Question
Liquids and gases are two states of matter.
(a) In both boiling and evaporation, a liquid changes into a gas.
(i) State two ways in which boiling differs from evaporation.
1.
2.
(ii) Before injecting a patient, a doctor wipes a small amount of a volatile liquid on to the patient’s skin.
Explain, in terms of molecules, how this procedure cools the patient’s skin.
(b) Gases can be compressed but liquids are incompressible.
Explain, in terms of molecules, why liquids are incompressible.
Answer/Explanation
Answer:
(a) (i) any two from:
occurs throughout the liquid OR bubbles formed
occurs at one temperature / boiling point
does not produce cooling OR unaffected by draught / surface area / humidity
(ii) (more) energetic molecules escape (from the liquid) OR molecules gain energy and escape OR molecules
overcome intermolecular forces / break bonds
average speed decreases OR molecules with less (kinetic) energy left behind
temperature of liquid decreases
(thermal) energy conducted / gained from skin / body OR (thermal) energy lost by skin / body
(b) molecules touching OR no space between molecules
large (repulsive / intermolecular) forces (when moved closer)
Question
(a)State what happens to the molecules of a gas in a sealed container when the temperature of the gas is increased.
(b) A quantity of gas is contained in a sealed container of fixed volume. The temperature of the gas is increased.
State, in terms of molecules, two reasons why the pressure of the gas increases.
1.
2.
(c)A helium-filled weather balloon is held at ground level. The volume of the balloon is 4800 \(m^3\).
The pressure of the helium is 98 kPa.
The balloon is released and rises to a height where the volume of the balloon is 7200 \(m^3\).
(i) Calculate the new pressure of the helium. Assume that the temperature stays constant.
pressure =
(ii) Suggest why it may be necessary to release helium from the balloon as it rises even higher.
Answer/Explanation
Answer:
(a) One of 1, 2 or 3:
1 Molecules move faster OR have more k.e./momentum
2 Molecules hit walls more often/ more frequently
3 Molecules hit walls with greater force/ impulse/ harder
(b) 1 mark for each of 1, 2 and 3 in (a) not given as answer to (a)
(c)(i) PV = constant OR \(P_1V_1 = P_2V_2\) OR 98 × 4800 = P × 7200
65 kP
(c)(ii) To prevent the balloon bursting (as its volume increases)
OR to reduce the pressure inside the balloon
OR pressure difference between inside and outside balloon rises