Silk and wool are both natural proteins but differ significantly in their chemical composition and properties. Here are some ways to differentiate them by solubility and chemical tests:


Solubility

  1. Solubility in Water:

    • Silk: Generally insoluble in water.
    • Wool: Also insoluble in water, but it can absorb significant amounts of water without dissolving.

  2. Solubility in Solvents:

    • Silk: Dissolves in concentrated sulfuric acid, hydrofluoric acid, and certain strong alkaline solutions.
    • Wool: Dissolves in hot concentrated sulfuric acid but not as readily as silk. It also dissolves in sodium hypochlorite (bleach) and caustic soda (sodium hydroxide).

Chemical Tests

  1. Burn Test:

    • Silk:
      • Burns slowly.
      • Smells like burning hair (similar to wool but usually less intense).
      • Leaves a black, brittle, and crushable residue.
    • Wool:
      • Burns slowly and tends to self-extinguish.
      • Smells like burning hair or feathers.
      • Leaves a black, crumbly ash.

  2. Xanthoproteic Test:

    • This test detects the presence of aromatic amino acids (like tyrosine and tryptophan) in proteins.
    • Procedure: Add concentrated nitric acid to the sample, then heat it.
      • Silk: Turns yellow, indicating the presence of aromatic amino acids.
      • Wool: Also turns yellow, but this test is not definitive for differentiating wool from silk as both will react similarly.

  3. Lead Acetate Test:

    • Procedure: Soak the fabric in a lead acetate solution.
      • Silk: Does not react significantly.
      • Wool: Forms a black precipitate due to the sulfur in the cystine amino acid reacting with lead ions to form lead sulfide.

  4. Alkaline Test:

    • Procedure: Treat the sample with a strong alkaline solution like sodium hydroxide.
      • Silk: Dissolves in strong alkaline solutions.
      • Wool: Also dissolves in strong alkaline solutions but more slowly compared to silk.

  5. Biuret Test:

    • Procedure: Add a few drops of dilute copper sulfate solution to an alkaline solution of the sample.
      • Silk: Produces a violet or purple color due to the presence of peptide bonds.
      • Wool: Also produces a violet or purple color for the same reason.

Summary

While both silk and wool are proteins and share some chemical properties, they differ in their reaction rates and solubility in certain solvents. The lead acetate test and the burn test are particularly useful for distinguishing between them due to wool's unique sulfur content and the distinct odor of their combustion products.