Distinguish temperature readings differences and ratios
Apply the offset to readings while keeping equal temperature intervals equal.
Exam scope and assessment notes
Mapped to the named 0973 section using teaching from the corresponding single-science course. A 0973-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade. This qualification reports two grades on the 9–1 scale; confirm the entry route for your zone and exam series.
Before you begin
- Convert between Celsius and kelvin using +273.
- Identify fixed gas mass, temperature and volume conditions.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which value is a temperature difference?
What you will learn
- Distinguish a Celsius or kelvin reading from a temperature difference.
- Reject Celsius-ratio and uncontrolled gas-comparison shortcuts.
An offset changes readings, not equal intervals
A sample warming from 20°C to 40°C changes from 293 K to 313 K under the selected approximation. Its temperature increase is 20°C or 20 K: the offset cancels when two readings are subtracted. Do not add 273 to a temperature difference. Saying an increase of 20°C is an increase of 293 K confuses an interval with an absolute reading.
The same numerical interval is useful because one Celsius degree and one kelvin have equal step size. This does not mean that a reading of 20°C equals 20 K. Record whether a number represents a reading or a change before applying a conversion or explaining an effect.
Use absolute readings and appropriate controls
0°C is 273 K in this model, not absolute zero. A Celsius reading doubling from 20 to 40 therefore does not mean that absolute temperature doubles; the corresponding readings are 293 and 313 K. If a numerical temperature ratio is required in a later supplied model, use its required absolute scale, not an invented Celsius ratio. No numerical pressure-temperature law is required for this section's Core comparison.
For qualitative gas predictions, identify what stays fixed as well as what changes. A warmer fixed-volume sample with more particles than another does not isolate the effect of temperature alone. Two samples with the same temperature but different masses and volumes do not isolate the specified volume effect. The correct response to insufficient controls is to state what information is missing.
Sources for this lesson
Pause and explain
How does an increase from −10°C to 5°C convert as an interval?
Worked example
A fixed gas mass warms at constant volume from 20°C to 40°C. Find the kelvin readings and temperature interval; decide whether absolute temperature doubles.
Show the worked solution
- Convert the readings to 293 K and 313 K.
- Subtract to obtain 20 K, the same step as 20°C.
- 313 K is not twice 293 K, so the doubling claim fails.
Answer 293 K to 313 K; increase 20 K; absolute temperature does not double.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Convert 20°C and 40°C to kelvin and find the temperature increase.
Give me a hint
Convert each reading, then subtract; do not add an offset to the interval.
Compare my reasoning
- 20°C becomes 293 K.
- 40°C becomes 313 K.
- The increase is 313 − 293 = 20 K.
293 K and 313 K; increase 20 K.
Look for these in your work
- Separate readings and interval.
- Keep the same offset.
Find the kelvin readings and temperature rise for −10°C to 5°C, then explain whether 0°C is absolute zero.
Give me a hint
Use the conversion for readings and equal step size for their difference.
Compare my reasoning
- The readings are 263 and 278 K.
- Their difference is 15 K.
- 0°C is 273 K under the selected approximation, so it is not absolute zero.
263 K to 278 K; rise 15 K; 0°C is not absolute zero.
Look for these in your work
- Handle the negative starting reading.
- Distinguish the zero points.
Two gas samples differ in temperature, mass and volume. A student uses only their Celsius readings to predict an exact pressure ratio. Explain two independent reasons the claim is unsupported.
Give me a hint
Check the temperature scale and the variables that are not controlled.
Compare my reasoning
- Celsius ratios are not absolute-temperature ratios.
- The samples do not hold gas mass and volume fixed.
- No appropriate numerical model and controls establish the asserted pressure ratio.
The wrong ratio scale and uncontrolled mass/volume make the exact pressure claim unsupported.
Look for these in your work
- Identify both issues.
- Avoid inventing a numerical relationship.
Common mistakes
- Do not add the offset to an interval.
- 0°C is not 0 K.
- Uncontrolled samples do not isolate one cause.
What can you explain now?
A temperature rises by 15°C. What is the equal kelvin interval?
Compare with the explanation
15 K.
The two scales have equal step sizes; their offset cancels in a difference.
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Make it stick
Convert readings and intervals correctly and explain why Celsius ratios and uncontrolled comparisons can mislead.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.