Question

a student carries out an investigation on a strain of bacteria that is used to decompose oil spills. she places equal numbers of bacteria in three different salt solutions: 2.5%, 3.5%, and 4.5%. she keeps all the solutions in a lighted incubator at a temperature of 22 degrees celsius. after three days, she removes the samples and counts the number of bacteria in each solution. what question is the student investigating by using this procedure? do oil - decomposing bacteria grow well in a warm, lighted environment? are the bacteria being studied the best strain to use in cleaning oil spills in ocean water? do bacteria in a saltwater solution grow best at 22 degrees celsius? does salt concentration affect the growth of this strain of bacteria?

Explanation:

1. Analyze the experiment: The student uses three different salt solutions (2.5%, 3.5%, 4.5%) and keeps other factors (temperature, light, initial number of bacteria) constant. She then counts the bacteria after three days.
2. Evaluate each option:

• Option 1: The experiment doesn't test different environments (warm/lighted vs. others), it keeps environment constant. Eliminate.
• Option 2: The experiment doesn't compare this strain to others, it focuses on salt concentration. Eliminate.
• Option 3: The temperature is kept constant (22°C), not varied, so it's not testing temperature's effect. Eliminate.
• Option 4: The independent variable is salt concentration (different percentages), and the dependent variable is bacterial growth (counted after 3 days). This matches the experiment's setup.

Answer:

D. Does salt concentration affect the growth of this strain of bacteria?