Pressure and Volume
Introduction
In this title we are going to explore how volume affects the pressure exerted by a gas.
We are going to do this by looking at a scenario of a volume of air in a container with a piston that can reduce the volume available to the gas.
Finally, we will look at a GCSE exam question where we can apply what we have learned.
In the centre you will see the container of gas with a plunger for reducing the volume.
There is a volume scale o the right. Three dials monitor properties of the gas.
At the top is the pressure in kilo Pascals. On the left at the bottom is a thermometer giving the temperature in degrees Kelvin. The gas is at room temperature of 20.0⁰C, this is 293⁰K.
On the left above is an indication of the rate of collisions made by the molecules of the gas.
Collision Rate
This is only counting the collisions of the molecules seen here in yellow. In reality, there is a huge number of molecules in such a volume, but this number will be sufficient for us to see how the molecules behave.
You should be seeing the molecules moving around at a moderate speed, they hit each other and the sides of the container. It is this striking of the sides of the container that is the cause of the pressure.
The pressure of the gas starts at normal atmospheric value of 293 kPa and the temperature is room temperature, indicated on the wall thermometer to the left of the scene.
You should notice that the collision rate varies considerably. This is due to the low number of molecules represented here. However, the pressure reading is what we would get from the actual gas with its huge number of molecules which averages out the variation completely.
Changing the volume
Drag the piston upwards by the handle, this will move the piston slowly and the temperature will remain constant.
As the volume decreases, the pressure will rise, and the rate of the collisions will also rise.
You should be able to observe the molecules density increasing, causing the collision rate to rise. This trend will also be visible on the collision rate dial.
For a range of volumes, 40-100 cl:
•Record the pressure;
Create a graph of volume on the x-axis and pressure on the y-axis. This will be a curve as shown below.
As pressure is proportional to 1 over Volume. If you plot this, you should find a straight-line or linear relationship between the pressure and 1/V.
Changing the volume
For the same range of volumes:
•Record the rate of collisions;
Create a graph of the volume on the x-axis and square of the collision rate on the y-axis.
You should find a linear relationship between the volume and the square of the collision rate. However, it unlikely to be a very good fit due to the random element of the collisions. To get a better fit take multiple readings of the collision rate at each volume and take the average.
Exam Questions
AQA May 2024 Foundation Paper 1
If you have difficulty answering this, try the e-scenario again taking note of the scale on the side of the gas container.
If you have difficulty answering this, try the e-scenario again taking note of the distance between the particles as the volume is decreased by moving the plunger upwards.
If you have difficulty answering this, try the e-scenario again taking note of the rate of the collisions when you heat the gas.
If you have difficulty answering this, try the e-scenario again taking note of the rate of the collisions when you reduce the volume of the gas.