Identify Variables and Controls Practice Test

Students investigate: “How does temperature affect the rate of the reaction between sodium thiosulfate and hydrochloric acid?” For each trial, they place 50.0 mL of sodium thiosulfate solution into a flask and warm or cool it to 15°C, 25°C, or 35°C using a water bath. They then add 5.0 mL of 1.0 M HCl, start a stopwatch, and stop timing when a black X under the flask is no longer visible due to sulfur formation. Which factor should be kept the same to make this a fair test?

To answer the question, How does stirring affect the dissolving rate of salt in water?, students add 5.0 g of table salt (NaCl) to 100 mL of water at 25°C in identical beakers. Trial 1 is not stirred. Trial 2 is stirred at 1 rotation per second. Trial 3 is stirred at 2 rotations per second. The same thermometer is used to confirm the water stays at 25°C, and the student records how many seconds it takes until the solution looks clear with no visible crystals.

Which factor is the independent variable?