Ideal Gas Molecules
5 marks · NormalA student places a glass bottle full of air with a tight lid in a refrigerator. The air inside was initially at room temperature (22 °C) and atmospheric pressure (100 kPa). After several hours, the bottle’s temperature has dropped to 4 °C.
Explain, using the idea of gas particles, what happens to the pressure of the air in the bottle as it cools.
Calculate the final pressure of the air in the bottle, in kPa. Give your answer to 2 significant figures. (You must use the Kelvin scale for temperature.)
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) As the temperature decreases, the air particles move more slowly and collide with the walls of the bottle less forcefully and less often, so the pressure inside the bottle decreases. · must show slower particle movement, reduced frequency and/or force of collisions
(b) 94 kPa · must show correct Kelvin conversion, use of
Common mistakes: Using Celsius instead of Kelvin; Forgetting to convert both temperatures
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Mentions that particles move more slowly when cooled.
M1 (a) · 1 mark · Explains that less frequent or less forceful collisions mean lower pressure.
M2 (b) · 1 mark · Converts temperatures from °C to K.
A1 (b) · 1 mark · Uses and rearranges for .
A2 (b) · 1 mark · Evaluates and gives final answer (94 kPa, 2 significant figures, with unit).
Worked solution
(a) When the temperature decreases, the average speed of air particles decreases. They strike the bottle walls less often and with less force, resulting in a lower pressure.
(b) Convert both temperatures to Kelvin:
Use :