Question

Is a calibrated thermometer more precise than one that has not been calibrated? What implications does this have for measuring temperature changes as contrasted with measuring individual temperatures?

Answer 1

If you want to know the temperature than a calibrated thermometer will give that to you and one that has not been calibrated will not. However, if you are interested only in the DIFFERENCE in temperatures, then it doesn't matter. WHY? Let's take an example.
With a calibrated thermometer:
Final reading = 31.0 C
initial reading = 25.0 C
difference = 6.0 degrees C.

With an uncalibrated thermometer that reads too high by 5 degrees C:
final reading = 36.0 C
initial reading = 30.0 C
difference = 6.0 C.
So it makes a huge difference (5.0C) if we want to know the exact temperature because we would read 31.0 as the correct T but the "bad" thermometer would tell us 36.0 C. Big difference. But if we want to know the difference in two temperatures, both thermometers give the same result.

Answer 2

what role does barometric pressure play in the boiling point of water?