The heavier elements of groups 13 and 14 besides the group oxidation state exhibit another oxidation state which is two units lower than the group oxidation state and the stability of this lower oxidation state increases down the group. This concept which is commonly called inert pair effect has been used to explain many physical and chemical properties of the element of these groups.
Which of the following statements is incorrect ?

Options

A. CO is used as a reducing agent
B. `Tl(III)` salts undergo disproportionation
C. `CO_(2)` is a greenhouse gas
D. `SiO_(2)` is a covalent solid

Answer

To solve the question regarding the incorrect statement about the heavier elements of groups 13 and 14 and the inert pair effect, we will analyze each statement provided in the options.

### Step-by-Step Solution:

1. **Understanding the Inert Pair Effect**:
- The inert pair effect refers to the tendency of the outermost s-electrons (the 'inert pair') to remain non-bonding in heavier elements of groups 13 and 14. This results in a lower oxidation state being more stable compared to the higher oxidation state as we move down the group.

2. **Analyzing Each Statement**:
- **Statement 1**: "CO is used as a reducing agent."
- Carbon monoxide (CO) is indeed a strong reducing agent. It can reduce metal oxides to metals, as shown in the reaction:
\[ \text{Fe}_2\text{O}_3 + 3\text{CO} \rightarrow 2\text{Fe} + 3\text{CO}_2 \]
- This statement is **correct**.

- **Statement 2**: "Tl³⁺ salts undergo disproportionation."
- Thallium (Tl) exhibits +1 and +3 oxidation states. The +1 state is more stable due to the inert pair effect, making Tl³⁺ less stable and unlikely to undergo disproportionation. Therefore, this statement is **incorrect**.

- **Statement 3**: "CO₂ is a greenhouse gas."
- Carbon dioxide (CO₂) is well-known as a greenhouse gas, contributing to the greenhouse effect by trapping heat in the atmosphere. This statement is **correct**.

- **Statement 4**: "SiO₂ is a covalent compound."
- Silicon dioxide (SiO₂) is indeed a covalent compound with a network structure, where silicon and oxygen are bonded covalently. This statement is **correct**.

3. **Conclusion**:
- After analyzing the statements, we find that the incorrect statement is **Statement 2**: "Tl³⁺ salts undergo disproportionation."

### Final Answer:
The incorrect statement is: **Tl³⁺ salts undergo disproportionation.**

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