How is a sample considered thoroughly dry?

• A. When it shows less than a 0.1% change in mass after additional heating
• B. When it feels dry to the touch
• C. When its temperature stabilizes at room temperature
• D. When it loses 5% of its original mass

Answer: (A)

A sample is considered thoroughly dry when it shows less than a 0.1% change in mass after additional heating. This measurement is critical in determining the completeness of the drying process. When a sample is heated, any remaining moisture will evaporate, causing a change in mass. The threshold of 0.1% is a stringent criterion that ensures any trace amounts of moisture have been effectively removed. This level of precision is particularly important in scientific and industrial contexts, where even a minuscule amount of water can affect chemical reactions, physical properties, or product quality.

Other methods of assessing dryness, such as feeling the sample or waiting for it to reach room temperature, are much less reliable because they do not provide quantitative data on moisture content. The option regarding mass loss of 5% may indicate that a sample has dried, but it does not guarantee that all moisture has been eliminated, especially if a small amount remains undetected. Therefore, relying on mass change measurement is the most accurate approach to ensure thorough drying.