# MHT-CET Physics Crash Course “Angle of Contact and its Characteristics” Class 12

## “Angle of Contact and its Characteristics” MHT-CET Class 12 Physics Crash Course | Chapter Mechanical Properties of Fluids

In this session G.S. Khairnar discussed the theory and MCQ’s of “Angle of Contact and its Characteristics”. The session will be useful for all the aspirants of MHT-CET 2022. The session will be in Marathi and English.

### Angle of Contact and its Characteristics

Hello dear you must have noticed that while performing the practical of titration in chemistry, liquid solution filled in burette and pipette forms concave or convex meniscus inside it. This is due to the force of surface tension that acts on the surface of liquid.

The shape of meniscus (concave/convex) depends upon the magnitude of cohesive and adhesive forces between molecules of liquid, solid and other surfaces in contact. The tangent drawn to concave/convex meniscus forms certain angles with the solid surface in contact which gives rise the concept of angle of contact.

In this article we are going to illustrate the concept of angle of contact on the basis of forces due to surface tension.

Let’s Discuss the Concept.……………!

Angle of Contact:

The angle between the tangent to the liquid surface at the point of contact & solid surface inside the liquid is called as the angle of contact between the solid & liquid.

Some important properties of angle of contact…………..!

♦ For a liquid which partially wets the solid, the angle of contact is acute.

e.g. acute angle θ = 40o(kerosene in contact with glass)

♦ For a liquid which does not wet the solid, the angle of contact is obtuse.

e.g. obtuse angle θ = 128o (mercury in contact with glass)

♦ For a liquid which completely wets the solid, angle of contact is equal to zero.

e.g. zero angle  = 0o (water in contact with glass)

♦ The angle of contact of a given liquid-solid pair, is depends on the nature of liquid & solid.

♦ Angle of contact varies with temperature. As temperature increases, angle of contact decreases and vice versa.

♦ Angle of contact varies with addition of impurity. Angle of contact increases with addition of soluble impurities and decreases when insoluble impurities are added.

Cause of angle of contact…..!

Suppose small quantity of liquid kept on plane solid surface, shape formed by liquid is depends upon forces at solid-liquid & air interface.

Let  be the angle of contact

T1– surface tension at liquid–solid interface.

T2-surface tension at air –solid interface.

T3-surface tension at liquid– air interface.

For drop to be at equilibrium force due to solid- air interface should have to balance other two forces.

In equilibrium of the drop,

∴ T1 + T3 cos  =  T2

∴  T3 cos  =  T2 – T1

→ T2> T1 then cos is positive then is acute. For a liquid which partially wets the solid, the angle of contact is acute.

e.g.  = 40o for kerosene – glass

When, T2< T1 then cos is negative then  is obtuse. For a liquid which does not wet the solid, the angle of contact is obtuse.

→ When, (T2 – T1) ≈ T3 then cosθ≈ 1 than θ than  is nearly equal to zero.

→ When, (T2 = T1), cosθ = 0, then = 90o. For a liquid which wets the solid.

θ = 90o for pure water – silver

→ When, (T2 – T1) > T3 then cosθ > 1. which is impossible & equilibrium will not formed & liquid spread over a solid surface & drop would not formed.

### Angle of Contact and its Characteristics

Lets solve some MCQ’s on angle of contact….

Q.1) Angle of contact _________with increase in temperature

a) remains same

b) increases

c) decreases

d) cannot be predicted

c) decreases

Q.2) The pressure above the free surface of liquid is increases, the angle of contact……..

a) remains same

b) increases

c) decreases

d) cannot be predicted

b) increases

Q.3) Sparingly soluble impurities are added in a liquid, then the angle of contact……..

a) remains same

b) increases

c) decreases

d) either ‘b’ or ‘c’

c) decreases

Q.4) If completely soluble impurities are added in a liquid, then the angle of contact……..

a) decreases

b) increases

c) remains same

d) either ‘b’ or ‘c’

b) increases

Q.5) If the adhesive force exerted on molecule in contact with solid is greater than cohesive force by liquid molecules then  the angle of contact is ……..

a) 00

b) less than 900

c) greater than 900

d) either ‘b’ or ‘c’

b) less than 900

Q.6) Angle of contact for the liquid which do not wet the solids is ……..

a) 00

b) less than 900

c) greater than 900

d) either ‘b’ or ‘c’

c) greater than 900

### ♦जिल्हा नुसार जाहिराती ♦

अहमदनगर अकोला अमरावती औरंगाबाद भंडारा बुलढाणा
चंद्रपुर धुले गढ़चिरौली गोंदिया हिंगोली जलगांव
जालना कोल्हापुर लातूर मुंबई नागपुर नांदेड़
नंदुरबार नाशिक उस्मानाबाद पालघर परभानी पुणे
रायगढ़ रत्नागिरि सांगली सातारा सिंधुदुर्ग सोलापुर
ठाणे वर्धा वाशिम यवतमाल बीड

### ♦शिक्षणानुसार जाहिराती ♦

७ वी (7th) दहावी (SSC) बारावी (HSC) डिप्लोमा आय.टी.आय पदवी
पदव्युत्तर शिक्षण बी.एड एम.एड एल.एल.बी / एल.एल.एम बीएससी एमबीए
बीसीए एमसीए बी.कॉम एम.कॉम GNM/ANM एमएससी
बी.फार्म एम.फार्म बी.ई एम.ई BAMS/BHMS एम.बी.बी.एस / एम.डी
बी.टेक एम.टेक MS-CIT