Final answer:
Nitrazine paper turning yellow indicates an acidic reaction, as the paper is a pH indicator that changes color in response to acidity, with yellow signifying a pH below 7.
Step-by-step explanation:
When nitrazine paper turns yellow upon testing a fluid, it indicates an acidic reaction. This is because nitrazine paper is a pH indicator, similar to litmus paper, that changes color depending on the acidity or alkalinity of the substance it is in contact with. An acidic substance will typically turn the paper yellow, showing that the pH level of the substance is below 7, which is the neutral point on the pH scale. In medical or biological contexts, this test can be particularly useful for examining body fluids, but it is also relevant in different branches of chemical analysis.