Gases & Absolute Temperature (Cambridge O Level Physics)

Revision Note

Test yourself
Dan MG

Author

Dan MG

Last updated

Absolute Temperature

  • The Kelvin temperature scale begins at absolute zero
    • Absolute zero, or 0 K, is equal to −273 °C 
    • An increase of 1 K is the same change as an increase of 1 °C

  • It is not possible to have a temperature lower than 0 K
  • This means a temperature in Kelvin will never have a negative value
  • To convert between temperatures θ in the Celsius scale, and T in the Kelvin scale, use the following conversion:

θ / °C = T / K − 273

T / K = θ / °C + 273

  

Kelvin Scale and Celsius Scale

K and Celsius conversion chart, downloadable AS & A Level Physics revision notes

Conversion chart relating the temperature on the Kelvin and Celsius scales

Worked example

Convert the following values between the Kelvin (absolute) and Celsius scales of temperature.

(a)

0 K = ...................... °C

(b)

0 °C = ...................... K

(c)

20 °C = ...................... K

Answer:

(a)

Step 1: Choose whether to add or subtract 273 to the value

  • The question is in Kelvin therefore subtract 273 to convert to Celsius

Step 2: Do the calculation

0 space minus space 273 space equals space minus 273 space degree straight C

Step 3: Write the answer with units

  • 0 K = −273 °C

(b)

Step 1: Choose whether to add or subtract 273 to the value

  • The question is in Celsius therefore add 273 to convert to kelvin

Step 2: Do the calculation

0 space plus space 273 space equals space 273 space straight K

Step 3: Write the answer with units

  • 0 °C = 273 K

   

Part (c)

Step 1: Choose whether to add or subtract 273 to the value

  • The question is in Celsius therefore add 273 to convert to kelvin

Step 2: Do the calculation

20 space plus space 273 space equals space 293 space straight K

Step 3: Write the answer with units

  • 20 °C = 293 K

Did this video help you?

The Gas Laws

  • The gas laws describe the relationships between the pressure, volume and temperature of a gas

Pressure & Volume at Constant Temperature

  • If the temperature of a gas remains constant, the pressure of the gas changes when it is:
    • Compressed – decreasing the volume causes pressure to increase
    • Expanded – increasing the volume causes pressure to decrease

Decreasing Volume Increases Pressure

Gas Volumes at Low Temperatures & High Pressures, downloadable IB Chemistry revision notes

Pressure increases when a gas is compressed

  • Similarly, a change in pressure can cause a change in volume
  • A vacuum pump can be used to remove the air from a sealed container
  • The diagram below shows the change in volume of a tied up balloon when the pressure of the air around it decreases:

Decreasing Pressure Increases Volume

Vacuum pump, downloadable IGCSE & GCSE Physics revision notes

Decreasing pressure from the air surrounding the balloon on the right allows the pressure from the air particles within to increase the volume of the balloon

  • When a gas is compressed, the molecules will hit the walls of the container more frequently
    • This creates a larger overall net force on the walls which increases the pressure

Volume & Temperature at Constant Pressure

  • If a gas is placed in a container which allows it to expand and compress (e.g. a piston) then its pressure can be kept constant
  • As a result, changes in temperature at constant pressure will affect the volume only
  • If the pressure of a gas remains constant, the volume of the gas changes when:
    • The gas gets hotter – increasing the temperature causes the gas to expand (volume increases)
    • The gas gets cooler – decreasing the temperature causes the gas to compress (volume decreases)

Relationship between Volume & Temperature

states-of-matter-volume-and-temperature

At constant pressure, an increase in the temperature of the gas causes it to expand

  • In the diagram above:
    • Diagram A shows molecules expanding into a greater volume as the temperature increases
    • Diagram B shows that the volume of the gas is directly proportional to the temperature

Pressure & Temperature at Constant Volume

  • If the temperature of a gas is increased, the particles move faster and gain kinetic energy
    • As a result, they will collide more often leading to an increase in pressure
  • If the volume of a gas remains constant, the pressure of the gas changes when:
    • The gas gets hotter – increasing the temperature causes the pressure to increase
    • The gas gets cooler – decreasing the temperature causes the pressure to decrease

Relationship between Pressure & Temperature

States of Matter Temperature and Pressure, downloadable IB Chemistry revision notes

At constant volume, an increase in the temperature of the gas increases the pressure due to more collisions on the container walls

  • In the diagram above:
    • Diagram A shows molecules in the same volume colliding with the walls of the container more often as the temperature increases
    • Diagram B shows that the temperature of a gas is directly proportional to the gas pressure

Examiner Tip

You are required to be able to describe the links between pressure & volume and pressure & temperature qualitatively. This means that the correct use of terms such as ‘collision’, ‘kinetic energy’ and ‘frequency’, will be really important.

Did this video help you?

Boyle's Law

  • For a fixed mass of a gas held at a constant temperature:

pV = constant

  • Where:
    • p = pressure in pascals (Pa)
    • V = volume in metres cubed (m3)
  • This means that the pressure and volume are inversely proportional to each other
    • When the volume decreases (compression), the pressure increases
    • When the volume increases (expansion), the pressure decreases
  • This relationship is known as Boyle’s Law and can also be written as:

P space proportional to space 1 over V

  • This means the pressure is inversely proportional to the volume of a gas

Graph Showing Boyle's Law

Boyles Law, downloadable AS & A Level Physics revision notes

Boyle's Law shows that pressure is inversely proportional to volume

  • The relationship between the pressure and volume for a fixed mass of gas at constant temperature can also be written as:

P subscript 1 V subscript 1 space equals space P subscript 2 V subscript 2

  • Where:
    • P1 = initial pressure (Pa)
    • P2 = final pressure (Pa)
    • V1 = initial volume (m3)
    • V2 = final volume (m3)
  • Notice that volume and pressure are measured in m3 and Pa respectively
  • In calculations, if units are given in cm3 or MPa this is a rare case where calculations can be done using the original units as long as answers are reported in the same, original units

Calculating Changes in Pressure & Volume

pressure-vs-volume-igcse-and-gcse-physics-revision-notes

Initial pressure and volume, P1 and V1, and final pressure and volume, P2 and V2

Worked example

A gas occupies a volume of 0.70 m3 at a pressure of 200 Pa.

Calculate the pressure exerted by the gas if it is compressed to a volume of 0.15 m3.

Assume that the temperature and mass of the gas stay the same.

Answer:

Step 1: List the known quantities

  • Initial volume, V1 = 0.70 m3
  • Initial pressure, P1 = 200 Pa
  • Final volume, V2 = 0.15 m3

Step 2: Write down the relevant equation

P subscript 1 V subscript 1 space equals space P subscript 2 V subscript 2

Step 3: Rearrange for the final pressure P2

P subscript 2 space equals space fraction numerator P subscript 1 V subscript 1 over denominator V subscript 2 end fraction

Step 4: Substitute the values into the equation

P subscript 2 space equals space fraction numerator 200 cross times 0.70 over denominator 0.15 end fraction space equals space 930 space Pa

Examiner Tip

It is an easy mistake to make to think that an inversely proportional graph will be a straight line sloping downwards. After all, a directly proportional graph is a straight line (through the origin) which slopes upwards!

The curve above which 'tends towards zero' (meaning the curve gets closer and closer but never touches the axis, or zero is an inversely proportional curve, as the graph below shows.

2-1-5-gases-and-abs-temp-exam-tip-1-cie-igcse-23-rn

You've read 0 of your 10 free revision notes

Unlock more, it's free!

Join the 100,000+ Students that ❤️ Save My Exams

the (exam) results speak for themselves:

Did this page help you?

Dan MG

Author: Dan MG

Expertise: Physics

Dan graduated with a First-class Masters degree in Physics at Durham University, specialising in cell membrane biophysics. After being awarded an Institute of Physics Teacher Training Scholarship, Dan taught physics in secondary schools in the North of England before moving to SME. Here, he carries on his passion for writing enjoyable physics questions and helping young people to love physics.