Research Summaries · Lesson 4

Putting two or three experiments together

A passage with two or three experiments is built like a set of building blocks. Each experiment changes one factor and holds the others fixed, and the fixed values often change from one experiment to the next. Questions that use more than one experiment ask you to put those blocks together.

Start with a quick grid: one row per experiment, one column per condition (temperature, concentration, mass, time). In each row, mark the column that varies and write the fixed value in every other column. Now you can see at a glance what changed between experiments and what stayed the same.

Then look for the shared condition: a trial whose settings appear in two experiments. It should give the same result in both, and it is the bridge that lets you compare across them. When a question asks how one factor changes the effect of another, find the matching trials in two experiments and compare the changes, not just the final numbers.

Rusting Steel Wool

Students studied conditions that affect how fast steel wool rusts. In every trial, a 2.0 g pad of steel wool was dipped for 10 seconds in a liquid, drained, and sealed in a glass jar with the same volume of air. After 24 hours, each pad was dried and weighed. As iron rusts, it combines with oxygen from the air, so the pad gains mass. The students recorded the mass each pad gained, in milligrams (mg).

Experiment 1

Pads were dipped in distilled water (0 g/L of salt), and the jars were kept at 4 different temperatures. The results are shown in Table 1.

Experiment 2

All jars were kept at 20 °C. Pads were dipped in salt water at 4 different salt concentrations, in grams of salt per liter of water (g/L). The results are shown in Figure 1.

Experiment 3

The procedure of Experiment 2 was repeated with the same 4 salt concentrations, except that all jars were kept at 40 °C. The results are shown in Figure 2.

Table 1
Jar temperature (°C)Mass gained in 24 h (mg)
1014
2022
3033
4047
Experiment 1, pads dipped in distilled water (0 g/L of salt): the mass gained in 24 h rose as the jar temperature rose. 10 °C: 14 mg; 20 °C: 22 mg; 30 °C: 33 mg; 40 °C: 47 mg.
Figure 1
Figure 1Experiment 2, all jars at 20 °C. One series, 20 °C: the mass gained in 24 h rose as the salt concentration rose. 0 g/L: 22 mg; 10 g/L: 35 mg; 20 g/L: 44 mg; 40 g/L: 52 mg.0102030405060010203040Salt concentration (g/L)Mass gained in 24 h (mg)
  • 20 °C
Show the data as a table
Figure 1: data
Salt concentration (g/L)20 °C (mg)
022
1035
2044
4052

Experiment 2, all jars at 20 °C. One series, 20 °C: the mass gained in 24 h rose as the salt concentration rose. 0 g/L: 22 mg; 10 g/L: 35 mg; 20 g/L: 44 mg; 40 g/L: 52 mg.

Figure 2
Figure 2Experiment 3, all jars at 40 °C. One series, 40 °C: the mass gained in 24 h rose as the salt concentration rose. 0 g/L: 47 mg; 10 g/L: 70 mg; 20 g/L: 86 mg; 40 g/L: 98 mg.020406080100010203040Salt concentration (g/L)Mass gained in 24 h (mg)
  • 40 °C
Show the data as a table
Figure 2: data
Salt concentration (g/L)40 °C (mg)
047
1070
2086
4098

Experiment 3, all jars at 40 °C. One series, 40 °C: the mass gained in 24 h rose as the salt concentration rose. 0 g/L: 47 mg; 10 g/L: 70 mg; 20 g/L: 86 mg; 40 g/L: 98 mg.

Combining experiments
StepWhat to doWhat you get
Grid the experimentsOne row per experiment; write the value of every conditionA picture of what changed and what stayed the same
Mark the varied factorIn each row, circle the condition that takes several valuesThe variable each experiment tests
Find the shared trialLook for one set of conditions that appears in two experimentsThe bridge between tables, with matching results
Compare acrossPick trials that differ only in the factor the question namesA fair comparison, even when the trials sit in different tables

Worked example Easy

Show the whole passage

Rusting Steel Wool

Students studied conditions that affect how fast steel wool rusts. In every trial, a 2.0 g pad of steel wool was dipped for 10 seconds in a liquid, drained, and sealed in a glass jar with the same volume of air. After 24 hours, each pad was dried and weighed. As iron rusts, it combines with oxygen from the air, so the pad gains mass. The students recorded the mass each pad gained, in milligrams (mg).

Experiment 1

Pads were dipped in distilled water (0 g/L of salt), and the jars were kept at 4 different temperatures. The results are shown in Table 1.

Experiment 2

All jars were kept at 20 °C. Pads were dipped in salt water at 4 different salt concentrations, in grams of salt per liter of water (g/L). The results are shown in Figure 1.

Experiment 3

The procedure of Experiment 2 was repeated with the same 4 salt concentrations, except that all jars were kept at 40 °C. The results are shown in Figure 2.

Table 1
Jar temperature (°C)Mass gained in 24 h (mg)
1014
2022
3033
4047
Experiment 1, pads dipped in distilled water (0 g/L of salt): the mass gained in 24 h rose as the jar temperature rose. 10 °C: 14 mg; 20 °C: 22 mg; 30 °C: 33 mg; 40 °C: 47 mg.
Figure 1
Figure 1Experiment 2, all jars at 20 °C. One series, 20 °C: the mass gained in 24 h rose as the salt concentration rose. 0 g/L: 22 mg; 10 g/L: 35 mg; 20 g/L: 44 mg; 40 g/L: 52 mg.0102030405060010203040Salt concentration (g/L)Mass gained in 24 h (mg)
  • 20 °C
Show the data as a table
Figure 1: data
Salt concentration (g/L)20 °C (mg)
022
1035
2044
4052

Experiment 2, all jars at 20 °C. One series, 20 °C: the mass gained in 24 h rose as the salt concentration rose. 0 g/L: 22 mg; 10 g/L: 35 mg; 20 g/L: 44 mg; 40 g/L: 52 mg.

Figure 2
Figure 2Experiment 3, all jars at 40 °C. One series, 40 °C: the mass gained in 24 h rose as the salt concentration rose. 0 g/L: 47 mg; 10 g/L: 70 mg; 20 g/L: 86 mg; 40 g/L: 98 mg.020406080100010203040Salt concentration (g/L)Mass gained in 24 h (mg)
  • 40 °C
Show the data as a table
Figure 2: data
Salt concentration (g/L)40 °C (mg)
047
1070
2086
4098

Experiment 3, all jars at 40 °C. One series, 40 °C: the mass gained in 24 h rose as the salt concentration rose. 0 g/L: 47 mg; 10 g/L: 70 mg; 20 g/L: 86 mg; 40 g/L: 98 mg.

How did the procedure of Experiment 3 differ from that of Experiment 2?

  1. In Experiment 3, no salt was used, and the jar temperature was varied from 10 °C to 40 °C.
  2. In Experiment 3, the jars were kept at 40 °C, not 20 °C, with the same salt concentrations.Answer
  3. In Experiment 3, the pads of steel wool were heavier than the 2.0 g pads used in Experiment 2.
  4. In Experiment 3, the mass gained was held constant while the salt concentration was varied.

How to solve it

  1. Grid the two experiments. Experiment 2: jars at 20 °C, salt varied (0, 10, 20, 40 g/L). Experiment 3: "the same 4 salt concentrations, except that all jars were kept at 40 °C."
  2. So both experiments vary salt, and the only difference is the fixed temperature: 20 °C in Experiment 2, 40 °C in Experiment 3.
  3. Check the other choices against the intro: every pad was 2.0 g, and mass gained is the measured result. The key is choice B.

Why each choice is right or wrong

  • A. Incorrect. Using no salt and varying temperature describes Experiment 1 (Table 1, 10 °C to 40 °C). Experiment 3 used 0, 10, 20, and 40 g/L of salt (Figure 2).
  • B. Correct. Experiment 3 repeated Experiment 2 with the same 4 salt concentrations (0 to 40 g/L), but at 40 °C instead of 20 °C, as Figures 1 and 2 show.
  • C. Incorrect. The intro says every trial used a 2.0 g pad, so the pad mass was the same in both experiments.
  • D. Incorrect. The mass gained was the measured result, and in Experiment 3 it changed from 47 mg to 98 mg (Figure 2); it was never held constant.

Worked example Medium

Show the whole passage

Rusting Steel Wool

Students studied conditions that affect how fast steel wool rusts. In every trial, a 2.0 g pad of steel wool was dipped for 10 seconds in a liquid, drained, and sealed in a glass jar with the same volume of air. After 24 hours, each pad was dried and weighed. As iron rusts, it combines with oxygen from the air, so the pad gains mass. The students recorded the mass each pad gained, in milligrams (mg).

Experiment 1

Pads were dipped in distilled water (0 g/L of salt), and the jars were kept at 4 different temperatures. The results are shown in Table 1.

Experiment 2

All jars were kept at 20 °C. Pads were dipped in salt water at 4 different salt concentrations, in grams of salt per liter of water (g/L). The results are shown in Figure 1.

Experiment 3

The procedure of Experiment 2 was repeated with the same 4 salt concentrations, except that all jars were kept at 40 °C. The results are shown in Figure 2.

Table 1
Jar temperature (°C)Mass gained in 24 h (mg)
1014
2022
3033
4047
Experiment 1, pads dipped in distilled water (0 g/L of salt): the mass gained in 24 h rose as the jar temperature rose. 10 °C: 14 mg; 20 °C: 22 mg; 30 °C: 33 mg; 40 °C: 47 mg.
Figure 1
Figure 1Experiment 2, all jars at 20 °C. One series, 20 °C: the mass gained in 24 h rose as the salt concentration rose. 0 g/L: 22 mg; 10 g/L: 35 mg; 20 g/L: 44 mg; 40 g/L: 52 mg.0102030405060010203040Salt concentration (g/L)Mass gained in 24 h (mg)
  • 20 °C
Show the data as a table
Figure 1: data
Salt concentration (g/L)20 °C (mg)
022
1035
2044
4052

Experiment 2, all jars at 20 °C. One series, 20 °C: the mass gained in 24 h rose as the salt concentration rose. 0 g/L: 22 mg; 10 g/L: 35 mg; 20 g/L: 44 mg; 40 g/L: 52 mg.

Figure 2
Figure 2Experiment 3, all jars at 40 °C. One series, 40 °C: the mass gained in 24 h rose as the salt concentration rose. 0 g/L: 47 mg; 10 g/L: 70 mg; 20 g/L: 86 mg; 40 g/L: 98 mg.020406080100010203040Salt concentration (g/L)Mass gained in 24 h (mg)
  • 40 °C
Show the data as a table
Figure 2: data
Salt concentration (g/L)40 °C (mg)
047
1070
2086
4098

Experiment 3, all jars at 40 °C. One series, 40 °C: the mass gained in 24 h rose as the salt concentration rose. 0 g/L: 47 mg; 10 g/L: 70 mg; 20 g/L: 86 mg; 40 g/L: 98 mg.

Which two trials should be compared to find how raising the temperature from 20 °C to 40 °C affects rusting of pads dipped in salt water at 20 g/L?

  1. The 20 °C trial and the 40 °C trial in Experiment 1, which both used distilled water
  2. The 20 g/L trial and the 40 g/L trial in Experiment 2, which were both kept at 20 °C
  3. The 40 °C trial in Experiment 1 and the 20 g/L trial in Experiment 2
  4. The 20 g/L trial in Experiment 2 and the 20 g/L trial in Experiment 3Answer

How to solve it

  1. The question names two conditions: the salt must be 20 g/L in both trials, and the temperature must be 20 °C in one and 40 °C in the other.
  2. Experiment 2 has a 20 g/L trial at 20 °C (44 mg, Figure 1). Experiment 3 has a 20 g/L trial at 40 °C (86 mg, Figure 2). These differ only in temperature.
  3. Reject any pair with 0 g/L of salt (Experiment 1) or with two different salt values. The key is choice D.

Why each choice is right or wrong

  • A. Incorrect. These trials do differ only in temperature, but Experiment 1 used distilled water (0 g/L of salt), not 20 g/L salt water.
  • B. Incorrect. These two trials were both at 20 °C and differ in salt (20 g/L and 40 g/L), so they show the effect of salt, not of temperature.
  • C. Incorrect. This pair differs in two factors: the Experiment 1 trial had 0 g/L of salt at 40 °C, and the Experiment 2 trial had 20 g/L at 20 °C.
  • D. Correct. Both trials used 20 g/L salt water; the Experiment 2 trial was at 20 °C (44 mg, Figure 1) and the Experiment 3 trial at 40 °C (86 mg, Figure 2), so only temperature differs.

Trap Comparing trials that differ in two factors

When trials come from different experiments, it is easy to pair two that look related but differ in two conditions, such as a 40 °C trial with no salt and a 20 °C trial with salt. The difference in their results mixes both effects together. Before you accept a pair, write every condition for both trials and check that only one of them differs.