When an organic compound is subjected to complete combustion in excess oxygen, name the products formed that confirm the presence of (i) carbon and (ii) hydrogen in the compound. Describe the chemical test used to detect each product, and write the general equation for the complete combustion of a hydrocarbon.
Generated by claude-sonnet-4-6 · 2026-06-26 01:08 · grounding rag
Model Answer
(i) Carbon is confirmed by the formation of CO₂ gas. When this gas is passed through lime water [Ca(OH)₂], it turns milky due to formation of calcium carbonate (CaCO₃). This confirms the presence of carbon.
(ii) Hydrogen is confirmed by the formation of water (H₂O). Water droplets appear on a cold surface/cobalt chloride paper turns from blue to pink, confirming hydrogen.
General equation for complete combustion of a hydrocarbon:
$$C_xH_y + O_2 \rightarrow CO_2 + H_2O + \text{heat and light}$$
Source: Chapter 4, Section 4.3.1 Combustion
---
Explanation
- Examiners expect two named products with one test each — lime water test for CO₂ and cobalt chloride paper / condensation for water.
- The general equation need not be balanced but writing it symbolically scores the mark.
- Keep the lime water test wording precise: "turns milky" is the key phrase; writing "white precipitate of CaCO₃" adds value.
- This answer covers all three marks: product for C (1), product for H (1), general equation (1).