Which of the following oxide is thermaly least stable?

Options

A. CaO
B. `Al_2O_3`
C. `Fe_2O_3`
D. `Ag_2O`

Answer

To determine which oxide is thermally least stable, we need to analyze the thermal stability of various metal oxides. Here’s a step-by-step solution:

### Step 1: Understand Thermal Stability
Thermal stability refers to the ability of a compound to resist decomposition when subjected to heat. The less thermally stable an oxide is, the more likely it is to decompose into its constituent elements or other compounds when heated.

### Step 2: Identify the Oxides
The question provides a list of oxides, which may include:
- Calcium oxide (CaO)
- Magnesium oxide (MgO)
- Aluminum oxide (Al2O3)
- Iron(III) oxide (Fe2O3)
- Mercury(II) oxide (HgO)

### Step 3: Classify the Metals
The metals in the oxides can be classified based on their reactivity:
- S-block metals (Group 1 and Group 2) are generally more reactive and their oxides tend to be less stable.
- Transition metals (like Fe) and post-transition metals (like Al) generally have more stable oxides.

### Step 4: Analyze Each Oxide
- **Calcium Oxide (CaO)**: As an S-block metal oxide, CaO is relatively reactive and can decompose at high temperatures.
- **Magnesium Oxide (MgO)**: More stable than CaO, it requires higher temperatures to decompose.
- **Aluminum Oxide (Al2O3)**: Very stable, it does not decompose easily even at high temperatures.
- **Iron(III) Oxide (Fe2O3)**: Also stable, it requires significant heat to decompose.
- **Mercury(II) Oxide (HgO)**: Known to decompose upon heating to produce mercury and oxygen, making it less stable than the others.

### Step 5: Conclusion
Among the oxides listed, **Mercury(II) oxide (HgO)** is the thermally least stable oxide, as it readily decomposes when heated.

### Final Answer
**Mercury(II) oxide (HgO)** is the thermally least stable oxide.

---