Question

From where did these water droplets appear ? Explain.

Answer 1

Step 1: Identify the compound being heated:
The compound is ferrous sulphate crystals with the chemical formula:
\[ \text{FeSO}_4·7\text{H}_2\text{O} \]
This means each molecule of ferrous sulphate is associated with 7 water molecules, known as water of crystallization.

Step 2: What happens during heating:
- When ferrous sulphate crystals are heated, the water of crystallization is released as water vapor.
- The reaction is:
\[ \text{FeSO}_4·7\text{H}_2\text{O} \xrightarrow{\text{Heat}} \text{FeSO}_4 + 7\text{H}_2\text{O} \]

Step 3: Formation of water droplets:
- The released water vapor rises inside the test tube or boiling tube.
- It comes in contact with the cooler upper parts of the tube.
- Upon cooling, the water vapor condenses and forms visible water droplets.

Step 4: Conclusion:
The water droplets seen during the heating of ferrous sulphate crystals appear due to the evaporation of water of crystallization, which then condenses on the cooler inner walls of the tube.

Answer 2

Step 1: Identify the compound:
The compound in question is ferrous sulphate crystals, which have the chemical formula:
\[ \text{FeSO}_4·7\text{H}_2\text{O} \]
This is the hydrated form of ferrous sulphate, meaning it contains water of crystallization.

Step 2: Observe the initial color:
- The crystals of FeSO₄·7H₂O appear light green in color.
- This green color is due to the presence of the 7 water molecules attached to each molecule of ferrous sulphate.

Step 3: Effect of heating:
- When the crystals are heated, they lose their water of crystallization.
- The chemical change is:
\[ \text{FeSO}_4·7\text{H}_2\text{O} \xrightarrow{\text{Heat}} \text{FeSO}_4 + 7\text{H}_2\text{O} \]
- As the water evaporates, the color of the substance changes.

Step 4: Final observation after heating:
- The substance left behind is anhydrous ferrous sulphate, which is white or colorless in appearance.

Step 5: Conclusion:
- The color change observed during heating is:
Green → White
This change occurs because the green hydrated crystals lose their water of crystallization and turn into white anhydrous powder.

Answer 3

Step 1: Understand water of crystallization:
- Water of crystallization refers to the fixed number of water molecules that are chemically bound within a crystal structure of a compound.
- These water molecules are essential for maintaining the crystalline form and properties of the compound.

Step 2: Example – Ferrous sulphate:
- Each molecule of ferrous sulphate contains 7 water molecules of crystallization.
- Its chemical formula is: FeSO₄·7H₂O
- This form is known as hydrated ferrous sulphate.

Step 3: Molecular formula of other crystalline salts:
(I) Copper sulphate:
- Contains 5 water molecules of crystallization.
- Chemical formula: CuSO₄·5H₂O

(II) Sodium carbonate:
- Contains 10 water molecules of crystallization.
- Chemical formula: Na₂CO₃·10H₂O

Step 4: Conclusion:
The crystalline forms of the given salts are:
- Ferrous sulphate: FeSO₄·7H₂O
- Copper sulphate: CuSO₄·5H₂O
- Sodium carbonate: Na₂CO₃·10H₂O
These water molecules give the compounds their characteristic crystalline shapes and colors.

Answer 4

Step 1: Understand the source material – Gypsum:
- Gypsum is a naturally occurring mineral with the chemical formula CaSO₄·2H₂O (calcium sulfate dihydrate).
- It contains two molecules of water of crystallization per formula unit.

Step 2: Heating gypsum to form Plaster of Paris:
- When gypsum is heated at 373 K (or 100°C), it loses water molecules.
- The chemical reaction is:
\[ \text{CaSO}_4·2\text{H}_2\text{O} \xrightarrow{373\,\text{K}} \text{CaSO}_4·\tfrac{1}{2}\text{H}_2\text{O} + \tfrac{3}{2}\text{H}_2\text{O} \]
- The product formed is Plaster of Paris (CaSO₄·½H₂O).

Step 3: Properties of Plaster of Paris:
- It is a white powder.
- When mixed with water, it sets into a hard mass due to rehydration back to gypsum.

Step 4: Uses of Plaster of Paris:
1. Medical use: It is used for making casts for broken bones because it sets quickly and holds the shape firmly.
2. Decorative use: It is widely used for making decorative materials such as statues, designs, and false ceilings in buildings.

Step 5: Conclusion:
- Plaster of Paris (CaSO₄·½H₂O) is formed by heating gypsum at 373 K.
- It is used for orthopedic casts and in interior decoration.