True or false statements :
(i) Sodium cyanide is used as a depresent to separate lead sulphide from zinc sulphide.

Explanation

To determine whether the statement "Sodium cyanide is used as a depressant to separate lead sulfide from zinc sulfide" is true or false, we can analyze the role of sodium cyanide in the froth flotation process.

**Step 1: Understanding the Froth Flotation Process**
Froth flotation is a method used to separate and concentrate ores, particularly sulfide ores. In this process, different reagents are added to the ore slurry to selectively separate the desired minerals from the unwanted ones.

**Hint:** Froth flotation is a common technique used in mineral processing.

**Step 2: Identifying the Role of Depressants**
Depressants are chemicals used in the froth flotation process to selectively inhibit the flotation of certain minerals while allowing others to float. This is crucial when dealing with mixed ores.

**Hint:** Depressants help in selectively separating minerals in a mixture.

**Step 3: Specific Use of Sodium Cyanide**
Sodium cyanide (NaCN) is specifically used as a depressant in the separation of lead sulfide (PbS) from zinc sulfide (ZnS). When sodium cyanide is added to the mixture, it selectively prevents zinc sulfide from floating, allowing lead sulfide to be collected in the froth.

**Hint:** Sodium cyanide selectively inhibits the flotation of zinc sulfide.

**Step 4: Formation of Complexes**
When sodium cyanide is introduced, it reacts with zinc sulfide to form a complex known as tetra-cyano-zincate. This reaction further ensures that zinc sulfide does not float, while lead sulfide does.

**Hint:** The formation of tetra-cyano-zincate is key to the separation process.

**Step 5: Conclusion**
Since sodium cyanide is indeed used as a depressant to separate lead sulfide from zinc sulfide in the froth flotation process, the statement is true.

**Final Answer:** True.

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