(e) absolute zero (0 K) as the lowest limit for temperature; the…

The idea

Absolute zero is 0 K, the lowest temperature on the thermodynamic scale. It is the lower limit: no temperature below 0 K is attainable, so 0 K is a boundary rather than a value reached in practice. At this limit a substance has minimum internal energy. Internal energy is the sum of the random kinetic and potential energies of its particles, so cooling towards 0 K reduces particle motion and the associated energy. Convert with T/K = θ/°C + 273.15, so 0 K is about −273.15 °C. A question may ask which value is the lowest possible temperature, or which statement about internal energy at 0 K is correct.

What earns the marks

• Absolute zero is 0 K, the lowest limit for temperature.
• Internal energy is the sum of random kinetic and potential energies of the particles.
• 0 K corresponds to about −273.15 °C, using T/K = θ/°C + 273.15.
• No temperature below 0 K is attainable, so 0 K is a limiting value.

Common mistakes and how to put them right

• Thinking 0 K can be reached easily in a laboratory.

  Put it right: It is a lower limit approached but not attained, so treat it as a boundary.

• Confusing 0 K with 0 °C.

  Put it right: 0 °C is 273.15 K, whereas 0 K is about −273.15 °C.

• Saying internal energy is zero at 0 K.

  Put it right: The statement is minimum internal energy, not zero.

• Treating temperature as a measure of total internal energy.

  Put it right: Temperature relates to the average random kinetic energy of particles.

Exam tips

• Read the options for the exact wording: lowest limit and minimum internal energy are the key phrases.
• Convert between kelvin and degrees Celsius using T/K = θ/°C + 273.15 before comparing values.
• Eliminate options that claim a temperature below 0 K or that internal energy is exactly zero at 0 K.