Before any organic compound can be properly studied, it first has to be separated from whatever mixture it was made or extracted in, and this chapter covers the practical laboratory methods that make that possible, along with how to then confirm exactly what elements the purified compound actually contains.
Each purification method in this chapter exploits one specific physical difference between the target compound and its impurities — boiling point for distillation, solubility for crystallisation, and differential adsorption for chromatography. Once purified, qualitative analysis identifies which elements are present at all, while quantitative analysis (Liebig's method, Kjeldahl's method, and others) determines exactly how much of each element the compound contains, which is the essential first step toward determining its molecular formula.
Purification methods and their basis
| Method | Used when |
|---|---|
| Simple distillation | separating a liquid from a non-volatile solid, or liquids with a large boiling point difference |
| Fractional distillation | separating liquids with close boiling points |
| Steam distillation | purifying substances that decompose near their normal boiling point |
| Distillation under reduced pressure | for compounds that decompose at their normal boiling point |
| Crystallisation | purifying a solid based on differing solubility at different temperatures |
| Sublimation | separating a sublimable solid from a non-sublimable impurity |
| Column chromatography | separating compounds based on differential adsorption on a stationary phase |
| Kjeldahl's method | quantitative estimation of nitrogen |
| Liebig's method | quantitative estimation of carbon and hydrogen |
Topics covered
- Purification techniques overview — and choosing the right method based on the physical properties involved
- Distillation and its variants — simple, fractional, steam, and reduced pressure distillation, and when each is appropriate
- Crystallisation — and how to select an appropriate solvent for a given compound
- Sublimation — and its specific use for compounds that transition directly between solid and gas
- Chromatography — column, thin layer and paper chromatography, and the underlying principle of differential adsorption
- Qualitative elemental analysis — detection tests for nitrogen, sulfur, halogens and phosphorus
- Quantitative elemental analysis — Liebig's method for carbon and hydrogen, and the Kjeldahl and Duma's methods for nitrogen
- Estimation of halogens, sulfur and phosphorus — the Carius method and its variations
- Calculating empirical formula from analysis data — connecting this chapter directly back to the mole concept chapter
Why this chapter matters
In NEET and JEE, questions identifying the correct purification method for a given scenario, and quantitative analysis numericals calculating percentage composition from Liebig's or Kjeldahl's method data, are a reliable, low-difficulty source of marks. This chapter is often underprepared relative to its actual frequency in exams, since it feels procedural rather than conceptual.
For Class 11 boards, expect a question identifying an appropriate purification technique for a given mixture and a numerical based on quantitative elemental analysis.
Mistakes students make in this chapter
- Choosing distillation for compounds with very close boiling points. Simple distillation only effectively separates liquids with a significant boiling point difference; close boiling points require fractional distillation instead, which uses a fractionating column for repeated vaporisation-condensation cycles.
- Applying steam distillation without checking the substance is actually steam-volatile and immiscible with water. It specifically works for substances that are volatile in steam and don't mix with water, allowing them to distil at a temperature below their normal boiling point.
- Confusing qualitative and quantitative analysis. Qualitative analysis only detects which elements are present; quantitative analysis (Liebig's, Kjeldahl's methods) determines exactly how much of each element is present, expressed as a percentage.
- Forgetting Kjeldahl's method doesn't work for all nitrogen-containing compounds. It fails for compounds where nitrogen is present in a ring structure or as a nitro/azo group, since the method depends on nitrogen being convertible to ammonium sulfate — Duma's method is used instead for these cases.
- Ignoring solvent choice in crystallisation. An effective solvent should dissolve the compound readily when hot but only sparingly when cold, and should not react with the compound itself — choosing a poor solvent gives poor purification.
Worked example: from percentage composition to molecular formula
- Treat the percentages as masses in a 100 g sample. This is the step that makes the whole calculation work — percentages cannot be divided by atomic masses, but grams can:C = 40.0 g H = 6.67 g O = 53.33 g
- Convert each mass to moles by dividing by the atomic mass. Moles tell you the ratio of atoms, which is what a formula records:C : 40.0/12 = 3.33 molH : 6.67/1 = 6.67 molO : 53.33/16 = 3.33 mol
- Divide every value by the smallest to reduce the ratio to whole numbers:C : 3.33/3.33 = 1H : 6.67/3.33 = 2O : 3.33/3.33 = 1
- Write the empirical formula — the simplest whole-number ratio of atoms — and find its formula mass:empirical formula = CH₂O, mass = 12 + 2 + 16 = 30
- Scale up to the molecular formula using the separately measured molar mass. The multiplier n must come out a whole number:n = 180 / 30 = 6molecular formula = (CH₂O)₆ = C₆H₁₂O₆
What the notes contain
The PDF explains each purification method with the specific physical principle it exploits and a labelled diagram of the apparatus, works through qualitative detection tests for common elements, and covers quantitative analysis calculations including full worked numericals connecting analysis data back to percentage composition and empirical formula.
How to use this: build the concept from these notes first, then do one quick NCERT pass before the exam for exact wording and diagram labels, and spend whatever time is left on previous year questions.
Questions
How do I know which purification method to use for a given mixture?
It depends on the specific physical property difference between the compound and its impurities — boiling point difference suggests distillation, solubility difference suggests crystallisation, and volatility difference suggests sublimation. The PDF walks through this decision process with several worked scenarios.
Why does Kjeldahl's method fail for some nitrogen compounds?
It requires nitrogen to be converted into ammonium sulfate during the digestion step, which doesn't happen efficiently for nitrogen present in a ring (like pyridine) or as a nitro or azo group — Duma's method, which converts nitrogen directly to nitrogen gas, is used for these specific cases instead.
Does this connect quantitative analysis back to finding a molecular formula?
Yes — the PDF shows the complete chain from raw quantitative analysis data, through percentage composition, to empirical formula, directly building on the mole concept methods from the Some Basic Concepts of Chemistry chapter.
Is chromatography covered in enough detail for board exams?
Yes, including the underlying principle of differential adsorption between the stationary and mobile phase, with column, thin layer and paper chromatography each explained.
All other chapters
Every chapter of Class 11 and Class 12 Physics, Chemistry and Biology — each with its own notes and PDF.