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Today I decided to check my thermometer and measured the temperature of boiling tap water. The thermometer showed 98.5°C. The atmospheric pressure now is 742 mmHg. According to the Clausius–Clapeyron equation, at this pressure the water should boil at 99.3°C. Is my thermometer faulty or there are some other factors that could have affected the boiling temperature?

Thank you.

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UPDATE: Measuring ice in water gave me + 0.6°C degrees. Still not enough information to conclude how accurate (faulty) the thermometer is. You guys are probably right: the only way to tell is to use distilled water. Unless somebody measures tap water with a thermometer that is known to be accurate.

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    $\begingroup$ Tap water contains impurities and hence boils at a lower temperature than that calculated for pure water at a given pressure. $\endgroup$ – Lelouch Jul 3 '16 at 19:58
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    $\begingroup$ Measuring the temperature of ice water (should be 0°C) would probably be a more reliable means of calibration. Also, what type of thermometer is it? An error of about 1% is entirely reasonable for most types. $\endgroup$ – lemon Jul 3 '16 at 20:03
  • $\begingroup$ @user47033 Is it your guess or your experience? Because I know that some salts, when dissolved in water, increase the boiling temperature, rather than decrease it. Also, they say that even for the pure water the temperature of boiling water should be slightly above the calculated boiling temperature. $\endgroup$ – GreenBear Jul 3 '16 at 20:13
  • $\begingroup$ @lemon That is the next thing I was going to do. I am still freezing the water. The thermometer is laboratory-type mercury-in-glass thermometer. I have no idea how exactly accurate it is supposed to be. Hence the question. I thought it was supposed to be pretty accurate. $\endgroup$ – GreenBear Jul 3 '16 at 20:13
  • $\begingroup$ Precise temperature measurement is actually pretty hard. If you are within a couple of degrees, it's OK for most types of thermometers. You would have to buy a precision lab thermometer with calibration certificate to be guaranteed to be much better than that. $\endgroup$ – CuriousOne Jul 3 '16 at 20:14
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Tap water has various impurites , which lowers the vapor pressure of the resulting mixture. This leads to a lower boiling temperature.

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    $\begingroup$ If the impurities lower the vapour pressure then a higher temperature (to raise the vapour pressure to atmospheric) is needed to boil the water. $\endgroup$ – Farcher Jul 3 '16 at 20:13
  • $\begingroup$ Should not the lowering of vapor pressure result in a higher boiling temperature? $\endgroup$ – GreenBear Jul 3 '16 at 20:14
  • $\begingroup$ Sugar syrup boils at notably higher temperatures than pure water. $\endgroup$ – GreenBear Jul 3 '16 at 20:21

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