Surface tension T = (r·h·ρ·g)/2 where r = capillary radius, h = liquid rise, ρ = density, g = gravity. Travelling microscope reads the meniscus height. Detergent reduces T sharply (lowered capillary rise) — demonstrates surfactant action. Account for the meniscus correction r/3 added to h for narrow tubes.
-- NCERT Physics Lab Manual Class 11, Part 6, p. 95Experiment 06 Surface Tension
8 MCQs1 revision card9-step worked example
Source: NCERT Experimental SkillsOfficial key: NTA-verifiedLast updated: 24 Sep 2026
Experiment 06 Surface Tension, explained for NEET
The reading that decides this experiment is the capillary's radius, not its diameter. A travelling microscope gives you the bore diameter; halving it is a step aspirants skip under time pressure, and the reported surface tension then comes out exactly twice the true value — a plausible-looking wrong number that no sanity check catches.
The experiment itself is short. A clean glass capillary is dipped vertically in water. Water wets glass, so the meniscus is concave and the liquid climbs until the upward pull of surface tension around the circle of contact balances the weight of the raised column. With the angle of contact for clean water on clean glass taken as nearly zero, that balance gives
T = r h ρ g / 2
where r is the internal radius of the bore, h the rise measured from the flat outside water level to the bottom of the meniscus, ρ the density of water and g the acceleration due to gravity. This is the working relation set out in the NCERT Physics Lab Manual Class 11, Part 6, page 95.
The detergent half of the experiment is the part NEET likes. A detergent is surface-active: its molecules crowd into the water surface and weaken the cohesive pull there. Surface tension falls, and with r, ρ and g unchanged, h falls in the same proportion. Lower rise is the evidence of lower surface tension, not a separate effect.
For the error question, note how T depends on its inputs. It is first power in r and first power in h, so the two percentage errors simply add — neither is squared, and neither can be dropped because it looks small.
Watch-out: h is measured to the bottom of the concave meniscus, and the tube must be clean. A greasy bore changes the angle of contact, and every number after that is wrong for a reason your arithmetic will never reveal.
Can you answer these Experiment 06 Surface Tension MCQs?
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
MCQ 1Easy RecallPractice
The SI unit of surface tension is:
Show answer and why every option is right or wrong
Answer: B. B is correct: surface tension is force per unit length of the contact line, so its SI unit is N m⁻¹ (equivalently J m⁻², surface energy per unit area), as stated in the NCERT Physics Lab Manual Class 11, Part 6, page 95.
Why A is wrong: A is wrong: N m⁻² is force per unit area, which is pressure — the excess pressure across a curved surface, not the surface tension itself.
Why C is wrong: C is wrong: N m is the unit of torque or of work; surface tension is a force divided by a length, not multiplied by one.
Why D is wrong: D is wrong: N m⁻³ is a force per unit volume, which is not what acts along the circle of contact in a capillary.
MCQ 2Easy RecallPractice
A little detergent is dissolved in the water used in the capillary-rise experiment. Compared with pure water in the same clean tube, the result is:
Show answer and why every option is right or wrong
Answer: D. D is correct: a detergent is surface-active, collecting at the water surface and weakening the pull there, so T falls and with r, ρ and g fixed the rise h falls in proportion — the effect the second half of this experiment is designed to show (NCERT Physics Lab Manual Class 11, Part 6, page 95).
Why A is wrong: A is wrong: it reverses the effect. Detergents lower surface tension; that is precisely why they wet and penetrate fabric rather than beading on it.
Why B is wrong: B is wrong: it confuses two properties. Surface tension belongs to the surface layer, and adding a surface-active substance changes it even when the bulk density barely moves.
Why C is wrong: C is wrong on both halves: T does not rise, and if T changed while r, ρ and g stayed fixed the rise would have to change with it.
MCQ 3Easy RecallPractice
In deriving T = r h ρ g / 2 for water in a clean glass capillary, the angle of contact is taken to be:
Show answer and why every option is right or wrong
Answer: A. A is correct: water wets clean glass, the meniscus is concave, and the contact angle is close to zero, so cos θ ≈ 1 and the general relation h = 2T cos θ / (r ρ g) collapses to the working form used here (NCERT Physics Lab Manual Class 11, Part 6, page 95).
Why B is wrong: B is wrong: at exactly 90° cos θ = 0 and there would be no rise at all — the liquid in the tube would stay level with the outside.
Why C is wrong: C is wrong: an angle near 180° describes mercury on glass, which is non-wetting and gives a convex meniscus and a depression, not a rise.
Why D is wrong: D is wrong: 45° is not a physical value here; the contact angle is set by the liquid–solid pair, not by averaging two limits.
MCQ 4Direct ApplicationPractice
Water rises to a height of 3.0 × 10⁻² m in a clean capillary of internal radius 5.0 × 10⁻⁴ m. Taking the density of water as 1.0 × 10³ kg m⁻³ and g = 9.8 m s⁻², the surface tension of water is closest to:
Show answer and why every option is right or wrong
Answer: C. C is correct: T = r h ρ g / 2 = (5.0 × 10⁻⁴ × 3.0 × 10⁻² × 1.0 × 10³ × 9.8) / 2 = 7.4 × 10⁻² N m⁻¹, the expected order for water at room temperature (NCERT Physics Lab Manual Class 11, Part 6, page 95).
Why A is wrong: A is wrong: it is what you get by dividing by 2 a second time, or by using half the given radius — the factor of 2 belongs in the formula once only.
Why B is wrong: B is wrong: it is r h ρ g with the division by 2 omitted, giving exactly double the correct value.
Why D is wrong: D is wrong by a factor of ten — a decimal slip when the powers of ten are multiplied out rather than collected first.
MCQ 5Direct ApplicationPractice
In a capillary-rise determination, the bore radius is measured to within 1.0% and the rise to within 2.0%, while density and g are treated as exact. The maximum percentage error in the calculated surface tension is:
Show answer and why every option is right or wrong
Answer: B. B is correct: in T = r h ρ g / 2 both r and h appear to the first power, so the maximum fractional error is the sum |1|(1.0%) + |1|(2.0%) = 3.0% (NCERT Physics Lab Manual Class 11, Part 6, page 95).
Why A is wrong: A is wrong: it keeps only the radius error and discards the rise error, but no independent measurement in a product may be dropped from a maximum-error estimate.
Why C is wrong: C is wrong: it keeps only the larger of the two errors. Worst-case combination adds them; it does not select the biggest.
Why D is wrong: D is wrong: it averages the two percentage errors. Fractional errors in a product add — they are never averaged.
MCQ 6Direct ApplicationPractice
The same water is used with a second clean capillary whose internal radius is half that of the first. The height to which the water rises becomes:
Show answer and why every option is right or wrong
Answer: D. D is correct: rearranging gives h = 2T / (r ρ g), so with T, ρ and g fixed h varies as 1/r — halving the radius doubles the rise (NCERT Physics Lab Manual Class 11, Part 6, page 95).
Why A is wrong: A is wrong: it makes h proportional to r. The radius sits in the denominator of h = 2T / (r ρ g), so a narrower tube lifts water further, not less far.
Why B is wrong: B is wrong: T is indeed unchanged, but h is not T — it also depends on the tube, which is exactly why a narrow bore is chosen for a measurable rise.
Why C is wrong: C is wrong: it treats h as varying with 1/r², as though only the column weight scaled. The contact length and the cross-section both change, leaving a single power of r.
MCQ 7CalculationPractice
Water rises 4.0 × 10⁻² m in a clean capillary. Detergent is then added, reducing the surface tension to 0.60 of its original value, and the same tube is used again at the same temperature. The new rise is:
Show answer and why every option is right or wrong
Answer: A. A is correct: h = 2T / (r ρ g), so with r, ρ and g unchanged h ∝ T, giving h' = 0.60 × 4.0 × 10⁻² m = 2.4 × 10⁻² m — the lowered rise that demonstrates the detergent effect (NCERT Physics Lab Manual Class 11, Part 6, page 95).
Why B is wrong: B is wrong: it divides by 0.60 instead of multiplying, which would mean a detergent makes water climb higher — the opposite of what the experiment shows.
Why C is wrong: C is wrong: it assumes the rise is fixed by the tube alone. The tube fixes r, but h also carries T, and T is what the detergent changed.
Why D is wrong: D is wrong: it subtracts 0.60 of the original height, treating the change as a 60% reduction instead of a scaling to 0.60 of the value.
MCQ 8CalculationPractice
For a given clean capillary and clean water, the calculated equilibrium rise is 5.0 × 10⁻² m, but the tube available projects only 3.0 × 10⁻² m above the water surface. When the tube is dipped, the water:
Show answer and why every option is right or wrong
Answer: C. C is correct: the meniscus radius of curvature adjusts to whatever value balances the column actually present, so the water reaches the top and the surface flattens instead of overflowing (NCERT Physics Lab Manual Class 11, Part 6, page 95).
Why A is wrong: A is wrong: overflow would be a perpetual flow with no energy supplied. Surface tension cannot pump water out of the tube.
Why B is wrong: B is wrong: a shorter tube does not weaken the pull. Nothing in h = 2T / (r ρ g) refers to the tube's length, so the water is not held back below the top.
Why D is wrong: D is wrong: it accepts the impossible overflow and then invents a settling height greater than the projecting tube itself, which the tube cannot contain.
Free NEET study resources
Get a structured 30-day Mechanics plan and a complete formula booklet — delivered to your inbox instantly.
Experiment 06 Surface Tension: quick recall before you leave
How do you solve a Experiment 06 Surface Tension question? A worked example
- 1
Given.
A clean glass capillary is dipped vertically in water. Internal radius of the bore r = 5.0 × 10⁻⁴ m, measured to within 1.0%. Rise of water h = 3.0 × 10⁻² m, measured to within 2.0%. Density of water ρ = 1.0 × 10³ kg m⁻³ (exact, problem-defined). g = 9.8 m s⁻² (exact, problem-defined). Angle of contact taken as nearly zero.
- 2
Required.
The surface tension of water, and the maximum percentage error in that value.
- 3
Concept.
At equilibrium the upward component of the surface-tension force acting around the circle of contact supports the weight of the raised liquid column. Because r and h are each measured quantities entering to the first power, their fractional errors combine additively in a worst-case estimate.
- 4
Formula.
T = r h ρ g / 2 (with cos θ ≈ 1), and ΔT/T = |1|(Δr/r) + |1|(Δh/h).
- 5
Substitution.
T = (5.0 × 10⁻⁴ m × 3.0 × 10⁻² m × 1.0 × 10³ kg m⁻³ × 9.8 m s⁻²) / 2. Error: ΔT/T = 1.0% + 2.0%.
- 6
Calculation.
5.0 × 10⁻⁴ × 3.0 × 10⁻² = 1.5 × 10⁻⁵; × 1.0 × 10³ = 1.5 × 10⁻²; × 9.8 = 1.47 × 10⁻¹. Divide by 2: 7.35 × 10⁻² N m⁻¹. Error: 1.0% + 2.0% = 3.0%. The 2 in the denominator is a counting factor from the geometry, and ρ and g are problem-defined exact values; none of the three contributes to the significant-figure count or to the error sum.
- 7
Final answer.
T = 7.4 × 10⁻² N m⁻¹, with a maximum error of 3.0% (about ± 0.2 × 10⁻² N m⁻¹). Both measured inputs carry two significant figures, so the answer is reported to two.
- 8
Common trap.
Feeding the microscope's bore diameter into the formula in place of the radius. It doubles T silently, to a value that still looks like a physical surface tension. Halve the diameter the moment it is read, and label it r before it goes anywhere near the calculation.
- 9
Similar NEET-style question.
The same tube and the same water are used after a detergent is dissolved, and the rise drops to 1.8 × 10⁻² m. Find the new surface tension, and state without recalculating why its percentage error is still 3.0%.
What to remember before solving Experiment 06 Surface Tension questions
Which Experiment 06 Surface Tension formulas do you need for NEET?
1 formula — click to collapse
Percentage error in derived quantity
Combines fractional errors of independent measurements. Adds in absolute value (worst-case).
| Symbol | Quantity | SI Unit |
|---|---|---|
| Z | derived quantity | - |
| x_i | measured | - |
| n_i | exponent | - |
Valid when
- Independent measurements
- Maximum-error analysis
More in Experimental Skills: 6 exam traps and mistakes · 2 formulas · 1 question pattern from its other lessons.
Experiment 06 Surface Tension questions from past NEET papers
No question in our NEET 2020–2025 set targets this topic directly.
Sources
Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.
Test yourself on this topic with real past-paper questions:
Practice this topic →