Benzoic acid is a organic compound. Its crude sample can be purified by crystallisation from hot water. What c

NCERT Exemplar · Q62

Q. Benzoic acid is a organic compound. Its crude sample can be purified by crystallisation from hot water. What characteristic differences in the properties of benzoic acid and the impurity make this process of purification suitable?

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Concept understanding — Separation Techniques

Separation Techniques: The Intuition

Imagine you're making chai. You boil tea leaves in water, then pour the liquid through a strainer. The strainer catches the leaves; the tea flows through. You just performed a separation technique — you took a mixture (tea leaves + water) and isolated one component (the liquid tea) from the other (the solid leaves).

Every separation technique answers one question: How do I pull apart things that are mixed together, using their differences?

The key insight: You cannot separate things that are identical. Separation works only because the components of a mixture differ in at least one physical or chemical property — size, density, boiling point, solubility, magnetic behaviour, or something else.

Note

Separation techniques are physical processes. They do not change the chemical identity of the substances. The tea leaves remain tea leaves; the water remains water. No chemical reaction occurs.

The Precise Statement

Separation techniques are methods used to isolate individual components from a mixture by exploiting differences in their physical or chemical properties. The goal is to obtain one or more pure substances from a mixture, without altering the chemical nature of the components.

The Core Idea in One Sentence

Important

Every separation technique works because the components of the mixture differ in at least one measurable property. The method you choose depends entirely on which property differs.

A Quick Map of Common Separation Techniques

Here is how the choice of technique follows from the property difference:

Property Difference	Technique	Example
Particle size	Sieving, Filtration	Separating stones from sand; tea leaves from water
Density	Centrifugation, Decantation, Sedimentation	Separating cream from milk; mud from water
Boiling point	Distillation	Separating alcohol from water; crude oil into fractions
Solubility	Evaporation, Crystallisation	Obtaining salt from seawater
Magnetic property	Magnetic separation	Separating iron filings from sulphur powder
Volatility	Sublimation	Separating camphor from sand
Attraction to a stationary phase	Chromatography	Separating pigments in ink or plant leaves

How to Think About Any Separation Problem

When you see a mixture and need to decide how to separate it, ask yourself:

What are the components? (List them.)
What property is different between them? (Size? Density? Boiling point? Solubility? Magnetism?)
Which technique exploits that difference? (Match the property to the method above.)

That three-step chain — mixture → property difference → technique — is the entire logic of separation.

A Concrete Walkthrough

Mixture: Sand and iron filings.

Components: Sand (silicon dioxide) and iron (metal).
Property difference: Iron is magnetic; sand is not.
Technique: Magnetic separation. Pass a magnet over the mixture. Iron filings stick to the magnet; sand is left behind.

Mixture: Salt dissolved in water.

Components: Salt (sodium chloride) and water.
Property difference: Water boils at $100^\circ\text{C}$ and evaporates; salt does not boil or evaporate — it remains as a solid.
Technique: Evaporation (or distillation, if you also want to collect the water). Heat the solution. Water turns to vapour and leaves; salt crystals remain in the dish.

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Benzoic acid is purified by crystallisation from hot water because its solubility increases significantly with temperature, allowing it to dissolve in hot water and then selectively crystallise out upon cooling, while impurities either remain dissolved or are filtered off.

Crystallisation is a powerful purification technique that relies on differences in solubility between the desired compound and its impurities, particularly how their solubilities change with temperature. For this method to be effective, the substance to be purified must exhibit a significant increase in solubility in a chosen solvent as the temperature rises, while the impurities should behave differently.

Here's a breakdown of the characteristic differences that make crystallisation from hot water suitable for purifying benzoic acid:

Principle of Crystallisation:

Crystallisation is a separation technique where a solid substance is purified by dissolving it in a suitable hot solvent to form a saturated solution, and then allowing it to cool. As the solution cools, the solubility of the desired compound decreases, causing it to precipitate out as pure crystals. Impurities, ideally, either remain dissolved in the cold solvent (because they are much more soluble) or do not dissolve in the hot solvent at all (and can be filtered off).
Solubility of Benzoic Acid in Water:

Benzoic acid exhibits a distinct solubility profile in water:
- It is sparingly soluble in cold water. This means only a small amount dissolves at room temperature.
- It is highly soluble in hot water. A much larger quantity of benzoic acid can dissolve in boiling water. This significant difference in solubility with temperature is crucial for its purification by crystallisation.
Solubility of Impurities in Water:

For effective purification, the impurities present in the crude benzoic acid sample must have different solubility characteristics compared to benzoic acid:
- Highly Soluble Impurities: Many common impurities are much more soluble in both hot and cold water than benzoic acid. When the solution is cooled, these impurities remain dissolved in the mother liquor (the remaining solution) even as benzoic acid crystallises out.
- Insoluble Impurities: Some impurities might be completely insoluble in hot water. These can be removed by hot filtration before the solution is allowed to cool.
The Purification Process Explained:
- The crude benzoic acid is dissolved in the minimum amount of hot water. Benzoic acid, being highly soluble in hot water, dissolves completely, forming a concentrated solution. Insoluble impurities, if any, are filtered off at this stage. …

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Q.Benzoic acid is a organic compound. Its crude sample can be purified by crystallisation from hot water. What characteristic differences in the properties of benzoic acid and the impurity make this process of purification suitable?

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