What are properties of Liquid?

 

Physical Properties of Liquids

 

Liquid is the state of matter which has definite volume but has no definite volume e.g., water, Alcohol, glycerine, oils etc.

Physical properties of liquids are associated with physical changes and are used to distinguish among various substances.

Classification of Physical Properties

Physical properties of a system are divided into three groups.

Additive Properties

These properties are the sum of the properties of individual components of a system. These properties do not change with change in the physical state of the system. e.g., mass, molecular weight.

Constitutive Properties

These are the properties which depend upon the arrangement of atoms in the molecules, e.g., surface tension, optical activity.

Colligative Properties

These are the properties which depend upon the number of molecules and are independent of nature of molecules, e.g., lowering of vapour pressure, elevation of boiling point, depression of freezing point.

·        Some properties are partly additive and partly constitutive, e.g., parachor, molecular refraction.

·        The additive character of these properties is affected by the arrangement and environment of the bonded atoms.

 

Vapour Pressure

The pressure exerted by the vapours of a liquid when they are in equilibrium with the pure liquid at a given temperature, is called Vapour Pressure.

Explanation

Consider a liquid placed in a closed vessel. The liquid molecules change into vapours due to evaporation process. These vapours get accumulated in the space above the liquid. After some time, the vapours also start changing into liquid due to condensation process. At certain stage the rates of evaporation and condensation processes become equal. This stage is called equilibrium state.

                             Liquid                                     Vapours

The pressure exerted by the vapours at this equilibrium state is called vapour pressure

of the liquid.        

 

Measurement of Vapour Pressure

Various methods used to measure the vapour pressure of a liquid include Static methods and Dynamic methods.

Static methods involve evaporation of a liquid in vacuum. Examples of these methods include Barometric method, Smith, and Menzies method (or isoteniscopic method).

Dynamic methods involve the boiling of a liquid under a definite pressure.

Ramsay and Young’s method is example of this method.

Manometric Method

The liquid whose vapour pressure is to be determined, is taken in a flask placed in a thermostat. One end of the tube from the flask is connected to a manometer and the other end is connected to a vacuum pump. The liquid is frozen with the help of a freezing mixture and the space above the frozen liquid is evacuated.

In this way the air is removed from the surface of the liquid along with the vapours of the liquid. The frozen liquid is then melted to release any entrapped air. Liquid is again frozen and released air is evacuated. This process is repeated many times till almost all the air is removed. Now the liquid is warmed in the thermostat o that temperature at which vapour pressure of the liquid in the flask is to be determined. Difference in the heights of the columns of mercury in the two limbs of the manometer determines the vapour pressure of the liquid.

 

Surface Tension

It is defined as the energy required to expand the surface of a liquid by a unit area.

It is also defined as the force acting along the surface of the liquid at right angle downwards on a unit length.

It is represented by γ (gamma).

 

Units of Surface Tension

Surface tension of a liquid is expressed in the following units.

Joule per square meter (J.m^-2)

Newton per meter (N.m^-1)

Explanation of Surface Tension

Consider two molecules A and B of a liquid. The molecule B is present inside the body of the liquid and is subjected to balanced attractive forces. It can therefore move freely inside the liquid. The molecule A is present on the surface of the liquid. It is subjected to unbalanced attractive forces due to the absence of liquid molecules on its upper side (above the surface of the liquid). This unbalanced attraction acting downwards make the surface of the liquid like a stretched rubber sheet, trying to contract and is responsible for the surface tension.

 

Factors Affecting Surface Tension

Two factors affect the surface tension of a liquid.

Strength of intermolecular attractive forces

Stronger are these forces, greater will be the surface tension.

Temperature

Rise in temperature decreases the surface tension due to weakening of intermolecular attractive forces.

Measurement of Surface Tension

Surface tension of a liquid can be determined by one of the following methods.

The Capillary Rise Method

The Drop Method

The Torsion Balance Method

Only Drop method is discussed here.

The Drop Method

This method is also called Stalagmometer Method. It is based on the principle that the size of the drop coming out from a capillary tube is controlled by the surface tension of the liquid. At the time of falling the drop, the force of surface tension pulling it upward is proportional to the weight of drop. Mathematically

                             w = v d g = γ 2 π r                                      ….. 1                             

Where v is volume of drop, d is density of the liquid, g is force of gravity, γ is surface tension of the liquid and r is the radius of capillary tube.

This equation being the basis of drop method, is used for the comparison of the surface tension.

If γ₁ and γ₂ are the surface tensions, w₁ and w₂ are the weights of drops of two liquids, then we can write

                             w₁ / w₂  =  γ₁ 2 π r  /  γ₂ 2 π r     or      w₁ / w₂  =  γ₁ / γ₂

                             or      γ₁  =  γ₂ x w₁ / w₂

Thus, knowing the values of γ₂ (surface tension of the reference liquid, usually water), w₁ and w₂, the value of γ₁ can be calculated.

In actual practice, it is difficult to determine the weight of a single drop, hence the no. of drops formed by a definite volume of the liquid are counted.

If a known volume, V, of the two liquids produce n₁ and n₂ drops respectively, under the same conditions, then the above equation becomes

                             n₁ v₁ d₁ g  =   γ₁ n₁ 2 π r                     ….. 2

                             n₂ v₂ d₂ g  =   γ₂ n₂ 2 π r                     ….. 3

The volume V of the liquid is given by

                                      V = nv

Hence n₁v₁ = V     and n₂v₂ = V

Thus equations 2 & 3 can be represented as

                             γ₁ n₁ 2 π r = V d₁ g                                      ….. 4

                             γ₂ n₂ 2 π r = V d₂ g                                      ….. 5

Dividing equation 4 by equation 5, we get

                             γ₁ n₁ 2 π r / γ₂ n₂ 2 π r = V d₁ g / V d₂ g

or                         γ₁ x n₁ / γ₂ x n₂ = d₁ / d₂

or                         γ₁ = γ₂ x n₂ x d₁ / n₁ x d₂                    ….. 6

Equation 6 is used to calculate the surface tension of the given liquid.

          The apparatus used for the measurement of surface                            tension by drop method is called stalagmometer. It consists                                        of a glass bulb which has a capillary tube at its lower end                                       and a simple glass tube at its upper end. Two points A and                                 B are marked above and below the bulb. The liquid is                                      sucked in the dry, clean stalagmometer up to the mark A.                                               It is allowed to flow down slowly in the form of drops.                                           The number of drops is counted as the liquid flows from                                      point A to B. Now the liquid is taken out of the stalagmo-                                             meter   and water is sucked. The no. of drops formed by water                                        are also counted as it flows from A to B. Water is used as the                         reference liquid. Knowing the surface tension of water, the                           surface tension of the given liquid can be calculated by equation 6.

 

Viscosity

The measure of the resistance which a liquid cause to its flow is called viscosity. It is denoted by the symbol η (eata).

Explanation

Consider a liquid flowing through a tube. The flowing liquid is considered to be made up of a number of concentric layers sliding past one another. The layer of the liquid in contact with the wall of the tube is almost stationary whereas the layer in the centre has the maximum velocity. Each layer exerts a drag on the next layer due to internal friction which causes resistance. The measure of this resistance is called viscosity.

Coefficient of Viscosity

It is defined as the force per unit area needed to maintain a unit difference of one meter per second velocity between two parallel layers of a liquid one meter apart.

 

Units of Viscosity

SI units of viscosity are kg. m^-1. s^-1

Viscosity of a liquid may also be expressed in the unit called Poise.

                             Poise = 10^-1 kg. m^-1. s^-1

Sub-multiples of poise are centipoise and milli poise.

Measurement of Viscosity

The direct measurement of viscosity is not possible. Only relative viscosity can be determined by comparing the viscosity of given liquid with that of a reference liquid, usually water. The apparatus used for measurement of viscosity is called Ostwald’s viscometer.

It consists of a capillary tube ‘bc’ connected                                                                   at its upper end with a bulb ‘X’ which is                                                        provided with a glass tube. The capillary tube                                                    at its lower end is connected to a U-shaped                                                                 tube having a bulb ‘Y’ provided with a glass                                                              tube. A point ‘a’ is marked on the glass tube                                                        above the bulb.

A definite volume of the liquid whose viscosity                                                      is to be measured, is added to the bulb ‘Y’.                                                           The liquid is sucked from the bulb ‘X’ till it                                                                  rises to point ‘a’. The time of flow of liquid                                                               from point ‘a’ to point ‘b’. The liquid is then                                                  taken out and an equal volume of water is added.                                                     The flow time of water from point ‘a’ to point ‘b’.                                                    The relative viscosity of the liquid is then calcu-                                               lated by applying the relationship

                             η_l = d_l t_l η_w / d_w t_w

where η_l is viscosity of liquid, η_w is the viscosity of water, d_l is density of liquid, d_w is density of water, t_l is time to flow the liquid and t_w is the time to flow water.

 

Refractive Index

It is the ratio of the velocity of light in the air to the velocity of light in a medium.

Mathematically

                   n = velocity of light in the air / velocity of light in a medium

where ‘n’ is refractive index of the medium.

Explanation

When a ray of monochromatic light passes from a rarer medium (air) to a denser medium (liquid), it is bent or refracted towards normal. The angle of refraction ‘r’ is less than the angle of incidence ‘i’. The relation between these two angles and the refractive indices (Plural of index) of the media is given by the law of refraction.

                             Sin i / Sin r = N / n

Where N = Refractive index of the denser medium

And     n = Refractive index of the rarer medium

 

 

According to Snell’s law, the ratio of sine of angle of incidence to that of angle of refraction is a constant quantity and is characteristic of the liquid used.

                             n = sin i / sin r

Factors Affecting Refractive Index

Two factors mainly affect the refractive index of a liquid.

·        Temperature.

·        Wavelength of the light.

 

Measurement of Refractive Index

The instrument used for the measurement of refractive index is called Refractometer.

Mostly used refractometers are Abbe’s refractometer and Pulfrich refractometer.

The construction and working of Abbe’s refractometer are discussed here.

Abbe’s Refractometer

An outline of the Abbe’s refractometer is shown here.

Construction                                                                                                       The main optical components of the instruments include Mirror M, Illuminating prism, P₁, Refracting prism P₂, a Telescope, T and an eye piece, E.

Light from a source, L is reflected by means of an adjustable mirror M on the illuminating prism P₁. The surface of the prism P1 is ground so that it spreads light in all directions. The upper prism P₂ is called refracting prism and its surface is polished. A small quantity of the liquid whose refractive index is to be determined, is placed between the prisms P₁ and P₂. The rays are totally reflected. When seen through the telescope, the view field appears divided into bright and dark portions.

 

Procedure

The prism box containing prisms P₁ and P₂ is opened. A few drops of the liquid are placed on the cleaned ground glass surface of the prism P₁. The prism box is now closed. The telescope is focused, and position of the mirror is adjusted for maximum illumination. The prism box is slowly moved forward and backward until the view field appears partly illuminated (bright) and partly dark. If white light is used, the coloured bands so observed are removed by rotating another prism called compensating prism. A sharp line will divide the field view into bright and dark portions. The prism box is rotated in such a way that the sharp boundary coincides with the point of intersection of the cross wires of telescope. The refractive index is noted directly from the scale ‘S’ with the help of eye-piece E.

 

Optical Activity

The property of a substance to rotate the plane of polarized light is called optical activity. The substance itself is called optically active.

The optically active substance which rotates the plane of polarized light towards left, is called Levo-rotatory, whereas the optically active substance which rotates the plane of polarized light towards right, is called Dextro-rotatory.

Plane Polarized Light

The light radiation moving only in one plane is forms plane polarized light. It is obtained by passing light from source through a ‘Nicol prism’.

 

Plane polarized light

 

Angle of Rotation

It is the angle through which an optically active substance rotates the plane polarized light. It is represented by ‘α’ (alpha).

The angle of rotation depends upon many factors such as nature of the substance, the length of the column through which light passes, temperature, wavelength of the light used, the concentration of the substance.

Measurement of the Optical Activity

The instrument used for measuring the angle of rotation of an optically active is called Polarimeter.

An outline of different parts of Polarimeter are shown below.

 

Layout of Polarimeter

Monochromatic light from the source S is allowed to pass through the lens L. The lens makes the rays of light parallel. The rays of light are then passed through a fixed Nicol prism P called Polarizer which converts the rays into plane polarized light. The plane polarized light then passes through a cylindrical tube PT. The emerging light is now allowed to pass through a moveable Nicol prism called Analyser (A). The analyser is connected to a circular metallic disc which has a graduated scale GS. The view field is seen by the telescope T.

The analyser is rotated for complete darkness and reading on the graduated scale is noted. Now the polarimeter tube is filled solution of optically active substance and placed in its position. When seen through the telescope, the field will appear bright since the solution has rotated the plane of polarized light. The analyser is now turned through some angle for complete darkness again. This is the angle of rotation. The sign of rotation is determined by observing the direction of rotation. For dextro (right), the sign is positive (+) and for levo (left), the sign is negative (-).

 

Dipole Moment

It is the measure of the separation of charges in a molecule. It is equal to the product of the size of the charge and the distance between the centres of positive and negative charges.

Mathematically                        µ = e x d

Where µ (mu) is dipole moment, e is size of the charge and d is the distance between the centres of positive and negative charges.

Representation of Dipole Moment

Dipole moment is a vector quantity. It is generally represented by an arrow showing the direction from positive to negative charge.

 

 

                                                                                      

                                                                                            

In H₂O, the net dipole moment of the molecule is the resultant of the vector addition of the individual bond moments.

Units of Dipole Moment

The SI unit of dipole moment is Coulombmeter (C.m). However, commonly used unit is ‘debye’ represented by D.

                                                D = 3.34 x 10 ^-30 C.m

Dielectric Constant

It is defined as the power of a medium to break the residual forces of attraction present in a compound. It is represented by the symbol C (epsilon).

Mathematically                        F = q₁ q₂ / C r²

Where F is the force between two bodies, q₁ and q₂ are the charges on 1^st & 2^nd bodies respectively and r is the distance between the centres of two charges, and C is the dielectric constant of the medium (sometimes called Permittivity of the medium).

If the medium used is vacuum, then its value is unity (C = 1).

Water has a dielectric constant value equal to 80.