Carbon and Its Compounds
1. Introduction to Carbon
Carbon (C) is a NON-METAL with atomic number 6. It is the FOURTH most abundant element in the universe.
'Carbon is the FOUNDATION of all organic chemistry. It forms MILLIONS of compounds because it can form LONG CHAINS and RINGS with itself — a property called CATENATION.'
2. Allotropes of Carbon
Allotropy: The property of an element to exist in TWO or MORE DIFFERENT physical forms.
Diamond
- Each carbon bonds with FOUR other carbons (tetrahedral structure)
- HARDEST natural substance (10 on Mohs scale)
- Does NOT conduct electricity
- HIGH refractive index → SPARKLING appearance
- Uses: Jewellery, cutting tools, drilling
Graphite
- Each carbon bonds with THREE other carbons (LAYERED structure)
- SOFT (layers SLIDE over each other)
- GOOD conductor of electricity (free electrons between layers)
- Uses: Pencil leads, LUBRICANT, electrodes
Fullerenes
- Carbon atoms arranged in SPHERES (C₆₀ — buckminsterfullerene) or tubes
- HOLLOW cage-like structure
- Discovered in 1985 by Kroto, Curl, and Smalley (Nobel Prize 1996)
3. Comparison — Diamond vs Graphite
| Property | Diamond | Graphite |
|---|---|---|
| Structure | Tetrahedral | Layered (hexagonal) |
| Hardness | HARDEST | SOFT |
| Electrical conductivity | INSULATOR | CONDUCTOR |
| Density | 3.5 g/cm³ | 2.3 g/cm³ |
| Bonding | Each C bonded to 4 C atoms | Each C bonded to 3 C atoms |
| Uses | Cutting, jewellery | Pencils, lubricant |
4. Carbon Dioxide (CO₂)
Preparation in the Lab: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂↑
Apparatus: Woulfe's bottle, thistle funnel, delivery tube.
Collection: By UPWARD DISPLACEMENT of air (CO₂ is HEAVIER than air).
5. Properties of CO₂
Physical Properties
- Colourless, ODOURLESS gas
- About 1.5 times HEAVIER than air
- SOLUBLE in water
- Can be LIQUEFIED under pressure
Chemical Properties
| Reaction | Equation |
|---|---|
| With WATER | CO₂ + H₂O → H₂CO₃ (carbonic acid — WEAK acid) |
| With LIME WATER | Ca(OH)₂ + CO₂ → CaCO₃↓ + H₂O (MILKY white) |
| With EXCESS CO₂ | CaCO₃ + H₂O + CO₂ → Ca(HCO₃)₂ (SOLUBLE — milkiness DISAPPEARS) |
| With BURNING Mg | 2Mg + CO₂ → 2MgO + C (Mg BURNS in CO₂) |
| Photosynthesis | 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ |
Test for CO₂
- Pass gas through LIME WATER (calcium hydroxide solution).
- Lime water turns MILKY (due to formation of CaCO₃).
- With EXCESS CO₂, milkiness DISAPPEARS (Ca(HCO)₃ forms).
6. Uses of CO₂
| Use | Reason |
|---|---|
| Fire EXTINGUISHERS | Heavier than air — forms a BLANKET that cuts off oxygen |
| Carbonated BEVERAGES | CO₂ dissolves under pressure → FIZZ |
| REFRIGERANT (dry ice) | Solid CO₂ SUBLIMES at —78°C → cooling |
| GREENHOUSES | Plants use CO₂ for PHOTOSYNTHESIS → growth |
| MANUFACTURE of urea, washing soda | Chemical feedstock |
7. Carbon Cycle
The NATURAL process by which carbon is EXCHANGED between the atmosphere, oceans, living things, and the Earth's crust.
Key processes:
- Photosynthesis: Plants ABSORB CO₂ from the atmosphere.
- Respiration: Animals and plants RELEASE CO₂ back.
- Decomposition: Dead organisms release CO₂.
- Combustion: Burning fossil fuels RELEASES CO₂.
- Ocean absorption: Oceans DISSOLVE CO₂.
Common Mistakes and Fixes
| Mistake | Fix |
|---|---|
| 'Diamond conducts electricity' | Diamond is an INSULATOR — all 4 valence electrons are used in bonding. Graphite CONDUCTS because one electron per atom is FREE |
| 'Diamond and graphite are compounds' | They are ALLOTROPES of the SAME element (carbon) — pure carbon in DIFFERENT forms |
| 'CO₂ is collected by downward displacement of water' | CO₂ is SOLUBLE in water — use UPWARD displacement of AIR (not water) |
| 'Burning Mg cannot burn in CO₂' | Mg is HIGHLY reactive — it CAN burn in CO₂, extracting oxygen from it |
ICSE Exam Focus (6–8 marks)
- 2-mark questions: Define allotropy, give allotropes of carbon
- 3-mark questions: Distinguish diamond and graphite
- 4-mark questions: Laboratory preparation of CO₂ with diagram
- 6-mark questions: Carbon cycle or reactions of CO₂ with lime water
Self-Test
Q1. What are allotropes? Name the allotropes of carbon. A1. Allotropes are DIFFERENT physical forms of the SAME element. Carbon allotropes: Diamond, graphite, fullerene.
Q2. Why does graphite conduct electricity but diamond does not? A2. In graphite, each carbon has ONE FREE electron (used for bonding between layers). This electron CAN move, allowing conduction. In diamond, all 4 electrons are STRONGLY bonded — no free electrons.
Q3. Write the equation for the laboratory preparation of CO₂. A3. CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂↑
Q4. How would you test for CO₂ gas? A4. Pass the gas through LIME WATER (Ca(OH)₂ solution). If CO₂ is present, the lime water turns MILKY due to formation of CaCO₃.
Q5. What happens when excess CO₂ is passed through lime water? A5. The white milky precipitate (CaCO₃) DISSOLVES to form soluble calcium hydrogen carbonate: CaCO₃ + H₂O + CO₂ → Ca(HCO₃)₂. The solution becomes CLEAR again.
Q6. Why is solid CO₂ called 'dry ice'? A6. Solid CO₂ SUBLIMES directly into gas (does NOT melt into liquid). It looks like ice but leaves NO wetness — hence 'DRY' ice.
