Introduction to capilarity

Capillarity is the elevation (or rise) or depression (or fall) of a liquid a narrow porous medium e.g tube.
Capillary action depends on cohesion and adhesion forces.

(i) When adhesion is greater than cohesion forces, e.g water and glass, then :
the liquid rises in the capillary tube
the meniscus curves upwards (concave);
The liquid wets glass

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(ii) When cohesion is greater than adhesion forces, e.g mercury and glass, then the :
liquid falls (or depresses) in the capillary tube

  • The meniscus curves downwards (convex);
  • The liquid forms spherical balls when spilled on a surface
  • does not wet glass
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Effect of size or diameter of the capillary tube on capillarity

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Uses of capillarity

It helps paraffin (fuel) to rise up in wicks of stoves and lamps.

It helps water to move up tree-trunks to the leaves

It helps blotting papers to absorb liquids

NOTE:

All the above uses are possible because of greater adhesion forces than cohesion forces between molecules of the two substances. (i.e Fuel and wick, water and tree-trunk, liquid and blotting paper respectively). Thus, the wet part of the solid goes on increasing upwards.

Damp proof material e.g polythene is put in the foundation course of a building to stop capillary action. This is because bricks, plaster and mortar are porous, so water can rise up through the narrow pores and weaken the wall.

SURFACE TENSION:

Surface tension is the force acting normally on a one-metre length of a line drawn on the surface of a liquid. Its S.I unit is Nm−1 .

Surface tension enables the surface of a liquid to behave like a stretched elastic skin (or membrane).
It is due to cohesion forces between the liquid molecules.

Surface tension is the force on the liquid surface that causes it to behave as if it is covered with a thin elastic membrane.
Molecular explanation of surface tension:

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According to Kinetic theory surface tension is explain as follow:-

Molecules on the surface experience a net down word force on them and surface of the liquid thus tends to contract and acquire a state of tension.

A molecule inside the surface of the liquid is surrounded by equal number of molecules on all sides. The intermolecular forces between it and the surrounding molecules is zero.

A molecule on the surface has very few molecules on the upper side compared to those bellow the liquid surface.

Thus if this molecule is displaced upwards, a resultant attractive force due to the large number of molecules bellow the liquid surface has to be overcome.

This force is trying to pull the molecule out of the surface into the bulk. It is trying to make the surface smaller, hence surface tension.

Experiment to demonstrate existence of Surface tension

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Procedures:

A clean beaker is filled with water.

A blotting paper is placed on the surface of the water.

A pin or needle is gently placed on the blotting papper and observed

Observation:

After some few minutes, the blotting paper absorbs water and sinks to the bottom.

The needle remains floating on the water surface.

Conclusion:

The needle is held by surface tension.

Effects of surface tension

(i) A needle (pin) floats on water surface.

(ii) Tents keep water, umbrellas and raincoats keep water off due to surface tension.

(iii) Insects e.g pond skater can walk across a water surface.

(iv) Water drops from a tap form spherical shapes. This is because; a free falling drop will take the shape that has the least (minimum) area.

Factors affecting surface tension

(i) Temperature: Increase in temperature (or heating a liquid) weakens or reduces surface tension.

(ii) Impurities: Impurities such as detergents, soap solution, alcohol e.t.c reduce surface tension.

Estimating the thickness (or Size) of an oil molecule:

A drop of oil is able to spread into a thin film of a large area when placed on a clean water surface. This is because the end of the oil molecule has greater adhesion forces than cohesion forces for neighboring molecules.

The size of an oil molecule is too small to be accurately measured. Its approximate size can only be estimated using an experiment.

Experiment to estimate the size of an oil molecule

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Fill a clean trough with clean water and sprinkle lycopodium powder on the surface of the water.

Drop a known volume, Vo of the oil onto the water surface covered with lycopodium powder.

The oil spreads forming a cylindrical oil film with a circular patch, whose diameter ‘’d’’ is measured.
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NOTE:

It is a common procedure to;

Dissolve a known volume of a solute (e.g cooking oil or oleic acid) V1 in a known volume of a solvent (e.g Petroleum, alcohol or ether) V2 to form a solute-solvent solution.

Then a known volume, V3 of the solute-solvent solution is dropped onto the water surface covered with lycopodium powder.

The solvent in the drop either dissolves in water or evaporates and the solute (oil) spreads forming a cylindrical oil film with a circular patch, whose diameter ‘’d’’ is measured.

The thickness or size of the oil film is estimated from:

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However, the volume of the oil in the solution which forms the oil film is calculated as follows:

image 198

Assumption

(i) All the solvent has evaporated or dissolved in water.

(ii) The oil patch is circular.

(ii) The oil film or molecule formed is cylindrical.

(iii) The oil film formed is one molecule thick. (i.e the spaces between the molecules in the oil film are assumed to be negligible).

(iii) The oil drop is spherical.

(iv) Volume of cylindrical film=Volume of oil in spherical drop

NOTE:
The water surface should be sprinkled with lycopodium powder so that:
(i) The film becomes stationary.
(ii) A clear circular patch for measuring diameter is formed.

Example 1:

A solution is made by dissolving 1cm3 of cooking oil in 1999cm3 of methanol. When 0.004 cm3 of the solution is dropped on the surface of water, an oil film of diameter 12cm is obtained.
(i) Calculate the volume of cooking oil in the film.
(ii) Estimate the thickness of a molecule of cooking oil.

(iii) State any two assumptions made in (i) above.

Solution:
Volume of solute or oil, (V1) = 1cm3
Volume of solvent (methanol), (V2) = 1999cm3
Volume of solution dropped, (V3) = 0.004cm3
Volume of solute or oil, in V3 = V

Example 2:
In an oil film, experiment to estimate the size of a molecule 0.005cm3 of oleic acid was dropped on lycopodium powder on a water surface. The mean diameter of the acid was 5cm. Calculate the thickness of a molecule of oleic acid.

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An oil drop of volume 1×10-9 m3 spreads on a water surface to form a patch of area 5×10-2 m2. If the patch is one molecule thick, find the approximate number of molecules in the drop.

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NOTE: Remember the oil drop is spherical thus its radius, r is equal to half the thickness, 𝑡2 of the oil film.
Thus the radius of the spherical oil drop, r is given by;,

image 201