Physical Properties of liquid Prof. Jadhav Swapnil S.

Introduction:- The physical properties of liquid are those which could be studied by determined without causing any chemical change in it. Physical properties of substance depend upon the intermolecular forces which originate in the internal structure of their molecule. Eg. Surface tension, viscosity, Refractive Index etc. Classification of physical properties:- 1) Additive Properties:- The properties of substance which depends on sum of corresponding properties of the constituents are called additive properties. Eg. Mass, molecular weight, molecular heat, dipole, dipole, radioactivity 2) Constitutive properties:- The properties which mainly depend on the arrangements of constituents and to a smaller extent on their number and nature are called constitutive properties. Eg. Optical activity, surface tension, viscosity 3) Additive and Constitutive Properties:- An additive property which depends on the intermolecular structure is called Additive and Constitutive Properties Eg. Parachor, viscosity, surface tension, molecular refractivity, atomic volume 4) Colligative properties:- The properties which depend on the number of particles in the solution and not on their nature are called Colligative properties. Eg. Vapour pressure, elevation in boiling point, depression in freezing point. Surface tension:- This property arises from the intermolecular forces of attraction. Consider a molecule A in the bulk of the liquid which is attracted equally in all direction by neighboring molecules. Consider molecule B on the surface of liquid which is attracted downward by neighboring molecules. Thus there is tendency of surface molecules to go into the bulk of the liquid. Thus the liquid surface is under tension and tends to reduce to minimum. This inward attraction gives rise to a force in the plane of surface called Surface tension. Def.:- Surface tension may be defined as “the force in dynes acting along the surface of liquid at right angle to any line 1 cm in length.” Unit:- S.I. = N/m or J/ m2 C.G.S = dyn/cm or ergs/ cm2

Parachor:- It is defined as the molecular volume of liquid when its surface tension is unit. Macleod (1923) gives relation between surface tension and density of liquid as γ = surface tension C = Macleod constant --- (1) D = density of liquid. d = density of sat. vapour of liquid. Sugden (1924) modified Macleod equation by multiplying both sides by M, M = molecular weight --- (2) M & C are constant for given liquid Hence, MC = constant = P --- (3) P is called Parachor Bellow critical temperature ‘d’ is negligible comparison with ‘D’ we get, --- (4) M/D = molecular volume. If γ = 1 then, P = M/D ie. P = molecular volume. For two liquids, we can write, and Taking ratio, we get, --- (5) Eq. (5) we say that, a comparison of parachors means, comparison of molecular volumes under the same condition of surface tension. parachor is both an additive and constitutive property. Ie. the parachor of an individual compound can be expressed as a sum of : (1) Atomic Parachors which are the contributions of each of the atoms present in the molecule. (2) Structural Parachors which are the contributions of the various bonds and rings present in the molecule.

Use of Parachor in Elucidating Molecular Structure:- (1) Structure of Benzene (Vogel) The Kekule’s formula for benzene is shown aside, the value of its parachor can be calculated by using Vogel’s data. 6 Carbon atoms : 6 × 8.6 = 51.6 6 Hydrogen atoms : 6 × 15.7 = 94.2 3 double bonds (=) : 3 × 19.9 = 59.7 6-membered ring : 1 × 1.4 = 1.4 Parachor of benzene = 206.9 The experimental value of the parachor of benzene is 206.2. Since the calculated parachor tallies with that determined by experiment, the Kekule’s structure for benzene is supported. (2) Structure of Quinone (Sugden) The two possible structural formulas proposed for quinine. The parachors calculated for the two structures are Structure A 6 C : 6 × 8.6 = 51.6 4 H : 4 × 15.7 = 62.8 2 O : 2 × 19.8 = 39.6 4 (=) : 4 × 19.9 = 79.6 1 six-membered ring : 1 × 1.4 = 1.4 Total = 235.0 Structure B 6 C : 6 × 8.6 = 51.6 4 H : 4 × 15.7 = 62.8 2 O : 2 × 19.8 = 39.6 3 (=) : 3 × 19.9 = 59.7 2 six-membered rings : 2 × 1.4 = 2.8 Total = 216.5. The experimental value of parachor for quinone is 236.8 which is near to structure A. Therefore, the structure A represents quinone correctly. (3) Structure of Nitro group (Sugden) The parachor has also been found useful in providing information regarding the nature of bonds present in certain groups. The nitro group (–NO2), for example, may be represented in four ways: The experimental value of parachor for – NO2 group has been found to be 73.0. Hence structure II is correct.

(4) Structure of phosphorus pentachloride (PCl5 ):- It may have two structural formulae The experimental value of parachor for PCl5 is 282.5. Hence structure II is correct. Viscosity:- Viscosity resists the flow of liquid. “The property of liquid which determine the rate of flow of liquid is called viscosity of liquid.” Let us examine a liquid flowing on a glass surface (Fig. 11.22). The molecular layer in contact with the stationary surface has zero velocity. The successive layers above it move with increasingly higher velocities in the direction of the flow. Now consider two adjacent moving layers of a liquid (Fig. 4.2). Let these be separated by a X = distance between two layer. v = velocity difference between two layer. F = force of friction Then

F α ௫௩ F = η × ௫

௩

η (eta) is known as the Coefficient of Viscosity. It may be defined as: the force of resistance per unit area required to maintain unit velocity difference between two layers of a liquid which are at a unit distance from each other. Unit:- CGS = Poise (P) or centipoise (10-2) and millipoise (10–3) or dyn/cm2 . SI = N/m2.s or Pa.s or Kg/m.s 1 poise = 1 g cm–1 s–1 = 0.1 kg m–1 s–1

The reciprocal of coefficient of viscosity is called Fluidity (φ). φ =

The Fluidity is a measure of the ease with which a liquid can flow. Viscosity of liquid decrease with increase in temperature.

Determination of Viscosity by Ostwald’s viscometer. The coefficient of Viscosity of a liquid can be determined with the help of Pioseulle’s equation as : Where, V = volume of the liquid flowing through capillary in time t, P = applied pressure, r = radius of the capillary. l= length of the capillary. The experimental measurement of P, r, l and V offers considerable difficulty. Hence, it is difficult to find the absolute coefficient of viscosity (η) from Poiseulle’s equation. Thus, the viscosity of a liquid is determined with respect to that of water by Ostwald Viscometer. Construction:- It consists of U shaped glass tube having two bulbs. The left-hand arm is essentially a capillary with two calibration marks A and B bellow & above the bulb. The right-hand arm is wide (pipette) and has a bulb C at the base. Experimental Procedure:- 1) A definite volume of liquid under examination is poured into the bulb C of viscometer. 2) This liquid is sucked through the left-arm slightly above the mark A. 3) The liquid is then allowed to flow back and the time of flow of liquid from A to B is noted. 4) Then the apparatus is cleaned and the process is repeated for 2nd with water whose viscosity is known. Calculation:- It is observed that the pressure P depends on (1) h = height of liquid level in the two arm. (2) d1 = density of liquid. (3) g = acceleration due to gravity. Then Pioseulle’s equation becomes For 1st liquid for 2 nd liquid

Tacking ratio of above two equations, we get, When η2 is known and by measuring densities & times of flow of the two liquid η1 of first liquid can be calculated. Advantages:- 1) It is very convenient apparatus for determination of viscosity. 2) Viscosity at different temperature can be determined as viscometer can be easily suspended in thermostat. Refractive Index (Snell’s Law):- When a beam of light passes from a less dense (rarer) like air to more dense medium like liquid, it is refracted (bent) towards normal. This is called refraction of light. “The ratio of sine of angle of incidence to the sine of angle of refraction is constant” This is known as Snell’s Law.

n = ୱ୧୬ ୱ୧୬

n is refractive index of 2nd medium w.r.t. 1st medium.

Where i = angle of incidence r = angle of refraction. The refractive index (n) of a substance is also defined as the ratio of the velocity of light in vacuum or air to that in the medium.

n = Velocity of light in vacuum

Velocity of light in medium

If i = 90° then sin i = sin 90° = 1 Then, n = ଵୱ୧୬

The refractive index of a liquid can be easily determined to a high degree of accuracy. It is a characteristic property of a liquid. It increase with temperature and decrease with wavelength of light used. D-line of sodium is used for standard measurement. Refractive index is a ratio, it has no units. If the refractive index of a liquid is measured at 20ºC and using D-line of sodium, it is represented by the following symbol. n20

D

Specific Refraction or Specific Refractivity:- Lorenz and Lorenz (1880), on the basis of electromagnetic theory of light derived the following relation for the refractive power of substance. Where, n = refractive index. d = density R = Specific Refraction. Molar (Molecular) Refraction or Molecular Refractivity (RM):- The product of molecular weight (M) and specific refractivity (R) is called molar or Molecular Refractivity (RM). RM = R × M Ie. The value of molar refraction is characteristic of a substance. It is Temperature-independent. Since the value of refractive index (n) is dimensionless, from equation it is evident that RM α M/d. ie. molar volume. Hence molar refractivity is expressed in cm3/mol. Measurement of refractive index by Abbe’s Refractometer:- Principle:- The refractive index of liquid is measure on the basis of critical angle principle. As ζi increases, ζr also increases, but to certain limit. When ζi > 900, ray suffers total internal reflection. When ζi < 900 (rays a, b)we observed light band. When ζi > 900 (ray d) we observed dark band. When ζi = 900 (ray c) we observed sharp edge which gives angle of refraction r’ and we get condition,

n = ୱ୧୬ ୱ୧୬

= ୱ୧୬ ଽୱ୧୬ ᇱ

= ଵୱ୧୬ ᇱ

r’ is called critical angle & this phenomenon is known as critical angle phenomenon. Note:- If angle of incidence (i) is greater than critical angle (r’), the light is reflected instead of refracted. This is called “total internal reflection.” Abbe’s Refractometer:- Construction and working:- Abbe’s Refractometer is as shown in fig. The optical system consist following part- (1) A mirror M. (2) A fixed telescope T. (3)The prism box PQ which can be rotate by adjusting screw R. A thin film of the liquid is placed between the two prisms P & Q. The surface of prism ‘P’ is polished white, while that of ‘Q’ is finely ground. P & Q are held in contact with each other in a metal box. Light from source ‘S’ is made to fall on prism ‘Q’ with the help of a mirror. The light enters the liquid at all angles of incidence. However, no ray can enter the upper prism with greater angle of refraction than the grazing incidence (i.e., at an angle) slightly less than 90º. Thus, the view in the telescope through eye-piece ‘O’ appears to be divided into two bands, one bright (light) and one dark. The prism box is rotated till the cross wire of the telescope coincides with the edge of the bright band. A pointer attached to the prism assembly indicates the refractive index directly on an engraved scale ‘Z’. When white light is employed, sharp edge is made by compensator ‘W’. Note:- In order to maintain temperature of liquid to be examine, two prism are enclosed in a water jacket.

Advantages:- 1) The instrument is easy to handle. 2) Only few drops of liquid are required. 3) Refractive Index can be measured directly and quickly. 4) Temperature can be controlled. 5) The Refractive Index range is from 1.3 to 1.7 with an accuracy of ± 0.0002 unit. Molecular Refractivity and Chemical Constitution:- Molecular Refractivity is proportional to molecular volume. Molecular Refractivity is also an additive and partly constitutive property. Hence it may be used to study the structure of molecules. Here calculated values of molecular refractivities are compared with the observed or experimental values.

b) For allyl alcohol:-

3 C : 3 × 2.42

6 H : 6 × 1.10

1 = bond : 1 × 1.73

1 O in OH : 1 × 1.53

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Total RM = 17.12

Since, calculated molecular refractivity of allyl alcohol (17.12) is close to experimental values (16.974). Hence the compound must be allyl alcohol rather than acetone.