When a delivery tube is involved in an experiment and heat is removed, what precaution should be taken?

• A. Remove the delivery tube from water to prevent suck-back.
• B. Leave the tube in water.
• C. Seal the system.
• D. Cool the tube with air.

Answer: (A)

When gas is collected over water and you stop heating, the gas inside the delivery tube cools and contracts, creating lower pressure. If the end of the tube is still submerged, water can be drawn back up into the apparatus—suck-back. Lifting the delivery tube out of the water removes the water’s contact at the end, so nothing is drawn back and the gas collection remains clean.