Shannon's brother thought that crushed ice would keep his soda cooler than whole ice cubes. To test this idea, Shannon divided a large bottle of soda equally among six glasses. Shannon added five whole ice cubes to each of the first three glasses while her brother crushed five ice cubes into small pieces before adding them to each of the other three glasses. Ten minutes after all the ice had been added to the glasses, Shannon used a thermometer to measure the temperature of the soda in each glass.

Which of the following was a dependent variable in this experiment?

Derek often saw woodpeckers perched in trees in his backyard, but he never saw them at his bird feeders. Derek usually filled his feeders with sunflower seeds. He wondered if he could attract woodpeckers by adding a different type of food to the feeders.

To test this idea, Derek gathered eight identical bird feeders. He filled four with sunflower seeds. He filled the remaining four feeders with crushed peanuts. Derek then placed the bird feeders in his backyard. Over the next five days, he counted the number of woodpeckers that visited each bird feeder each morning.

Which of the following was an independent variable in this experiment?

Ken wanted to grow cucumbers from seeds. He read that using a heating pad to heat up potting soil could help make seeds germinate, or sprout, faster. Ken wondered whether the temperature of the heating pad would affect how quickly the seeds germinated. Ken prepared two potting trays, each made up of ten small pots of soil. He planted one cucumber seed in each small pot and arranged the potting trays near a sunny window. He set an electric heating pad to 75°F and placed it under one potting tray. He set a second heating pad to 85°F and placed it under the other potting tray. Ken observed the pots daily, and he counted the number of days it took until a seed germinated in each pot.

Which of the following was a dependent variable in this experiment?

Dr. Tran collected two species of algae, Chlorella rotunda and Tetraselmis cordiformis. He wanted to know whether one species released more oxygen as it grew than the other species.

To find out, Dr. Tran prepared six culture tanks. He filled each tank with the same amount of a mixture of water and nutrients. Then, he added 1,000 live algae cells to each tank and sealed the tank. In three of the tanks, he added C. rotunda cells. In the other three, he added T. cordiformis cells. After five days, Dr. Tran measured the amount of oxygen in each culture tank.

Which of the following was an independent variable in this experiment?

Giardia is a microscopic parasite that lives in water and can infect humans. Dr. Dawson designed a drinking straw that contained a filter to remove Giardia from water. Dr. Dawson wanted to know if a longer filtering straw would remove more Giardia.

Dr. Dawson made six filtering straws: three that were five inches long and three that were ten inches long. She prepared six one-liter batches of water, each containing 10,000 Giardia. Then, Dr. Dawson passed one batch of water through each straw. After each batch passed through the straw, she used a microscope to count the number of Giardia that remained in a small sample of the water.

Which of the following was an independent variable in this experiment?