--%>
Assignment Help - homework help

Liquid surfaces

The surface between a liquid and a vapour distinguishes these fluids.


The surface tension of liquids can be looked upon as that the property which draws a liquid together and forms a liquid vapour interface, therefore, distinguishing liquids from gases.

The molecular basis for this property is suggested, where the unbalanced attractions experienced by the surface molecules are shown to lead to the amount of free liquid will pull it together to form a less spherical drop. The surface layer can be expected to have properties that differ from those of the bulk of the liquid.

The surface tension of the liquid can be defined with reference to where it is most easily pictured is a wire frame, arranged as a piston, used to expand a soap film. The definition also applies to the mechanically more difficult systems where the film is replaced by a layer of liquid of appreciable thickness. The force required stretching the film or liquid vapour is proportional to the length l of the piston. Since there are two surfaces of the film, the total length of the film is 2l, and the proportionality equation:

ƒ = γ(2l) can be written.

The proportionality constant γ is known as the surface tension, and according to the above equation it can be looked upon as the force by a surface of unit length.
Of more general use is the relation between surface tension and surface energy. The mechanical energy required to expand the surfaces by moving the piston a distance dx is f dx, or 2l dx. Since the area of new surface is 2l dx, the result:

Mechanical energy/change of surface area = 2lγ dx/ 2l dx = γ, can be obtained. This expression shows that the surface tension can be interpreted as the energy per unit surface area and that it is a mechanical rather than thermal energy. In these terms, the tendency of a surface to reduce its area is just another example of a system tending toward an arrangement of low free energy.

Surface tension of some liquids, N m-1:

Liquid 20°C 60°C 100°C Liquid t, °C Surface tension
H2O 0.07275 0.06618 0.05885 Hg 0 0.480
C2H5OH 0.0223 0.0223 0.0190 Ag 970 0.800
C6H6 0.0289 0.0237   NaCl 1080 0.094
(C2H5)2O 0.0170   0.0080 AgCl 452 0.125


Example: compare the heights to which water and carbon tetrachloride will rise as a result of capillary action in a tube with an internal diameter of 0.1 mm. at 20°C the surface tensions of water and carbon tetrachloride, respectively, are 0.0727 and 0.0268 N m-1, and their densities are 0.998 and 1.595 g mL-1.

Solution: we use to obtain:

L = 2 γ/rpg

The radius of the cube is 0.5 mm = 0.5 × 10-4 m, and the densities are 9.98 × 103 and 1.598 × 103 kg m-3.

For water: l = 2 (0.0727 N m-1)/(0.5 × 10-4 m) (9.98 × 103 kg m-3) (9.81 m s-1)

= 0.0297 m = 29.7 mm

For CCl4: l = 2 (0.0268 N m-1)/(0.5 × 10-4 m) (1.595 × 103 kg m-3) (9.81 m s-2)

= 0.00685 m = 6.85 mm.

   Related Questions in Chemistry

  • Q : Problem on relative humidity Relative

    Relative humidity is the ratio of the partial pressure of water in air to the partial pressure of water in air saturated with water at the same temperature, stated as a percentage: Relative  =

    Q : Molecular Properties Symmetry Molecular

    Molecular orbitals and molecular motions belong to certain symmetry species of the point group of the molecule.Examples of the special ways in which vectors or functions can be affected by symmetry operations are illustrated here. All wave functions soluti

    		•
  • Q : Colligative property associated question

    Give me answer of this question. Which of the following is not a colligative property : (a)Optical activity (b)Elevation in boiling point (c)Osmotic pressure (d)Lowering of vapour pressure

    		•
  • Q : Calculate molarity of a solution

    Provide solution of this question. Molarity of a solution prepared by dissolving 75.5 g of pure KOH in 540 ml solution is: (a) 3.05 M (b) 1.35 M (c) 2.50 M (d) 4.50 M

    		•
  • Q : Define the term oxidizing agent Briefly

    Briefly define the term oxidizing agent?

    		•
  • Q : What are biodegradable polymers?

      These are polymers that can be broken into small segments by enzyme-catalysed reactions. The required enzymes are produced by microorganism. It is a known fact that the carbon-carbon bonds of chain growth polymers are inert to enzyme-catalysed reactions, and hence they are non biod

    		•
  • Q : Question associated to vapour pressure

    Choose the right answer from following. The vapour pressure lowering caused by the addition of 100 g of sucrose(molecular mass = 342) to 1000 g of water if the vapour pressure of pure water at 25degree C is 23.8 mm Hg: (a)1.25 mm Hg (b) 0.125 mm Hg (c) 1.15 mm H

    		•
  • Q : Explain polyhalogen compounds with

    Carbon compounds containing more than one halogen atom are called polyhalogen compounds. Most of these compounds are valuable in industry and agriculture. Some important polyhalogen compounds are described as follows:

    Q : Molarity of Sodium hydroxide Select the

    Select the right answer of the question. Molarity of 4% NaOH solution is : (a) 0.1M (b) 0.5M (c) 0.01M (d) 0.05M

    		•
  • Q : Production of alcoholic drinks give all

    give all physical aspects in the production of alcohol

    		•