Q1. [3] medium initial-understanding
Carbon is found in very small amounts in the earth's crust and atmosphere, yet it is considered one of the most important elements. (i) In what two forms does carbon occur in the earth's crust? (ii) Give two reasons why carbon is considered so significant despite its limited abundance in nature.
Generated by claude-sonnet-4-6 · 2026-06-26 01:05 · grounding rag
Model Answer
(i) Carbon occurs in the earth's crust in two forms:
- As minerals such as carbonates and hydrogencarbonates
- As coal and petroleum
(The atmosphere contains carbon as carbon dioxide — 0.03%.)
(ii) Carbon is significant despite its limited abundance because:
- Catenation — Carbon can form bonds with other carbon atoms, producing long chains, branched chains, and rings. This gives rise to millions of stable compounds.
- Tetravalency — Carbon has a valency of four, allowing it to bond with many other elements (H, O, N, S, Cl), forming compounds with diverse and specific properties that are essential to all living organisms.
Source: Introduction and Section 4.2, Chapter 4 — Carbon and its Compounds
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Explanation
- Part (i) is directly from the introduction: "0.02% carbon in the form of minerals (like carbonates, hydrogencarbonates, coal and petroleum)." List exactly these — don't guess.
- Part (ii) expects the two keywords: catenation and tetravalency. Examiners look for these terms by name plus a brief explanation of each. Simply saying "forms many compounds" without naming the property will lose marks.
- Don't spend words on carbon dioxide for part (i) — the question asks about the crust, not the atmosphere. Mentioning it briefly as a bonus is fine but not required.