Liquids, unlike solids, have the ability to flow, that is, if a liquid is set in motion, when it moves, it tries to stay all together, and its virtue of being viscous is precisely attributed. The viscosity is the resistance having the molecules that make up a liquid to separate from each other, that is, is the resistance of a fluid to deform and this opposition is due to the adhesive forces having a molecules of a liquid or fluid with with respect to the other molecules of the same liquid.
It is important to highlight that viscosity is a characteristic that is present in liquids that are in motion, it cannot be seen reflected in a liquid that is static because if the liquid remains fixed, the molecules that compose it will not have the need to interact with each other to try to stay together. When the viscosity is shown in a fluid, it is trying to oppose its movement, which is given by the application of a force.
The larger the molecules of a liquid, the greater resistance they will put to their displacement, therefore in this case it is said that these fluids are more viscous due to the fact that the displacement that their molecules can present occurs in a slower way (the reason is that the intermolecular forces that are present in this liquid are stronger), otherwise, when the molecules that make it up are smaller, they will have less opposing force so their movement will be faster (they present weak intermolecular forces).
The fact that a fluid is more viscous than another means that it has greater opposition to its deformation, however, with the sole action of applying heat energy (increase in temperature) to a fluid, it causes its viscosity to decrease, which causes this can move much faster. Apart from liquids, gases also have the characteristic of viscosity because these are also fluids or can be set in motion, however in this case their effects are usually negligible since they are taken into account as ideal fluids.
