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Detection of Carbon and Hydrogen in Organic Compounds

Chemistry · General Principles Of Organic Chemistry · NEET

To detect carbon and hydrogen, you heat the organic compound with dry copper(II) oxide (CuO). The carbon is oxidised to carbon dioxide, which turns lime water milky, and the hydrogen is oxidised to water, which turns white anhydrous copper sulphate blue. Memory hook: CuO is the "burner" — milky lime water means carbon, blue copper sulphate means hydrogen.
Organic compound+ dry CuO, heatCarbon → CO₂C + 2CuO → 2Cu + CO₂Hydrogen → H₂O2H + CuO → Cu + H₂OLime water turns milkyCaCO₃ forms → CarbonWhite CuSO₄ turns blueCuSO₄·5H₂O → HydrogenMilky = carbon · Blue = hydrogen
Heating the compound with copper(II) oxide oxidises carbon to CO2 (turns lime water milky) and hydrogen to water (turns white anhydrous copper sulphate blue).

Your doubts, answered

Why is copper(II) oxide (CuO) used and not simple burning in air?

CuO is an oxidising agent that supplies oxygen in a controlled way when heated. It cleanly oxidises the carbon of the compound to CO2 and the hydrogen to H2O. Reactions: C + 2CuO → 2Cu + CO2 and 2H + CuO → Cu + H2O. Plain burning in open air can give soot and lets the CO2 and water escape before you can test them, so CuO inside a closed setup is more reliable.

How does the lime water test prove carbon is present?

The CO2 formed is passed into lime water, which is calcium hydroxide solution, Ca(OH)2. It reacts to form insoluble white calcium carbonate: CO2 + Ca(OH)2 → CaCO3 (white) + H2O. The milky or cloudy turbidity is the proof of carbon. No CO2 means no carbon was in the compound.

Why does white anhydrous copper sulphate turn blue?

Anhydrous CuSO4 is white. When it meets the water vapour produced from the hydrogen of the compound, it forms hydrated copper sulphate, CuSO4·5H2O, which is blue: CuSO4 + 5H2O → CuSO4·5H2O (blue). The blue colour confirms hydrogen. This is why the copper sulphate must be dry (anhydrous) before the test.

What is the difference between detection and estimation of carbon and hydrogen?

Detection is qualitative — it only answers 'is carbon or hydrogen present, yes or no' using the lime water and copper sulphate colour tests. Estimation (Liebig's method) is quantitative — it measures how much carbon and hydrogen by weighing the CO2 and H2O absorbed. NEET may test both, so keep the two ideas separate.

Can carbon be detected if the compound has no hydrogen?

Yes. Carbon detection depends only on CO2 turning lime water milky, so a compound like carbon tetrachloride (CCl4) still shows carbon. In that case the copper sulphate stays white because there is no hydrogen to form water. The two tests are independent of each other.

⚠️ The NEET trap
✗ Students write that CO2 turns anhydrous copper sulphate blue and that water turns lime water milky.
✓ CO2 (from carbon) turns lime water milky; water (from hydrogen) turns white anhydrous copper sulphate blue. Keep the pairs matched: carbon → CO2 → lime water; hydrogen → water → copper sulphate.
🧠 NTA loves swapping the two reagents. Remember: 'milk' for carbon, 'blue' for hydrogen.

Real NEET questions

NEET 2025

Which one of the following reactions does NOT belong to Lassaigne's test?

A · Na + X → NaX
B · 2CuO + C → 2Cu + CO2 ✓
C · Na + C + N → NaCN
D · 2Na + S → Na2S
Solution: Lassaigne's (sodium fusion) test detects nitrogen, sulphur and halogens by fusing the compound with sodium to give NaCN, Na2S and NaX. The reaction 2CuO + C → 2Cu + CO2 is the oxidation of carbon by copper(II) oxide — it belongs to the detection/estimation of carbon, not to Lassaigne's test. This is exactly the reaction used in the carbon and hydrogen detection method.

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Frequently asked

What reagent is used to detect both carbon and hydrogen at once?

Dry copper(II) oxide (CuO). On heating, it oxidises carbon to CO2 and hydrogen to water in a single setup, so one experiment can show both elements.

What confirms carbon in this test?

The gas produced turns lime water (calcium hydroxide) milky because insoluble white CaCO3 forms: CO2 + Ca(OH)2 → CaCO3 + H2O.

What confirms hydrogen in this test?

The water vapour produced turns white anhydrous copper sulphate blue by forming CuSO4·5H2O.

Why must the copper sulphate be anhydrous?

Only anhydrous (dry, white) CuSO4 can show a colour change. If it already contains water it is blue, so it could not react with the water from the compound to give a fresh blue colour.

Is this method detection or estimation?

It is detection — a qualitative test that only tells you whether carbon and hydrogen are present. Measuring the amount is done separately by Liebig's estimation method.

Related concepts:
Kjeldahl's Method for Nitrogen EstimationDumas Method for Nitrogen EstimationSublimation as a Purification TechniqueCrystallisation for Purifying Organic SolidsClassification of Organic Compounds (Acyclic, Cyclic)
Practise Chemistry for NEET:
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