Meat broth that is heated to remove microbes is placed into two sterilized flasks-a normal flask that is open to the air and a swan neck flask with a curved neck to limit air movement. care is made by the experimenter to prevent contamination of the flasks during the experiment. the broth is then allowed to sit. the open flask shows immediate signs of rotting and the presence of microorganisms. the swan neck flask does not. what is the mostly likely reason for the presence of microorganism in the open flask?

We all know that air contains lots of microbes in it and it can be proved by putting a plate containing broth media in open air. After some time microbes will appear on the plate.

In this question, there is two flask a normal flask that is open to the air and a swan neck flask with a curved neck to limit air movement. So in normal flask which is open to air microbes can easily be transmitted into the flask through the air but the swan neck flask does not allow air movement in the flask, therefore, no microbe can enter into the flask.

Therefore due to contamination through air open flask shows immediate signs of rotting and the presence of microorganisms. So contact with air is the reason for the presence of microorganism in the open flask.

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