हिंदी
Maharashtra State BoardSSC (Marathi Semi-English) 9th Standard [इयत्ता ९ वी]
पाठ्यपुस्तक उत्तर१०८२८
अवधारणा स्पष्टीकरण और वीडियो३४०
पाठ्यक्रम

Water of Crystallization

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Topics

  • Introduction
  • Experiment 1
  • Experiment 2

Introduction:

Water of crystallisation refers to the fixed number of water molecules that are an integral part of a compound's crystalline structure. These water molecules are chemically bonded within the lattice of the crystal and contribute to the compound's physical appearance, including its shape and colour.

For example:

  • Blue vitriol: CuSO₄·5H₂O (blue color is due to water of crystallisation)
  • Green vitriol: FeSO₄·7H₂O
  • Washing soda: Na₂CO₃·10H₂O
  • Alum: K₂SO₄·Al₂(SO₄)₃·24H₂O

When heated, compounds with water of crystallisation lose these water molecules, causing them to lose their crystalline structure and change in appearance. This property is common in hydrated salts and demonstrates the role of water in the stability of crystalline solids.

Experiment 1

1. Aim: To observe the water of crystallisation in blue vitriol (copper sulphate pentahydrate) and understand the changes on heating.

2. Requirements: Blue vitriol crystals (CuSO₄·5H₂O), two test tubes, distilled water, a Bunsen burner, and a test tube holder.

3. Procedure

  • Take blue vitriol crystals in two test tubes.
  • Add water to the first test tube, shake, and observe the colour.
  • Heat the crystals in the second test tube over a flame and observe changes.
  • After cooling, add water to the heated crystals and observe the colour.

Properties of water of crystallisation

\[\begin{array}{cc} & \\ \mathrm{CuSO}_4.\mathrm{5H}_2\mathrm{O} +Heat& \longrightarrow{}{CuSO}_4+\mathrm{5H}_2\mathrm{O}\\ \mathrm{(Blue)} & \mathrm{(White)}\end{array}\]

4. Observations: First test tube: blue crystals dissolved, forming a blue solution.

Second test tube:

  • On heating, blue crystals turned white.
  • Water droplets appeared on the test tube walls.
  • Adding water to the white powder restored the blue solution.

Conclusion: Blue vitriol contains water of crystallisation, giving it a blue colour. Heating removes this water, turning it into anhydrous copper sulphate (white). Adding water restores the original hydrated salt, demonstrating water of crystallisation in hydrated salts.

Experiment 2

1. Aim: To observe the water of crystallisation in alum and its effects on heating.

2. Requirements

  • Apparatus: evaporating dish, Bunsen burner, tripod stand, wire gauze.
  • Chemical: Alum.

3. Procedure

  • Place a small piece of alum in an evaporating dish.
  • Set the dish on a tripod stand over a wire gauze.
  • Heat the dish gently using a Bunsen burner.
  • Observe the changes in the alum during and after heating.

4. Observations: On heating, the alum stone loses its crystalline structure. Water droplets appear, indicating the release of water of crystallization. The alum becomes puffed and loses its solid, crystalline form, turning into a white, amorphous mass.

5. Conclusion: Alum contains water of crystallisation, which is an integral part of its crystalline structure. On heating, this water is released, causing the crystalline structure to break down, resulting in puffed alum. This shows that crystalline substances like alum contain water of crystallisation, which is lost upon heating, altering their physical properties.

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