Breaking a stink-bomb in a classroom demonstrates as the students at the front of the class will be the first to smell the produced odors although eventually all students throughout the class will smell the gas equally.

The correct word for the blank space is: diffusion.

Explanation:

In Biology and Chemistry, diffusion refers to the spread of particles from areas of higher concentration to areas of lower concentration. Eventually, the space where the particles were spread will be fully concentrated with it. Diffusion allows gas exchange in the atmosphere.

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