What characterizes efflorescent powders?

Efflorescent powders are characterized by their ability to lose water when exposed to air, which is often due to the presence of water of hydration or crystallization within their structure. This water is integral to the powder's crystalline form; when the powder is dry, it retains this water. Upon exposure to ambient conditions, these powders will release water vapor, which may result in the formation of a dry powder over time. This behavior is particularly important in various applications, such as in pharmaceuticals and construction materials, as it can influence the stability and effectiveness of these powders.

The other options do not accurately define efflorescent powders. While some efflorescent materials may dissolve in water, not all do. Additionally, they do not remain solid at all times; rather, they may change state due to the loss of hydration. Lastly, while specific substances can indeed be harmful, the classification of efflorescent powders does not necessarily imply they are harmful by nature.