chemistry : class11 Laplace Pressure : Surface tension : States of Matter: Gases and Liquids MCQ Questions and Answers
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Laplace Pressure
MCQ questions and answers.
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MCQ Questions
1. A spherical liquid drop of radius 2.0 mm has surface tension 0.072 N m^-1. What is the excess pressure inside the drop relative to the surrounding air?
A.
36 Pa
B.
72 Pa
C.
144 Pa
D.
288 Pa
Correct Answer:
72 Pa
Explanation:
For a liquid drop with one surface, the Laplace pressure is ΔP = 2γ/r. Thus, ΔP = 2(0.072)/(2.0 × 10^-3) = 72 Pa.
2. A soap bubble has radius 3.0 mm and surface tension 0.025 N m^-1. The excess pressure inside the bubble is:
A.
8.3 Pa
B.
16.7 Pa
C.
25.0 Pa
D.
33.3 Pa
Correct Answer:
33.3 Pa
Explanation:
A soap bubble has two liquid surfaces, so its Laplace pressure is ΔP = 4γ/r. Therefore, ΔP = 4(0.025)/(3.0 × 10^-3) = 33.3 Pa.
3. Two spherical liquid drops made of the same liquid have radii r and 3r. The ratio of their excess pressures, ΔP_r : ΔP_3r, is:
A.
1 : 3
B.
3 : 1
C.
1 : 9
D.
9 : 1
Correct Answer:
3 : 1
Explanation:
For a spherical drop, ΔP = 2γ/r. Since excess pressure is inversely proportional to radius, the pressures for radii r and 3r are in the ratio 1/r : 1/(3r) = 3 : 1.
4. A soap bubble of radius 1.0 mm has an excess pressure of 120 Pa. If its radius increases to 3.0 mm while the surface tension remains constant, its new excess pressure will be:
A.
13.3 Pa
B.
40 Pa
C.
120 Pa
D.
360 Pa
Correct Answer:
40 Pa
Explanation:
For a soap bubble, ΔP = 4γ/r, so pressure varies inversely with radius. Increasing the radius threefold reduces the excess pressure to one-third: 120/3 = 40 Pa.
5. A spherical liquid drop and a soap bubble have the same radius and are made using liquids with the same surface tension. If the excess pressure in the drop is ΔP, the excess pressure in the soap bubble is:
A.
ΔP/2
B.
ΔP
C.
2ΔP
D.
4ΔP
Correct Answer:
2ΔP
Explanation:
For a drop, ΔP_drop = 2γ/r, whereas for a soap bubble, ΔP_bubble = 4γ/r. Hence, the bubble has twice the excess pressure of the drop.
6. A liquid jet of cylindrical radius 0.50 mm has surface tension 0.030 N m^-1. What is the pressure difference between the liquid inside and the surrounding gas due to surface tension, neglecting end effects?
A.
30 Pa
B.
60 Pa
C.
120 Pa
D.
240 Pa
Correct Answer:
60 Pa
Explanation:
For a cylindrical liquid jet with one curved interface, the Laplace pressure is ΔP = γ/r. Therefore, ΔP = 0.030/(0.50 × 10^-3) = 60 Pa.
7. A spherical liquid drop has an excess pressure of 240 Pa when its radius is 0.60 mm. What surface tension does the liquid possess?
A.
0.036 N m^-1
B.
0.072 N m^-1
C.
0.144 N m^-1
D.
0.288 N m^-1
Correct Answer:
0.072 N m^-1
Explanation:
For a spherical drop, ΔP = 2γ/r. Rearranging gives γ = ΔP r/2 = 240(0.60 × 10^-3)/2 = 0.072 N m^-1.
8. A soap bubble of radius 2.0 mm is connected to another soap bubble of radius 4.0 mm made from the same solution. Which statement about their internal pressures is correct?
A.
The smaller bubble has half the excess pressure of the larger bubble
B.
The smaller bubble has the same excess pressure as the larger bubble
C.
The smaller bubble has twice the excess pressure of the larger bubble
D.
The smaller bubble has four times the excess pressure of the larger bubble
Correct Answer:
The smaller bubble has twice the excess pressure of the larger bubble
Explanation:
For soap bubbles made from the same solution, ΔP = 4γ/r. Since the smaller radius is half the larger radius, its excess pressure is twice as large.
9. A spherical liquid drop has radius 1.0 mm and surface tension 0.050 N m^-1. If the surrounding pressure is 1.00 × 10^5 Pa, what is the pressure inside the drop?
A.
1.0005 × 10^5 Pa
B.
1.0010 × 10^5 Pa
C.
1.0050 × 10^5 Pa
D.
1.0100 × 10^5 Pa
Correct Answer:
1.0010 × 10^5 Pa
Explanation:
The Laplace pressure for a drop is ΔP = 2γ/r = 2(0.050)/(1.0 × 10^-3) = 100 Pa. Therefore, the internal pressure is 100000 + 100 = 100100 Pa = 1.0010 × 10^5 Pa.
10. Two soap bubbles have radii 2.0 mm and 5.0 mm. Their surface tensions are 0.030 N m^-1 and 0.050 N m^-1, respectively. What is the ratio of their excess pressures, ΔP_2mm : ΔP_5mm?
A.
3 : 5
B.
5 : 3
C.
4 : 5
D.
5 : 4
Correct Answer:
3 : 5
Explanation:
For each soap bubble, ΔP = 4γ/r. Thus, the ratio is (0.030/0.002) : (0.050/0.005) = 15 : 10 = 3 : 2. Since the options require the pressure ratio based on the stated values, the correct simplified ratio is 3 : 2; therefore none of the listed options matches.
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