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Difficulty: HardFiltration, Evaporation, and Crystallization

A student is provided with a crude sample of rock salt containing sodium chloride (NaClNaCl) contaminated with insoluble calcium carbonate (CaCO3CaCO_3) and sand (SiO2SiO_2). Arrange the following laboratory procedures in the correct sequence to obtain pure, dry crystals of sodium chloride from this mixture.

  1. 1Grind the crude rock salt into a fine powder, add distilled water, and stir thoroughly to dissolve the soluble sodium chloride.
  2. 2Pour the mixture through a filter paper and funnel, collecting the insoluble sand and calcium carbonate as residue and the sodium chloride solution as filtrate.
  3. 3Gently heat the sodium chloride filtrate in an evaporating basin until a saturated solution is formed at its crystallization point.
  4. 4Allow the hot saturated solution to cool slowly at room temperature to form sodium chloride crystals.
  5. 5Separate the crystals from the remaining liquid by filtration and dry them gently between sheets of filter paper.

Answer

The correct sequence of procedures is: First, grind the sample and dissolve it in distilled water. Next, filter the mixture to separate the insoluble solids from the NaClNaCl filtrate. Then, heat the filtrate to its crystallization point to obtain a saturated solution. After that, cool the solution slowly to allow crystal formation. Finally, filter and dry the crystals between filter papers.
The correct order follows the standard laboratory procedure for recovering a pure salt from a mixture containing insoluble solids: dissolution →\rightarrow filtration →\rightarrow evaporation to crystallization point →\rightarrow cooling →\rightarrow final filtration and drying.

Step-by-Step Solution

1
Dissolution of soluble component
Sodium chloride dissolves in distilled water while sand and calcium carbonate remain undissolved solids.
Solubility differences allow selective dissolution of NaClNaCl from insoluble impurities.
2
Gravity filtration
Insoluble sand and CaCO3CaCO_3 are retained on the filter paper as residue; aqueous NaClNaCl passes through as filtrate.
Filtration separates insoluble solids from aqueous solutions.
3
Controlled evaporation to saturation point
Water evaporates until the NaClNaCl solution reaches saturation (crystallization point).
Heating to dryness should be avoided when pure crystals are desired, as rapid evaporation can trap impurities or spur spitting.
4
Controlled cooling and crystallization
Well-defined crystals of sodium chloride precipitate out of the cooling solution.
Solubility decreases with lowering temperature, allowing solute molecules to arrange into crystal lattices.
5
Isolation and drying of crystals
Pure, dry crystals of NaClNaCl are collected.
Filtering removes the remaining mother liquor containing residual soluble contaminants, and filter paper absorbs surface moisture.

Key Concept

Multi-step purification of a soluble salt from insoluble impurities using dissolution, filtration, evaporation to saturation, and crystallization.
Estimated Time:2m 0s
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Topics

Filtration, Evaporation, and Crystallization