ChemistryICSE Class 8 · Carbon and its Compounds

Allotropes of Carbon: Diamond, Graphite, Coal, Coke, Charcoal and Soot (ICSE Class 8 Chemistry)

Updated 17 October 2026 · Fact-checked

Allotropy is the existence of an element in different forms in the same physical state, with different physical properties but the same chemical properties. Carbon has crystalline allotropes (diamond, graphite, fullerenes) and amorphous ones (coal, coke, charcoal, soot). All burn to give only carbon dioxide.

Understand Allotropes of Carbon: Diamond, Graphite and Amorphous Forms

Allotropy. An element exists in two or more different forms in the same physical state, with different physical properties but the same chemical properties. The forms are allotropes.

Carbon allotropes.

Crystalline (regular repeating arrangement; definite shape): diamond, graphite, fullerenes (such as C₆₀, a football-shaped molecule).
Amorphous (no regular arrangement; no definite shape): coal, coke, charcoal (wood, animal, sugar), lamp black or soot, gas carbon.

Why different physical properties? The carbon atoms are arranged differently.

Why the same chemical properties? All are made of carbon atoms only. Equal masses of diamond, graphite or charcoal burnt in oxygen each give only carbon dioxide, and the same mass of it. $C + O_{2} \to C O_{2}$ ; 12 g of any allotrope gives 44 g of carbon dioxide.

Property	Diamond	Graphite
Arrangement	each C joined to 4 others in a rigid 3-D network	each C joined to 3 others in flat layers of hexagons, layers held weakly
Hardness	hardest natural substance	soft, slippery
Appearance	transparent, sparkling	greyish-black, opaque, shiny
Electricity	non-conductor	good conductor

(In graphite one electron per carbon atom is not used in bonding and is free to move, which is why it conducts.)

Uses. Diamond: cutting glass, drilling rocks, jewellery, fine surgical instruments. Graphite: pencil leads (mixed with clay), lubricant for hot machines, electrodes, crucibles.

Amorphous forms.

Coal: a fossil fuel; used as fuel and to make coke, coal gas and coal tar.
Coke: residue left when coal is heated in the absence of air; smokeless fuel and a reducing agent.
Charcoal: heating wood, bones or sugar in a limited supply of air; porous, adsorbs gases and colours (gas masks, water filters, removing colour from sugar).
Soot: from burning oil, kerosene or wax in limited air; printing ink, black paint, shoe polish, and in rubber tyres.

Allotropes of Carbon: Diamond, Graphite and Amorphous Forms: formulas and key facts

Allotropes of carbon: $C r y s t a l l in e : d iam o n d, g r a p hi t e, f u l l er e n e ∣ A m or p h o u s : co a l, co k e, c ha r co a l, soo t$; Same chemical properties.
Burning any allotrope: $C + O_{2} \to C O_{2}$; 12 g C gives 44 g CO₂.
Diamond and graphite: $D iam o n d : 4 b o n d s, ha r d es t, n o n - co n d u c t or ∣ G r a p hi t e : 3 b o n d s, l a y er s, so f t, co n d u c t or$; Arrangement differs.

How to solve Allotropes of Carbon: Diamond, Graphite and Amorphous Forms questions

Link each property or use to the arrangement of atoms, and link equal chemical behaviour to the single element.

1Name the allotrope and its type (crystalline or amorphous).
2Give the property from its table and the reason (arrangement).
3For uses, say which property the use depends on.
4For burning, use $C + O_{2} \to C O_{2}$ : mass of CO₂ = mass of C × 44/12.

Allotropes of Carbon: Diamond, Graphite and Amorphous Forms in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Pencil and drill bit

Suppose a pencil lead and a glass-cutting tip are both made of carbon.

Explain the very different uses.

Try it first, then show the solutionSolution

Graphite has layers that slide, so it is soft and leaves marks on paper.
Diamond has a rigid 3-D network, so it is the hardest and can cut glass.

Answer: Different arrangements of carbon atoms give soft graphite and hard diamond.

Memory hook: Same atom, different arrangement.

Burning proof

Imagine 3 g of diamond and 3 g of graphite are burnt separately in excess oxygen.

What mass of carbon dioxide forms in each case?

Try it first, then show the solutionSolution

$C + O_{2} \to C O_{2}$ : 12 g gives 44 g.
3 g gives $\frac{3 \times 44}{12} = 11$ g.

Answer: 11 g each, showing that both are carbon.

Memory hook: Same carbon, same CO₂.

Tricks to remember Allotropes of Carbon: Diamond, Graphite and Amorphous Forms

Short memory aids. Say each one aloud once, then use it on a question the same day.

Diamond 4, graphite 3
Diamond: each C has 4 neighbours; graphite: 3.
Use it when: Structure.
Crystalline have shape
Crystalline: diamond, graphite, fullerene; amorphous: coal, coke, charcoal, soot.
Use it when: Classification.
Sheets slide
Graphite's layers slide like cards, so it is soft and a lubricant.
Use it when: Properties.

Allotropes of Carbon: Diamond, Graphite and Amorphous Forms: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: The hardest · 1 mark · MCQBoard level

The hardest natural substance is:
(a) graphite
(b) coke
(c) diamond
(d) charcoal

Try it first, then show the solutionSolution

In diamond each carbon atom is joined to four others in a rigid 3-D network.

Answer: (c) diamond

Example 2: Graphite conducts · 2 marks · Short answerBoard level

Why does graphite conduct electricity while diamond does not? State one use that depends on this.

Try it first, then show the solutionSolution

In graphite each carbon is joined to three others and one electron is free to move; in diamond all four electrons are used in bonds. (1)
Use: electrodes in electrolysis and dry cells. (1)

Answer: Free electrons in graphite; electrodes.

Example 3: Proof of the same element · 3 marks · Short answerHarder board level

Equal masses of diamond and graphite are burnt separately in oxygen. 3 g of each is taken. (a) Write the equation. (b) Find the mass of carbon dioxide formed in each case. (c) What does it show?

Try it first, then show the solutionSolution

(a) $C + O_{2} \to C O_{2}$ . (1)
(b) 12 g gives 44 g; 3 g gives $\frac{3 \times 44}{12} = 11$ g in each case. (1)
(c) Both are forms of carbon, and allotropes have the same chemical properties. (1)

Answer: (b) 11 g each (c) same element

Example 4: Match property and use · 3 marks · Short answerHarder board level

Give the property that explains each use: (a) graphite in pencils, (b) diamond in glass cutters, (c) charcoal in gas masks, (d) soot in printing ink.

Try it first, then show the solutionSolution

(a) Soft and leaves marks (layers slip). (¾)
(b) Very hard. (¾)
(c) Porous, adsorbs gases. (¾)
(d) Fine black particles. (¾)

Answer: (a) softness (b) hardness (c) adsorption (d) fine black particles

Example 5: Three samples · 4 marks · Case-basedHOTS / competency

Sample A: transparent, extremely hard, does not conduct. Sample B: grey-black, soft, conducts. Sample C: black powder deposited on a cold plate held in a candle flame. (a) Identify A, B and C. (b) Which are crystalline? (c) Which one is amorphous? (d) If 1.2 g of each burns completely, what mass of CO₂ forms? Why the same?

Try it first, then show the solutionSolution

(a) A: diamond; B: graphite; C: soot. (1)
(b) A and B. (½)
(c) C. (½)
(d) $1.2 \times \frac{44}{12} = 4.4$ g each, because all three are carbon only. (2)

Answer: (a) diamond, graphite, soot (b) A, B (c) C (d) 4.4 g each

Where students lose marks in Allotropes of Carbon: Diamond, Graphite and Amorphous Forms

The slips examiners see most often, and what to write instead.

Mistake: Calling graphite amorphous
Graphite is crystalline with a regular layered arrangement.
Wrong working
"Graphite is amorphous."
Right working
"Graphite is crystalline."
Do this: Memorise the lists.
Mistake: Saying allotropes have different chemical properties
They have the same chemical properties.
Wrong working
"Diamond and graphite behave differently on burning."
Right working
"Both give only CO₂."
Do this: Give the burning proof.
Mistake: Wrong number of bonds
Diamond 4; graphite 3.
Wrong working
"Each carbon in graphite is joined to four others."
Right working
"Each carbon in graphite is joined to three others in layers."
Do this: Link bonds to hardness and conductivity.

Exam tips

Define allotropy completely: same state, different physical, same chemical.
Use a two-column table for diamond versus graphite.
Link each use to a property in one line.
Remember 12 g carbon gives 44 g carbon dioxide.

Practice questions on Allotropes of Carbon: Diamond, Graphite and Amorphous Forms

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · Very short answerBoard level
Name two crystalline and two amorphous allotropes of carbon.
Hint
Shape or no shape.
Answer
Crystalline: diamond and graphite. Amorphous: coal and charcoal (also coke, soot).
Q2 · 2 marks · Short answerHarder board level
Why does a diamond sparkle?
Hint
It bends and reflects light.
Answer
A diamond is transparent and bends and reflects light strongly inside it, so it sparkles.
Q3 · 3 marks · Short answerHarder board level
6 g of charcoal is burnt completely in plenty of air. Find the mass of carbon dioxide formed.
Hint
12 g gives 44 g.
Answer
$C + O_{2} \to C O_{2}$ ; $\frac{6 \times 44}{12} = 22$ g.
Q4 · 3 marks · Short answerHarder board level
Give one use each of coal, coke, charcoal and soot, and state which carbon property is used.
Hint
Fuel, reducing, adsorbing, pigment.
Answer
Coal: fuel (burns, gives heat). Coke: reducing agent for metal extraction (almost pure carbon). Charcoal: removing colour or gases (porous, adsorbs). Soot: printing ink (fine black particles).
Q5 · 4 marks · Long answerHOTS / competency
A student says: "Diamond and graphite are different elements because their properties are so different." Show with reasoning and one experiment that she is wrong.
Hint
Think of burning.
Answer
Both are carbon atoms arranged differently, which changes only the physical properties. Experiment: burn equal masses in oxygen; each gives only carbon dioxide and the same mass of it (12 g gives 44 g), showing the same chemical nature. So they are allotropes of one element.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Define allotropy.Show answer
Different forms of an element in the same state, with different physical but the same chemical properties.
2.Which form is hardest?Show answer
Diamond.
3.Which conducts electricity?Show answer
Graphite.
4.Name an amorphous form.Show answer
Charcoal (coal, coke, soot).
5.What does burning any allotrope give?Show answer
Only carbon dioxide.

Allotropes of Carbon: Diamond, Graphite and Amorphous Forms: frequently asked questions

What is allotropy?

Allotropy is the existence of an element in two or more forms in the same physical state, which differ in physical properties but have the same chemical properties.

What is the difference between diamond and graphite?

Diamond has each carbon joined to four others in a rigid network: it is hard, transparent and a non-conductor. Graphite has layers of carbon joined to three others: it is soft, grey-black and a good conductor.

Why do all allotropes of carbon give carbon dioxide on burning?

They are all made of carbon atoms only, so complete burning gives only carbon dioxide.