Six Groups, Two Thermometers
Mr. Delgado's six biology groups (24 students) have 30 minutes of a 50-minute period to run a catalase lab: blend potato in buffer, add hydrogen peroxide, and measure reaction rate at an assigned temperature. The room has two thermometers and three water baths, so one "temperature" must be run at room temperature. Each group chose its own substrate: two used 20 mL of 3% H₂O₂, three used 10 mL of 3%, and one used 10 mL of 1%. Some groups measured foam height in a graduated cylinder; others counted bubbles for 30 seconds. Timing came from phones, the wall clock, or a stopwatch. During share-out, the 60 °C group reports the highest foam, contradicting the prediction that 37 °C is optimal, and the class cannot agree whether temperature or substrate amount explains it. Mr. Delgado has 10 minutes left and no extra peroxide for a rerun.
1. Identify the confounded variables and state precisely what the class data can and cannot support.
Read the worked answer
Confounded with temperature:
- Substrate concentration (3% vs. 1%) and volume (10 vs. 20 mL). The 60 °C group is one of the two 20 mL / 3% groups, so its result confounds the highest temperature with roughly double the substrate of most groups.
- Measurement method. Foam height is a cumulative quantity that keeps growing with time; bubble counts are a rate over a fixed window. The two numbers cannot be placed on one axis.
- Timing source. Three different clocks produce three different reaction windows.
- Temperature itself is unverified for the room-temperature group, since no thermometer was assigned and the actual ambient temperature was never recorded.
What the data can support: almost nothing about temperature. A defensible reading is only "more concentrated hydrogen peroxide produced more foam."
What it cannot support: any ranking of temperatures, including the claim that 37 °C is optimal. The prediction is a kinetics expectation, not a finding of this lab, and the 60 °C result is not evidence against it - the 60 °C group also used more substrate.
This is the teachable point available in the last 10 minutes: a result that contradicts a prediction is only informative when everything except the tested variable was held constant.
2. Redesign the protocol so that data from all six groups can be pooled, given 30 minutes, two thermometers, and three water baths.
Read the worked answer
Fix the design, then fit it to the equipment.
Standardize the constants (post on the board; every group copies):
- 10 mL of 3% H₂O₂ for every group; 2 g of potato per group (pre-weighed).
- One metric: bubbles counted for 30 seconds beginning 10 seconds after mixing. Same window for all.
- One clock: a single stopwatch or phone per bench, and the teacher calls "start" for all groups simultaneously.
- Record actual temperature, never an assumed one.
Work the equipment shortage, don't ignore it:
- With two thermometers and four temperature conditions, do not try to monitor every bath continuously. Check and record all bath temperatures once, before the lab, on a class board; a water bath holds temperature well enough over 10 minutes.
- Assign the four conditions to four groups; the remaining two groups serve as replication of 25 °C and 37 °C, which is better science than inventing a fifth condition.
- Make the room-temperature condition legitimate by placing it in a beaker of room-temperature water and recording the measured value.
Pool only if constants held: at share-out, post results on a common graph of rate vs. temperature, and have students verify each other's constants before a point is plotted.
3. What should Mr. Delgado do with the 60 °C group's outlying result in the final 10 minutes - and what should he not do?
Read the worked answer
Do:
- Have the group read its own protocol aloud (20 mL, 3%) before any interpretation. The data are not discarded; they are annotated.
- Use the discrepancy as the debrief. Ask the class: "If we wanted to know what 60 °C does, which variable would we have to change back?" The answer - halve the substrate to 10 mL of 3% - is the controlled experiment they should propose.
- Plot only comparable data on the class graph (the 10 mL / 3% groups), and list the excluded groups in a margin with the reason. This teaches that exclusion is a documented design judgment, not a deletion.
- Assign the rerun as a short next-day task for one group using leftover materials, or as a written proposal if no peroxide remains.
Do not:
- Tell the group their result is "wrong." It is probably a real effect of substrate amount.
- Silently drop the point from the class graph, which teaches students that inconvenient data disappear.
- Conclude anything about enzyme denaturation at 60 °C from this run - the class has no evidence for it, only a prediction.