Samples under vacuum must be agitated by which device?

• A. Manual stirring only
• B. A mechanical device
• C. Ultrasonic bath
• D. No agitation required

Answer: (B)

Under vacuum conditions, mixing needs to be steady and reproducible to keep solids in suspension, prevent settling, and ensure the sample interacts uniformly with the solvent or atmosphere. A mechanical agitator provides continuous, controlled agitation that doesn’t depend on the operator’s effort, which is essential when air changes and pressure can make manual stirring unreliable. An ultrasonic bath isn’t suitable because it requires immersion and generates cavitation and heat that don’t fit a vacuum setup, and can disturb the sample or equipment. No agitation would let solids settle and create nonuniform results, while manual stirring alone can be inconsistent in a closed vacuum system. So a mechanical device is the best choice for reliable, representative agitation under vacuum.