Capillary Action and Viscosity | Chemistry Grade XI

Capillary Action and Viscosity

Capillary Action and Viscosity | Chemistry Grade XI

Capillary Action and Viscosity

CAPILLARY ACTION
The tube which has small internal diameters is called capillary tube. When capillary tube is dipped in a liquid, the liquid level either rises or falls in the capillary tube depending on cohesive and adhesive force. This phernomenon is known as capillary action.

When adhesive force is more than cohesive force, the liquid level rises in capillary tubes and forms concave meniscus. When capillary tube is dipped in water, the adhesive force between water molecule and glass molecule is more than cohesive force among the water molecules. This develops more affinity between water molecules and glass molecules. So water level rises up in the capillary tube.

When cohesive force is more than adhesive force, the liquid level falls in capillary tube and forms convex meniscus. When capillary tube is dipped in mercury, the cohesive force among mercury atoms is more than adhesive force between mercury atom and glass molecules. This develops more affinity among mercury atoms than that of mercury atom and glass molecule. So, mercury level falls in capillary tube.

Capillary Action in Water and MercuryVISCOSITY
When liquid flows through a narrow pipe, the velocity of different layers is not the same. The liquid layers which are contact with the inner wall of pipe are almost stationary. The rate of flow of the liquid layer gradually increases towards the center of the pipe. During the flow of liquid, the liquid layer opposes or resists the flow of adjacent liquid layer, when flows one over another.

The viscosity of a liquid can be defined as internal resistance or friction produced by one layer of the liquid flowing over an adjacent liquid layer. Viscosity is measured in terms of the coefficient of viscosity. The frictional force produced when one layer of liquid flows over another is directly proportional to the surface area of the liquid layer and velocity gradient.

F is proportional to A * dv/dx
or, F = x * Adv/dx
Where x=proportionality constant known as coefficient of viscosity
when, A = 1square cm
dv=1cm/s
dx=1cm
Then, F = x

So, coefficient of viscosity can be defined as the force required to maintain unit velocity difference of the liquid layer separated by unit distance with the unit surface area. The SI unit of coefficient of viscosity is Ns.m-2 and CGS unit of coefficient of viscosity is poise.

Factors affecting viscosity

1. Nature of liquid: The liquid which has greater strength of the intermolecular force of attraction provides more frictional force between the adjacent layers. So, the viscosity of liquid increases with an increase in strength of the intermolecular force of attraction. Presence of hydrogen bond also increases the viscosity of the liquid

2. Temperature: With the increase in temperature, the intermolecular force of attraction decreases. So, viscosity of the liquid decreases.

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