ACT Science Quiz: Identifying Sources Of Error

During a titration experiment, if the burette readings are consistently read from below eye level, the measured concentration of the solution would most likely:

A researcher uses a stopwatch to time a reaction that lasts approximately 2 seconds and records the time to the nearest hundredth of a second. If human reaction time affects the start and stop of the timing, the measured reaction time would most likely:

To determine the speed of sound, an experimenter used an echo method. Procedure: (1) Stand 50.0 m from a large wall (distance measured with a measuring tape). (2) Clap two wooden blocks together and start a stopwatch at the clap. (3) Stop the stopwatch when the echo is heard. (4) Compute speed as $v=2d/t$. The experiment was done outdoors on a windy day; wind direction changed during trials. The experimenter also sometimes anticipated the echo and stopped the stopwatch early. Expected result: similar speeds across trials near 340 m/s. The variability in the calculated speeds was most likely caused by:

An experimenter investigated how light intensity affects photosynthesis using aquatic plants. Steps: (1) Place equal-length plant sprigs in four beakers of water with baking soda. (2) Position a lamp at distances of 10 cm, 20 cm, 30 cm, and 40 cm. (3) After 2 minutes, count oxygen bubbles produced in 1 minute for each beaker. (4) Repeat twice. The lamp warmed the nearest beaker noticeably, and room lights were turned on and off as people entered. Bubble counting was done by eye, and some bubbles merged before reaching the surface. Expected pattern: closer lamp produces more bubbles. Which factor is the most significant source of error in this procedure?

A student measured the concentration of a sugar solution using a hydrometer. Steps: (1) Pour solution into a tall cylinder. (2) Lower the hydrometer gently until it floats freely. (3) Read the scale at the liquid surface and record specific gravity. (4) Repeat for three solutions. The student read the scale from slightly above the meniscus, and bubbles sometimes stuck to the hydrometer stem. The solutions were at different temperatures because some were freshly mixed with warm water. Expected pattern: higher sugar concentration gives higher specific gravity. Which procedural error would have the greatest effect on the results?

A student tested how fertilizer affects plant growth. Procedure: (1) Fill 12 pots with the same brand of potting soil. (2) Plant one bean seed per pot at 2 cm depth. (3) Assign 3 pots each to 0 g, 1 g, 2 g, or 3 g fertilizer mixed into the top layer. (4) Water each pot with "about 50 mL" daily using a cup without volume markings. (5) Measure plant height after 14 days using a ruler. The pots were placed on a windowsill; some received direct sunlight longer than others. Expected pattern: moderate fertilizer increases height, but too much may reduce growth. Which procedural error would have the greatest effect on the results?

To measure the concentration of a dye, an experimenter made a calibration curve with a spectrophotometer. Steps: (1) Prepare standards of 0.00, 0.20, 0.40, 0.60, 0.80 mM dye in identical cuvettes. (2) Wipe cuvette sides with a tissue and insert into the instrument in the same orientation. (3) Zero the spectrophotometer using a blank cuvette containing water. (4) Measure absorbance of each standard and plot absorbance vs. concentration. The blank cuvette accidentally contained a very dilute dye solution (not pure water). Expected pattern: absorbance increases linearly with concentration. If the blank contained dye, the measured absorbances of standards would most likely:

In an experiment to measure the viscosity of a liquid, a student uses a stopwatch to time the descent of a sphere. If the sphere is dropped inconsistently, the viscosity measurements would most likely:

To measure the period of a pendulum, an experimenter used a 1.00 m string and a metal bob. Steps: (1) Measure string length from the support clamp to the bottom of the bob using a meterstick. (2) Pull the bob to a small angle (~10°) and release. (3) Use a handheld stopwatch to time 20 oscillations; divide by 20 to get the period. (4) Repeat for three trials. The lab had an air vent blowing intermittently, and the experimenter started/stopped the stopwatch by watching the bob pass the center point. The meterstick's zero end was slightly chipped, but the experimenter aligned the chipped end to the clamp each time. Expected result: periods should be nearly identical across trials. The variability in the results was most likely caused by:

An experimenter compared melting points of two wax samples. Procedure: (1) Place a small wax piece in a thin glass capillary tube. (2) Attach the tube to a thermometer and immerse both in a hot water bath. (3) Heat the bath slowly and record the temperature when the wax first becomes transparent. (4) Repeat for the second wax. The water bath was heated on a hot plate that cycled on/off, causing temperature to rise in small jumps. The thermometer bulb sometimes touched the beaker wall. Expected pattern: each wax has a characteristic melting point. Which factor is the most significant source of error in this procedure?

The consistency of results in a viscosity measurement experiment would most likely be affected by:

A student measured the pH change during a neutralization reaction by adding 0.10 M NaOH to 25.0 mL of 0.10 M HCl. Steps: (1) Place HCl in a beaker with a magnetic stir bar. (2) Calibrate a pH probe using pH 4 and pH 7 buffers. (3) Add NaOH in 1.0 mL increments using a syringe, recording pH after each addition. (4) Continue until 35 mL NaOH is added. The syringe had worn markings and delivered 1.0 mL when the plunger was set to 0.9 mL (a consistent error). Expected pattern: pH rises slowly, then sharply near equivalence. If the syringe error occurred, the equivalence point volume would most likely be measured as:

A technician measured the conductivity of six salt solutions, each read once. Figure 1 shows the readings. The technician expects conductivity to rise steadily with salt concentration.

Which reading in Figure 1 most clearly does not fit the pattern of the others?

In an experiment to determine the speed of sound, students measure the time taken for an echo to return after a clap. If they do not account for temperature variations, the speed of sound calculated would most likely:

A weather station records air temperature every four hours. Two identical sensors were mounted a meter apart at the same height, one inside a louvered white screen that keeps sunlight off it while letting air through, the other clamped to a post with nothing above it. The day was clear, with sunrise near hour 6 and sunset near hour 19. Figure 1 shows what both sensors reported.

Which of the following best explains the relationship between the two records in Figure 1?

A student filtered a precipitate, then weighed the damp filter paper on an electronic balance every two minutes to see whether the reading was steady enough to record. Immediately before each weighing of the paper, a sealed steel mass was weighed on the same balance as a check on the balance itself; the pan held only one object at a time. Figure 1 plots both sets of readings as the change from that object's own first weighing, so both start at zero.

Which of the following best explains the readings in Figure 1?

Bacterial colonies growing on an agar plate are counted by eye, and a colony near the rim or touching a neighbor is easy to miss. Two observers each counted the same six plates, working separately and without seeing each other's totals, and each counted every plate once. Table 1 gives both counts for all six plates.

Which of the following does Table 1 most strongly suggest about the counting?

A chemist measured the absorbance of four standard solutions once each and fitted a calibration line through the four points. The chemist wants to know how much of the scatter around that line comes from the instrument itself. Table 1 shows the measurements.

What should the chemist do to answer that question?

A soil-moisture probe reports the percentage of water in the soil it is pushed into. To check that the probe was working, a researcher sealed a soil sample in a jar so that the sample could neither gain nor lose water, left the probe in the sealed sample, and recorded what the probe reported at the same hour every day for ten days. Figure 1 shows those readings.

Which of the following is the best explanation for the readings in Figure 1?

A glider was released four times along a 1.00 m air track, more gently each time, and each run was timed twice at once. A student started a stopwatch on the release signal and stopped it on seeing the glider reach the far end, so every hand-timed run runs long by roughly the student's reaction time. A photogate timed the same runs electronically as the glider passed. Speed is calculated by dividing the 1.00 m track length by the measured time. Table 1 gives both times for all four runs.

For which run does the hand timing throw off the calculated speed by the largest percentage?