Cartain materials such as turpentine oil, unsaturated organic compound, phosophorus, metals such as `Zn`, and `Pb`, etc., can absorb `O_(2)` from air in the presence of `H_(2)O`, which is converted to absorb `O_(2)` from air in the presence of `H_(2)O`, which is converted to `H_(2)O_(2)`. This is called autoxidation. Intermolecular redox reactions are those in which one molecule is oxidised and the other is reduced. Intramolecular redox reactions are those in which oen atom of a molecule is oxidised and the other atom is reduced.
Which of the following reactions is`//`are disproportionation reactions `(s)`?

Options

A. `Cl_(2)+2overset(ө)OHrarrCl^(ө)+ClO^(ө)+H_(2)O`
B. `2HCuCl_(2)rarrCu+Cu^(2+)4Cl^(ө)+2H^(o+)`
C. `HCHO+overset(ө)OHrarrCH_(3)OH+HCOO^(ө)`
D. `MgCO_(3)rarrMgO+CO_(2)`

Explanation

To determine which of the given reactions are disproportionation reactions, we need to understand the concept of disproportionation. A disproportionation reaction is a specific type of redox reaction in which a single substance is both oxidized and reduced, resulting in the formation of two different products with different oxidation states.

### Step-by-Step Solution:

1. **Identify the Reactions**: We need to analyze the reactions provided in the options to check if they fit the definition of disproportionation reactions.

2. **Analyze Each Reaction**:
- **Reaction 1**: Cl₂ → Cl⁻ + Cl⁺
- In this reaction, the oxidation state of chlorine (Cl) changes from 0 in Cl₂ to -1 in Cl⁻ (reduction) and +1 in Cl⁺ (oxidation). Since one species is oxidized and another is reduced, this is a disproportionation reaction.

- **Reaction 2**: 2Cu⁺ → Cu + Cu²⁺
- Here, the oxidation state of copper changes from +1 in Cu⁺ to 0 in Cu (reduction) and +2 in Cu²⁺ (oxidation). This also qualifies as a disproportionation reaction because one copper ion is oxidized while another is reduced.

- **Reaction 3**: HCHO → HCOOH + CO
- In this reaction, formaldehyde (HCHO) is oxidized to formic acid (HCOOH) and carbon monoxide (CO). The carbon in HCHO is oxidized, but it does not involve a single species being both oxidized and reduced. Therefore, this is not a disproportionation reaction.

- **Reaction 4**: MgCO₃ → MgO + CO₂
- In this reaction, magnesium carbonate decomposes into magnesium oxide and carbon dioxide. The oxidation states of magnesium and carbon do not change; thus, this is not a disproportionation reaction.

3. **Conclusion**: Based on the analysis, the reactions that are disproportionation reactions are:
- Reaction 1 (Cl₂ → Cl⁻ + Cl⁺)
- Reaction 2 (2Cu⁺ → Cu + Cu²⁺)

### Final Answer:
The disproportionation reactions are **Reaction 1 and Reaction 2**.