Consider the given statements regarding the behavior of ideal gases: 1. Ideal gas molecules have negligible volume compared to the total volume of the container. 2. There are strong intermolecular attractive forces between ideal gas molecules. 3. Collisions between ideal gas molecules are perfectly elastic. Which of the statements given above is/are correct?
- (a)1 and 2 only
- (b)1 and 3 only
- (c)2 and 3 only
- (d)1, 2 and 3
Answer
Why
Correct — B. An ideal gas is the kinetic-theory model built on a few assumptions:
Statement 1 is true: molecules are point masses, their own volume negligible beside the container's.
Statement 2 is false: ideal gas molecules exert no intermolecular forces, attractive or repulsive.
Statement 3 is true: collisions are perfectly elastic, so total kinetic energy is conserved.
1 and 3 only → option (b).
Why the others are wrong
- (a)1 and 2 only — Statement 2 fails. Strong attractions are exactly what the ideal gas model assumes away. They are why real gases deviate from ideal behaviour and can liquefy.
- (c)2 and 3 only — It keeps the false statement 2 and drops the true statement 1. Negligible molecular volume is a core ideal-gas assumption, while strong attraction is not.
- (d)1, 2 and 3 — Not all three hold. The ideal gas model assumes no forces between molecules, so strong attraction describes a real gas, not an ideal one.
Concept
The kinetic theory of gases explains pressure and temperature through the motion of molecules. Its ideal gas assumes negligible molecular volume, no forces between molecules except during collisions, perfectly elastic collisions and random motion.
Such a gas obeys PV = nRT exactly. Real gases come close at low pressure and high temperature, when molecules are far apart.
At high pressure and low temperature, molecular volume and attraction start to matter. The van der Waals equation corrects for both.
Key facts
- An ideal gas obeys PV = nRT at every pressure and temperature.
- Kinetic theory treats ideal gas molecules as point masses with no intermolecular forces.
- Collisions in an ideal gas are perfectly elastic, so total kinetic energy is conserved.
- Real gases behave most nearly ideally at low pressure and high temperature.
Study next
Common traps
- Carrying the attraction of real gases, which lets them condense, into the ideal gas model.
- Confusing elastic collisions, where kinetic energy is conserved, with inelastic ones, where only momentum is.
This is a statement-elimination item: once statement 2 is known to be false, every option that includes it goes, leaving option (b).
Related PYQs
No directly related past PYQ was found.