Which of the following should be checked before starting to heat a distillation set-up?

• A. Check that the thermometer is placed at the bottom of the distillation flask.
• B. Make sure the condenser is disconnected from the water supply.
• C. Verify that the distillation flask is completely filled with liquid.
• D. Ensure all glassware connections are secure and there are no cracks or leaks.

Answer: (D)

Verified step by step guidance

1. Understand the purpose of the distillation setup: it is used to separate components based on differences in boiling points by heating a liquid mixture.
2. Recognize that safety and proper functioning depend on the integrity of the apparatus, so checking for secure glassware connections and absence of cracks or leaks is essential before heating.
3. Evaluate the given options: placing the thermometer at the bottom of the distillation flask is incorrect because it should be positioned at the level of the vapor path to accurately measure vapor temperature.
4. Note that the condenser should be connected to the water supply to ensure efficient condensation of vapors, so disconnecting it is not advisable.
5. Understand that the distillation flask should not be completely filled; it should be partially filled to allow space for vapor formation and prevent boiling over.