Which technique is used to separate solids by size prior to metal analysis?

• A. Centrifugation
• B. Filtration
• C. Sieving
• D. Decantation

Answer: (C)

Separating solids by size relies on a barrier that only allows particles smaller than a chosen opening to pass through. Sieving uses a screen with defined mesh openings, so when the mixture is agitated, larger particles stay on top while smaller ones go through to form a finer, more uniform fraction. This is especially important before metal analysis because analytical methods often require a consistent particle size to dissolve evenly and produce representative results; uneven sizes can hinder digestion and lead to unreliable measurements. Other methods target different properties: centrifugation separates by density and mass under rotation; filtration removes liquid from solids based on pore size; decantation involves pouring off liquid while leaving solids behind. So sieving is the approach that directly controls size separation.