Experimental Design · Lesson 3

Why was this step done? Tools and procedures

Every sentence of a procedure is a choice the researchers made on purpose. “Why was this step done?” questions ask you to name that purpose. Almost every step does one of four jobs, shown in the table below.

Find the step in the passage and read the words around it. They usually point straight at the purpose: “checked that it read 0.0 °C” is about the thermometer; “left … until it returned to room temperature” is about the cup’s temperature; “exactly 200 g” is about keeping an amount the same. The right choice names a reason that fits this step in this procedure, and only one choice will.

What a step is usually for
JobTypical stepWhat it protects
Keep a condition the sameExactly 200 g of water in every trialOnly the tested variable differs
Remove an unwanted factorCup kept away from the window and ventDrafts or sunlight can’t change results
Calibrate an instrumentThermometer checked in ice water (0 °C)Readings are accurate
Make trials comparableCup returned to 22 °C; 3 trials averagedEvery trial starts alike; chance scatter shrinks

Lids and Cooling

Students studied how different lids affect how fast hot water cools in a cup.

The students used one foam cup for every trial. Before each trial, they placed the thermometer in a beaker of ice water and checked that it read 0.0 °C. They then poured exactly 200 g of water at 80.0 °C into the cup, covered it with a paper, plastic or foam lid (or left it uncovered), and set the cup in a corner of the room, away from the window and the air vent. They read the water temperature every 5 minutes for 20 minutes.

After each trial, the cup was emptied and left for 30 minutes until it returned to room temperature, 22 °C. Each lid condition was tested 3 times, and Figure 1 shows the average temperatures.

Figure 1
Figure 1Figure 1 plots average water temperature against time for four series, all starting at 80 °C. No lid: 70.5, 63, 57 and 52 °C at 5, 10, 15 and 20 min. Paper lid: 73.5, 68, 63.5, 59.5 °C. Plastic lid: 75, 70.5, 66.5, 63 °C. Foam lid: 77, 74, 71.5, 69 °C. The foam lid slows cooling most.4045505560657075808505101520Time (min)Water temperature (°C)
  • No lid
  • Paper lid
  • Plastic lid
  • Foam lid
Show the data as a table
Figure 1: data
Time (min)No lid (°C)Paper lid (°C)Plastic lid (°C)Foam lid (°C)
080808080
570.573.57577
10636870.574
155763.566.571.5
205259.56369

Figure 1 plots average water temperature against time for four series, all starting at 80 °C. No lid: 70.5, 63, 57 and 52 °C at 5, 10, 15 and 20 min. Paper lid: 73.5, 68, 63.5, 59.5 °C. Plastic lid: 75, 70.5, 66.5, 63 °C. Foam lid: 77, 74, 71.5, 69 °C. The foam lid slows cooling most.

Worked example Easy

Show the whole passage

Lids and Cooling

Students studied how different lids affect how fast hot water cools in a cup.

The students used one foam cup for every trial. Before each trial, they placed the thermometer in a beaker of ice water and checked that it read 0.0 °C. They then poured exactly 200 g of water at 80.0 °C into the cup, covered it with a paper, plastic or foam lid (or left it uncovered), and set the cup in a corner of the room, away from the window and the air vent. They read the water temperature every 5 minutes for 20 minutes.

After each trial, the cup was emptied and left for 30 minutes until it returned to room temperature, 22 °C. Each lid condition was tested 3 times, and Figure 1 shows the average temperatures.

Figure 1
Figure 1Figure 1 plots average water temperature against time for four series, all starting at 80 °C. No lid: 70.5, 63, 57 and 52 °C at 5, 10, 15 and 20 min. Paper lid: 73.5, 68, 63.5, 59.5 °C. Plastic lid: 75, 70.5, 66.5, 63 °C. Foam lid: 77, 74, 71.5, 69 °C. The foam lid slows cooling most.4045505560657075808505101520Time (min)Water temperature (°C)
  • No lid
  • Paper lid
  • Plastic lid
  • Foam lid
Show the data as a table
Figure 1: data
Time (min)No lid (°C)Paper lid (°C)Plastic lid (°C)Foam lid (°C)
080808080
570.573.57577
10636870.574
155763.566.571.5
205259.56369

Figure 1 plots average water temperature against time for four series, all starting at 80 °C. No lid: 70.5, 63, 57 and 52 °C at 5, 10, 15 and 20 min. Paper lid: 73.5, 68, 63.5, 59.5 °C. Plastic lid: 75, 70.5, 66.5, 63 °C. Foam lid: 77, 74, 71.5, 69 °C. The foam lid slows cooling most.

Why did the students place the thermometer in ice water before each trial?

  1. To bring the water in the cup down to room temperature
  2. To check that the thermometer gave accurate readingsAnswer
  3. To keep the starting water temperature at 80.0 °C
  4. To find the lid that slowed cooling the most

How to solve it

  1. Find the step: “they placed the thermometer in a beaker of ice water and checked that it read 0.0 °C.”
  2. Ice water has a known temperature, 0 °C. Checking a reading against a known value is calibration: it shows whether the instrument reads correctly.
  3. The step involves only the thermometer, not the cup’s water or the lids, so the answer is B.

Why each choice is right or wrong

  • A. Incorrect. The thermometer, not the cup’s water, went into the ice water, and the cup’s water started at 80.0 °C, not room temperature.
  • B. Correct. Ice water is at a known temperature, 0 °C, so a reading of 0.0 °C confirms the thermometer is accurate before temperatures like those in Figure 1 are recorded.
  • C. Incorrect. The 80.0 °C starting temperature came from the water poured into the cup; dipping the thermometer in ice water does not set it.
  • D. Incorrect. The lids are compared using the temperatures in Figure 1; the ice-water check happens before any lid is tested.

Worked example Medium

Show the whole passage

Lids and Cooling

Students studied how different lids affect how fast hot water cools in a cup.

The students used one foam cup for every trial. Before each trial, they placed the thermometer in a beaker of ice water and checked that it read 0.0 °C. They then poured exactly 200 g of water at 80.0 °C into the cup, covered it with a paper, plastic or foam lid (or left it uncovered), and set the cup in a corner of the room, away from the window and the air vent. They read the water temperature every 5 minutes for 20 minutes.

After each trial, the cup was emptied and left for 30 minutes until it returned to room temperature, 22 °C. Each lid condition was tested 3 times, and Figure 1 shows the average temperatures.

Figure 1
Figure 1Figure 1 plots average water temperature against time for four series, all starting at 80 °C. No lid: 70.5, 63, 57 and 52 °C at 5, 10, 15 and 20 min. Paper lid: 73.5, 68, 63.5, 59.5 °C. Plastic lid: 75, 70.5, 66.5, 63 °C. Foam lid: 77, 74, 71.5, 69 °C. The foam lid slows cooling most.4045505560657075808505101520Time (min)Water temperature (°C)
  • No lid
  • Paper lid
  • Plastic lid
  • Foam lid
Show the data as a table
Figure 1: data
Time (min)No lid (°C)Paper lid (°C)Plastic lid (°C)Foam lid (°C)
080808080
570.573.57577
10636870.574
155763.566.571.5
205259.56369

Figure 1 plots average water temperature against time for four series, all starting at 80 °C. No lid: 70.5, 63, 57 and 52 °C at 5, 10, 15 and 20 min. Paper lid: 73.5, 68, 63.5, 59.5 °C. Plastic lid: 75, 70.5, 66.5, 63 °C. Foam lid: 77, 74, 71.5, 69 °C. The foam lid slows cooling most.

Why did the students pour exactly 200 g of water into the cup in every trial?

  1. So any difference in cooling comes from the lid, not the amount of waterAnswer
  2. So that the water would cool at the same rate no matter which lid was used
  3. Because a larger mass of water would cool faster than 200 g
  4. So that the temperature could be read every 5 minutes

How to solve it

  1. The step keeps something the same in every trial: the mass of water.
  2. In this experiment only the lid changes from trial to trial. Holding the mass at 200 g stops the amount of water from affecting the temperatures in Figure 1.
  3. So the mass is a controlled variable, and any difference in cooling can be credited to the lid. The answer is A.

Why each choice is right or wrong

  • A. Correct. The lid is the only factor meant to change. Using the same 200 g every time keeps the amount of water from affecting how fast it cools, so differences in Figure 1 can be credited to the lid.
  • B. Incorrect. The purpose is to see whether cooling rates differ, and Figure 1 shows they do (52 °C with no lid versus 69 °C with the foam lid at 20 min). Fixing the mass does not make the rates equal.
  • C. Incorrect. This brings in an outside claim the passage never makes, and it does not explain why every trial used the same mass.
  • D. Incorrect. Readings every 5 minutes depend on the clock, not on the mass of water; the mass has nothing to do with when readings are taken.

Trap True, but not why *this* step was done

Wrong choices on “why” questions are often good lab advice in general: dry your equipment, use a good insulator, read the scale carefully. The question asks why the students in this passage did this step.

Find the step, read the words around it (until it returned to room temperature, 22 °C), and pick the reason those words point to. If a choice would be sensible in any lab but the passage never connects it to the step, skip it.