A teaching article in J. Chem. Educ. describes incidents where, “on removing the heat source too quickly from a stillpot, students observed the contents of the receiving flask being drawn back into the distillation flask due to the rapid contraction of vapour.” It notes that “this backflow into a very hot flask has occasionally resulted in fractured glassware” and recommends design changes and procedural instructions to keep the condenser outlet above the liquid and to vent the system before cooling.

The article explains that thermal shock occurs when one part of the glass changes temperature faster than the rest, creating internal stress that can crack the vessel without warning. It gives examples such as "a hot flask is placed on a cold benchtop" and "hot glassware is rinsed under cold tap water" as situations where rapid cooling of hot glass can lead to cracking. It emphasizes: "Do not place hot glassware directly on a cold metal surface" and "Never rinse hot glassware with cold water" as key rules to prevent thermal shock.

Ensure that the outlet of the condenser is not under the surface of the distillate in the collecting cylinder. On cooling, the apparatus can suck liquid back along the delivery tube into the flask. As the flask is still hot, the inrush of cold liquid can crack the glass and may also cause the hot mixture to spit.

This explanation notes that laboratory glassware must withstand extreme temperature changes and describes thermal shock (thermal fracturing) as occurring "when large temperature differences are present in the glassware." It states that thermal shock "causes the glass to crack and break when the heated area expands and the cool area contracts," directly linking sudden temperature changes in a hot vessel exposed to a cooler region with cracking.

The page describes gas-collection "suck back" when a heated test tube connected to a delivery tube is cooled: "When the test tube is strongly heated the air within it expands, and can be seen bubbling out of the delivery tube and through the water. If the heat is removed, or becomes less intense, then the air in the test tube contracts and sucks cold water back along the delivery tube." It then warns: "A Pyrex test tube can withstand limited thermal shock - it should easily cope with cooling from 100 degrees centigrade rapidly to room temperature. However, the glass in a strongly heated test tube is at several hundred degrees centigrade, and it is unlikely that the glass will survive the sudden cooling by the cold water. There is a high risk that the glass will shatter…" The page presents this as an example of why suck back into hot glassware is dangerous and must be avoided.

The article explains that when a glass container is filled with a hot liquid, there is an immediate temperature difference between the inside and outside surfaces, denoted as ΔT, and "due to this difference, stresses develop in the glass." It notes that "the higher the temperature differential, the greater the stresses and this can result in breakage" and that this mechanism also applies when the inside is hot and the outside is suddenly cooled. It further states that thermal shock breakage typically shows "a crack that circles the base and rises up the body of the container."

Under safety notes the page warns: “Never immerse the delivery tube from a condenser below the surface of the liquid in the collecting vessel. As the system cools or if the heat source is removed, a partial vacuum can form in the apparatus and the liquid will be sucked back into the hot distillation flask (suck‑back). This is dangerous with flammable or reactive liquids.” It further notes that such sudden inrushes can cause violent boiling and thermal stress on the glassware.

This technical note defines thermal shock as "damage or failure caused by a rapid change in temperature" where the outer surface may expand or contract faster than inner layers, creating internal stress that can lead to cracking or failure. Among the listed don'ts is: "Avoid sudden exposure to large temperature changes, such as adding cold liquid to a hot vessel," underscoring that introducing a cold liquid into a hot glass vessel is a known cause of thermal-shock cracking.

The safety section on stopping a vacuum distillation notes: “It is also important to first allow air back into the system before turning off the vacuum source. If the vacuum is turned off first, sometimes changes in pressure inside the apparatus (as it cools) cause back-suction. If a water aspirator is used, this may cause water from the sink to be pulled into the vacuum line. The vacuum trap prevents this back suction from ruining the distillate.” This describes the same pressure‑drop mechanism that can pull liquid back into hot apparatus if outlets are submerged.

This educational lesson describes that materials expand when heated and contract when cooled, and that if this expansion is uneven, "the expansion can create forces within an object that might cause it to crack if the temperature changes too quickly." It states that "this cracking is what we call thermal shock," clarifying that rapid temperature changes within glass or ceramic objects can lead to cracking due to internal stresses.

In the section on distillation hazards, the guide states: “Ensure that the outlet of the condenser remains above the surface of the distillate in the receiver. Immersion of the outlet may result in back‑siphoning of the distillate into the stillpot if heating is stopped or the apparatus cools, causing a sudden inrush of liquid into hot glassware.” It warns that the resulting rapid cooling and possible overfilling of the boiling flask increases the risk of violent boiling and glass breakage.

The handbook cautions: “Do not allow the condenser outlet to dip below the surface of liquid in the receiving flask. If the distillation is stopped and the apparatus cools, a partial vacuum can develop and the liquid in the receiver can be sucked back into the hot distilling flask (suckback). This can cause sudden boiling, contamination, and in extreme cases thermal shock to the glassware.” The document presents this as a standard safety concern in distillation work.

This safety training video, used on CLEAPSS courses, demonstrates "suck back" when heating calcium carbonate with gas collection over water. The description notes: "How not to heat calcium carbonate, showing the effects of 'suckback'". In the demonstration, when heating stops, water is visibly drawn back through the delivery tube into the hot reaction tube, illustrating that cooling of the gas in the system can cause external liquid to be sucked back. The video is used in school laboratory safety training to show why delivery tubes should not be left immersed and why suck back into hot glassware is hazardous.

In this safety demonstration, the presenter shows what happens when a glass carboy undergoes rapid temperature change from cold to hot, leading to cracking and failure. The video warns: "Please do not pour hot wort into your carboy when it is cold, or place a glass jar full of hot liquid on a cold surface. Not only could you experience cracking glass, but the glass could shatter violently," illustrating the risk of thermal shock when hot glassware contacts a cooler environment.

Standard laboratory safety guidance for distillation (as found in undergraduate organic chemistry lab manuals) warns that if the delivery tube or condenser outlet is left immersed in the collected distillate when heating is stopped, a partial vacuum can form in the apparatus as vapour condenses. This can suck the cooled distillate back into the still pot, where it contacts the much hotter distillation flask. Many manuals note that this sudden introduction of relatively cold liquid into hot glassware can cause thermal stress and potentially crack the flask, even though modern borosilicate glass reduces the likelihood compared to ordinary glass.

A user asks for videos showing suck back during distillation "to show some people the effect of rapid cooling of hot glassware by solvent or water when it gets sucked back." Respondents discuss that when vacuum or pressure drops, liquid can be drawn back from a receiver into the hot flask. One comment describes that this can quench very hot glass and is a recognized hazard: it "can easily crack the boiling flask" and is something "everyone is warned about in basic lab safety." Although informal, the thread reflects practitioners’ awareness that suck back during distillation can cause thermal shock to glassware.

This ACS safety guide, widely used in teaching labs, discusses thermal shock to glassware in a section on glass apparatus. It notes that hot glass can be fractured by contact with cold liquids: "Glass may break if a hot vessel is cooled rapidly; avoid directing cold water on hot glassware or allowing cold liquids to be drawn into hot flasks through connecting tubing." It also mentions hazards from gas collection and distillation setups: "Prevent 'suckback' of cooling water or receiving liquids into hot reaction flasks by proper design and by removing delivery tubes from liquids before interrupting heating." The guide warns that such accidents can shatter glass and cause burns or cuts.