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General ScienceEasy

A hot air balloon rises because warm air

  1. A.Becomes denser than the surrounding air
  2. B.Expands and becomes less dense than the surrounding air
  3. C.Loses its mass on heating
  4. D.Has a lower specific heat than cold air

Correct answer

B. Expands and becomes less dense than the surrounding air

Explanation

The correct answer is B, expands and becomes less dense than the surrounding air. By Charles's law the volume of a fixed mass of gas at constant pressure is proportional to its absolute temperature, so heating the air inside the envelope makes it expand; some of it escapes from the open mouth, the mass of air left inside falls, its density becomes lower than that of the outside air, and the buoyant force of Archimedes lifts the balloon. A, becomes denser is the opposite of what happens, and a denser gas would sink. C, loses its mass on heating is wrong as a reason: mass is lost only because heated air spills out of the envelope, and heat by itself does not destroy mass. D, has a lower specific heat is irrelevant to buoyancy, which depends on density alone. The same law explains why a balloon shrinks in cold weather.

Read the full article: Gas Laws and Kinetic Theory of Gases: Formulas and PYQs

Practice Questions

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Q1.General ScienceEasy

Boyle's law states that at constant temperature, the volume of a fixed mass of gas is

  1. A.Directly proportional to its pressure
  2. B.Inversely proportional to its pressure
  3. C.Independent of its pressure
  4. D.Proportional to the square of its pressure
Show answer
Correct answer: B. Inversely proportional to its pressure

Boyle's law, established in 1662, says that for a fixed mass of gas at constant temperature the product of pressure and volume stays constant, so doubling the pressure halves the volume.

Q2.General ScienceEasy

Absolute zero of temperature is equal to

  1. A.0 degrees Celsius
  2. B.-100 degrees Celsius
  3. C.-273.15 degrees Celsius
  4. D.-373.15 degrees Celsius
Show answer
Correct answer: C. -273.15 degrees Celsius

Extending the straight line of Charles's law backwards shows that the volume of an ideal gas would become zero at minus 273.15 degrees Celsius, and this point is taken as the zero of the kelvin or absolute scale, at which molecular motion carries its least possible energy.

Q4.General ScienceMedium

The volume occupied by one mole of an ideal gas at 273.15 K and one atmosphere pressure is about

  1. A.11.2 litres
  2. B.22.4 litres
  3. C.44.8 litres
  4. D.1 litre
Show answer
Correct answer: B. 22.4 litres

Substituting one mole, 273.15 kelvin and one atmosphere in the ideal gas equation gives a volume of 22.4 litres, the molar volume at standard temperature and pressure, and this is the figure used in mole calculations; with the modern standard pressure of one bar the value works out to about…

Q5.General ScienceMedium

According to Graham's law, the rate of diffusion of a gas is inversely proportional to the

  1. A.Square of its molar mass
  2. B.Square root of its molar mass
  3. C.Cube root of its molar mass
  4. D.Absolute temperature
Show answer
Correct answer: B. Square root of its molar mass

Graham's law of diffusion states that at the same temperature and pressure the rate at which a gas diffuses is inversely proportional to the square root of its density, and therefore of its molar mass.