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Representative sampling of a powder for size analysis is difficult chiefly because

  • A.the particles change their size while they are being stored
  • B.no instrument is able to measure a sufficiently small sample
  • C.powders react with the air during the sampling operation
  • D.powders segregate by size when handled or poured

Answer: D

Pouring a heap sends coarse particles rolling to the outside, so a scoop from one place is not the batch. A spinning riffler or a thief sampler is used to obtain a genuinely representative portion. Sampling error frequently exceeds instrument error by a wide margin.

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