Which of the following is NOT a source of error in a melting point experiment?

In a melting point experiment, it is crucial to obtain an accurate measurement of the sample's melting point to infer purity and identity. Using a calibrated thermometer ensures that the temperature readings are accurate, which is essential for reliable results. Therefore, employing a calibrated thermometer does not introduce error; instead, it minimizes potential inaccuracies in the measurement process.

Conversely, moisture in the air can affect the melting point by introducing water into the sample, which may alter its behavior during heating. Contamination of the sample with other substances can also lead to erroneous melting point readings, as impurities generally depress or elevate the melting point depending on their nature. Additionally, determining the melting point too hastily can result in not accurately observing the end point of the melting transition, leading to an incorrect assessment of the temperature. All these factors can introduce significant error, making the use of a calibrated thermometer the only option that does not contribute to experimental error.