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I saw my mother washing thermos flask with warm water. I ask to her why she do so. She replied: " because if you wash it with cold water, then may it get broken." I wondered how but she had no reply. Rather, she promissess that she had such experiences. This was my problem. Why the glass inside thermos flask get broken when treat with cold water? ( cold water means not so cold. I mean at $27^0$ ). I have no well knowledge about the properties of materials used to make thermos flask, but my problem is that, if it can work well in hot, then how it get broken when subjected to cold water?

I search it in web and there shows manual of some thermos flasks tell that to use warm water to wash it and avoid cold water, as my mother said.

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The main reason why a thermos flask may break is when when you pour cold water into it while the glass is still hot. E.g. you empty a thermos flask that still contains some hot liquid, then immediately follow this up with cold water.

The phenomenon is called thermal shock. When the cold water hits the hot glass inner wall of the flask the glass starts cooling and shrinking due to thermal expansion (in reverse). But heat conduction takes time, so for a brief moment the inside of the glass wall is colder than the outside of the glass wall.

As a result the inside is contracting while the outside is not yet. This causes stress in the glass which in some cases leads to the glass cracking.

You can even demonstrate this using a small, cheap drinking glass. Heat it up in an over to about $150^\circ$ (Celsius) and prepare a water/ice bath. Gently drop the hot glass into the ice/water bath: chances are that it will shatter.

A cold thermos flask will not break when you pour cold water into it.

Note that it is by no means guaranteed that a thermos flask will break in these conditions. Local flaws, scratches or pre-existing micro-fissures in the glass probably initiate/exacerbate the breakage. Severity of the thermal shock (temperature difference), glass thickness and innate glass tensile strength also play a role.

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    $\begingroup$ Wouldn't the reverse not also break the glass? Poring hot liquid in while the glass is still cold? $\endgroup$ – Gyro Gearloose Jan 2 '16 at 16:20
  • $\begingroup$ @GyroGearloose: yes, it would, the phenomenon is the same. But either way, far from every thermos flask will break. It depends on temperature differences, thickness and glass strength. $\endgroup$ – Gert Jan 2 '16 at 16:21
  • $\begingroup$ Then why are there no warnings about filling hot liquids in? I feel there must be an additional, different effect at work. $\endgroup$ – Gyro Gearloose Jan 2 '16 at 16:24
  • $\begingroup$ @GyroGearloose: the phenomenon is well understood and people who work a lot with lab glassware (like me) have usually experienced it. Local flaws, scratches or pre-existing micro-fissures probably initiate/exacerbate it. $\endgroup$ – Gert Jan 2 '16 at 16:26
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    $\begingroup$ The experiment you suggest can be rather dangerous: dropping glass at 150°C into (cold) water without appropriate protective gear may result in the glass not simply cracking, but bits of glass flying into your eyes, cutting your hands, ... I am all for recommending hand-on experiments: but PLEASE give some warnings about the dangers of the experiment and appropriate precautions to take. $\endgroup$ – Floris Jan 2 '16 at 19:13

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