Which of the following properties is/are common to both `K_2Cr_2O_7 & KMnO_4` ?

Options

A. Both on heating form an oxosalt, a metal oxide & oxygen gas.
B. Both have their colour because of charge transfer spectrum.
C. Both behave as primary titrants as well as self-indicators in redox titrations.
D. Both from a slightly coloured precipitate with hydrogen sulphide in acidic medium.

Explanation

To determine which properties are common to both potassium dichromate (K₂Cr₂O₇) and potassium permanganate (KMnO₄), we will analyze each of the given statements step by step.

### Step 1: Analyze the first statement
**Statement:** Both on heating form an oxosalt, a metal oxide, and oxygen gas.

- **Potassium Dichromate (K₂Cr₂O₇):** Upon strong heating, it decomposes to form potassium chromate (K₂CrO₄), chromium(III) oxide (Cr₂O₃), and oxygen gas (O₂).

\[ 2 K_2Cr_2O_7 \rightarrow 2 K_2CrO_4 + Cr_2O_3 + O_2 \]

- **Potassium Permanganate (KMnO₄):** Upon heating, it decomposes to form potassium manganate (K₂MnO₄), manganese(IV) oxide (MnO₂), and oxygen gas (O₂).

\[ 2 KMnO_4 \rightarrow K_2MnO_4 + MnO_2 + O_2 \]

**Conclusion:** Both compounds form an oxosalt, a metal oxide, and oxygen gas upon heating. Thus, this statement is **true**.

### Step 2: Analyze the second statement
**Statement:** Both have their color because of charge transfer spectra.

- **Potassium Dichromate (K₂Cr₂O₇):** It is orange in color due to the presence of Cr⁶⁺ ions, which exhibit charge transfer spectra.

- **Potassium Permanganate (KMnO₄):** It is purple in color due to the presence of Mn⁷⁺ ions, which also exhibit charge transfer spectra.

**Conclusion:** Both compounds are colored due to charge transfer spectra. Thus, this statement is **true**.

### Step 3: Analyze the third statement
**Statement:** Both behave as primary titrants as well as self-indicators in redox reactions.

- **Potassium Dichromate (K₂Cr₂O₇):** It is a primary titrant in redox reactions.

- **Potassium Permanganate (KMnO₄):** It is typically used as a secondary titrant, not a primary titrant.

**Conclusion:** This statement is **false** because only K₂Cr₂O₇ behaves as a primary titrant.

### Step 4: Analyze the fourth statement
**Statement:** Both form slightly colored precipitates with hydrogen sulfide in acidic medium.

- **Potassium Dichromate (K₂Cr₂O₇):** In acidic medium, it reacts with hydrogen sulfide (H₂S) to produce chromium(III) sulfate (Cr₂(SO₄)₃) and sulfur (S), forming a green precipitate.

- **Potassium Permanganate (KMnO₄):** In acidic medium, it reacts with hydrogen sulfide (H₂S) to produce manganese(IV) oxide (MnO₂) and sulfur (S), forming a black precipitate.

**Conclusion:** Both compounds do form colored precipitates with hydrogen sulfide in acidic medium. Thus, this statement is **true**.

### Final Conclusion
The properties that are common to both K₂Cr₂O₇ and KMnO₄ are:
- They both form an oxosalt, a metal oxide, and oxygen gas upon heating (Statement 1).
- They both have their color due to charge transfer spectra (Statement 2).
- They both form slightly colored precipitates with hydrogen sulfide in acidic medium (Statement 4).

Therefore, the correct answer is **A, B, and D**.

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