When a delivery tube is used in an experiment and heat is removed, what precaution should you take?

• A. Remove the delivery tube from water to prevent suck-back.
• B. Keep the tube submerged to maintain pressure.
• C. Tilt the apparatus to allow air in.
• D. Seal the end of the delivery tube.

Answer: (A)

When heat is removed in a gas-collection setup, the gas inside contracts and the pressure inside the apparatus drops. If the end of the delivery tube is still submerged in the collecting water, this lower pressure pulls water back up the tube into the apparatus, causing suck-back. To prevent this, lift the delivery tube so its end is not underwater; this lets air enter and stops water from being drawn in as the gas cools, keeping the gas collection dry and accurate.

Keeping the tube submerged would still allow water to be sucked back; tilting the apparatus to let air in isn’t reliable and can disrupt the setup; sealing the end would trap gas and not address the issue of water being drawn into the delivery tube.