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From the ideal gas equation, PV is proportional to T. Now if V is kept constant, we'll get Gay-Lussac's law.

Now my question is:

According to observations, we know that substances undergo a change in dimensions or they expand on exposure to heat. So wouldn't that mean the fixed volume of gas must expand on exposure to heat? But if this happens the pressure must also increase, and wouldn't that break the glass thermometer?

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  • $\begingroup$ There is no "must". The gas will want to expand, that's right. But whether we let that happen or not is another question. $\endgroup$ – Ivan Neretin Jan 10 '18 at 6:21
  • $\begingroup$ When a basketball is inflated, pressure increases and it must be accompanied by volume decrease right?. The volume is the space occupied by the molecules of the gas and it's increasing as the no.of molecules of gas pumped into the body increases? $\endgroup$ – susan J Jan 10 '18 at 6:43
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If the temperature is high enough, yes, the glass thermometer will indeed break.

However, the glass (or other material) used to construct a thermometer should be strong enough to withstand the internal pressure in the temperature range for which it is intended. Else the thermometer would not be safe to use.

According to wikipedia, glass typically has a tensile strength (ability to withstand being pulled apart) of 7 MPa (approximately 70 atm), which should be sufficient for most applications.

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  • $\begingroup$ Or the inability of the glass to deform ensures that the gas has a constant volume? $\endgroup$ – susan J Jan 10 '18 at 5:54
  • $\begingroup$ You are aware that a tensile strength of 7 MPa does not mean that a given equipment geometry can survive a pressure of 70 atm, correct? There is a big difference between the radial pressure within a cylinder, and the tensile stress in the cylindrical shell. $\endgroup$ – Chet Miller Jan 10 '18 at 19:26
  • $\begingroup$ Yes I am aware! But the number serves as an indication. $\endgroup$ – helen Jan 11 '18 at 5:59

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