Variables, controles, condiciones constantes y cómo diseñar una prueba.
Claves
Un buen experimento cambia UNA variable y mantiene las demás.
Control = igual que los demás ensayos pero sin el factor estudiado.
Estrategia paso a paso
Haz una lista: qué cambia y qué se mantiene en cada experimento.
Trampas habituales
Diseños que cambian dos variables a la vez.
Ejemplo resuelto
¿La catalasa produce el O₂? Ensayo con agua en lugar de catalasa.
Preguntas de ejemplo
Intenta responder antes de abrir la solución. Las preguntas están en el idioma del examen.
Pregunta 1
Dificultad fácil≈ 1 min
Passage VI — Wrapping and cooling (data representation)
Students tested how well different materials slow the cooling of hot water. They filled 5 identical metal cans with 400 mL of water at 80°C. One can was left unwrapped. Each of the other cans was wrapped in a different material: aluminum foil (a single layer), or a 1.0 cm thick layer of cotton, wool, or plastic foam. The cans were placed on a wooden table in a room kept at 21°C, and the water temperature in each can was recorded every 10 minutes for 40 minutes. The results are shown in Figure 1.
The unwrapped can was most likely included in the study in order to:
AA. provide a baseline for judging how much each wrapping slowed cooling.
BB. measure the temperature of the room during the study.
CC. show that metal cans keep water hot longer than glass or plastic containers do.
DD. keep the water in the other cans from cooling below 21°C.
Ver respuesta y explicación
Respuesta correcta: A. provide a baseline for judging how much each wrapping slowed cooling.
Explicación
The unwrapped can shows how fast the water cools with no insulation. Comparing each wrapped can with it shows how much each material helped, so it serves as the control.
The room temperature was kept at 21°C and was not measured with the can. Only metal cans were used, so containers were not compared. An unwrapped can cannot affect the cooling of the other cans.
Pregunta 2
Dificultad media≈ 1 min
Passage VII — Solubility of 3 salts (research summary)
The solubility of a substance is the greatest mass of it that can dissolve in a given amount of water at a given temperature. A solution that holds that greatest mass is saturated.
Students measured the solubility of 3 salts (Salts X, Y, and Z) in 100 g of water at 5 temperatures. At each temperature, they added a salt to the water in small portions, stirring for 5 minutes after each addition, until some of the salt remained undissolved. They then filtered the mixture, dried and weighed the undissolved salt, and subtracted that mass from the total mass of salt added. The results are shown in Figure 1.
In this study, the dependent variable was the:
AA. mass of salt that dissolved in 100 g of water.
BB. temperature of the water.
CC. mass of water used.
DD. time spent stirring after each addition.
Ver respuesta y explicación
Respuesta correcta: A. mass of salt that dissolved in 100 g of water.
Explicación
The students measured how much salt dissolved (total added minus undissolved): that is the result that depends on the conditions.
Temperature was the variable they chose (independent). The mass of water (100 g) and the stirring time (5 minutes) were kept the same.
Pregunta 3
Dificultad difícil≈ 1 min
Passage V — Salt and seed germination (research summary)
Soil that contains a high concentration of salt can prevent seeds from germinating (beginning to grow). Students studied how salt affects the germination of radish seeds.
Experiment 1: The students placed 100 radish seeds on filter paper in each of 5 dishes. They moistened the paper in each dish with 10 mL of a sodium chloride (NaCl) solution of a different concentration, in millimoles per liter (mM). The dishes were covered and kept in the dark at 22°C. The students counted the germinated seeds after 3 days and after 7 days (see Table 1).
Experiment 2: The students repeated the procedure of Experiment 1, except that every dish received the 100 mM NaCl solution and each dish was kept at a different temperature. They counted the germinated seeds after 7 days (see Table 2).
Table 1 (Experiment 1, 22°C) and Table 2 (Experiment 2, 100 mM NaCl)
Experiment
NaCl concentration (mM)
Temperature (°C)
Seeds germinated after 3 days (%)
Seeds germinated after 7 days (%)
1
0
22
78
96
1
50
22
64
93
1
100
22
41
85
1
150
22
17
62
1
200
22
4
31
2
100
14
—
52
2
100
18
—
71
2
100
22
—
85
2
100
26
—
88
2
100
30
—
74
The students want to find out whether potassium chloride (KCl) affects radish germination in the same way as NaCl. Which of the following experiments would best answer this question?
AA. Repeat Experiment 2 using the 100 mM NaCl solution, but count the seeds after 10 days.
BB. Repeat Experiment 1 at 30°C, using the same NaCl solutions as before.
CC. Repeat Experiment 1, but use KCl in some dishes and twice as many seeds in the others.
DD. Repeat Experiment 1, but use KCl in place of NaCl at the same concentrations.
Ver respuesta y explicación
Respuesta correcta: D. Repeat Experiment 1, but use KCl in place of NaCl at the same concentrations.
Explicación
To compare KCl with NaCl, the new experiment should change only the kind of salt: same concentrations, same temperature, same seeds and volumes. Repeating Experiment 1 with KCl in place of NaCl does exactly that, and the results can be compared directly with Table 1.
Counting later or changing the temperature does not involve KCl at all. Changing both the salt and the number of seeds at once would make it impossible to tell which change caused any difference.