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SCIENCE

The Claim

When stopping a distillation, if the delivery tube is immersed in the distillate, the cooled liquid can be sucked back into the hot distillation flask, and the sudden temperature change can cause the glass to crack.

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The Short Version

The described hazard is well established in laboratory distillation guidance. A submerged delivery tube can allow distillate to be drawn back into a hot flask as the apparatus cools, and that cold-liquid contact can thermally shock the glass and crack it. Breakage is not inevitable, but the risk and mechanism are real and widely documented.

Caveats

  • This is a conditional hazard: suck-back is more likely when the outlet is submerged and the apparatus cools without being vented properly.
  • Glass cracking is a possible consequence, not an automatic one; contamination, bumping, and sudden boiling are also common hazards of suck-back.
  • Risk varies with glass type, temperature difference, and liquid volume, so prevention still requires correct setup and shutdown technique.

The Receipts

  1. Preventing "Suckback" in Student Distillations

    Journal of Chemical Education (ACS Publications)

  2. Thermal Shock Explained: Simple Rules To Prevent Glass Breakage In Daily Lab Work

    LabChoice Australia

  3. Fractional distillation of an ethanol–water mixture – student sheet

    Royal Society of Chemistry Education

  4. Why Is Thin Glassware Used in Labs for Heating?

    At-Mar Glass

  5. Suck back

    ISSR / Halesowen College (educational lab safety material)

  6. Thermal Shock - An Explanation of Why Glass Breakage Occurs and How to Prevent It

    Aegg

  7. Distillation Techniques and Safety

    University of St Andrews, School of Chemistry

  8. What is Thermal Shock - and the Do's and Don'ts to Avoid It

    De Dietrich Process Systems

  9. 5.4C: Step-by-Step Procedures for Vacuum Distillation

    LibreTexts Chemistry

  10. Thermal Shock Lesson

    Ceramic and Glass Industry Foundation

+ 6 more sources — see the full list on Lenz

Filed Under

Distillation