Teach lesson
Gay-Lussac: why Kelvin matters
Observe a real run and compare what happens when P/T uses temperature in Celsius and in kelvin.
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Learning Outcomes
Convert Celsius temperatures to kelvin in real lab data.
Calculate and compare several P/T(K) values.
Explain why the gas-law ratio uses absolute temperature.
Student activity preview
Activity Content
Preview only. In a class session, students can fill in responses and submit their work to the teacher.
The scale changes the meaning
5 min
Two teams analyze the same heated container. One divides pressure by degrees Celsius; the other divides by kelvin. Could they reach very different conclusions only because they chose a different temperature zero? The Celsius zero was chosen from a property of water; it does not mean that thermal motion has stopped. Kelvin uses another starting point: 0 K is the lower limit of temperature. You will compare both calculations before deciding which one better represents the relationship.
From Celsius to kelvin
Use the figure as a conversion reminder. Each pair of marks represents the same temperature on the two scales. In the table, use K = °C + 273.15.
The ideal model would treat P/T(K) as constant. What do you predict for real readings?
Justify your choice in one sentence. You may refer to the ideal model or uncertainty in real measurements.
Observe where the data come from
12 min
Open the lab and observe the heating of the same fixed-volume ethanol sample. Before starting, prepare two rows on paper or in a note with columns for time, temperature, and pressure. Near 00:30 and 03:30, record temperature in °C and pressure in kPa, then convert both temperatures to kelvin. The activity also includes rows from the same run for comparison.
Open the Gay-Lussac lab
The sealed sample always contains the same total amount of ethanol, and the container keeps a fixed volume. The ideal model also treats the gas-phase amount as fixed; the lab records temperature and pressure, not the amount in each phase.
Open the lab from this activity's lab button.
On the configuration screen, select the only available sample: 0.014 mol of ethanol.
Start one observation, which takes about 4 minutes. During the run, record approximate temperature and pressure at the times shown in the following table.
Return to TEACH and complete that table before continuing with the analysis.
Lab readings
The times are already provided. Copy observed readings into the °C and kPa columns. Units appear in the headings. Then convert each temperature to K and round to one decimal place. Finally, calculate each P/T(K) and keep four decimal places.
| Run time (min:s) | Temperature °C | Pressure kPa | Temperature K | P/T(K) kPa/K |
|---|---|---|---|---|
After completing the table, what do you observe when comparing your two P/T(K) values?
Compare the ratios
13 min
The list contains three pre-calculated reference points from the same run. Celsius temperatures were rounded to one decimal first, then 273.15 was added and kelvin was rounded to one decimal; P/T is shown to four decimals. Use it for reference. Your own two ratios are still the ones you calculated from the lab readings.
Reference data
- Measurement 1: T = 23.9 °C = 297.1 K; P = 87.76 kPa; P/T = 0.2954 kPa/K.
- Measurement 41: T = 34.9 °C = 308.1 K; P = 91.36 kPa; P/T = 0.2965 kPa/K.
- Measurement 61: T = 44.6 °C = 317.8 K; P = 95.48 kPa; P/T = 0.3004 kPa/K.
Celsius calculation. Complete this format: Initial ratio = ___ kPa/°C; final ratio = ___ kPa/°C; relative change = ___ %. Use 87.76 / 23.9, 95.48 / 44.6, and |last - first| / first × 100. Round ratios to two decimals and the percentage to one decimal place.
Kelvin change. Use the first and last ratios in the reference list to calculate |last - first| / first × 100. Show the operation and percentage, rounded to one decimal place.
Which temperature scale produces the more stable calculated ratio?
Justify the previous choice by comparing the relative change you calculated with Celsius and the one you calculated with kelvin from the reference list. Do not calculate them again.
Decide which scale better represents the relationship
8 min
You have calculated ratios using two scales that represent the same temperatures but place zero at different points. Use ratio stability and the position of each zero to build your conclusion. Limit the response to comparing temperature scales; small differences between real readings mean that you should not claim an exact constant.
Conclusion. In 2-3 sentences, state which scale is appropriate for the law and explain why its physical zero matters. Use at least one percentage as evidence.
Compare the result of your two P/T(K) ratios with your initial prediction. What do you do with the prediction?