1. The thermodynamics scale of temperature is called an absolute scale because it is based on molecular kinetic energy and is independent of the properties of the substance being measured. 2. There are two main pieces of evidence for the molecular theory of gases: 1) gases behave as if their particles are in constant, random motion and 2) when gases are compressed or expanded, the process is elastic, indicating the particles are not destroyed during collisions. 3. Lighter molecules like hydrogen can escape Earth's atmosphere more easily than heavier molecules because they have higher average speeds due to their lower mass, allowing some to reach escape velocities much greater than the root-mean-square speed of the molecules. However, large quantities of

1. Qualitative Questions on Thermal Physics
1. State why the thermodynamics scale of temperature is called an absolute scale. [2] J90/II/6
2. State two pieces of evidence for believing that matter is made up of molecules.
The assumptions of the simple kinetic theory of gases include:
• gases are made up of many molecules moving at random;
• the collisions of the molecules with the walls of the container are elastic.
What experimental evidence is there that these assumptions are reasonable? N84/I/17
3. In the upper atmosphere, a large fraction of the hydrogen molecules travel so fast that they
are able to escape from the Earth. There may also be some loss of helium, but for all other
heavier molecules escape is negligible. Explain:
• Fast molecules are able to escape from the Earth’s atmosphere but slow molecules are
not;
• Molecules of smaller mass are able to escape whereas more massive ones are not;
• Some molecules escape even though the speed required for escape is very much
greater than their root-mean-square speed;
• Large quantities of hydrogen have remained indefinitely on the Earth as a constituent of
water. [7] J89/II/10
4. If two sufficiently energetic sodium atoms collide, an electron in one of them may be raised to
a higher energy level.
• Explain why such a collision is called an inelastic collision.
• Briefly explain why such inelastic collisions are not consistent with basic assumptions of
the kinetic theory of gases. J90/III/3
5. An ideal gas is allowed to expand suddenly with no thermal energy entering or leaving the
gas. The temperature of the gas is observed to change.
• Explain, by reference to the first law of thermodynamics, what happens to the internal
energy of the gas.
• Explain why the change in the internal energy of the gas will give rise to a change in the
root-mean-square speed of the molecules of the gas.
• Hence explain whether the temperature of the gas will rise or fall as a result of this
expansion. [8] N95/III/5
6. When water changes to steam at 100 o
C, heat (the latent heat of vaporisation) must be
supplied to the system. Is work done on or by the system as the water vaporises? Taking into
account the fact that the temperature remains constant during the change of phase, state
whether or not the internal energy changes, giving the reason for your answer.[6]
N85/II/5, N97/III/5, J00/III/5
7. An adiabatic change is one in which no heat is supplied or extracted. When a gas expands
adiabatically, what can you deduce about the temperature of the gas? Explain. N89/II/4
8. Explain the meaning of internal energy. Explain why a rise in the temperature of an ideal gas
takes place during an adiabatic (no heat exchange) compression. [2] J92/II/6
9. Explain what is meant by an isothermal change. How may an isothermal compression of a gas
be achieved in practice? Explain why, in the isothermal compression of an ideal gas, the
internal energy is unchanged even though mechanical work is done. N78/I/15
10. The densities of water and steam are 1000 kg m-3
and 0.590 kg m-3
respectively. 1 kg of water
evaporates at 100 o
C at atmospheric pressure.
• State and explain the internal energy change of the molecules of water.
• State, with a reason, the increase in the potential energy of the molecules. N92/II/5

2. 11. Explain why the total change in the internal energy of the gas during a complete cycle must be
zero. N88/II/10
12. Explain, in terms of the energies of atoms, conditions under which it is possible to increase the
total energy of the atoms of a substance without any changes of the temperature of that
substance. J94/III/6
13. State qualitatively and explain in molecular terms, what happens to the internal energy of a
fixed mass of an ideal gas when, separately,
• the temperature of the gas is raised,
• the volume is decreased at constant temperature,
• the gas as a whole is moving at a certain speed. [6] J95/III/5
14. In a space, such as a swimming pool enclosure, water at 30 o
C and water vapour, also at
30 o
C, coexist.
• Compare the pattern of movement and the speed of molecules in water and water
vapour at the same temperature. [4]
• Compare the internal energy per unit mass of water and water vapour at the same
temperature. [3]
• Explain, in terms of internal energy, why the specific latent heat of vaporisation of a
substance is greater than its specific latent heat of fusion. [2] N06/III/2
15. An ideal gas is contained in a cylinder by means of a piston. The cylinder and piston are made
of a thermal insulator. An atom of the gas collides with the piston.
• State, with a reason, whether the momentum of the atom is conserved in the collision.
[2]
• The piston is lowered so that the volume of the gas is reduced.
o State what change, if any, occurs in the speed of a gas atom during an elastic
collision with the moving piston. [1]
o Use the kinetic theory of gases to explain why the temperature of the gas rises. [2]
N03/II/3