1. Which of the following variables was intentionally held constant in the experiment that produced the data in Table 1?
2. During gel electrophoresis, which property of the DNA fragments primarily determines how far they migrate through the gel?
3. A microbiology student tested whether disinfectant concentration affects bacterial survival on plastic. She prepared four spray bottles containing the same disinfectant at 0%, 25%, 50%, and 75% concentration (diluted with sterile water). Identical plastic squares were inoculated with the same volume of bacterial culture and left for 5 minutes. Each square was sprayed with exactly 2.0 mL of solution from its bottle and left for 2 minutes before being swabbed. The swab was streaked onto agar plates that were incubated at 37°C for 24 hours. The student counted the number of colonies that grew on each plate. In this experiment, the dependent variable is:
4. A student investigated how salt concentration affects the boiling point of water. She prepared four 250 mL beakers, each containing 200 mL of distilled water. She added table salt to make solutions of 0%, 2%, 4%, and 6% salt by mass. Each beaker was heated on the same hot plate setting, starting at room temperature (22°C). The student stirred each solution at a constant rate with the same glass rod and recorded the temperature when the liquid reached a steady rolling boil for 30 seconds. She used the same thermometer for all trials and measured boiling point to the nearest 0.1°C. Which of the following is the independent variable?
5. A student tested how the temperature of water affects the time it takes a tablet to dissolve. She placed one identical effervescent tablet into 200 mL of water at each of four temperatures: 10°C, 20°C, 30°C, and 40°C. She used the same beaker type, did not stir the water in any trial, and used the same brand of tablets. She started a timer when the tablet touched the water and stopped the timer when bubbling ceased and no solid remained. Which of the following is the independent variable?
6. A student investigated how fertilizer amount affects algae growth in small aquariums. She set up four identical 2.0 L tanks, each filled with 1.5 L of the same pond water and the same initial algae sample. She added fertilizer at 0 g, 0.5 g, 1.0 g, or 1.5 g per tank. All tanks were kept under the same lamp, at the same distance from the light, and at 24°C. After 7 days, she measured algae growth by filtering each tank and recording the dry mass of algae (g). The experimenter manipulated <u>which</u> factor and measured <u>which</u> factor?
7. A chemist studied how stirring speed affects the time required for 5.0 g of sugar to dissolve in 100 mL of water. She used a magnetic stir plate and tested four stirring speeds: 0 rpm, 200 rpm, 400 rpm, and 600 rpm. The water temperature was kept constant at 25°C using a water bath, and the same beaker type and sugar brand were used each time. For each trial, the chemist started a stopwatch when sugar was added and stopped it when no visible crystals remained. Which of the following is the independent variable?
8. A food science student tested how baking temperature affects cookie spread. She prepared one batch of dough and used a scoop to place identical 30 g dough balls on the same type of baking sheet lined with parchment. She baked trays at 160°C, 180°C, 200°C, and 220°C for exactly 10 minutes in the same oven, allowing the oven to preheat fully each time. After cooling for 5 minutes, she measured cookie diameter (cm) with a ruler. Which variable was held constant?
9. A chemistry student examined whether stirring rate changes how quickly sugar dissolves. She added 10.0 g of granulated sugar to 200 mL of water at $25^\circ\text{C}$ in identical beakers. Using the same magnetic stirrer, she tested four stirring settings: 0 rpm (no stirring), 200 rpm, 400 rpm, and 600 rpm. She recorded the time (in seconds) until no visible sugar crystals remained. The sugar mass, water volume, water temperature, beaker type, and observation method were kept the same each trial. The experimenter manipulated <u>_____</u> and measured <u>_____</u>.
10. A student tested the hypothesis that increasing salt concentration lowers the freezing point of water. She prepared four 100 mL samples of distilled water in identical cups and added NaCl to make solutions of 0%, 2%, 4%, and 6% (by mass). Each cup contained a thermometer, and all cups were placed in the same freezer set to $-15^\circ\text{C}$. The student stirred each sample for 10 s every minute and recorded the temperature at which the first ice crystals appeared (freezing point). The cup type, water volume, freezer setting, and stirring schedule were kept the same for all trials. Which of the following is the independent variable?
11. A physics student tested how ramp angle affects the distance a toy car travels after leaving the ramp. The student built a ramp with an adjustable angle and tested angles of 10°, 20°, 30°, and 40°. The car was released from rest from the same marked starting point on the ramp each time. The ramp surface, car, and floor surface were kept the same for all trials. After the car rolled down and left the ramp, the student measured the horizontal distance from the end of the ramp to where the car first stopped. Which factor was controlled in this experiment?
12. A student tested how ramp angle affects the speed of a toy car. She built a ramp of fixed length (1.0 m) and released the same car from rest at the top. She tested four ramp angles: $10^\circ$, $20^\circ$, $30^\circ$, and $40^\circ$. For each angle, she measured the time for the car to travel from the top to the bottom using the same stopwatch and the same start/finish marks. The ramp surface, car, release position, and travel distance were kept constant. Which of the following is the independent variable?
13. A student studied how surface type affects how far a coin slides. She pushed the same coin with a spring-loaded device set to the same notch each time so the initial push was consistent. She tested four surfaces placed on a level table: glass, wood, sandpaper, and felt. She measured the sliding distance (cm) from the release point until the coin stopped. The coin, push setting, table level, and measurement method were held constant. Which of the following is the independent variable?
14. A student tested the hypothesis that fertilizer amount affects bean plant growth. She planted one bean seed per pot in identical pots containing the same mass of potting soil. Each pot received one of four fertilizer amounts mixed into the soil: 0 g, 2 g, 4 g, or 6 g. All pots were placed on the same windowsill and rotated daily. Each pot received 50 mL of water every day at the same time. After 21 days, the student measured plant height (cm) from soil surface to the top leaf. Which factor was controlled in this experiment?
15. A student studied how the length of a pendulum affects its period. She used the same metal bob and the same string type, but adjusted the pendulum length to 20 cm, 40 cm, 60 cm, and 80 cm (measured from pivot to bob center). For each length, she displaced the bob by the same small angle and measured the time for 10 swings, then calculated the period (s). The room conditions were unchanged. In this experiment, the dependent variable is:
16. A biology student tested how salinity affects brine shrimp survival. She prepared four tanks with salinities of 0 ppt, 10 ppt, 20 ppt, and 30 ppt using the same brand of salt and the same total water volume. Each tank received 25 shrimp of similar size. All tanks were kept under the same light cycle and at $24^\circ$C. After 48 hours, she counted the number of living shrimp. Which of the following is the independent variable?
17. Researchers tested whether light color affects basil growth. Basil seedlings of the same age were placed in identical pots with the same soil mass and watered with 50 mL daily. Pots were kept in the same growth chamber at $22^\circ$C. Four groups received LED light for 12 hours/day, but the LEDs differed by color: red, blue, green, or white. After 21 days, the researchers measured the height of each plant in centimeters. In this experiment, the dependent variable is:
18. A student tested whether the number of paper clips affects how far a rubber band launches a small paper ball. The same rubber band was stretched to the same length each time using a marked ruler line. The student attached 0, 2, 4, or 6 paper clips to the paper ball before launching it from the same height. The distance traveled along the floor was measured in centimeters with a tape measure. The room floor type and launch angle were kept the same. In this experiment, the dependent variable is:
19. A student investigated how stirring speed affects how quickly sugar dissolves in water. She added 10.0 g of sugar to 200 mL of water in identical beakers. A magnetic stirrer was set to 0 rpm, 200 rpm, 400 rpm, or 600 rpm. Water temperature was kept at $25^\circ$C and the same sugar brand was used. The student measured the time (s) until no visible crystals remained. To isolate the effect of stirring speed, the experimenter controlled:
20. A study examined how the thickness of insulation affects heat loss from a building. Insulation thickness was varied, while the building's size and external temperature were kept constant. Heat loss was measured over a fixed period. The experimenter manipulated ________ and measured ________.
21. A physics class tested how the angle of a ramp affects the speed of a toy car at the bottom. The same toy car was released from rest at the top of a 1.00 m ramp set at angles of 10°, 20°, 30°, and 40° relative to the floor. The ramp surface was kept the same for all trials, and the car was released from the same starting line each time. Students measured the car’s speed at the bottom using a photogate timer placed at the ramp’s end.
The experimenter manipulated <u>which</u> factor and measured <u>which</u> factor?
22. A student examined whether fertilizer amount changes the number of flowers produced by marigold plants. She grew identical seedlings in 12 identical pots with the same soil type. Pots were assigned to receive 0 g, 5 g, 10 g, or 15 g of fertilizer per week (3 pots per group). All plants received the same volume of water each day, were kept in the same greenhouse with the same light schedule, and were grown for 6 weeks. At the end, she counted the number of flowers on each plant.
Which factor was controlled in this experiment?
23. A student investigated how the number of ice cubes affects cooling of a drink. Four identical cups each received 250 mL of the same soda at $22^\circ$C. The student added 0, 2, 4, or 6 identical ice cubes to the cups and did not stir. After 3 minutes, she measured the soda temperature with the same thermometer. Which variable was held constant?
24. In a study to observe the effect of pH on enzyme activity, researchers altered the pH levels of the reaction environment and measured the rate of reaction. The concentration of enzyme and substrate were kept constant. Which variable was held constant?
25. A student tested how the amount of sunlight affects the rate of photosynthesis in an aquatic plant. Sprigs of the same plant species were placed in identical containers with the same volume of water and the same concentration of dissolved baking soda. A lamp was placed at four distances from the plant: 10 cm, 20 cm, 30 cm, and 40 cm. Over 5 minutes, the student counted the number of oxygen bubbles released. Which of the following is the independent variable?
26. An engineering team tested how blade pitch affects the electrical output of a small wind turbine. In a wind tunnel set to a constant wind speed of 8.0 m/s, they tested blade pitch angles of $5^\circ$, $10^\circ$, $15^\circ$, and $20^\circ$. The same turbine, generator, and blade material were used each time. Output was measured as voltage (V) across a resistor using the same multimeter after 60 s of steady operation. Which variable was held constant?
27. An experiment was done to determine the effect of different concentrations of bleach on fabric color fading. The concentration of bleach was altered, and the degree of color fading was assessed. The type of fabric and washing temperature were kept constant. Which was the dependent variable?
28. To evaluate the effect of different teaching methods on student performance, educators used various teaching strategies and measured student test scores. The curriculum content and class size were kept constant. What was the dependent variable?
29. A physics student investigated whether wire length affects electrical resistance. He built a circuit with a battery, a switch, and a digital multimeter set to measure resistance. He tested copper wires of lengths 0.5 m, 1.0 m, 1.5 m, and 2.0 m. All wires had the same diameter and were made from the same spool of copper. The room temperature was kept constant, and the same multimeter and battery were used each time. He recorded the resistance (in ohms) for each wire length. In this experiment, the dependent variable is:
30. A study aimed to find the effect of exercise duration on weight loss. Participants engaged in different lengths of exercise routines while their diet and sleep were kept constant. The change in weight was recorded. Which is the dependent variable?
31. In a lab study, the effect of exercise on memory retention was tested. Participants performed different exercise routines, and their ability to recall information was assessed. The difficulty of the information and the study time were kept constant. Which factor is the dependent variable?
32. Researchers conducted an experiment to test the effect of different fertilizers on tomato yield. The type of fertilizer was varied while the amount of sunlight and water received by the plants was kept constant. The number of tomatoes produced was recorded. In this experiment, the dependent variable is:
33. To determine the effect of soil pH on plant growth, researchers grew plants in soils with different pH levels and measured plant biomass after 8 weeks. Sunlight and water were kept constant. Which of the following was measured?
34. In a research study, the effect of varying water pH on fish survival rate was tested. The pH of water was changed, and the survival rate of fish was recorded. Fish species and tank conditions were controlled. Which factor was manipulated?
35. A team of scientists investigated the influence of caffeine on heart rate. They administered different doses of caffeine to participants and recorded their heart rates. The age and gender of participants were kept constant. The experimenter manipulated ________ and measured ________.
36. An experiment was performed to study the effect of temperature on the solubility of a salt in water. The temperature of the water was varied, and the amount of salt dissolved was measured. The volume of water and type of salt remained the same throughout the experiment. Which factor was controlled in this experiment?
37. In an experiment to test the effect of sugar concentration on yeast fermentation, different sugar concentrations were used while the type of yeast and temperature were kept constant. The amount of carbon dioxide produced was measured. What was the independent variable?
38. Researchers aimed to discover the impact of different exercise intensities on cardiovascular health. The intensity of exercise was varied, and blood pressure was measured. Diet and age of participants were kept constant. Which factor was controlled?
39. Scientists aimed to determine how the concentration of salt in water affects its boiling point. They varied the salt concentration from 0% to 20% while maintaining a constant atmospheric pressure and used the same pot for each trial. The boiling point was recorded. The experimenter manipulated ________ and measured ________.
40. In a study on how different cooking oils affect food crispiness, the type of oil was varied while keeping cooking temperature and time constant. The crispiness of the food was assessed. Which variable was held constant?
41. A study was conducted to investigate the effect of exercise duration on energy expenditure. Exercise duration varied across trials, while participant age and gender were kept constant. Energy expenditure was measured. Which factor was controlled in this experiment?
42. In an experiment measuring how soil pH affects plant leaf color, the pH of the soil was altered while light exposure and water availability were kept constant. The color intensity of the leaves was measured. Which variable was held constant?
43. To test how water temperature affects the time needed for a sugar cube to dissolve, a student filled 4 identical 250 mL beakers with 200 mL of water. The water temperatures were adjusted to 10°C, 20°C, 30°C, or 40°C. For each trial, the student dropped one standard sugar cube into the beaker and started a stopwatch. He gently stirred each beaker at a constant rate of 30 revolutions per minute using the same stirring rod and stopped timing when the cube was no longer visible. He used the same brand of sugar cubes and the same water volume each time.
Which of the following is the independent variable?
44. A lab group examined how ramp angle affects the time for a toy car to travel 1.5 m. They set a wooden ramp to three angles: $10^\circ$, $20^\circ$, and $30^\circ$ using a protractor. The same car was released from rest at the same starting line each time, and the travel distance was kept at 1.5 m measured along the ramp. The surface of the ramp was not changed between trials. Students timed the car’s motion with the same stopwatch and recorded the time in seconds. Which of the following is the independent variable?
45. Which of the following variables was kept constant in Study 3?
46. A materials science student tested whether wire thickness affects electrical resistance. She cut copper wires of four diameters (0.25 mm, 0.50 mm, 0.75 mm, 1.00 mm), each to the same length (1.0 m). She connected each wire to the same power supply set to 3.0 V and measured current using the same ammeter. The room temperature was held at 21°C. For each wire, she calculated resistance using $R=\frac{V}{I}$. In this experiment, the dependent variable is:
47. A biology class tested whether light color affects plant growth. Identical seedlings were placed in four boxes lined with reflective foil. Each box contained one LED lamp: red, blue, green, or white. The lamps were adjusted so each box received the same light intensity (measured in lux) for 12 hours per day. All plants were watered with 20 mL of water every morning, kept at 23°C, and grown in the same type and mass of potting soil. After 14 days, students measured the height of each plant (cm) from soil surface to the tip of the tallest leaf. In this experiment, the dependent variable is:
48. A student tested how the length of a pendulum affects its period. She used the same metal bob and the same string, but adjusted the pendulum length to 20 cm, 40 cm, 60 cm, and 80 cm (measured from the pivot to the center of the bob). For each length, she released the bob from a small angle (about $10^\circ$) and timed 10 full swings with a stopwatch, then calculated the period (seconds per swing). The bob mass, release angle, and timing method were kept constant. Which variable was held constant?
49. A microbiology student tested whether incubation temperature affects yeast fermentation. She prepared four identical flasks, each with 100 mL of the same 5% glucose solution and 1.0 g of dry yeast. Each flask was fitted with a balloon to collect gas. The flasks were incubated for 30 minutes at $15^\circ\text{C}$, $25^\circ\text{C}$, $35^\circ\text{C}$, or $45^\circ\text{C}$. After 30 minutes, she measured the circumference of each balloon (cm) as an indicator of CO$_2$ produced. The glucose concentration, yeast mass, solution volume, incubation time, and flask type were kept constant. To isolate the effect of temperature, the experimenter controlled:
50. A food scientist tested the hypothesis that higher oven temperature shortens cookie baking time. She baked identical cookie dough balls (20 g each) on the same metal tray lined with the same parchment paper. She used three oven settings: $160^\circ\text{C}$, $180^\circ\text{C}$, and $200^\circ\text{C}$. For each temperature, she timed how long it took for the cookie edges to turn golden brown, using the same oven rack position and the same stopwatch. Which variable was held constant?
51. To study how sound frequency affects a tuning fork’s vibration duration, a student used 4 tuning forks labeled 256 Hz, 320 Hz, 384 Hz, and 512 Hz. Each fork was struck with the same rubber mallet using the same force technique and then held by the handle above a foam pad to reduce external vibrations. Using a stopwatch, the student measured how long (seconds) the fork remained audible to the same listener in a quiet room. The room, mallet, and listening distance were kept the same.
Which variable was held constant?
52. A neuroscience student tested whether caffeine dose affects reaction time. Volunteers drank one of four beverages containing 0 mg, 50 mg, 100 mg, or 150 mg of caffeine. Each drink had the same volume (250 mL) and flavoring, and participants waited 30 minutes before testing. Reaction time was measured as the average time (in milliseconds) to press a button after a light flashed during 20 trials on the same computer. The room lighting and noise level were kept constant. In this experiment, the dependent variable is:
53. A student investigated how the number of paper towel layers affects water absorption. She cut paper towels into identical $10\text{ cm} \times 10\text{ cm}$ squares and stacked them to make pads with 1, 2, 3, or 4 layers. She poured exactly 50 mL of water onto each pad placed in the same type of tray. After 60 seconds, she measured how much water remained in the tray (in mL) and calculated water absorbed by the pad. All trials used the same brand of paper towel and were performed at room temperature. Which of the following is the independent variable?
54. An environmental scientist tested how the amount of motor oil affects plant germination. She filled 15 identical cups with the same mass of soil and planted 5 bean seeds in each cup. Cups were assigned to receive 0 mL, 2 mL, 4 mL, 6 mL, or 8 mL of motor oil mixed into the soil (3 cups per level). All cups were kept under the same grow light, watered with the same volume of water every day, and maintained at the same temperature. After 10 days, she recorded the number of seeds that germinated in each cup.
In this experiment, the dependent variable is:
55. A scientist studied the effect of acidity on the corrosion rate of metal. The acidity of the solution was varied, and the rate of corrosion was measured. The type of metal and exposure time were constant. What was the dependent variable?
56. An experiment was set up to test the effect of varying concentrations of salt water on seed germination. The concentration of salt was changed, and the number of seeds that germinated was recorded. Light and temperature were kept constant. Which factor is the dependent variable?
57. An experiment was conducted to test the effect of ambient temperature on the battery life of smartphones. Different temperatures were applied, and the duration the phone stayed on was recorded. The brand of smartphone and battery type were constant. Which factor was manipulated?
58. A study was conducted to determine the effect of different light intensities on the growth rate of a specific plant species. The experimenter varied the light intensity (low, medium, high) and measured the height of the plants after 4 weeks. Soil type and water amount were kept constant. Which of the following is the independent variable?
59. Scientists conducted an experiment to evaluate the impact of water hardness on soap effectiveness. The hardness of the water was varied, and the amount of lather produced was measured. The type of soap and temperature of water were controlled. Which of the following is the manipulated variable?
60. Researchers examined the effect of different types of music on study efficiency. Participants listened to various music genres while studying, and their test scores were measured. The study environment and time spent studying were controlled. What was controlled in this experiment?
61. A team of biologists investigated how different diets affect the growth rate of a species of fish. The type of diet was varied, and the growth rate was measured. Water quality and tank size were kept constant. What was the independent variable?
62. Scientists tested how different colors of light affect algae growth. Light color was varied while keeping light intensity and water temperature constant. Algae biomass was measured. In this experiment, the dependent variable is:
63. A study investigated how sugar type affects cake texture. The type of sugar was varied while keeping baking time and oven temperature constant. The texture of the cake was evaluated. Which factor was controlled in this experiment?
64. Which of the following variables was kept constant in Study 2?
65. Researchers conducted an experiment to see how water salinity affects seed germination rate. Salinity levels were varied while keeping light exposure and temperature constant. The number of seeds germinated was counted. Which of the following is the independent variable?
66. In an investigation of how pressure affects gas solubility in liquids, pressure levels were varied while temperature and type of liquid were kept constant. The amount of gas dissolved was measured. Which variable was held constant?
67. Scientists examined how the angle of sunlight affects solar panel efficiency. Sunlight angle was varied while keeping panel type and environmental conditions constant. The electricity generated was measured. In this experiment, the dependent variable is:
68. Researchers explored how different soil types affect plant root depth. Soil type was varied while keeping water and light levels constant. Root depth was measured. The experimenter manipulated ________ and measured ________.
69. In an experiment to determine the effect of wind speed on evaporation rate, wind speed was varied while keeping ambient temperature and humidity constant. The rate of water evaporation was recorded. Which of the following is the independent variable?
70. In an experiment to determine the effect of sugar concentration on yeast fermentation, sugar concentration was varied while keeping temperature and yeast type constant. The amount of carbon dioxide produced was measured. In this experiment, the dependent variable is:
71. Researchers studied the impact of humidity on paper strength. Humidity levels were varied while keeping paper type and sample size constant. The paper's tensile strength was tested. Which factor was controlled in this experiment?
72. A study was conducted to examine the effect of light wavelength on photosynthesis rate. Light wavelength was varied while keeping carbon dioxide concentration and water supply constant. The oxygen production was measured. Which variable was held constant?
73. An experiment was designed to test how acidity affects metal corrosion rate. The acidity of the solution was varied, while keeping the metal type and solution temperature constant. The weight loss of the metal was measured. In this experiment, the dependent variable is:
74. Which of the following variables was kept constant in Study 3?
75. Researchers investigated the effect of light intensity on plant growth. They used three levels of light intensity: low, medium, and high, while keeping the type of plant and soil conditions constant. They measured the height of the plants after four weeks. Which of the following is the independent variable?
76. A study was conducted to see how different fertilizers affect plant growth. The type of fertilizer was varied while the amount of water and sunlight were kept constant. Plant growth was measured in terms of height. Which factor was controlled in this experiment?
77. An experiment was designed to test how different levels of caffeine affect heart rate. The caffeine dose was varied across trials while keeping age and gender of participants constant. Heart rate was measured. Which variable was held constant?
78. Researchers explored how time of day affects bird singing behavior. They recorded bird songs at dawn, noon, and dusk, while keeping location and weather conditions consistent. The number of songs was counted. Which factor was controlled in this experiment?
79. A study was conducted to examine how different types of music affect concentration levels. The type of music played was varied while keeping the noise level and task difficulty constant. The time taken to complete a task was recorded. Which of the following is the independent variable?
80. In an experiment to determine the effect of sound frequency on human reaction time, frequency was varied while keeping volume and task complexity constant. Reaction time was recorded. Which factor was controlled in this experiment?
81. In an experiment to see how air pressure affects balloon size, air pressure was varied while keeping balloon material and room temperature constant. The size of the balloon was measured. Which of the following is the independent variable?
82. Researchers wanted to determine how soil type affects water absorption rate. Soil type was varied while keeping the amount of water and container size constant. The rate of water absorption was measured. The experimenter manipulated ________ and measured ________.
83. In a study to determine how sugar concentration affects the density of a solution, the sugar concentration was varied while keeping temperature and volume constant. The density of the solution was measured. Which variable was held constant?
84. A research team investigated the effect of soil moisture on root development in plants. Soil moisture levels were varied while plant species and nutrient levels were kept constant. Root length was measured. The experimenter manipulated ________ and measured ________.
85. In a study examining the effect of temperature on bacterial growth, temperature was varied between 20°C, 25°C, and 30°C, while nutrient levels and humidity were kept constant. The number of bacteria was counted after 48 hours. The experimenter manipulated ________ and measured ________.
86. In an experiment to determine the effect of pH on enzyme activity, researchers varied the pH of the solution from 4 to 8, while keeping the temperature and enzyme concentration constant. They measured the rate of reaction. In this experiment, the dependent variable is:
87. In Study 1, the incubation time was kept constant at 10 minutes for all trials. This variable is best described as a:
88. Which of the following variables was a controlled variable in Study 2?
89. Which of the following variables was held constant in Study 2 but varied in Study 1?
90. In Study 2, which variable was intentionally kept constant across all trials?
91. Consider Tray 2 in Study 1 and Tray 8 in Study 2. Which of the following statements comparing these two trays is most accurate?
92. Which of the following variables was intentionally varied in Study 1?
93. In Study 2, the students tested the effect of the spring compression distance on the horizontal distance. Which variable was kept constant in Study 2?
94. An experiment was conducted to see how temperature affects the viscosity of a liquid. Temperature was varied while keeping the type of liquid and container size constant. The viscosity was measured. In this experiment, the dependent variable is:
95. Based on the chemical equation provided in the Introduction, the bubbles counted in these experiments consisted primarily of:
96. Which of the following variables was the independent (manipulated) variable in Study 2?
97. A physics class tested how the number of batteries affects the brightness of a small bulb. Students built the same circuit using the same bulb and wires. They powered the circuit with 1, 2, 3, or 4 identical AA batteries in series. Brightness was quantified by measuring light intensity (lux) with the same light sensor placed 10 cm from the bulb in a dark room. Which of the following is the independent variable?
98. Researchers examined how exercise intensity affects heart rate. Volunteers walked on a treadmill at four speeds: 2 km/h, 4 km/h, 6 km/h, and 8 km/h for 5 minutes each. The treadmill incline was kept at 0% and the room temperature was held at $21^\circ$C. Heart rate (beats per minute) was recorded at the end of each 5-minute interval using the same monitor. In this experiment, the dependent variable is:
99. A student tested how the amount of soap affects the size of bubbles produced. She mixed soap solutions by adding 0 mL, 5 mL, 10 mL, or 15 mL of liquid soap to water to make a total volume of 100 mL each time. Using the same bubble wand and the same blowing time, she produced one bubble per trial and measured bubble diameter (cm) with a ruler held behind the bubble. The room airflow was minimized by closing windows. Which factor was controlled in this experiment?
100. A student investigated how salt concentration affects the boiling point of water. She prepared four 200 mL samples of distilled water in identical beakers and added NaCl to make concentrations of 0%, 2%, 4%, and 6% (mass/volume). Each beaker was heated on the same hot plate set to the same power setting, and the water was stirred at a constant rate with the same magnetic stir bar. For each trial, the student recorded the temperature when vigorous boiling was first sustained for 30 s using the same calibrated thermometer. Which of the following is the independent variable?
101. To test whether sound volume affects concentration, a researcher asked participants to complete the same 20-question math worksheet while listening to white noise through headphones. The noise volume was set to 30 dB, 50 dB, 70 dB, or 90 dB. Each participant had exactly 10 minutes, used the same room, and wore the same headphone model. The researcher recorded the number of correct answers. The experimenter manipulated <u>blank</u> and measured <u>blank</u>.
102. A chemistry student studied how reaction temperature affects the rate of a reaction between magnesium and hydrochloric acid. She placed identical 2.0 cm magnesium strips into 50 mL of 1.0 M HCl in the same type of flask. The acid concentration and volume were kept constant. The flask was placed in water baths at $10^\circ$C, $25^\circ$C, and $40^\circ$C. The student measured reaction rate by recording the time (s) for the magnesium to completely disappear. The experimenter manipulated <u>blank</u> and measured <u>blank</u>.
103. Scientists examined how pH affects enzyme activity. They prepared four buffer solutions at pH 4, 6, 8, and 10. In each trial, they added the same volume of enzyme solution and the same concentration of substrate to a test tube, then incubated the mixture for 5 minutes at $37^\circ$C. Enzyme activity was measured as the change in absorbance at 420 nm using the same spectrophotometer. Which variable was held constant?
104. A student tested how the type of metal affects corrosion rate in saltwater. Equal-sized strips of iron, copper, aluminum, and zinc were cleaned the same way and then placed into separate beakers containing 200 mL of 3.5% saltwater. All beakers were kept at the same temperature and left undisturbed for 7 days. Corrosion was measured by the mass lost (g) from each strip after drying. Which of the following is the independent variable?
105. A physics student investigated how the number of rubber bands affects the launch distance of a small foam projectile. She built a launcher and pulled the projectile back to the same marked position each time. She tested 1, 2, 3, and 4 identical rubber bands attached in parallel. The projectile type, pullback distance, launch angle ($45^\circ$), and floor surface were kept constant. After each launch, she measured the horizontal distance from the launcher to the first point of contact with the floor. In this experiment, the dependent variable is:
106. A biology class investigated whether light color affects photosynthesis rate in an aquatic plant. Groups placed equal-length sprigs of Elodea in separate beakers containing 200 mL of the same sodium bicarbonate solution. Each beaker was illuminated by an LED lamp covered with a colored filter: red, blue, green, or clear (white). Lamps were positioned 20 cm from the beakers, and the room temperature was maintained at $22^\circ\text{C}$. After 5 minutes of acclimation, students counted the number of oxygen bubbles released in 3 minutes for each beaker. The beaker size, solution concentration, plant length, lamp distance, and counting time were held constant. In this experiment, the dependent variable is:
107. In Study 3, the students used the same mass (0.5 g) of Magnesium in both trials. This was necessary to ensure that:
108. A biology class investigated whether light color affects the growth of bean seedlings. Identical seedlings were placed in 4 boxes lined with colored plastic: clear (white light), red, blue, or green. Each box contained 10 seedlings in the same potting soil. All boxes were kept at $22^\circ\text{C}$, watered with 20 mL daily, and placed the same distance from identical lamps for 12 hours per day. After 14 days, students measured the average seedling height in each box using a ruler. In this experiment, the dependent variable is:
109. An environmental science student tested whether pH affects how quickly limestone dissolves. He placed identical 5.0 g limestone chips into 100 mL of solution at pH 3, pH 5, pH 7, and pH 9. Each solution was kept at $25^\circ\text{C}$ and stirred at the same rate. After 30 minutes, he removed, dried, and weighed the remaining limestone to determine mass lost. To isolate the effect of pH, the experimenter controlled:
110. An engineer tested the hypothesis that increasing ramp angle increases the acceleration of a toy car. She released the same toy car from rest at the top of a 1.5 m ramp set to 3 different angles: $10^\circ$, $20^\circ$, and $30^\circ$. The ramp surface material stayed the same, the starting position was marked and used each time, and the car’s wheels were checked to ensure they spun freely. She measured the time to travel the full 1.5 m using a photogate timer and then calculated acceleration from distance and time. Which factor was controlled in this experiment?
111. A student tested the hypothesis that increasing salt concentration lowers the freezing point of water. She prepared 4 beakers, each with 100 mL of distilled water, and added NaCl to reach concentrations of 0%, 2%, 4%, or 6% (mass/volume). Each beaker was placed in the same freezer set to $-10^\circ\text{C}$ for 45 minutes. The student stirred each beaker for 10 seconds every 5 minutes using the same stirring rod and then recorded the temperature at which the first ice crystals appeared (freezing point). All beakers were identical and started at $20^\circ\text{C}$. Which of the following is the independent variable?
112. A chemist examined how temperature affects the rate of an effervescent tablet reaction. She placed one 2.0 g tablet into 200 mL of water in the same 400 mL beaker at four water temperatures: $10^\circ\text{C}$, $20^\circ\text{C}$, $30^\circ\text{C}$, and $40^\circ\text{C}$. She used the same brand of tablet, the same water volume, and stirred at the same speed with a magnetic stirrer. She measured reaction rate as the time (in seconds) until bubbling stopped. Which of the following is the independent variable?
113. A student tested the hypothesis that increasing salt concentration slows yeast fermentation. She prepared 4 sugar-yeast solutions, each with 10.0 g sugar, 1.0 g dry yeast, and 100 mL water in identical 250 mL flasks. She added table salt to reach salt concentrations of 0%, 1%, 2%, or 3% (mass/volume). Each flask was kept in a 35°C water bath for 20 minutes. After sealing each flask with a balloon, she measured the circumference of the balloon (in cm) as an indicator of CO$_2$ produced. All flasks used the same yeast brand, same sugar mass, same water volume, and the same incubation time.
Which of the following is the independent variable?
114. A nutrition student tested whether the concentration of vitamin C solution affects how quickly it reduces a blue dye (DCPIP) to colorless. She prepared vitamin C solutions at 0.10%, 0.20%, 0.30%, and 0.40% (mass/volume). For each trial, she added 5.0 mL of a vitamin C solution to 5.0 mL of DCPIP in the same type of test tube, gently swirled for 5 seconds, and measured the time (s) until the mixture became colorless. The dye volume, total liquid volume, swirl time, and room lighting were kept constant.
To isolate the effect of vitamin C concentration, the experimenter controlled:
115. A student tested the hypothesis that increasing the number of paper towel layers improves water absorption. She cut paper towels from the same brand into identical 10 cm × 10 cm squares and stacked them to make pads with 1 layer, 2 layers, 3 layers, or 4 layers. Each pad was placed in a shallow tray containing 100 mL of water for exactly 30 seconds. After removal, the pad was allowed to drip for 5 seconds, then weighed on the same balance. The tray size, water volume, soak time, and towel brand were kept constant.
Which of the following is the independent variable?
116. A chemist tested whether the concentration of hydrochloric acid (HCl) affects the reaction rate with magnesium ribbon. She prepared 4 beakers containing 50.0 mL of HCl at 0.5 M, 1.0 M, 1.5 M, or 2.0 M. In each trial, she added a 2.0 cm strip of magnesium ribbon (same thickness and mass) and recorded the time (seconds) until bubbling stopped. The same beaker size, solution volume, and room temperature were used for all trials.
In this experiment, the dependent variable is:
117. A researcher investigated whether different LED colors affect radish seedling growth. She planted 10 radish seeds in each of 4 identical trays containing the same mass of potting soil and the same amount of water. The trays were placed in separate light boxes that differed only in LED color: red, blue, green, or white. All boxes were set to the same light intensity (measured at tray level), the same 12-hour light/12-hour dark cycle, and the same temperature. After 7 days, she measured the average seedling height (cm) in each tray.
In this experiment, the dependent variable is:
118. To test how fertilizer amount affects tomato yield, a gardener grew tomato plants in 12 identical pots. Each pot received the same soil type, the same pot size, and the same watering schedule. The only difference was the amount of fertilizer added weekly: 0 g, 5 g, 10 g, or 15 g per pot. After 10 weeks, the gardener counted the number of tomatoes produced per plant. Which factor was controlled in this experiment?
119. A food science class tested how baking temperature affects cookie spread. Students used the same cookie dough recipe and portioned 30 g of dough for each cookie. Cookies were baked for exactly 10 minutes on the same type of baking sheet, but at three oven temperatures: $160^\circ$C, $180^\circ$C, and $200^\circ$C. After cooling for 5 minutes, cookie diameter was measured in millimeters with the same ruler. Which of the following is the independent variable?
120. Researchers tested whether different fabrics retain heat differently. Squares of cotton, wool, polyester, and denim (each 20 cm $\times$ 20 cm) were wrapped around identical 250 mL glass bottles filled with water initially at $60^\circ$C. Bottles were placed in the same room with no direct sunlight. After 30 minutes, the water temperature in each bottle was measured with the same thermometer. In this experiment, the dependent variable is:
121. To test the effect of sugar type on cookie texture, bakers used different sugars while keeping the baking time and oven temperature the same. The resulting texture of the cookies was evaluated. Which variable was manipulated?
122. An environmental science student tested whether the amount of activated carbon changes how well it removes dye from water. She poured 100 mL of a blue dye solution into each of four identical jars. She added activated carbon in amounts of 0 g, 0.5 g, 1.0 g, and 2.0 g, then sealed and shook each jar for exactly 60 s. After letting the carbon settle for 5 minutes, she measured the solution’s color intensity using the same colorimeter (reported in absorbance units). The dye concentration, solution volume, shaking time, settling time, jar type, and instrument were kept constant. In this experiment, the dependent variable is:
123. Researchers studied how the concentration of a pollutant affects fish respiration rates. Pollutant levels were varied, while water temperature and fish species were kept constant. The respiration rate was recorded. The experimenter manipulated ________ and measured ________.
124. Which of the following variables was kept constant in Study 2?
125. Based on the chemical equation provided in the Introduction, the bubbles counted in these experiments consisted primarily of: