Water of crystalisation

• What is water of crystallisation?


  Water molecules that are incorporated into the crystal structure of a compound but not chemically bonded to it.
• What is a hydrated compound?

  A compound that contains a fixed number of water molecules within its crystal lattice.
• What is an anhydrous compound?

  A compound that does not contain water of crystallisation.
• How is water of crystallisation represented in a chemical formula?

  By a dot between the formula of the compound and the number of water molecules.
• What happens to a hydrated compound when it is heated?

  It loses water of crystallisation and becomes anhydrous.
• What are the steps to determine water of crystallisation experimentally?

  1. Weigh the hydrated compound.
  2. Heat to remove water.
  3. Weigh the anhydrous compound.
  4. Calculate moles of water lost and anhydrous compound to find the ratio.
• How do you calculate the number of moles of water lost from a hydrated compound?

  Divide the mass of water lost by the molar mass of water (18.0 g/mol).
• How do you calculate the number of moles of the anhydrous compound?

  Divide the mass of the anhydrous compound by its molar mass.
• How do you determine the number of water molecules per formula unit of a salt?

  By finding the ratio of moles of water to moles of the anhydrous compound.
• What color change occurs with cobalt(II) chloride when it is hydrated?

  It changes from blue (anhydrous) to pink (hydrated).
• What color change occurs with copper(II) sulfate when it is hydrated?

  It changes from white (anhydrous) to blue (hydrated).
• What is the purpose of heating a hydrated salt in an experiment?

  To drive off water of crystallisation and convert the salt into its anhydrous form.
• What happens during the thermal decomposition of hydrated salts?

  Hydrated salts lose water of crystallisation in stages upon heating, and further decomposition may occur at higher temperatures.
• What is the molar mass of water (H₂O)?

  18.0 g/mol.
• What does the dot in Na₂CO₃•10H₂O signify?

  The association of 10 water molecules with one formula unit of sodium carbonate in the crystal structure.
• What is the experimental evidence that a compound contains water of crystallisation?

  The compound loses mass when heated, corresponding to the mass of water driven off.
• Why is it important to reweigh the compound after cooling in a water of crystallisation experiment?

  To ensure that all water has been driven off and prevent errors due to moisture absorption from the air.
• How can you test for the presence of water of crystallisation in a salt?

  Heat the salt and observe if it changes color or loses mass, indicating the loss of water.
• What does the term "efflorescence" refer to?

  The process by which hydrated salts lose water of crystallisation when exposed to air, turning into the anhydrous form.
• Give an example of a salt commonly used to demonstrate water of crystallisation in the lab.

  Copper(II) sulfate (CuSO₄•5H₂O).