Question:
During a picnic on a warm, dry day, a metal can of lemonade is wrapped in a damp cloth.
Evaporation cools the water in the cloth.
(a) Explain, in terms of molecules, how evaporation cools the water in the cloth.
Answer/Explanation
Ans: ast(er) / high(er) speed / (more) energetic molecules escape (into air)
average speed / average kinetic energy of molecules decreases
temperature related to speed / energy of molecules or slow(er) / low(er) speed / less energetic molecules remain (in water)
(b) As the water in the cloth cools, so does the lemonade.
Explain how electrons transfer thermal energy through the metal of the can.
Answer/Explanation
Ans: any three from:
atoms / ions vibrate
(vibrating) atoms / ions hit electrons
electrons propelled / travelling through metal / moving through metal
electrons hit (distant) atoms
free electrons / delocalised electrons mentioned
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)