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Surface Tension (Part - III)

Pressure on liquid surface

1.

2.

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3.

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Capillary

The rise or fall of the liquid level in the capillary,

above or below the liquid level in the container is

called capillary action or capillarity.

e.g.

1. Oil in the wick lamp rises due to capillarity.

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2. It is due to the capillary action that the plants get

their sap absorbed by the roots, up to the leaves.

Explanation of Capillary Action

PA = PC as both points are air at the same level.

PD = PC as the liquid surface at theses points is

plane.

∴ PD = PA

But as the liquid surface is concave, PA > PB

∴ PD > PB

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Points B and D are inside the liquid, at the same level.

So, the liquid cannot remain in equilibrium

When PD > PB.

Hence liquid rushes in to capillary to increase PB.

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Rise of a liquid in a capillary tube

the total vertical force acting upwards is 2πrT cosθ.

Volume V of the liquid inside the capillary

= Area of cross - section x height

of liquid column.

∴ V = πr2 x h

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If ρ is the density of the liquid,

Mass M of the liquid inside the capillary =

Volume x Density.

M = π r2hρ

Weight (W) of the liquid in capillary =

mass x acceleration due to gravity

∴ W = πr2h ρ g

This is the gravitational force acting on the liquid

column inside the capillary, tending to pull it

downwards. Thus, it the total downward force acting on

the liquid. But, the liquid inside the capillary is in

equilibrium.

i.e. Total upward force = Total downward force

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This is the formula for surface tension of a liquid which

paritally or completely wets glass.

1. The height ‘h’ of the liquid column is measured from

the liquid level outside the capillary, in the

container, to the lower level of the liquid meniscus

in the capillary.

2. While measuring height of the liquid column, the

vertical scale of the travelling microscope must be

perfectly vertical.

3. For a given solid liquid pair, T, cosθ, ρ and g are

constant. Hence, product ‘hr’ is also constant.

Hence, different capillaries, of same material, of

different diameters are dipped in same liquid, the

liquid rises more in a capillary of lesser diameter

and vice-versa.

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Effect of impurities on surface tension

If the liquid contains any impurity, its surface tension

changes by large amount. The effect of soluble and

insoluble impurities is different.

1. When a highly soluble substance is added to a

liquid, the impurity molecules exert strong adhesive

forces in addition to the cohesive forces among the

liquid molecules. This increases the surface

tension.

e.g. when common slat is added to water, surface

tension of the salt solution is more than that of pure

water.

2. When a substance which is not readily soluble in

the liquid is added to the liquid, the liquid molecules

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are separated by the impurity molecules. This

decreases the intermolecular forces between the

liquid molecules and decreases the surface

tension. e.g. when soap is added to water, the soap

solution has less surface tension then pure water.

Thus, it becomes thin and can reach to every part

of the cloth and remove dirt from that. Hence, soap

solution is used to wash dirty clothes.

3. When a completely insoluble impurity is added to a

liquid, the intermolecular forces become very weak,

which reduces the surface tension to a greater

extent. But this happens only when it is added.

Once the substance gets settled, the surface

tension of the liquid regains its original value. e.g.

when sand is added to water, as long as sand is

poured in water, its surface tension decreases.

When pouring stops and the sand gets settled at

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the bottom of the container, the surface tension of

water gets its original value.

1. A capillary tube is dipped into a liquid. If the levels

of liquid inside and outside the tube are the same,

then the angle of contact is

(a) 45 º (b) 0º

(c) 90º (d) 60º

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Ans:

(c) 90 º

The levels of the liquid inside and outside the tube

are the same.

∴ h = 0 but

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5. Farmers plough their fields to retain some soil

water. How does ploughing help in this process?

(a) Capillaries are created

(b) Capillaries are broken

(c) By turning the soil upside down, lower layers of

soil are heated

(d) None of the above

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Ans:

(b) Capillaries are broken

In the process of ploughing, the natural capillaries

in the soil are broken and hence the water in the

soil does not rise upto the surface and is not lost

due to evaporation and the soil does not become

dry.

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6. Surface tension of a liquid is independent of the

(a) Temperature of the liquid

(b) Area of the liquid surface

(c) Nature of the liquid

(d) Impurities present in the liquid

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Ans:

(b) Area of the liquid surface

T depends upon the nature the liquid, temperature

of the liquid and the impurities present in the liquid.

But it does not depend upon the area. Because, if

area is changed, the work done also changes

proportionately.