Why is freshly prepared saturated solution of ferrous sulphate used in the Lassaigne's test for nitrogen.

Explanation

### Step-by-Step Solution:

1. **Understanding Lassaigne's Test**: Lassaigne's test is a qualitative analysis method used to detect the presence of nitrogen in organic compounds. It involves the formation of a colored complex that indicates the presence of nitrogen.

2. **Role of Ferrous Sulphate**: In this test, ferrous sulphate (FeSO₄) is used as a reagent. When an organic compound containing nitrogen is heated with sodium metal, it forms sodium cyanide (NaCN) or sodium thiocyanate (NaSCN), depending on the nitrogen source.

3. **Freshly Prepared Saturated Solution**: The use of a freshly prepared saturated solution of ferrous sulphate is crucial because it ensures that the ferrous ions (Fe²⁺) are in their most reactive state. Over time, ferrous sulphate can undergo oxidative hydrolysis, converting Fe²⁺ ions into ferric ions (Fe³⁺), which do not react in the same way.

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