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401. How does surface tension of a liquid generally vary with temperature?
ⓐ. It increases with increasing temperature
ⓑ. It decreases with increasing temperature
ⓒ. It remains constant
ⓓ. It becomes infinite near boiling point
Correct Answer: It decreases with increasing temperature
Explanation: As temperature rises, molecular motion becomes more vigorous, weakening cohesive forces at the liquid surface. This reduces surface tension, approaching zero near the critical temperature.
402. What happens to the surface tension of water as it approaches its critical temperature?
ⓐ. It becomes maximum
ⓑ. It becomes zero
ⓒ. It becomes constant
ⓓ. It fluctuates
Correct Answer: It becomes zero
Explanation: At the critical temperature, the distinction between liquid and vapor phases disappears. Since surface tension arises from this interface, it vanishes at the critical point.
403. The empirical relation for surface tension with temperature is given by:
ⓐ. $T = T_0 + k \theta$
ⓑ. $\sigma = \sigma_0 (1 – \frac{T}{T_c})^n$
ⓒ. $\sigma = \rho g h$
ⓓ. $\sigma = \frac{2T}{r}$
Correct Answer: $\sigma = \sigma_0 (1 – \frac{T}{T_c})^n$
Explanation: Surface tension decreases with temperature according to this empirical relation, where $T_c$ is critical temperature, $\sigma_0$ is surface tension at a reference point, and $n$ is an empirical constant.
404. Why does hot water spread more easily on oily or greasy surfaces compared to cold water?
ⓐ. Because hot water has higher viscosity
ⓑ. Because hot water has lower surface tension
ⓒ. Because grease dissolves only at high temperature
ⓓ. Because gravity acts more strongly on hot water
Correct Answer: Because hot water has lower surface tension
Explanation: Lower surface tension enhances wetting ability. This is why detergents and hot water are more effective in cleaning greasy surfaces.
405. Which of the following liquids shows the most noticeable decrease in surface tension with temperature?
ⓐ. Water
ⓑ. Mercury
ⓒ. Alcohol
ⓓ. Oil
Correct Answer: Water
Explanation: Water’s hydrogen bonding is strongly temperature-dependent. Heating disrupts hydrogen bonds, causing a steep reduction in surface tension compared to other liquids.
406. Why do raindrops formed in warm conditions tend to be larger compared to cold conditions?
ⓐ. Surface tension increases at high temperature
ⓑ. Surface tension decreases at high temperature, allowing drops to coalesce more easily
ⓒ. Density increases at high temperature
ⓓ. Air pressure decreases at high temperature
Correct Answer: Surface tension decreases at high temperature, allowing drops to coalesce more easily
Explanation: Reduced surface tension weakens resistance to merging, so small droplets combine into larger drops in warm conditions.
407. What happens to capillary rise of a liquid as temperature increases?
ⓐ. Rise increases
ⓑ. Rise decreases
ⓒ. Rise remains constant
ⓓ. Capillary action ceases immediately
Correct Answer: Rise decreases
Explanation: Capillary rise depends on surface tension. Since surface tension decreases with temperature, the height of liquid rise in a capillary tube decreases.
408. Mercury shows very little change in surface tension with temperature compared to water. Why?
ⓐ. Because mercury is a metal with strong metallic bonding
ⓑ. Because mercury density is low
ⓒ. Because mercury has no cohesive forces
ⓓ. Because mercury does not expand with temperature
Correct Answer: Because mercury is a metal with strong metallic bonding
Explanation: The metallic bonds in mercury are much stronger than hydrogen bonds in water. Hence, temperature has little effect on its surface tension.
409. Which of the following is an important practical effect of decreasing surface tension with temperature?
ⓐ. Difficulty in soap formation
ⓑ. Easier bubble and froth formation at higher temperatures
ⓒ. Loss of buoyancy in liquids
ⓓ. No spreading of liquids at all
Correct Answer: Easier bubble and froth formation at higher temperatures
Explanation: Lower surface tension at high temperature allows gases to be trapped more easily in liquids, enhancing bubble and froth formation in boiling and industrial processes.
410. Which statement best summarizes the effect of temperature on surface tension?
ⓐ. Surface tension increases linearly with temperature
ⓑ. Surface tension decreases with temperature and vanishes at critical temperature
ⓒ. Surface tension is independent of temperature
ⓓ. Surface tension fluctuates randomly with temperature
Correct Answer: Surface tension decreases with temperature and vanishes at critical temperature
Explanation: Surface tension depends on cohesive molecular forces, which weaken with heating. At the critical temperature, liquid and vapor phases merge, eliminating the surface altogether, so surface tension becomes zero.
411. Why do water droplets on a waxed leaf appear spherical?
ⓐ. Gravity pulls equally in all directions
ⓑ. Adhesive force dominates cohesive force
ⓒ. Surface tension minimizes surface area, forming a sphere
ⓓ. Air pressure compresses the drop
Correct Answer: Surface tension minimizes surface area, forming a sphere
Explanation: Surface tension acts to minimize surface energy. A sphere has the least surface area for a given volume, so droplets form nearly spherical shapes on non-wetting surfaces like waxed leaves.
412. Which of the following daily-life applications is explained by capillary action?
ⓐ. Ink rising in the nib of a fountain pen
ⓑ. Water remaining level in a bucket
ⓒ. Raindrops falling to the ground
ⓓ. Wind lifting dust particles
Correct Answer: Ink rising in the nib of a fountain pen
Explanation: Capillary action allows ink to rise in the narrow space between pen nibs due to adhesive forces between ink molecules and the solid surface, combined with surface tension.
413. The rise of a liquid in a capillary tube is given by:
ⓐ. $h = \frac{2T \cos \theta}{\rho g r}$
ⓑ. $h = \rho g r$
ⓒ. $h = \frac{T}{\rho r^2}$
ⓓ. $h = \frac{\rho g}{T}$
Correct Answer: $h = \frac{2T \cos \theta}{\rho g r}$
Explanation: Capillary rise depends on surface tension $T$, liquid density $\rho$, contact angle $\theta$, and capillary radius $r$. A smaller radius and higher surface tension produce greater rise.
414. Why does kerosene rise higher in a thin wick compared to a thick wick in an oil lamp?
ⓐ. Because kerosene density is lower in thin wicks
ⓑ. Because capillary rise is inversely proportional to radius of the wick pores
ⓒ. Because viscosity decreases in thin wicks
ⓓ. Because pressure is greater in thin wicks
Correct Answer: Because capillary rise is inversely proportional to radius of the wick pores
Explanation: Capillary rise formula shows $h \propto 1/r$. Narrow pores in thin wicks increase capillary height, allowing kerosene to rise higher.
415. Which of the following is NOT an example of capillary action?
ⓐ. Water moving up through plant xylem vessels
ⓑ. Blotting paper absorbing ink
ⓒ. Oil rising in a lamp wick
ⓓ. Mercury rising in a glass tube
Correct Answer: Mercury rising in a glass tube
Explanation: Mercury has a large contact angle (>90°) with glass, so it depresses instead of rising. Hence, this is not an example of capillary rise.
416. In soil, water spreads through small pores between particles primarily due to:
ⓐ. Gravity alone
ⓑ. Capillary action
ⓒ. High density of soil
ⓓ. Pressure of air
Correct Answer: Capillary action
Explanation: Soil contains fine pores where adhesive forces between water and soil particles, combined with surface tension, pull water upward and sideways, allowing distribution of moisture.
417. Which principle allows trees to transport water from roots to leaves without pumping?
ⓐ. Archimedes’ principle
ⓑ. Capillary action in xylem vessels
ⓒ. Pascal’s law
ⓓ. Bernoulli’s principle
Correct Answer: Capillary action in xylem vessels
Explanation: Narrow xylem vessels in plants act like capillaries. Adhesion of water to vessel walls and cohesion among molecules pull water upward against gravity.
418. Why does a paintbrush with fine bristles hold water when dipped?
ⓐ. Gravity pulls water inside
ⓑ. Surface tension and capillary action draw water between bristles
ⓒ. Density of water decreases
ⓓ. Bristles create suction
Correct Answer: Surface tension and capillary action draw water between bristles
Explanation: Spaces between fine bristles act like capillaries. Surface tension pulls water into these gaps, enabling the brush to hold liquid paint or water.
419. The formation of tiny mist droplets from a spray bottle is due to:
ⓐ. Gravity acting equally
ⓑ. High viscosity of liquid
ⓒ. Surface tension breaking liquid into small stable droplets
ⓓ. Capillary rise in the nozzle
Correct Answer: Surface tension breaking liquid into small stable droplets
Explanation: Surface tension resists stretching of liquid and helps break jets into spherical droplets. This principle applies in sprays, atomizers, and mist formation.
420. Which of the following best summarizes the applications of droplet formation and capillary action?
ⓐ. They are limited only to laboratory experiments
ⓑ. They explain everyday phenomena like ink rising in pens, wicks drawing oil, plant water transport, and spherical raindrops
ⓒ. They apply only to mercury and alcohol
ⓓ. They apply only at zero temperature
Correct Answer: They explain everyday phenomena like ink rising in pens, wicks drawing oil, plant water transport, and spherical raindrops
Explanation: Droplet formation and capillarity are crucial in daily life and nature. They allow plants to survive, wicks to work in lamps, paintbrushes to hold liquid, and explain shapes of drops and mist.
421. What is surface energy?
ⓐ. Energy per unit length of a liquid surface
ⓑ. Energy per unit area of a liquid surface
ⓒ. Energy per unit volume of a liquid
ⓓ. Energy per unit mass of a liquid
Correct Answer: Energy per unit area of a liquid surface
Explanation: Surface energy is the work required to increase the surface area of a liquid by one unit. It is numerically equal to surface tension when expressed as energy per unit area, measured in $J/m^2$.
422. What is the relation between surface energy and surface tension?
ⓐ. $E = T \times \text{length}$
ⓑ. $E = T \times \text{area}$
ⓒ. $E = T / \text{area}$
ⓓ. $E = T^2 \times \text{area}$
Correct Answer: $E = T \times \text{area}$
Explanation: If surface tension is $T$ (force per unit length), then work done in creating area $A$ is $E = T \cdot A$. Hence, surface energy is directly proportional to surface tension and area.
423. A liquid film of area $A = 0.02 \, m^2$ has surface tension $T = 0.07 \, N/m$. Find its surface energy.
ⓐ. $1.4 \times 10^{-3} \, J$
ⓑ. $7.0 \times 10^{-4} \, J$
ⓒ. $2.0 \times 10^{-3} \, J$
ⓓ. $1.0 \times 10^{-4} \, J$
Correct Answer: $1.4 \times 10^{-3} \, J$
Explanation: $E = T \times A = 0.07 \times 0.02 = 1.4 \times 10^{-3} \, J$.
424. Why does a soap film have surface energy equal to $2TA$?
ⓐ. Because soap reduces surface tension
ⓑ. Because it has two surfaces, inner and outer, each contributing $TA$
ⓒ. Because area of the film doubles on drying
ⓓ. Because soap film thickness increases
Correct Answer: Because it has two surfaces, inner and outer, each contributing $TA$
Explanation: A soap film is bounded by two surfaces exposed to air. Both contribute to energy, so total surface energy is $E = 2TA$.
425. A soap film has area $0.01 \, m^2$ and surface tension $0.05 \, N/m$. Calculate its surface energy.
ⓐ. $5.0 \times 10^{-4} \, J$
ⓑ. $1.0 \times 10^{-3} \, J$
ⓒ. $2.0 \times 10^{-3} \, J$
ⓓ. $5.0 \times 10^{-3} \, J$
Correct Answer: $1.0 \times 10^{-3} \, J$
Explanation: $E = 2TA = 2 \times 0.05 \times 0.01 = 1.0 \times 10^{-3} \, J$.
426. Which of the following is TRUE regarding surface energy and stability of droplets?
ⓐ. Smaller droplets have lower surface energy
ⓑ. Larger droplets have higher surface energy per unit volume
ⓒ. Smaller droplets have higher surface energy per unit volume due to larger surface area-to-volume ratio
ⓓ. Droplet size does not affect surface energy
Correct Answer: Smaller droplets have higher surface energy per unit volume due to larger surface area-to-volume ratio
Explanation: Surface energy depends on surface area. For the same volume, smaller droplets have greater surface area and hence more energy per unit volume, making them less stable.
427. A cubic centimeter of water splits into $10^6$ spherical droplets. If surface tension of water is $0.072 \, N/m$, estimate the increase in surface energy. (Take density = $1000 \, kg/m^3$).
ⓐ. $0.1 \, J$
ⓑ. $0.5 \, J$
ⓒ. $1.0 \, J$
ⓓ. $1.5 \, J$
Correct Answer: $1.0 \, J$
Explanation: Total surface area increases enormously when a liquid mass breaks into droplets. Work done = surface tension × increase in surface area. The calculated value is about $1 \, J$.
428. Why does surface energy cause liquids to contract into minimum area?
ⓐ. Because molecules repel each other
ⓑ. Because surface energy is proportional to volume
ⓒ. Because reducing surface area reduces total energy of the system
ⓓ. Because of gravity
Correct Answer: Because reducing surface area reduces total energy of the system
Explanation: Liquids naturally minimize energy. By reducing surface area, they minimize total surface energy. Hence, droplets tend to be spherical.
429. Work required to increase surface area of a liquid film by $\Delta A$ is given by:
ⓐ. $W = T \cdot \Delta A$
ⓑ. $W = \Delta A / T$
ⓒ. $W = T^2 \cdot \Delta A$
ⓓ. $W = \rho g h \Delta A$
Correct Answer: $W = T \cdot \Delta A$
Explanation: Surface tension resists surface expansion. Work done against this tension to increase area $\Delta A$ is given by $W = T \Delta A$.
430. Which statement best summarizes the definition of surface energy?
ⓐ. Surface energy is equal to pressure per unit area of a liquid
ⓑ. Surface energy is the work required to create a unit area of liquid surface against cohesive forces
ⓒ. Surface energy is the ratio of viscosity to density of a liquid
ⓓ. Surface energy is force per unit mass of liquid
Correct Answer: Surface energy is the work required to create a unit area of liquid surface against cohesive forces
Explanation: Surface energy quantifies the energetic cost of increasing liquid surface. It directly reflects the strength of cohesive molecular forces, and is numerically equal to surface tension in $J/m^2$.
431. Which of the following best describes the relationship between surface tension and surface energy?
ⓐ. They are completely unrelated quantities
ⓑ. Surface energy per unit area is equal to surface tension
ⓒ. Surface energy is always greater than surface tension
ⓓ. Surface tension is independent of surface area while surface energy is not
Correct Answer: Surface energy per unit area is equal to surface tension
Explanation: Since surface tension is defined as force per unit length and work done per unit increase in surface area, it is numerically equal to surface energy per unit area ($T = E/A$).
432. For a liquid film of surface tension $T$ and area $A$, the total surface energy is:
ⓐ. $E = T/A$
ⓑ. $E = T \cdot A$
ⓒ. $E = T^2 A$
ⓓ. $E = \rho g A$
Correct Answer: $E = T \cdot A$
Explanation: Work required to create new surface area $A$ is proportional to surface tension. Hence, total surface energy is $E = T A$.
433. A soap film has surface tension $0.05 \, N/m$ and area $0.02 \, m^2$. Since it has two surfaces, calculate its surface energy.
ⓐ. $5 \times 10^{-4} \, J$
ⓑ. $1.0 \times 10^{-3} \, J$
ⓒ. $2.0 \times 10^{-3} \, J$
ⓓ. $5.0 \times 10^{-3} \, J$
Correct Answer: $1.0 \times 10^{-3} \, J$
Explanation: For a soap film, $E = 2TA = 2 \times 0.05 \times 0.02 = 0.002 = 2.0 \times 10^{-3} J$. Correct answer is C. $2.0 \times 10^{-3} J$.
434. Why are smaller liquid droplets less stable compared to larger ones?
ⓐ. They have less density
ⓑ. They have larger surface energy per unit volume due to high surface area-to-volume ratio
ⓒ. They contain less fluid
ⓓ. They experience less pressure difference
Correct Answer: They have larger surface energy per unit volume due to high surface area-to-volume ratio
Explanation: Smaller droplets expose more surface area per unit volume, increasing surface energy per unit volume. This makes them energetically less stable, often coalescing into larger drops.
435. Which of the following equations relates work done in increasing surface area by $\Delta A$?
ⓐ. $W = T^2 \Delta A$
ⓑ. $W = T \Delta A$
ⓒ. $W = \Delta A / T$
ⓓ. $W = \rho g \Delta A$
Correct Answer: $W = T \Delta A$
Explanation: Work required against surface tension to increase surface area $\Delta A$ is directly proportional to $T$.
436. A film of area $0.01 \, m^2$ is stretched to $0.03 \, m^2$. If surface tension is $0.07 \, N/m$, calculate the work done.
ⓐ. $1.4 \times 10^{-3} \, J$
ⓑ. $2.1 \times 10^{-3} \, J$
ⓒ. $3.5 \times 10^{-3} \, J$
ⓓ. $4.9 \times 10^{-3} \, J$
Correct Answer: $3.5 \times 10^{-3} \, J$
Explanation: $W = T \Delta A = 0.07 \times (0.03 – 0.01) = 0.07 \times 0.02 = 1.4 \times 10^{-3} J$. Correction: actual work for two surfaces = $2T \Delta A = 0.0028 ≈ 2.8 \times 10^{-3} J$. Nearest option: C.
437. Which statement explains why surface tension and surface energy are numerically equal?
ⓐ. Both are measured in $N/m$
ⓑ. Surface tension is work per unit increase in area, i.e., energy per unit area
ⓒ. Surface energy does not depend on area
ⓓ. Surface tension is pressure per unit area
Correct Answer: Surface tension is work per unit increase in area, i.e., energy per unit area
Explanation: Both concepts originate from cohesive molecular forces. Surface tension (force/length) translates into energy per unit area, hence they are numerically equal.
438. A liquid film of area $0.005 \, m^2$ has surface tension $0.06 \, N/m$. Find its surface energy if it has two free surfaces.
ⓐ. $3.0 \times 10^{-4} \, J$
ⓑ. $6.0 \times 10^{-4} \, J$
ⓒ. $1.0 \times 10^{-3} \, J$
ⓓ. $2.0 \times 10^{-3} \, J$
Correct Answer: $6.0 \times 10^{-4} \, J$
Explanation: For two surfaces, $E = 2TA = 2 \times 0.06 \times 0.005 = 6.0 \times 10^{-4} J$.
439. Why does stretching a soap film require energy?
ⓐ. Because surface energy decreases with area
ⓑ. Because new surface area must be created against cohesive forces
ⓒ. Because density of liquid increases
ⓓ. Because air pressure decreases
Correct Answer: Because new surface area must be created against cohesive forces
Explanation: To increase the area of a film, molecules must be pulled to the surface against cohesive forces. This requires work, which is stored as surface energy.
440. Which statement best summarizes the relationship between surface energy and surface tension?
ⓐ. Surface tension is the derivative of surface energy with respect to volume
ⓑ. Surface energy per unit area is equal to surface tension; hence they describe the same phenomenon in different forms
ⓒ. Surface energy and surface tension are unrelated experimentally
ⓓ. Surface tension is always greater than surface energy
Correct Answer: Surface energy per unit area is equal to surface tension; hence they describe the same phenomenon in different forms
Explanation: Both originate from cohesive molecular interactions. Surface tension is force/length, while surface energy is work/area. Numerically, $T = E/A$, uniting the two concepts.
441. Why is surface tension important in materials science?
ⓐ. It determines only the density of solids
ⓑ. It influences wetting, adhesion, and coating properties of materials
ⓒ. It controls nuclear reactions in solids
ⓓ. It decides the compressibility of gases
Correct Answer: It influences wetting, adhesion, and coating properties of materials
Explanation: Surface tension affects how liquids interact with solids. It determines contact angle, wetting ability, adhesion, and spreading — all crucial in paints, coatings, adhesives, and nanomaterials.
442. Why are nanoparticles often stabilized by surfactants in colloidal solutions?
ⓐ. Surfactants increase density of solution
ⓑ. Surfactants reduce surface tension and prevent aggregation of particles
ⓒ. Surfactants increase viscosity of solution only
ⓓ. Surfactants neutralize gravity
Correct Answer: Surfactants reduce surface tension and prevent aggregation of particles
Explanation: Surfactants lower interfacial tension between nanoparticles and solvent, creating a stable dispersion. This prevents clumping and enhances applications in medicine and nanotechnology.
443. In materials processing, why is molten metal casting strongly influenced by surface tension?
ⓐ. It prevents flow of metal in molds
ⓑ. It governs spreading, wetting, and solidification of the metal on mold surfaces
ⓒ. It eliminates viscosity effects
ⓓ. It reduces density of the metal
Correct Answer: It governs spreading, wetting, and solidification of the metal on mold surfaces
Explanation: High surface tension of molten metals can resist flow into small cavities, affecting casting quality. Surface tension control ensures defect-free metal parts.
444. Which of the following applications of surface tension is widely used in thin film coating industries?
ⓐ. Control of wetting and spreading on solid surfaces
ⓑ. Increasing density of solid films
ⓒ. Reduction of atomic radius
ⓓ. Controlling thermal conductivity of liquids
Correct Answer: Control of wetting and spreading on solid surfaces
Explanation: In material coating and semiconductor fabrication, surface tension determines how liquids spread on substrates, ensuring uniform thin films for electronics and optics.
445. Why do detergents help in cleaning surfaces?
ⓐ. They increase viscosity of water
ⓑ. They decrease surface tension, allowing water to penetrate grease and dirt
ⓒ. They evaporate water quickly
ⓓ. They reduce adhesion between dirt particles
Correct Answer: They decrease surface tension, allowing water to penetrate grease and dirt
Explanation: By lowering surface tension, detergents enhance wetting of fabrics and utensils, loosening grease and making cleaning easier.
446. In surface chemistry, adsorption of gases on solids is influenced by:
ⓐ. Density of solid only
ⓑ. Surface energy of the solid surface
ⓒ. Gravitational acceleration
ⓓ. Magnetic properties of gases
Correct Answer: Surface energy of the solid surface
Explanation: Higher surface energy increases adsorption capacity. Molecules are attracted to minimize system energy, which is the basis of catalysts and adsorption materials.
447. Which of the following is an application of capillarity and surface tension in materials science?
ⓐ. Diffusion in metals
ⓑ. Liquid infiltration in porous ceramics
ⓒ. Electrical conductivity of alloys
ⓓ. Magnetic alignment of particles
Correct Answer: Liquid infiltration in porous ceramics
Explanation: Capillary action allows liquids to penetrate porous materials. This property is exploited in ceramic processing, fuel cells, and filtration membranes.
448. Why does paint spread better on a surface treated with primer?
ⓐ. Primer increases viscosity of paint
ⓑ. Primer reduces surface tension mismatch, improving wetting and adhesion
ⓒ. Primer decreases density of paint
ⓓ. Primer increases surface roughness only
Correct Answer: Primer reduces surface tension mismatch, improving wetting and adhesion
Explanation: A primer adjusts surface energy of a material, enabling uniform spreading and strong adhesion of paint, critical in materials engineering.
449. In nanotechnology, why is control of surface energy critical?
ⓐ. Because nanoparticles are free from surface effects
ⓑ. Because nanoparticles have very high surface area-to-volume ratio, making surface forces dominant
ⓒ. Because nanoparticles ignore intermolecular forces
ⓓ. Because nanoparticles always repel each other
Correct Answer: Because nanoparticles have very high surface area-to-volume ratio, making surface forces dominant
Explanation: At nanoscale, surface energy governs stability, reactivity, and aggregation of particles, making it crucial for drug delivery, catalysis, and material design.
450. Which of the following best summarizes the role of surface tension and surface energy in surface chemistry?
ⓐ. They only explain buoyancy of liquids
ⓑ. They determine interactions at interfaces, controlling wetting, spreading, adhesion, emulsification, and colloid stability
ⓒ. They eliminate viscosity effects in solutions
ⓓ. They explain only gravitational forces in fluids
Correct Answer: They determine interactions at interfaces, controlling wetting, spreading, adhesion, emulsification, and colloid stability
Explanation: In surface chemistry, surface tension and surface energy are key to understanding how phases interact. These principles are applied in detergents, emulsions, coatings, material science, and nanotechnology.
451. What is the angle of contact?
ⓐ. The angle formed between liquid surface and air surface
ⓑ. The angle between the tangent to the liquid surface at the point of contact and the solid surface inside the liquid
ⓒ. The angle between liquid molecules in bulk
ⓓ. The angle at which liquid evaporates from the surface
Correct Answer: The angle between the tangent to the liquid surface at the point of contact and the solid surface inside the liquid
Explanation: The angle of contact is defined at the junction of liquid, solid, and air. It indicates the balance between adhesive and cohesive forces and determines whether the liquid wets the solid.
452. For water in a clean glass tube, the angle of contact is approximately:
ⓐ. $0^\circ$
ⓑ. $30^\circ$
ⓒ. $90^\circ$
ⓓ. $120^\circ$
Correct Answer: $0^\circ$
Explanation: Water wets clean glass strongly due to greater adhesive forces than cohesive forces. Thus, the meniscus is concave, and the angle of contact is nearly $0^\circ$.
453. For mercury in a clean glass tube, the angle of contact is approximately:
ⓐ. $0^\circ$
ⓑ. $60^\circ$
ⓒ. $90^\circ$
ⓓ. $135^\circ$
Correct Answer: $135^\circ$
Explanation: Mercury does not wet glass because cohesive forces between mercury molecules dominate over adhesive forces with glass. Hence, the meniscus is convex, and the angle of contact is obtuse ($\sim135^\circ$).
454. Which of the following statements is TRUE regarding angle of contact?
ⓐ. Acute angle of contact means liquid wets the solid
ⓑ. Obtuse angle of contact means liquid wets the solid
ⓒ. Angle of contact has no relation to wetting
ⓓ. Angle of contact is always $90^\circ$
Correct Answer: Acute angle of contact means liquid wets the solid
Explanation: When adhesive forces > cohesive forces, the angle of contact is acute (<$90^\circ$), and the liquid wets the surface (e.g., water on glass).
455. Which formula is used to measure surface tension using capillary rise when angle of contact is considered?
ⓐ. $T = \frac{h \rho g r}{2 \cos \theta}$
ⓑ. $T = \frac{2 \cos \theta}{h \rho g r}$
ⓒ. $T = \frac{2T}{r}$
ⓓ. $T = \frac{h}{\rho g r}$
Correct Answer: $T = \frac{h \rho g r}{2 \cos \theta}$
Explanation: The balance of surface tension and weight of liquid column gives this formula, where $\theta$ is the angle of contact between the liquid and tube wall.
456. Which of the following liquids exhibits an obtuse angle of contact with glass?
ⓐ. Water
ⓑ. Alcohol
ⓒ. Mercury
ⓓ. Kerosene
Correct Answer: Mercury
Explanation: In mercury, cohesive forces dominate adhesive forces with glass, producing an obtuse angle of contact ($>90^\circ$), hence a convex meniscus.
457. How can angle of contact be experimentally measured?
ⓐ. Using a thermometer
ⓑ. By observing the shape of the meniscus in a capillary tube with a traveling microscope
ⓒ. Using a hydrometer
ⓓ. By measuring liquid density
Correct Answer: By observing the shape of the meniscus in a capillary tube with a traveling microscope
Explanation: The curvature of the meniscus provides the angle of contact, which can be measured accurately using optical instruments like a traveling microscope.
458. If adhesive forces between liquid and solid are stronger than cohesive forces, the angle of contact is:
ⓐ. $>90^\circ$
ⓑ. $=90^\circ$
ⓒ. $<90^\circ$
ⓓ. Independent of forces
Correct Answer: $<90^\circ$
Explanation: Stronger adhesion causes the liquid to spread along the solid surface, forming a concave meniscus and acute angle of contact.
459. If cohesive forces in a liquid are stronger than adhesive forces with the container, the liquid will:
ⓐ. Form a concave meniscus with acute angle of contact
ⓑ. Form a convex meniscus with obtuse angle of contact
ⓒ. Form a flat meniscus with $0^\circ$ angle
ⓓ. Not interact with the container
Correct Answer: Form a convex meniscus with obtuse angle of contact
Explanation: Dominant cohesive forces pull liquid molecules inward, curving the meniscus downward and creating an obtuse angle of contact.
460. Which statement best summarizes the importance of angle of contact?
ⓐ. It only defines surface tension
ⓑ. It determines whether a liquid wets a solid or not, and is crucial in capillarity and material coating processes
ⓒ. It applies only to mercury in glass
ⓓ. It has no role in practical applications
Correct Answer: It determines whether a liquid wets a solid or not, and is crucial in capillarity and material coating processes
Explanation: Angle of contact governs wetting properties of liquids, important in applications like detergents, ink flow, oil recovery, waterproofing, and coatings.
461. What is meant by a zero angle of contact?
ⓐ. The liquid surface makes an obtuse angle with the solid
ⓑ. The liquid completely spreads over the solid surface
ⓒ. The liquid forms a convex meniscus
ⓓ. The liquid does not wet the solid at all
Correct Answer: The liquid completely spreads over the solid surface
Explanation: A zero contact angle ($\theta = 0^\circ$) indicates perfect wetting, where adhesive forces are much stronger than cohesive forces. Water on clean glass nearly shows this behavior.
462. If the angle of contact $\theta = 0^\circ$, what will be the value of $\cos \theta$?
ⓐ. 0
ⓑ. 1
ⓒ. –1
ⓓ. Infinity
Correct Answer: 1
Explanation: Since $\cos 0^\circ = 1$, this condition maximizes capillary rise $h = \frac{2T \cos \theta}{\rho g r}$, leading to maximum upward movement of liquid in a capillary.
463. A liquid rises to a height $h$ in a capillary tube. If angle of contact is $0^\circ$, the expression becomes:
ⓐ. $h = \frac{2T}{\rho g r}$
ⓑ. $h = \frac{T}{\rho g r}$
ⓒ. $h = \frac{2T}{\rho g r \cos \theta}$
ⓓ. $h = \rho g h r$
Correct Answer: $h = \frac{2T}{\rho g r}$
Explanation: For $\theta = 0^\circ$, $\cos \theta = 1$. Thus, capillary rise is maximum, given by $h = \frac{2T}{\rho g r}$.
464. What type of meniscus is formed when angle of contact is acute ($< 90^\circ$)?
ⓐ. Flat meniscus
ⓑ. Convex meniscus
ⓒ. Concave meniscus
ⓓ. Irregular meniscus
Correct Answer: Concave meniscus
Explanation: In an acute angle, adhesive forces dominate, pulling liquid upward along the solid surface. This produces a concave meniscus, as with water in glass.
465. Mercury in a glass tube shows a convex meniscus because:
ⓐ. Its angle of contact is obtuse ($> 90^\circ$)
ⓑ. Its angle of contact is acute ($< 90^\circ$)
ⓒ. Its angle of contact is zero
ⓓ. Adhesive and cohesive forces are equal
Correct Answer: Its angle of contact is obtuse ($> 90^\circ$)
Explanation: For mercury, cohesive forces are stronger than adhesive forces, giving an angle of contact $\theta \approx 135^\circ$. Hence, the meniscus is convex.
466. If the angle of contact $\theta = 120^\circ$, what is the value of $\cos \theta$?
ⓐ. 0.5
ⓑ. –0.5
ⓒ. –0.866
ⓓ. –1
Correct Answer: –0.5
Explanation: For obtuse angles, cosine is negative. $\cos 120^\circ = -0.5$. Substituting this into the capillary rise formula gives negative rise, meaning depression of liquid.
467. A capillary tube of radius $0.5 \, mm$ is inserted in mercury ($T = 0.46 \, N/m, \, \theta = 135^\circ, \, \rho = 13,600 \, kg/m^3$). Find the height of mercury column.
ⓐ. 1.0 cm rise
ⓑ. 0.5 cm rise
ⓒ. 2.5 mm depression
ⓓ. 3.2 mm depression
Correct Answer: 3.2 mm depression
Explanation: Using $h = \frac{2T \cos \theta}{\rho g r}$. Here, $\cos 135^\circ = -0.707$. Substituting: $h = \frac{2 \times 0.46 \times -0.707}{13,600 \times 9.8 \times 0.0005} \approx -3.2 \, mm$. Negative sign shows depression.
468. If adhesive and cohesive forces are equal, what will be the angle of contact?
ⓐ. $0^\circ$
ⓑ. $45^\circ$
ⓒ. $90^\circ$
ⓓ. $180^\circ$
Correct Answer: $90^\circ$
Explanation: When adhesive and cohesive forces balance, the meniscus is flat. This corresponds to an angle of contact of $90^\circ$.
469. Which of the following correctly pairs liquid–solid systems with their angle of contact?
ⓐ. Water–glass: $135^\circ$
ⓑ. Mercury–glass: $0^\circ$
ⓒ. Water–glass: $0^\circ$
ⓓ. Mercury–glass: $60^\circ$
Correct Answer: Water–glass: $0^\circ$
Explanation: Water wets glass completely due to strong adhesive forces, so angle of contact is nearly $0^\circ$. Mercury–glass instead shows $\sim 135^\circ$.
470. Which statement best summarizes the types of contact angles?
ⓐ. Zero angle → perfect wetting, acute angle → partial wetting, obtuse angle → non-wetting (capillary depression)
ⓑ. Zero angle → non-wetting, obtuse angle → perfect wetting, acute angle → neutral wetting
ⓒ. Zero, acute, and obtuse angles all indicate same wetting condition
ⓓ. Types of contact angle apply only to mercury in glass
Correct Answer: Zero angle → perfect wetting, acute angle → partial wetting, obtuse angle → non-wetting (capillary depression)
Explanation: Zero angle means complete wetting (e.g., water on clean glass). Acute (<90°) means partial wetting (concave meniscus). Obtuse (>90°) means poor wetting (convex meniscus, like mercury in glass).
471. Why does water spread easily on a clean glass surface?
ⓐ. Because cohesive forces are greater than adhesive forces
ⓑ. Because adhesive forces between water and glass are greater than cohesive forces of water molecules
ⓒ. Because surface tension of glass is zero
ⓓ. Because glass repels water molecules
Correct Answer: Because adhesive forces between water and glass are greater than cohesive forces of water molecules
Explanation: Strong adhesion causes water to wet glass and spread out, forming a concave meniscus with nearly zero contact angle.
472. Why does mercury not spread on glass but form spherical drops instead?
ⓐ. Because glass is hydrophilic
ⓑ. Because cohesive forces in mercury are stronger than adhesive forces with glass
ⓒ. Because mercury has no surface tension
ⓓ. Because glass density is too high
Correct Answer: Because cohesive forces in mercury are stronger than adhesive forces with glass
Explanation: Mercury molecules bind more strongly to each other than to glass, resulting in poor wetting, convex meniscus, and obtuse angle of contact.
473. Which of the following conditions favors good wetting of a solid by a liquid?
ⓐ. Adhesive forces < cohesive forces
ⓑ. Adhesive forces > cohesive forces
ⓒ. Adhesive forces = cohesive forces
ⓓ. No molecular interaction at all
Correct Answer: Adhesive forces > cohesive forces
Explanation: Stronger adhesion causes liquid molecules to cling to the solid surface, spreading out over it. This is desirable in coating, painting, and lubrication.
474. Which of the following daily-life examples illustrates poor wetting?
ⓐ. Rainwater spreading on windshield with detergent
ⓑ. Mercury drops on glass
ⓒ. Ink spreading in blotting paper
ⓓ. Kerosene rising in lamp wick
Correct Answer: Mercury drops on glass
Explanation: Mercury shows poor wetting of glass due to its high cohesion. Drops remain spherical with large contact angle.
475. The spreading coefficient $S$ of a liquid on a solid is defined as:
ⓐ. $S = \gamma_{SV} + \gamma_{LV} – \gamma_{SL}$
ⓑ. $S = \gamma_{SV} – \gamma_{LV} + \gamma_{SL}$
ⓒ. $S = \gamma_{LV} – \gamma_{SL}$
ⓓ. $S = \gamma_{SV} + \gamma_{SL}$
Correct Answer: $S = \gamma_{SV} + \gamma_{LV} – \gamma_{SL}$
Explanation: Here, $\gamma_{SV}$ = solid–vapor surface energy, $\gamma_{LV}$ = liquid–vapor surface tension, $\gamma_{SL}$ = solid–liquid interfacial energy. If $S > 0$, the liquid spreads completely.
476. Which application of wetting is used in detergents and soaps?
ⓐ. Increasing viscosity of water
ⓑ. Decreasing surface tension to allow water to spread and penetrate fabric
ⓒ. Reducing evaporation rate of water
ⓓ. Making water more cohesive
Correct Answer: Decreasing surface tension to allow water to spread and penetrate fabric
Explanation: Detergents lower water’s surface tension, enabling it to wet greasy and fibrous surfaces effectively, thus aiding cleaning.
477. Why is waterproof coating applied on fabrics?
ⓐ. To increase adhesive forces with water
ⓑ. To decrease adhesive forces with water so that water cannot spread
ⓒ. To increase the density of fabric
ⓓ. To increase surface tension of water
Correct Answer: To decrease adhesive forces with water so that water cannot spread
Explanation: Hydrophobic coatings reduce wetting by increasing the contact angle (>90°). Water beads up and rolls off instead of spreading.
478. Why does oil spread easily on water surface?
ⓐ. Because density of oil is less than water
ⓑ. Because adhesive forces between oil and water are greater than oil’s cohesive forces
ⓒ. Because oil has lower surface tension than water
ⓓ. Both B and C
Correct Answer: Both B and C
Explanation: Oil’s lower surface tension and strong adhesive interaction with water molecules make it spread into a thin film on water surfaces.
479. Which of the following is a technological application of wetting and spreading phenomena?
ⓐ. Lubrication of machine parts by oils
ⓑ. Condensation of steam on cooled surfaces
ⓒ. Coating of metals with paint or protective films
ⓓ. All of the above
Correct Answer: All of the above
Explanation: Wetting is essential in lubrication (oil spreading on metal), condensation (water film formation), and coatings (paint spreading). It ensures efficiency and durability in engineering.
480. Which statement best summarizes the applications of wetting and spreading?
ⓐ. They are useful only in physics experiments
ⓑ. They explain important natural and industrial processes such as detergent action, lubrication, waterproofing, coatings, and oil spreading on water
ⓒ. They only apply to mercury and glass
ⓓ. They only occur at absolute zero temperature
Correct Answer: They explain important natural and industrial processes such as detergent action, lubrication, waterproofing, coatings, and oil spreading on water
Explanation: Wetting and spreading phenomena are central to everyday life and materials science. Their control is applied in detergents, textile treatments, surface coatings, lubrication, and even pollution studies (oil films on water).
481. Why do liquid drops tend to form spherical shapes in the absence of external forces?
ⓐ. Because gravity pulls equally in all directions
ⓑ. Because surface tension minimizes surface area for a given volume
ⓒ. Because adhesive forces are stronger than cohesive forces
ⓓ. Because pressure inside the drop is zero
Correct Answer: Because surface tension minimizes surface area for a given volume
Explanation: A sphere has the minimum surface area for a given volume. Surface tension acts like a contractive skin, pulling molecules inward and making droplets spherical.
482. The excess pressure inside a spherical liquid drop of radius $r$ is given by:
ⓐ. $\Delta P = \frac{T}{r}$
ⓑ. $\Delta P = \frac{2T}{r}$
ⓒ. $\Delta P = \frac{3T}{r}$
ⓓ. $\Delta P = \frac{4T}{r}$
Correct Answer: $\Delta P = \frac{2T}{r}$
Explanation: The pressure inside a drop exceeds outside pressure due to surface tension. For a single surface (liquid drop), the relation is $\Delta P = \frac{2T}{r}$.
483. The excess pressure inside a soap bubble of radius $r$ is:
ⓐ. $\Delta P = \frac{T}{r}$
ⓑ. $\Delta P = \frac{2T}{r}$
ⓒ. $\Delta P = \frac{3T}{r}$
ⓓ. $\Delta P = \frac{4T}{r}$
Correct Answer: $\Delta P = \frac{4T}{r}$
Explanation: A soap bubble has two surfaces (inner and outer). Each contributes $2T/r$, so the total excess pressure is $\Delta P = 4T/r$.
484. A water droplet of radius $0.001 \, m$ has surface tension $0.072 \, N/m$. Find the excess pressure inside the drop.
ⓐ. $72 \, Pa$
ⓑ. $144 \, Pa$
ⓒ. $288 \, Pa$
ⓓ. $720 \, Pa$
Correct Answer: $144 \, Pa$
Explanation: $\Delta P = \frac{2T}{r} = \frac{2 \times 0.072}{0.001} = 144 \, Pa$.
485. A soap bubble of radius $2 \, mm$ has surface tension $0.04 \, N/m$. Find the excess pressure inside it.
ⓐ. $20 \, Pa$
ⓑ. $40 \, Pa$
ⓒ. $80 \, Pa$
ⓓ. $160 \, Pa$
Correct Answer: $160 \, Pa$
Explanation: $\Delta P = \frac{4T}{r} = \frac{4 \times 0.04}{0.002} = 80 \, Pa$. Correction: Actually $\Delta P = 80 \, Pa$, so correct answer is C.
486. Why are small soap bubbles less stable than larger ones?
ⓐ. Because they contain less air
ⓑ. Because they have higher internal pressure due to smaller radius
ⓒ. Because surface tension vanishes at small radius
ⓓ. Because their density decreases
Correct Answer: Because they have higher internal pressure due to smaller radius
Explanation: From $\Delta P = \frac{4T}{r}$, smaller radius gives higher excess pressure. This makes small bubbles unstable, causing them to collapse or merge.
487. Two soap bubbles of radii $r_1$ and $r_2$ are connected by a tube. Air flows from:
ⓐ. Larger bubble to smaller bubble
ⓑ. Smaller bubble to larger bubble
ⓒ. Both directions equally
ⓓ. No flow occurs
Correct Answer: Smaller bubble to larger bubble
Explanation: Smaller bubbles have higher internal pressure ($\Delta P \propto 1/r$). Thus, air flows from high-pressure smaller bubble to low-pressure larger bubble.
488. Which factor does NOT affect the stability of a soap bubble?
ⓐ. Surface tension of liquid
ⓑ. Radius of bubble
ⓒ. Atmospheric pressure outside
ⓓ. Color of soap film
Correct Answer: Color of soap film
Explanation: Bubble stability depends on pressure difference and surface tension. Film color is due to light interference and has no role in stability.
489. Why does adding soap to water stabilize bubbles?
ⓐ. It increases water’s density
ⓑ. It decreases water’s surface tension and makes thin films more elastic
ⓒ. It increases viscosity infinitely
ⓓ. It eliminates air inside bubbles
Correct Answer: It decreases water’s surface tension and makes thin films more elastic
Explanation: Soap molecules reduce surface tension, allowing bubbles to form more easily and last longer by preventing rupture.
490. Which of the following best summarizes the role of surface tension in drops and bubbles?
ⓐ. It decreases density of liquids
ⓑ. It controls shape, pressure difference, and stability of drops and bubbles
ⓒ. It increases viscosity of the liquid
ⓓ. It eliminates cohesive forces in molecules
Correct Answer: It controls shape, pressure difference, and stability of drops and bubbles
Explanation: Surface tension tends to minimize surface area, creating spherical drops and bubbles. It also causes internal excess pressure that governs their stability, growth, and collapse.
491. Why do rain drops appear spherical while falling through air?
ⓐ. Gravity pulls equally in all directions
ⓑ. Surface tension minimizes surface area for a given volume, leading to a spherical shape
ⓒ. Air pressure shapes the drop into a sphere
ⓓ. Density of water forces spherical geometry
Correct Answer: Surface tension minimizes surface area for a given volume, leading to a spherical shape
Explanation: A sphere has the smallest surface area for a fixed volume. Surface tension acts to reduce surface energy, so drops in the absence of strong external forces form spheres.
492. Why do large raindrops deviate from perfect spherical shape as they fall?
ⓐ. Because surface tension increases with drop size
ⓑ. Because air resistance distorts the drop, flattening its bottom
ⓒ. Because adhesive forces with air molecules increase
ⓓ. Because gravity becomes weaker for large drops
Correct Answer: Because air resistance distorts the drop, flattening its bottom
Explanation: For small drops, surface tension dominates and keeps them spherical. For larger drops, air drag distorts the shape, leading to a flattened or parachute-like form.
493. Which of the following best explains why soap bubbles are spherical?
ⓐ. Pressure outside is uniform
ⓑ. Gravity is absent inside bubbles
ⓒ. Surface tension pulls equally in all directions, minimizing surface energy
ⓓ. Cohesion is zero in soap films
Correct Answer: Surface tension pulls equally in all directions, minimizing surface energy
Explanation: The thin film of soap bubble is under uniform tension. The equal inward pull of surface tension minimizes area, giving a spherical shape.
494. The excess pressure inside a spherical droplet of radius $r$ due to surface tension is given by:
ⓐ. $\Delta P = \frac{T}{r}$
ⓑ. $\Delta P = \frac{2T}{r}$
ⓒ. $\Delta P = \frac{3T}{r}$
ⓓ. $\Delta P = \frac{4T}{r}$
Correct Answer: $\Delta P = \frac{2T}{r}$
Explanation: A droplet has one liquid–air interface. Surface tension creates an inward pull that increases internal pressure by $\frac{2T}{r}$.
495. The excess pressure inside a soap bubble of radius $r$ is:
ⓐ. $\Delta P = \frac{2T}{r}$
ⓑ. $\Delta P = \frac{3T}{r}$
ⓒ. $\Delta P = \frac{4T}{r}$
ⓓ. $\Delta P = \frac{T}{r}$
Correct Answer: $\Delta P = \frac{4T}{r}$
Explanation: A soap bubble has two liquid–air interfaces (inside and outside). Each contributes $\frac{2T}{r}$, so total is $\frac{4T}{r}$.
496. A soap bubble in air is always spherical because:
ⓐ. Atmospheric pressure is zero
ⓑ. Surface tension acts tangentially to minimize surface energy
ⓒ. Cohesion vanishes in bubbles
ⓓ. Adhesion dominates over cohesion
Correct Answer: Surface tension acts tangentially to minimize surface energy
Explanation: The liquid film is pulled equally inward in all directions by surface tension, so the bubble assumes a spherical shape.
497. A droplet of radius $1 \, mm$ has excess pressure $\Delta P = 150 \, Pa$. If the radius is halved, what will be the excess pressure?
ⓐ. 75 Pa
ⓑ. 150 Pa
ⓒ. 300 Pa
ⓓ. 600 Pa
Correct Answer: 300 Pa
Explanation: For a drop, $\Delta P \propto \frac{1}{r}$. Halving $r$ doubles excess pressure. Hence, $150 \times 2 = 300 \, Pa$.
498. Which of the following is NOT true about the shape of liquid drops and bubbles?
ⓐ. Small drops are more spherical than large ones
ⓑ. Large drops get distorted due to air resistance
ⓒ. Bubbles in zero gravity still form spherical shapes
ⓓ. Shape of drops is independent of surface tension
Correct Answer: Shape of drops is independent of surface tension
Explanation: Surface tension is the dominant factor in determining droplet shape. Without it, drops would not be spherical.
499. Why does adding soap make water bubbles more stable and colorful?
ⓐ. Soap increases water density
ⓑ. Soap reduces surface tension, allowing larger bubbles with thinner films
ⓒ. Soap increases gravitational force inside bubbles
ⓓ. Soap removes adhesion between water molecules
Correct Answer: Soap reduces surface tension, allowing larger bubbles with thinner films
Explanation: Lower surface tension makes it easier to blow big bubbles. Thin soap films also produce light interference, giving colorful patterns.
500. Which statement best summarizes the effect of surface tension on drop and bubble shape?
ⓐ. Surface tension destroys droplet stability
ⓑ. Surface tension minimizes surface area, leading to spherical shapes; external forces can distort larger drops or bubbles
ⓒ. Surface tension only affects solids, not liquids
ⓓ. Surface tension maximizes surface area of liquids
Correct Answer: Surface tension minimizes surface area, leading to spherical shapes; external forces can distort larger drops or bubbles
Explanation: Surface tension acts like a skin, pulling drops and bubbles into spheres. For small drops, this dominates. For larger ones, external forces like air drag compete, causing distortions.
The fifth section of Thermal Properties of Matter focuses on advanced and tricky problems that combine multiple concepts. According to the NCERT/CBSE Class 11 syllabus, this section includes questions from latent heat, energy conservation in phase changes, conduction through composite rods, convection in daily life, and radiation balance in nature. These MCQs prepare students for higher-order problem-solving which is often required in board exams and national-level competitive exams like JEE and NEET. Out of the complete set of 600 MCQs, this part provides another 100 solved questions with step-by-step explanations for practice and self-assessment.
- 👉 Total MCQs in this chapter: 600.
- 👉 This page contains: Fifth set of 100 solved MCQs with answers.
- 👉 Includes mixed-concept problems for JEE/NEET aspirants.
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- 👉 To finish the chapter, click the Part 6 button above.