What should be avoided when weighing reagent chemicals on a balance?

Weighing reagent chemicals directly on the balance pan should be avoided because it can lead to contamination of the balance and inaccurate measurements. Reagents can often be corrosive, hygroscopic, or can leave residues that might affect future measurements. If any material spills or adheres to the pan, it can also alter the balance's calibration or precision over time.

Using a separate container, such as a weighing boat or paper, minimizes these risks by ensuring that only the reagent is in contact with the container. This practice maintains the integrity of both the chemicals being weighed and the balance itself, leading to more accurate and reliable results. Additionally, using appropriate containers as suggested in other options can help facilitate the weighing process while protecting the balance.