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InkFlaskCambridge IGCSE

Hold, mix and react · Biology · Chemistry

Delivery tube and bung

Delivery tube and bung, with labels: bung, delivery tube, the solid is heated here, limewater, the tube’s end below it.

What it is for

Carries gas from a stoppered flask or tube to a syringe or test.

Mistake to avoid Leaving the delivery tube in limewater or water when heating stops: the cold liquid is sucked back into the hot tube (suck-back).

How it is asked

  • Complete a diagram: the delivery tube runs from an airtight bung into the limewater, with its end below the surface.
  • Test for carbon dioxide: bubble the gas through limewater, which turns milky.
  • Explain suck-back, and how to stop it: lift the tube out of the liquid before the heating stops.

Exam-style question

A student heats copper(II) carbonate in a test-tube fitted with a bung and delivery tube, and passes the gas into limewater.

  1. What is seen in the limewater, and what does it show?
  2. Why must the bung fit tightly?
  3. Why does the student lift the delivery tube out of the limewater before stopping the heating?
Show what a good answer says
  1. The limewater turns milky (cloudy white): the gas is carbon dioxide.
  2. Otherwise the gas escapes round the bung instead of passing through the limewater.
  3. As the hot tube cools, the gas in it contracts and limewater is sucked back into the hot tube, which can crack it.

Teaching points for checking your own answer, not official Cambridge marks.

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