Viscosity is a property of a liquid which is based on its flow in different layers. When a liquid moves with a steady speed over a solid and the motion is quite slow then it is observed that the layer of the liquid in contact with the solid is more or less stationary. In other words the velocity of the liquid along the surface of the solid is zero and the velocity of any other layer of the liquid will be proportional to its distance from the stationary layer and it will be maximum for the topmost layer. If a specific layer of a liquid is taken, then it is observed that a layer above it is moving faster and a layer immediately below is moving slower.
It is seen that the upper layer tends to increase the velocity of the lower layer while a lower layer tends to decrease the velocity of the upper one. In other words, the two layers try to destroy their relative motion in a similar way as if some backward force is acting tangentially on the layers. So if relative velocity is to be maintained between the two layers of a liquid, an external force is required to overcome the backward force. The liquids such as kerosene oil, alcohol, water etc, flow easily and are called mobile while liquids like tar, pitch, glycerin etc, move with difficulty and are called viscous.
An element is a substance that is made entirely from one type of atom. For example element hydrogen is made from atoms containing single proton and single electron. The purity of an element affects the viscosity values as the increase in concentration of Ti, Ni, Cr, Mn, Mg tends to increase the viscosity of Aluminum whereas the viscosity decreases with increases Zn and Si concentration. So the viscosity of an element depends on its purity also.