When is a sample considered dry according to the mass change criteria?

• A. After it loses at least 2% of its mass
• B. When it shows no loss in water content
• C. When it shows less than a 0.1% change in mass after heating
• D. Only after resting at room temperature for 24 hours

Answer: (C)

A sample is considered dry according to the mass change criteria when it shows less than a 0.1% change in mass after heating. This criterion is significant because it indicates that the sample has reached a point where any further weight loss during heating is negligible and that most of the moisture has been removed. In practice, this low threshold signifies that the majority of the water has evaporated, making the sample acceptable for further analysis or processing.

In comparison to other options, a 2% mass loss might not be sufficiently indicative of complete drying, as variations in sample composition could lead to a larger mass change due to other factors unrelated to moisture content. The lack of visual change in water content does not quantify the exact moisture level effectively, leaving room for ambiguity. Waiting for 24 hours at room temperature, on the other hand, may not accurately determine dryness as environmental conditions could still influence moisture retention or absorption. Thus, reliance on a precise mass change measurement after heating provides a clearer standard for evaluating the dryness of the sample.