Which of the following best describes the key step in Pasteur's experiment that disproved spontaneous generation?

• A. Adding chemicals to the broth to kill all microbes, then exposing it to air.
• B. Sealing broth in a regular flask without boiling, resulting in no microbial growth.
• C. Boiling broth in a swan-neck flask and leaving it exposed to air, preventing microbial growth due to trapped dust.
• D. Boiling broth in an open flask, allowing microbes from the air to enter and grow.

Answer: (C)

Verified step by step guidance

1. Understand the historical context: Pasteur's experiment was designed to test the theory of spontaneous generation, which claimed that life could arise spontaneously from non-living matter.
2. Identify the key experimental design: Pasteur used a swan-neck flask, which allowed air to enter but trapped dust and microbes in the bend of the neck, preventing them from reaching the broth.
3. Recognize the critical step: He boiled the broth to kill any existing microbes, ensuring the broth was initially sterile before exposure to air.
4. Observe the outcome: Despite exposure to air, no microbial growth occurred in the broth because the microbes were trapped in the swan-neck, disproving spontaneous generation.
5. Conclude that the key step was the combination of boiling the broth to sterilize it and using the swan-neck flask to allow air in but prevent microbial contamination.