Teach lesson
Gay-Lussac's Law: pressure and temperature from a real gas run
Students use the UNED Gay-Lussac remote lab to test whether pressure is approximately proportional to absolute temperature at fixed volume.
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Learning Outcomes
Use a real remote Gay-Lussac lab observation to collect pressure-temperature evidence.
Convert Celsius temperatures to kelvin before using a gas-law ratio.
Calculate P/T(K) for selected reference rows and judge whether it is approximately constant.
Create or describe a pressure vs temperature(K) graph from real data.
Explain why a narrow real-data extrapolation should not be overclaimed.
Write a claim-evidence-reasoning conclusion about the pressure-temperature relationship.
Student activity preview
Activity Content
Preview only. In a class session, students can fill in responses and submit their work to the teacher.
Predict the pressure-temperature pattern
8 min
Imagine gas trapped inside a rigid container. When it is heated, its particles move faster and hit the walls more often and harder. Because the container cannot expand, those collisions can make the pressure rise.
Gay-Lussac's law turns that idea into a model: if the amount of gas and the volume stay fixed, pressure rises with absolute temperature. In the lab, you will observe a real ethanol heating run and decide whether the data follow that pattern. You will begin with the simplified model of a fixed gas amount, then consider why a real sample and real sensors need not produce a perfect proportion.
Your route is to observe pressure and temperature, convert Celsius to kelvin, compare the P/T(K) ratio, build a graph, and write a conclusion limited by the evidence.
Key ideas for this activity:
- Pressure is how strongly the gas pushes on the container walls. The lab reports it in kilopascals (kPa).
- Temperature is recorded in degrees Celsius, but gas-law ratios use kelvin (K) because it is a scale that starts at absolute zero. For example, 24 °C is 297.15 K. Convert with T(K) = T(°C) + 273.15.
- Direct proportionality means two quantities change together while keeping an almost constant ratio. If pressure is proportional to absolute temperature, P/T(K) should change very little.
- On a pressure versus kelvin-temperature graph, temperature goes on the horizontal axis and pressure on the vertical axis. An increasing, nearly straight pattern supports the model within the measured interval; it does not prove the model works identically at every temperature.
The real Gay-Lussac setup
The lab video shows a fixed-volume setup and a pressure/temperature display. The lab records a full pressure-temperature series for this run.
Model to test
\frac{P}{T_K} \approx \text{constant}
\qquad
T_K = T_C + 273.15
Before opening the lab, predict what will happen as temperature rises from about 24 °C to 44 °C. Write three short ideas: (1) whether pressure will rise, fall, or stay the same; (2) how fixed volume and comparing temperature in kelvin affect your prediction; and (3) why real data might not follow the model exactly.
Why does this activity use kelvin temperature for the gas-law ratio?
Use the one-sample lab deliberately
10 min
The Gay-Lussac lab shows one heating run for the ethanol sample. That is enough for a strong pressure-temperature analysis. Your job is to observe the run, use the included rows, check units, and judge the model.
Lab workflow
The activity works with one pressure-temperature series for ethanol #1.
Open the Gay-Lussac lab
Open the Gay-Lussac lab with this activity's lab button.
If the lab does not open on the first click, use the
Open the lab againlink.In configuration, choose the available sample. The lab may display it as
0.014 moles ethanol #1.Start observing. Watch the pressure and temperature display during the heating run.
Use the activity data table to work with rows spread across the run.
Check how pressure and temperature rise, then use the included rows to calculate the ratios.
Which plan builds a useful table for this Gay-Lussac run?
Build a reference data table
16 min
Use at least six rows spread across the heating run. The rows included below come from the same Gay-Lussac run you observe in the lab, so do not change the sample, gas amount, or volume.
Pressure-temperature evidence table
Use the included rows from this run. The kelvin column is already prepared so you can check the conversion and calculate P/T(K). Keep enough significant figures to judge the pattern.
| Data row or time note | Temperature C | Temperature K | Pressure kPa | P/T(K) kPa/K | Observation note |
|---|---|---|---|---|---|
Check your table. What temperature range and pressure range did your rows cover, and how did you make sure you did not mix Celsius and kelvin in the same calculation?
Using one row near 30.8 C and 90.0 kPa, calculate P/T(K). Show the Celsius-to-kelvin conversion in your explanation. In the numeric field, enter the decimal with a point, for example 0.296.
Graph the relationship
14 min
Create a pressure vs temperature(K) graph from your table. Use your own graph to decide whether the relationship is approximately linear; you do not need to know the result before plotting it.
Set up the graph
Use this guide to check axes, units, and row spread before interpreting your points.
Pressure-temperature graph evidence
Attach a graph, a spreadsheet file, a link, or a clear text/image reference showing pressure on the vertical axis and temperature in kelvin on the horizontal axis. A text reference is acceptable if it identifies the data points used, both axes, units, and the pattern. The graph must use the table rows, not invented example data.
Describe the graph artifact you attached or referenced. State the x-axis, y-axis, units, data source, and whether the points look roughly linear.
Use your graph to decide whether pressure is approximately proportional to absolute temperature in this run. Mention at least two pieces of evidence: shape, slope direction, P/T values, R2 (if your graph tool shows it), or any systematic drift or scatter.
Avoid two common overclaims
10 min
Two mistakes are common in this lab: using Celsius in the gas-law ratio, and treating a short real-data trend as a perfect proof of absolute zero.
Kelvin conversion warning
P/T has gas-law meaning only when T is absolute temperature. A Celsius ratio near room temperature is not a valid substitute.
If you use 30.8 C in the denominator instead of 303.95 K for P/T, what happens to the ratio?
The full reference data fit is very linear over about 24-44 C, but its straight-line extrapolation (extending that line far beyond the measured points) reaches zero pressure roughly around -210 C, not -273 C. Why should a high-school conclusion not claim that this one run proves absolute zero exactly?
Write the scientific claim
10 min
Use your table, ratio calculation, graph, and limitation discussion to write a final claim.
Write a claim-evidence-reasoning conclusion of 6-8 sentences. Include: your claim about pressure and absolute temperature, two numerical evidence details, why kelvin is required, and one limitation of the real lab data.
If you were explaining this lab to a younger student, what would you say is the difference between "the data support Gay-Lussac's law" and "the data are perfect"?
Optional extension: connect gas-law models
12 min
If your class has already studied Boyle's law or Charles's law, compare what is held constant in each law.
Compare Gay-Lussac's law with one other gas law. For each law, identify the dependent variable, independent variable, and controlled variables.