Which purification method is generally used for a high boiling organic liquid compound, which decompose below its boiling point?
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Match the distillation technique to the properties of the substance:
- \textbf{Simple Distillation:} Separate liquids with large differences in boiling points (> 25 °C).
- \textbf{Fractional Distillation:} Separate liquids with small differences in boiling points.
- \textbf{Vacuum Distillation:} Purify liquids that decompose at or near their normal boiling point.
- \textbf{Steam Distillation:} Purify liquids that are immiscible with water and volatile in steam.
Let's analyze the properties of the compound and the purification methods.
The compound has a high boiling point and decomposes *below* its normal boiling point. This means that if we try to boil it at atmospheric pressure, it will break down before it turns into a vapor. Therefore, simple distillation or fractional distillation are not suitable.
The principle of boiling is that a liquid boils when its vapor pressure equals the external pressure. Distillation under reduced pressure (also called vacuum distillation) is a technique used specifically for this situation.
By reducing the external pressure above the liquid, we lower the temperature at which the liquid's vapor pressure equals the external pressure.
This allows the liquid to boil at a much lower temperature than its normal boiling point.
If this lower boiling temperature is below the decomposition temperature of the compound, the compound can be successfully distilled and purified without decomposing. Steam distillation is used for compounds that are immiscible with water and are volatile in steam. It is not the general method for thermally unstable liquids. Fractional distillation is used to separate a mixture of liquids with close boiling points and is not suitable for a thermally unstable compound.
Therefore, the correct method is distillation under reduced pressure.
