What is the primary characteristic of a side arm flask?

A side arm flask is specifically designed with a side tube that allows for the exit of gases during chemical reactions or processes. This feature is crucial when carrying out experiments that involve gases, as it provides a means to safely release gaseous byproducts without disturbing the main solution in the flask.

The side arm is particularly useful in processes like distillation, where it's essential to control the escape of vapors. The unique structure of this flask enhances the flexibility and safety of experimental setups, making it a valuable tool in many laboratory practices. The presence of the side tube is what primarily distinguishes a side arm flask from other types of laboratory glassware, which may not offer this functionality.

In contrast, other characteristics mentioned do not define a side arm flask specifically or pertain to incorrect attributes. Some flasks have varying shapes, openings, and materials that do not align with the defining feature of the side arm.