Which flask is primarily used for heating substances that need even distribution of heat?

The Florence flask is specifically designed for applications that require even distribution of heat. Its round shape provides a large surface area which promotes uniform heating, making it ideal for boiling liquids and facilitating reactions that require consistent temperature control. This shape also allows for easy swirling of liquids without the risk of spills, which is particularly useful in processes like distillation and in situations where gentle, even heating is required.

In contrast, beakers, while commonly used, have straight sides and are more suited for holding and mixing solid and liquid samples but may not provide the same level of consistent heat distribution. The Erlenmeyer flask, while useful for mixing and storing solutions, has a narrower neck which limits its effectiveness for heating purposes. Test tubes are designed for small-scale reactions and do not offer the stability or surface area needed for evenly heating larger volumes of liquids. Thus, the Florence flask is the optimal choice for tasks that require even heat distribution.