# Class 11 Physics Chapter 10 Important Questions Mechanical Properties of Fluid

Q 1:- What is pressure? give its SI unit and dimensions. Ans:- Pressure at a point is defined as the limiting value of the normal force acting per

It is important for the students that all the concepts should be very clear for better marks in future. Here, we are providing important conceptual questions and answers for class 11 physics chapter 10 Mechanical properties of fluid. In this lesson, students will learn about Mechanical properties of fluid. This will not only help the students to know the important questions but will also help them during revision.

Q 1:- What is pressure? give its SI unit and dimensions.
Ans:- Pressure at a point is defined as the limiting value of the normal force acting per unit surface area when the surface area tends to zero.

Thus,

pressure p=\lim_{ΔA \rightarrow 0}\frac{∆F}{∆A}

The SI unit of pressure is a pascal (Pa) and its dimension is [ML-1T-2].

Q 2:- Is pressure scalar or vector? Why?
Ans:- Pressure is a scalar because in the definition of force, we consider only component of the force normal to the area under consideration and not the vector force.

Q 3:- Define an atmospheric pressure.what its value?
Ans:- An atmospheric pressure is the pressure exerted by the atmosphere at sea level. Its value is equal to the pressure exerted by a mercury column of height 760mm or (0.760m ), i.e.,1.013×105 Pascal.

Q 4:- What do you mean by the term specific gravity of a substance question how is it related to the relative density of that substance?
Ans:- Specific gravity of a given substance is defined as the ratio of the weight of a given volume of the substance to the weight of equal volume of water at 4 ℃. Specific gravity of a substance is a unit less and dimension less term. For a given substance, value of specific gravity is numerically equal to its relative density.

Q 5:- What are the factors which affect the atmospheric pressure at a place?
Ans:- Atmospheric pressure at a place depends on the height of the atmosphere, density of atmosphere and the value of g at that place.

Q 6:- What is gauge pressure at a point?
Ans:- The gauge pressure at a point is the difference between total pressure at that point and the atmospheric pressure.

Q 7:- Name the factor on which hydraulic pressure at a point in a vessel containing a liquid depends. Does it depend on the shape of the vessel?
Ans:- The hydraulic pressure at a point in a given vessel containing a liquid depends upon:

• (a) The height of liquid column above that point.
• (b) The density of the liquid, and
• (c) Value of g.

The hydraulic pressure does not depend upon the shape of the vessel containing liquid and the amount of liquid in the vessel.

Q 8:- Distinguish between hydrostatics and hydrodynamics.
Ans:- Hydrostatics is the branch of mechanics which deals with the study of fluids at rest. On the other hand, hydrodynamics is the branch of mechanics which deals with the study of fluids in motion.

Q 9:- What is a streamline?
Ans:- Streamline in a fluid flow is the path, straight or curved, the tangent to which at any point gives the direction of fluid flow at that point.

Q 10:- In a streamlined flow, what is the velocity of fluid flow in contact with the fixed surface (or the wall of containing vessels)?
Ans:- Fluid particles in contact with a fixed surface are at rest or zero flow velocity.

Q 11:- What is meant by critical velocity of a liquid?
Ans:- Critical velocity of a liquid is the maximum velocity of liquid flow up to which the flow remains a streamline flow. Beyond this velocity, flow does not remain steady and becomes unsteady of turbulent.

Q 12:- What is a laminar flow?
Ans:- Laminar flow is that steady (streamline) flow of a fluid in which magnitude of velocity (i.e., speed ) of fluid flow at different points may differ but direction of fluid velocity remains same.

Q 13:- Is a laminar flow always streamline? Is every streamline flow laminar?
Ans:- Yes, a laminar flow is always a streamline flow. however every streamline flow needs not to be a laminar flow.

Q 14:- Give few examples of turbulent motion.
Ans:- Smoke rising from a burning stack of wood, the wakes in air left by moving cars and aeroplanes wakes in water left by moving boats and ships are turbulent in nature. Oceanic current are also turbulent and twinkling of stars too is a result of atmospheric turbulence.

Q 15:- Write the properties of an ideal fluid.
Ans:- An ideal fluid is that in which there is no dissipation of energy due to viscous drag or any other cause,when the fluid is flowing.

Q 16:- State Torricelli's law regarding speed of efflux of a fluid.
Ans:- According to Torricelli's law the speed of efflux from an orifice in an open tank is exactly the same as the speed acquired by a particle falling freely through a distance equal to the depth of a orifice below the free liquid surface in open tank.

Q 17:- Does the speed of efflux depends on density of fluid?
Ans:- No, the speed of efflux of a fluid does not depend on its density.

Q 18:- Why are like roofs blown off during a certain wind storm?
Ans:- According the applications of bernoulli's principles.

Q 19:- What do you mean by strain rate for a flowing liquid? How is it measured?
Ans:- The increase in shearing strain per unit time for a flowing liquid is called its strain rate. if a particular liquid layer situated at a normal distance l from a fixed layer flows with a linear speed v, then v/l is a measure of the strain rate.

Q 20:- Define coefficient of viscosity of a liquid. what is the SI unit?
Ans:- The ratio of shearing stress to the strain rate of a fluid is called its coefficient of viscosity. coefficient of viscosity of a fluid is also equal to the tangential force required per unit surface area so as to maintain a unit velocity gradient between two adjacent fluid layers. The SI unit of coefficient of viscosity is 1 poiseuille (1PI) and it is also referred to as 1 Pascal second (1 Pa s) or 1 N second m-2.

Q 21:- Which is more viscous,water or blood?
Ans:- Blood is more viscous than water.

Q 22:- How does coefficient of viscosity of a liquid change with change in temperature?
Ans:- The coefficient of viscosity of a liquid decreases on increasing its temperature.

Q 23:- How does the flow rate of laminar flow of a liquid in a cylindrical tube or pipe depend on its radius?
Ans:- For laminar flow of a liquid through a cylindrical tube or pipe, the flow rate V is proportional to fourth power of its radius r.

Q 24:- What is terminal velocity ? on what factor does it depends?
Ans:- Terminal velocity of a body falling freely in a viscous medium is the maximum value of uniform velocity acquired by it in the given viscous fluid.terminal speed of a small-sized spherical body depends upon:

• (a) radius of the body,
• (b) coefficient of viscosity of fluid, and
• (c) densities of the given body and the viscous fluid.

Q 25:- A small spherical body is is freely falling through a long column of a viscous liquid. draw a graph showing variation of its velocity with time.
Ans:- See figure 10.30 initially, the velocity increases but gradually the value of acceleration goes on decreasing and finally the body starts moving with a constant terminal velocity.

Q 26:- The Blade of a kitchen mixer induce turbulence. Explain why.
Ans:- Turbulent promotes mixing of a fluid and increase the rate of transfer of mass, momentum and energy. the turbulence created by the blades of a kitchen mixer helps in providing a thick, homogeneous milk shake.

Q 27:- Why does free surface of a liquid behave like a stretched membrane?
Ans:- The liquid molecules in the surface film of a liquid experience a net inward force and ,thus, have extra energy in comparison to molecules in the interior of a liquid. as any system in its stable equilibrium position has minimum potential energy, the free liquid surface behaves like a stretched membrane and is under a tension tending to acquire least possible surface area.

Q 28:- What do you mean by force of cohesion and force of adhesion?
Ans:- The attractive intermolecular force between neighbouring molecules of a given substance is called the force of cohesion. the intermolecular attractive force between molecules of different substances is called the force of adhesion for example molecular force between two water molecules is the force of cohesion and attractive force between a water molecule and neighbouring glass molecule is the force of adhesion.

Q 29:- Is viscosity (viscous force) a vector? is surface tension a vector?
Ans:- No both viscous force and force of surface tension are scalars because they do not have a fixed direction of their own.

Q 30:- How is surface tension related to surface energy of a liquid?
Ans:- Surface tension of a given liquid is numerically equal to its surface energy per unit surface area.

Q 31:- On what factors does value of angle of contact depend?
Ans:- Angle of contact for a given pair of liquid and solid depends both on the nature of liquid and the nature of solid. Besides these, angle of contact also depends on the temperature. In general value of angle of contact for a given solid-liquid pair increases with increase in temperature.

Q 32:- What is a wetting agent ? What is its use ? give an example of a wetting agent.
Ans:- A wetting agent is a material which if dissolved in water reduces the angle of contact of water. As a result, the solution may easily penetrate through the solid surface and becomes effective. detergents and dyes are wetting agents.

Q 33:- What is a waterproofing agent? What is its function? give an example.
Ans:- A waterproofing agent is a compound which if added to water makes its angle of contact obtuse. As a result,water cannot penetrate the surface with which it comes in contact.waterproof school bags, raincoats and tarpaulins make use of waterproofing agents.

Q 34:- Water rises in a capillary tube but Mercury level falls in a capillary tube as compared to the surrounding.explain why?
Ans:- Water wets the glass capillary tube and angle of contact of a water glass pair is very small,hence water meniscus is of concave shape. consequently,pressure on liquid side of meniscus is less than atmospheric pressure and its leads to rise of water in capillary tube.

Mercury does not wet glass and angle of contact of Mercury glass-pair is obtuse,hence Mercury meniscus is concave. consequently, pressure on liquid side of meniscus is greater than the atmospheric pressure. it leads to fall of Mercury level in capillary tube as compared to the surroundings.

Q 35:- A long capillary tube ( say 1 m long) is dipped in a beaker containing water in an artificial satellite orbiting around the earth. To what height will water rise in the capillary tube, and why?
Ans:- Water will rise throughout the long capillary to and at its upper end,the water meniscus becomes flat. that is angle of contact, theta is equals to 90 degree in accordance with the formula given by

h=\frac{2scos}{rpg}.

Q 36:- Generally, soccer players use boots having studs on their soles. Why?
Ans:- The studs in the soles of shoes reduce the area of contact between the shoe and the ground. As a result, weight of the player acts on a smaller area and consequently, player presses the ground with an increased pressure. therefore, feet of players sink into the turf laid on ground and grip the surface firmly. This enables the player to run about on the ground without a risk of slipping.

Q 37:- We can cut an apple easily with a sharp knife than a blunt knife. explain how
Ans:- In case of sharp knife, the cross- section area of the knife edge is small. therefore, for a given force applied on the knife, the pressure exerted by the edge on the apple is large and apple is cut easily. For a knife of blunt edge, the cross section area of edge is comparatively more. Hence, for the same force applied on the knife, the pressure exerted by the edge on the apple is less. hence, it is difficult to cut the apple.

Q 38:- Wooden or concrete sleepers are provided below the rails while laying a railway track. why?
Ans:- Broad wooden or concrete sleepers are provided below the rails while laying a railway track so that pressure exerted on the ground by the weight of moving train through rails become less due to large surface area of base. As a result, the railway track does not get depressed.

Q 39:- Briefly explain the action of a straw while taking a soft drink.
Ans:- When we suck cold drink through the straw, the pressure inside the straw becomes less than the atmospheric pressure. Due to difference in pressure, soft drink rises in the straw and we can conveniently take the soft drink.

Q 40:- In a dropper, water does not come out unless it's rubber bulb is pressed. why?
Ans:- Water is held inside the dropper against the atmospheric pressure.On pressing the rubber bulb, the pressure on water becomes more than the atmospheric pressure and hence water comes out.

Q 41:- Although normal atmospheric pressure is about 105 Pascal, but we do not feel it. why?
Ans:- All the value of atmospheric pressure is quite high, having a value of about 105 Pascal, we do not feel its effect. It is because atmospheric pressure acts on us from all directions and at all the points uniformly. therefore, the net force acting on a human being due to atmospheric pressure is zero and he does not feel it.

Q 42:- To empty an oil tin, two holes are made in it. why?
Ans:- If only one hole is made in an oil tin, the oil will not come out continuously. As the oil starts coming out of tin, the pressure inside the tin becomes less than the atmospheric pressure and oil can not come out. however, when two holes are made in an oil tin, air keeps on entering the tin through second hole and the pressure inside the tin remains constant. As a result, oil continues to come out of first hole.

Q 43:- It is observed that the speed of water emerging from a garden hose increases when one partially closes the hose opening with his thumb or fingers. explain why?
Ans:- As per equation of continuity for study flow of a non compressible liquid

Av = a constant,

v = slow speed of liquid. If a person partially closes the hose opening with his thumb fingers, the cross section area A is reduced and consequently, slow speed v increases.

Q 44:- Why a jet attached to the head of water pipe of a firefighter?
Ans:- generally, a jet is attached to the head of water pipe used by a firefighter so that in accordance with the equation of continuity speed of water coming out of Jet is large and consequently a range of water stream is greater.

Q 45:- Why does the pressure exerted by a liquid flowing horizontally decrease when it it passes from a broader region to a narrow reason (constriction)?
Ans:- When a liquid flowing horizontally through a pipe of varying cross section passes from a broader region to a constriction, its flow speed v increases in accordance with the equation of continuity. due to increase in flow of speed, the kinetic energy per unit volume of of flowing liquid increase, consequently, as per Bernoulli's theorem pressure of liquid is decreased.

Q 46:- It is dangerous to stand on the edge of a railway platform when a train is passing by. explain why?
Ans:- When a train is passing, it drags neighbouring air with a high speed. As a result, Air pressure near the train (that is, on the basis of a railway platform) is reduced by a large extent.due to this low pressure, if a person is standing near the edge of a platform, he may lose his balance and may fall towards the train and may get hurt.

Q 47:- what is the cause of viscous drag force in fluids?
Ans:- When we consider laminar flow of a fluid over a fixed surface at rest, the fluid layer in contact with that surface is at rest. As we normally move away from the fixed surface, velocity of fluid goes on increasing. thus, there is relative motion between different fluid layers. For any fluid layer, the upper neighbouring layer moves with a greater velocity and tends to pull it forward. But the lower neighbouring layer of fluid moves with a smaller velocity and tends to to pull it backward. It results in a force acting between in the adjacent layers which tends to oppose relative motion between them. this is the viscous force.

Q 48:- Viscous force is generally referred to as a fluid friction. in what respect viscous force resembles with friction force present in solids?
Ans:- viscous force and friction force both oppose relative motion and come into play whenever relative motion tends to take place. Moreover, both these forces act tangentially to the surfaces in contact in a direction opposite to the direction of relative motion both. The forces are intermolecular forces and non conservative in nature.

Q 49:- Do gases also exhibit the property of viscosity?
Ans:- Yes, the gases too exhibit the property of viscosity. it is on account of viscous drag of air offers resistance to the motion of automobiles, trains, aeroplanes, etc., raindrops falls with small terminal velocities on account of viscous drag of air. Of course, the coefficient of viscosity of gases is comparatively smaller than that of liquids.

Q 50:- Why is dirt deposited on the side walls of a flowing waste water open drain?
Ans:- In an open drain, speed of flow of waste water is extremely small near the walls of drain and is more along its central axis. As there is relative motion, a viscous force acts on the waste water.consequently, dirt particles, being heavier, are unable to flow with water and get stuck to the side walls of drain.

Q 51:- Lubricating oils used in machinery are generally of high value of coefficient of viscosity?
Ans:- Due to high value of coefficient of viscosity, a lubricating oil does not flow easily. As a result, the thin-film of lubricating oil formed between different part of machinery remains intact for longer time and machine works efficiently.

Q 52:- sometimes machine parts are in extreme winter why?
Ans:- Due to extremely low temperature in winter season, especially in early morning time, the coefficient of viscosity of the engine oil as well as lubricants increases. Due to high value of viscous force, the machine parts are sometimes jammed.

Q 53:- Why do water liquids flow easily than cold liquids!
Ans:- we know that coefficient of viscosity of a liquid decreases with increase in temperature. As a result, a hotter liquid offers less viscous force and flows easily whereas a colder liquid offers more viscous force and does not flow easily.

Q 54:- As per bernoulli's principle, the pressure of water should throughout remains same for water flowing in a horizontal pipe of uniform cross section area. But in reality, the pressure goes on decreasing with length of the pipe. Explain why.
Ans:- Bernoulli's principle is strictly true for a non viscous fluid flow only. however, in practice, water offers some viscous force. when water flows through a horizontal pipe of uniform cross section, work is being done against the viscous force. as a result, pressure of water falls gradually with onword moment in the pipe.

Q 55:- Raindrops falls under gravity from a height of about 1 km. Yet their speed is very small. Explain now?
Ans:- As raindrops fall under gravity through atmosphere, a viscous force act on them as per stokes' law and soon the raindrops start falling with a constant terminal speed. value of terminal speed is proportional to the square of the radius of raindrops. As radius of raindrop is generally very small, the value of their terminal speed too is quite small. During a downpour, the terminal speed of raindrops is about 7 m per second.

Q 56:- Dust particles slowly settle down in a closed room. why?
Ans:- Dust particles are of spherical shape having very small value of radii. hence, while falling under gravity, they soon acquire terminal speed which is very very small. Thus, dust particles fall through air with a small and constant terminal speed and take quite some time before settling down in a closed room.

Q 57:- Small air bubbles rise slower than the bigger ones through a liquid.why?
Ans:- When an air bubble rises through a liquid, it soon starts rising with a constant terminal speed due to viscous nature of the liquid. the terminal speed is proportional to the square of the radius of bubble. consequently, terminal speed of a small air bubble is less than a bigger one and hence, smaller air bubble rises slowly through the liquid.

Q 58:- Why is a parachute invariably used while jumping from an aeroplane in flight?
Ans:- When a paratrooper jumps from an aeroplane in flight, he opens his parachute. Being of large size, a parachute experiences enlarge viscous force. As a result, acceleration of paratrooper goes on decreasing and soon he requires a constant terminal speed.magnitude of terminal speed is small. As a result, the paratrooper reaches the ground with a small speed and lands safely.

Q 59:- Why are raindrops spherical in shape?
Ans:- On account of the property of surface tension, the free surface of a raindrop tends to acquire the least possible surface area. since for a given volume the surface area of a sphere is minimum, hence the raindrop invariably acquires a spherical shape.

Q 60:- Small drops of Mercury rs vehicle but bigger drops are of oval shape?
Ans:- For a small droplet of Mercury, the force of surface tension is quite large as compared to its weight and hence the droplet is spherical in shape. When size of Mercury drop increases, its weight increases much more rapidly than that of surface tension. Consequently, the shape of the drop becomes oval.

Q 61:- What awaits the class but Mercury does not explain why?
Ans:- In case of water, the water molecules are strongly attracted the glass molecules. consequently, water wets the glass and does not form globules. in case of mercury, the mercury molecules are strongly at attracted to themselves and very weakly attracted to glass molecule. consequently, mercury does not wet glass but forms globules on glass surface.

Q 62:- Why should the antiseptic be of low surface tension?
Ans:- If surface tension of an antiseptic is low, then even a small amount of antiseptic will easily spread over the the entire surface of wound or cut.

Q 63:- Why does the addition of flux (soldering paste) to the tin make the soldering process easy?
Ans:- When the flux is added to molten tin, it reduces the surface tension of the molten tin. As a result, the molten tin spreads easily over the junction, where the soldering is to be done. so, the soldering process becomes easy.

Q 64:- why is teflon coated on the surface of a nonsticking fry pan?
Ans:- When teflon is coated on the surface of aluminium fry pan, the angle of contact between the cooking oil and pan becomes an obtuse angle. as a result, now the cooking oil does not stick to the surface of fry pan. So, we we have to use a small amount of oil for frying.

Q 65:- How does a cotton wick in a kerosene lantern (or an oil filled lamp) keep on burning and emitting light?
Ans:- The cotton wick contains a large number of fine capillaries. Due to capillary action, oil (or kerosene) continuously rises to the top end of the wick and hence, the wick keeps burning and limiting light.

Q 66:- A mercury barometer always reads slightly less than the true value of atmospheric pressure. why?
Ans:- Due to phenomenon of capillarity, the mercury level in the barometer tube gets slightly depressed and hence reads less pressure. however, for a barometer tube of wind bore, this error is negligible.

Q 67:- A new earthen pot keeps water cooler than the old one. explain why?
Ans:- A new earthen pot has large number of fine holes which behave as capillaries. Water from inside the pot, rises in these capillaries and comes out. this water then takes heat from the earthen pot and evaporates. As a result, the water in the pot gets cooled. however, in an old earthen pot the capillaries get blocked and hence cooling mechanism of water fails.

Q 68:- Why do we prefer cotton dresses during summer season?
Ans:- The cotton clothes have fine pores which behave as a capillaries. during, summer season when a lot of sweat is formed on our body,cotton dresses help in easily sucking the sweat formed due to capillaries action. so, we feel comfortable.

Q 69:- A glass rod coated with wax does not become wet when dipped in water. why?
Ans:- The angle of contact between wax and water is an obtuse angle ,that is water molecules are very weakly attracted to wax molecules and strongly attracted to themselves. As a result, water does not wet the glass rod coated with wax.

Q 70:- While skiing on snow, a skier wears skis in addition to the shoes. explain why.
Ans:- when a skier wears skis, the force due to his weight acts over a much larger surface area than the area of the soles of the shoes. As a result, pressure on the soft surface of the snow is reduced by a large extent and the skier can slide over snow easily without sinking in it.

Q 71:- The force required by a person to move his limbs immersed in water is less than the force needed for the same movement in air. explain why.
Ans:- When a person has limbs immersed in water, the net weight of limbs is less than their true weight in air as an upthrust is acting on them due to weight of water displaced, in accordance with Archimedes principle. consequently, the person has to apply less force for movement of his limbs inside water.

Q 72:- Explain why does a balloon filled with helium gas not rise in atmosphere indefinitely but halts after a certain height is achieved.
Ans:- A balloon filled with helium gas rises in atmosphere in accordance with Archimedes principle because its weight is less than the weight of air displaced by it. However, with increase in height the density of air goes on decreasing gradually. If at a certain height, the density of air decreases to such a value that the weight of balloon becomes just equal to weight of air displaced by it, the balloon will be in equilibrium there and will not rise further.

Q 73:- In a flowing stream slow water runs deep why?
Ans:- If we consider a flowing stream of water and at some place observe flow speed to be quite slow, then water runs deep there it is in accordance with equation of continuity, according to which flow speed v is inversely proportional to cross section area A. If v is less at a place, then value of A must be large and for a given breath of steam, its dept should be large.

Q 74:- why does a flag flutter when strong winds are blowing on a certain day?
Ans:- When strong winds blow over the top of a flag, the speed and consequently, the kinetic energy of the wind is more at the top than that of air blow the flag.as per bernoulli's principle, pressure of air at the top becomes less than that below the flag. Due to the difference in pressure, the flag flutters.

Q 75:- The accumulation of ice on an aeroplane wing may change its shape such that its upward lift is greatly reduced. comment.
Ans:- Yes, it is true. The upper left on aeroplane wing is on account of its special shape due to which speed of air streamlines below the wing is less and above the wing is more. the accumulation of snow on the wing may reduce the curvature of the wing and flattens it. As a result, speed of air lines below and above the wing becomes almost same. as a result, pressures below and above the wing of aeroplane are nearly same and there is practically no uplift.

Q 76:- Why is it that we need a constant driving force for maintenance of the flow of crude oil through an oil pipeline?
Ans:- On account of viscous nature of oil, its motion is being continuously retarded down along the length of pipe line carrying crude oil. therefore, in order to maintain the flow of oil at a constant speed, a constant driving force need to be applied on the oil. For this purpose, booster pumps are used at regular distances along the pipeline.

Q 77:- Why are racing cars and aeroplanes are highly streamlined?
Ans:- when our vehicles moves through air, its motion is opposed by the force of viscosity due to air. the viscous force increases with increase in speed of the vehicle. Work done against the viscous force dissipates kinetic energy of the vehicle usually in the form of heat. therefore, racing cars and aeroplanes are streamlined and engineered to precision so as to minimise dissipation of their kinetic energy into heat.

Q 78:- Why do the clouds seem floating in the sky?
Ans:- Extremely small sized waterdrops falling under gravity acquire terminal speed much before reaching the earth.such extremely small sized water drops are seen to float in the sky in the shape of clouds.

Q 79:- Small piece of camphor moves erratically (or dance about) on the water surface. explain why.
Ans:- When a small piece of camphor is put on water surface, it slowly dissolves in water and reduces the surface tension of water. Due to irregular shape of camphor piece, its rate of dissolution in water is different at different points, thereby reducing surface tension of water by unequal amounts so at different point. so, a net resultant force due to surface tension of water acts on the camphor piece and it moves in that direction. As direction of net force on camphor goes on changing, it moves erratically on the water surface.

Q 80:- Why do we need to apply a large force to draw apart two plane glass plates enclosing a thin water film normally?
Ans:- the thin water film formed between two plane glass plates, due to its property of surface tension, tends to acquire minimum surface area and pulls the glass plates towards itself. if we want to draw a part normally these glass plates, then we shall have to apply a force so as to counter the force of surface tension. So, a large force is to be applied.

Q 81:- Drop of olive oil when poured on cold water, spread so as to form a thin film over water surface but when poured on hot water, remains as a drop floating on water. why?
Ans:- Surface tension of olive oil is less than the surface tension of cold water. as a result, drop of olive oil, if poured on cold water, spreads over water surface forming a thin film. however, surface tension of hot water is less. as a result, now the drop of olive oil remains as a drop, and being lighter than water, floats as a drop on water.

Q 82:- Water is depressed in a glass tube whose bore is coated with paraffin wax. Explain why.
Ans:- When a bore of a glass tube is coated with paraffin wax, then molecular force on a water molecule due to wax molecules is very small as compared to force on a water molecule due to to other water molecules. As a result, water does not wet wax coated capillary and its angle of contact becomes an obtuse angle. So, the water level is depressed in the tube.

Q 83:- Why does an object entering the earth's atmosphere at a high speed catch fire?
Ans:- When an object enters the earth's atmosphere at a high speed, a large viscous force acts on it due to atmospheric air. due to high speed of object, the magnitude of viscous force is extremely high. While overcoming the large viscous force, the kinetic energy of the object is dissipated in in the form of heat and temperature of object rises. As a result, the object is liable to catch fire and burn.

Q 84:- End of a glass tube becomes round on heating. Explain why.
Ans:- On heating, the end of a glass tube, glass melts. on account of the property of surface tension, the liquid glass acquires symmetrical circular (round) shape.

Q 85:- A piece of chalk immersed into water emits bubble in all directions. Why?
Ans:- The chalk piece has fine capillaries. when it is immersed into water, water rises in these capillaries and the air present in the capillaries is forced out. As a result bubbles are emitted from the chalk piece in all directions.

Q 86:- Hot soup taste better than cold one. Why?
Ans:- The surface tension of hot soup is comparatively less than that of cold soup. as a result, a sip of hot soup inside the mouth of a person spreads to larger surface area. As a result, the person concerned can experience its taste in a better way.