and these are less viscous when cold.

5. Nov 12, 2015

256bits

Newtonian fluids are simplest to descibe, where viscosity, ie the shear strsss is proportional to the velocity of adjacent layers. Other non-Newtonian fluids have more complex relationships.

You don't relalize it, but you have mentioned 2 variables that could have an influence upon viscosity.
1. temperature
2. pressure

Pressure -
There would be an affect upon molecular spacing, ot rather how far apart the molecules are from one another. This is usually considered to not greatly affect the viscosity of a particular fluid, so molecular spacing cannot be the answer.

Temperature
Molecules move around a lot in liquids and gases, but even so, some interaction will have to occur. This action can be a change in kinetic energy, or velocity of the molecules; or a brief molecular bond between the molecules.

Liquids
Lower temperatures mean lower molecular jostling, meaning more bonds and the bonds can act longer. Higher temperatures results in the converse.

Gases
Gases have less intermolecular bonds forming between them, so the velocity of the molecules must be important.

Viscosity
For a liquid, as a result of temperature, the formation of more or less bonds between molecules in the adjacent layers, affects the viscosity.
Higher temp -> less bonding-> less viscosity

For gases, molecules themselves move between layers. With higher temperatures the molecules will have more interactions, affecting the viscosity.
Higher temp --> more kinetic interactions -> increased viscosity