Q.The fragrance of flowers is due to the presence of some steam volatile organic compounds called essential oils. These are generally insoluble in water at room temperature but are miscible with water vapour in vapour phase. A suitable method for the extraction of these oils from the flowers is:

Concept: Separation Techniques

Essential oils are organic compounds with two key properties: they are steam volatile (vaporise with steam) and immiscible with water at room temperature but mix with water vapour. The challenge is to extract them without decomposition, since many organic compounds decompose at their normal boiling points.

Steam distillation works by passing steam through the flower material. The steam carries the volatile oil vapours along with it; both condense together, and since the oil is immiscible with liquid water, it separates as a distinct layer that can be collected. Crucially, the mixture boils below 100 °C (below the boiling point of either pure component), preventing thermal decomposition of heat-sensitive oils. …

The key to this problem lies in understanding what essential oils are and why ordinary separation methods fail.

Essential oils are delicate organic molecules responsible for floral fragrance. They have two critical properties: they decompose at high temperatures (often below their own boiling points), and while insoluble in liquid water, they are miscible with water vapour. This second property is the breakthrough—it means that when water vapour passes through flower petals, the oil molecules evaporate along with it, forming a mixed vapour even though the oil's boiling point might be $200\text{–}300\,°\text{C}$.

The principle behind steam distillation is immiscible liquid behaviour under Dalton's law. When two immiscible liquids are heated together, each exerts its own vapour pressure independently. The mixture boils when the sum of their partial pressures equals atmospheric pressure:

This happens at a temperature below the boiling point of either pure component. The water contributes most of the vapour pressure (since it's volatile), allowing the oil to distil at perhaps $95\text{–}100\,°\text{C}$ instead of its decomposition-prone boiling point.

Now let's see why each option does or doesn't work:

1. Simple distillation (i) would require heating the flowers to the oil's boiling point—typically $200\,°\text{C}$ or higher. At these temperatures the fragile organic molecules break apart (oxidation, polymerisation, charring). You'd destroy the very compounds you want to extract.

2. Crystallisation (ii) separates solids from solution based on differential solubility. Essential oils are liquids at room temperature and don't crystallise from the plant matrix. This method is irrelevant here.

3. Distillation under reduced pressure (iii) lowers boiling points by reducing atmospheric pressure. While this helps thermally sensitive compounds, it still requires heating the oil itself to vaporisation. For complex plant material soaked in cellular water, you can't selectively reduce pressure on just the oil. More importantly, you lose the advantage of water as a carrier vapour. …