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?
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.