Rhombic and monoclinic sulphur are _____.

Answer

**Step-by-Step Solution:**

1. **Understanding Allotropes:**
- Both rhombic and monoclinic sulfur are forms of sulfur that exist in different physical structures but have the same chemical composition. This phenomenon is known as allotropy.

2. **Defining Rhombic and Monoclinic Sulfur:**
- Rhombic sulfur, also known as alpha sulfur, has a crystalline structure that forms rhombic octahedral crystals.
- Monoclinic sulfur, also known as beta sulfur, has a needle-like crystalline structure.

3. **Comparing Physical Properties:**
- **Density:**
- Rhombic sulfur has a density of approximately 2.06 g/ml.
- Monoclinic sulfur has a density of about 1.98 g/ml.
- **Melting Point:**
- The melting point of rhombic sulfur is 112.8 °C.
- The melting point of monoclinic sulfur is 119 °C.

4. **Preparation Methods:**
- Rhombic sulfur can be prepared by dissolving sulfur powder in carbon disulfide.
- Monoclinic sulfur is prepared by evaporating sulfur powder in a dish until it melts.

5. **Conversion Between Forms:**
- When rhombic sulfur is heated to a temperature of 369 K (or higher), it converts to monoclinic sulfur.

6. **Solubility:**
- Rhombic sulfur is insoluble in water but soluble in organic solvents like ether and carbon disulfide.
- Monoclinic sulfur has the opposite solubility characteristics.

**Final Answer:**
Rhombic and monoclinic sulfur are **allotropes** of sulfur.

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