Diffusion in gases
4 marks · NormalA teacher sets up the experiment shown below to demonstrate diffusion in gases. Two gas jars are stacked as shown: the lower jar contains brown bromine gas, and the upper jar contains clear air. The lids are removed at time t = 0, and observations are made over the next 10 minutes.
The observations are recorded in the table:
| Time after removing lids (min) | Description of appearance |
|---|---|
| 0 | Lower jar brown, upper jar clear |
| 2 | Brown colour starts to move up |
| 5 | Upper jar pale brown |
| 10 | Both jars evenly light brown |
Describe how the appearance of the gas jars changes over time. Use information from the table.
Explain why the brown colour eventually becomes evenly spread in both jars.
Give one reason, using the particle model, why the process takes several minutes rather than happening instantly.
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.
Show answer
(a) At first only the lower jar is brown, the upper jar is clear; the brown colour gradually spreads upwards until both jars are evenly light brown.
(b) The brown colour becomes evenly spread because bromine particles and air particles move randomly, so bromine diffuses from high concentration (lower jar) to low concentration (upper jar), creating an even mixture. · must show reference to movement from high to low concentration, random movement of particles
(c) Because gas particles collide with air particles and change direction, which slows their progress. · must show particles constantly bounce/collide
Common mistakes: thinking bromine rises due to density (instead of diffusion); missing random movement as cause of delay
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Describes that initially only the lower jar is brown, brown spreads upwards, both jars become evenly pale brown using points from the table.
M1 (b) · 1 mark · States that bromine diffuses from an area of high concentration to low concentration.
M2 (b) · 1 mark · Mentions that particles move randomly until even spread is reached.
B2 (c) · 1 mark · States that particles take time as they bounce off/collide with other particles, so do not move straight.
Worked solution
a) At 0 minutes, only the lower jar is brown. After 2 minutes, the brown colour starts to move up. By 5 minutes, the upper jar is pale brown. After 10 minutes, both jars are evenly light brown.
b) The brown colour becomes evenly spread because bromine gas particles move from where there is a high concentration (lower jar) to where there is a low concentration (upper jar). The particles mix due to random movement until the concentration is equal everywhere.
c) The process takes several minutes because gas particles do not travel in straight lines; they repeatedly collide with air particles and change direction, which slows their diffusion.