Class 8 - Chapter 6: Composition of Matter - Notes

Chapter 6 — Composition of Matter (Class 8)

Clear, concise notes — mobile friendly. Questions in red, answers in green.

20 Important Words & Meanings (Hindi)

Matter — पदार्थ
Atom — परमाणु
Molecule — अणु
Element — तत्त्व
Compound — यौगिक
Mixture — मिश्रण
Solid — ठोस
Liquid — द्रव
Gas — गैस
Density — घनत्व
Intermolecular force — अणुओं के बीच आकर्षण बल
Solution — विलयन
Suspension — निलंबन
Colloid — कोलॉइड
Phase — अवस्था (चरण)
Valency — संयोजकता (वालेंसि)
Alloy — मिश्रधातु
Organic compound — कार्बन युक्त यौगिक
Inorganic compound — अकार्बनिक यौगिक
Metalloid — अधातु/अर्धधातु

Important Notes (Quick Summary)

All matter consists of tiny particles (atoms/molecules). Their arrangement and intermolecular forces decide the physical state (solid, liquid, gas).
Solid: Particles closely packed, vibrate in fixed positions ⇒ definite shape & volume, high density, nearly incompressible.
Liquid: Particles have moderate attraction, can flow ⇒ definite volume but shape of container, less dense than many solids.
Gas: Particles far apart, free motion ⇒ no definite shape or volume, highly compressible.
Classification by composition: Elements (one type of atom), Compounds (chemically combined different atoms in fixed ratio), Mixtures (physical combination).
Solution is a homogeneous mixture (solute + solvent). Suspension has large particles (filterable). Colloid has intermediate-sized particles (Tyndall effect).
Molecular formula shows number of atoms (e.g., \(H_2O\), \(CO_2\), \(MgCl_2\)). Use MathJax for correct display.
Valency: combining capacity. Use cross-multiplication method to write simple molecular formulas.
Organic compounds contain carbon (many give residue on strong heating); inorganic compounds usually do not (or give different residues).

20 Important One-Word / Very Short Answers (1–2 lines)

Q1: Smallest particle of matter? Atom.

Q2: Smallest particle of a compound? Molecule.

Q3: Example of liquid element? Mercury (Hg).

Q4: Example of gaseous element? Oxygen (O₂).

Q5: Example of solid element? Iron (Fe).

Q6: What is brass? An alloy of copper and zinc.

Q7: Formula of water? \(H_2O\).

Q8: Formula of methane? \(CH_4\).

Q9: Valency of oxygen? 2.

Q10: Valency of carbon? 4.

Q11: Type of matter - Air? Mixture (homogeneous gaseous mixture).

Q12: Type of matter - Water? Compound (H₂O).

Q13: Type of matter - Iron? Element (Fe).

Q14: Milk is which type? Colloid (liquid dispersed solid particles).

Q15: State with definite shape? Solid.

Q16: State with definite volume only? Liquid.

Q17: State with neither definite shape nor volume? Gas.

Q18: Example of organic compound? Glucose (C₆H₁₂O₆).

Q19: Example of inorganic compound? Sodium chloride (NaCl).

Q20: Example of complex compound? Hemoglobin (contains Fe in complex structure).

20 Very Short Answer Type Questions (answers 1–2 lines)

Q1: Why solids have definite shape? Particles are tightly packed and only vibrate about fixed positions, so shape stays fixed.

Q2: Why liquids have definite volume but not definite shape? Interparticle forces hold particles close enough to maintain volume but allow them to move, so shape conforms to container.

Q3: Why gases are compressible? Particles are far apart with large spaces; pressure reduces these spaces, compressing gas.

Q4: Define solvent and solute. Solvent: component present in larger amount; Solute: component dissolved in solvent.

Q5: What is suspension? A heterogeneous mixture with large particles (>10⁻⁴ m) that settle and can be filtered.

Q6: What is colloid? A heterogeneous mixture with intermediate particle size (~10⁻⁵ m) showing Tyndall effect and not separated by ordinary filter paper.

Q7: How is a compound different from mixture? Compounds have constituents chemically bonded in fixed ratio; mixtures are physical combination with variable proportions.

Q8: Give one example of a homogeneous mixture. Salt dissolved in water (salt solution).

Q9: Give one example of a heterogeneous mixture. Sand in water (suspension).

Q10: What does molecular formula show? Number and type of atoms present in one molecule of a compound.

Q11: What is valency? Number of bonds an atom can form (combining capacity).

Q12: Why does water extinguish fire though it contains hydrogen and oxygen? Properties of compound differ from constituent gases; water is stable, nonflammable and absorbs heat, extinguishing fire.

Q13: What is alloy? A solid solution of one metal in another, e.g., brass (Cu + Zn).

Q14: What is meant by 'phase'? A part of a system that is homogeneous in composition and properties (e.g., oil phase, water phase).

Q15: Why does milk look translucent? Tiny dispersed particles scatter some light (colloidal nature), making milk translucent.

Q16: How can components of a suspension be separated? By filtration or by letting particles settle (sedimentation) followed by decantation.

Q17: What happens when iron and sulphur are heated together? They chemically combine to form iron sulphide, a compound with new properties (no magnetic attraction).

Q18: How to write molecular formula by cross-multiplication? Write symbols, write valencies below, cross-multiply valencies to become subscripts, simplify to smallest whole numbers.

Q19: Give example of organic compound from chapter experiments. Camphor, sugar, glucose and urea are organic compounds mentioned.

Q20: Which mixtures show Tyndall effect? Colloids (e.g., milk, fog) show the Tyndall effect—scattering of light by dispersed particles.

20 Short Answer Type Questions (about 2–3 lines each)

Q1: Explain why gases have neither definite shape nor definite volume.

In gases, intermolecular forces are very weak and particles move freely in all directions, filling available space; thus gases take the shape and volume of their container.

Q2: How can you distinguish between a solution and a suspension?

Solutions are homogeneous, transparent, particles are molecular size and pass through filter paper; suspensions are heterogeneous, particles large, opaque and can be separated by filtration.

Q3: Describe an activity to show that a mixture retains properties of its components.

Mix iron filings and sulphur powder — iron remains magnetic and sulphur yellow; properties of components are retained indicating a mixture.

Q4: What observations show that a compound is different from its constituents?

On forming a compound (e.g., heating iron with sulphur), colour changes and magnetic property may disappear — new properties indicate a compound different from constituents.

Q5: Define homogeneous and heterogeneous mixtures with examples.

Homogeneous: uniform composition throughout (salt solution). Heterogeneous: non-uniform; components visible as phases (sand in water, milk as colloid).

Q6: What is meant by 'organic compound'? Give two examples.

Organic compounds contain carbon atoms usually with H/O/N etc. Examples: glucose (C₆H₁₂O₆), urea (CH₄N₂O).

Q7: Explain why alloys are considered solutions.

Alloys like brass are homogeneous solid mixtures where one metal is dissolved in another at atomic level, forming a single solid phase (solid solution).

Q8: How can you separate components of a mixture of sand and salt?

Dissolve mixture in water (salt dissolves), filter sand, evaporate water to recover salt — combination of filtration and evaporation.

Q9: Why does common salt not leave residue on strong heating but carbon compounds do?

Inorganic salt like NaCl is thermally stable and does not decompose at moderate heating; organic compounds contain carbon and may decompose or char leaving carbonaceous residue on incomplete combustion.

Q10: Explain interparticle distance in solids, liquids and gases.

Solids: minimum distance — closely packed. Liquids: moderate distance — particles can slide past. Gases: large distance — particles widely separated and move freely.

Q11: How is valency of an element deduced from simple compounds?

From known molecular formulas with hydrogen (valency 1), or by counting bonds in molecule; e.g., in H₂O, O combines with 2 H so valency of O is 2.

Q12: What are complex compounds? Give one example.

Complex compounds have a large molecule with metal atom(s) at center bonded to organic ligands; example: chlorophyll (contains Mg) or hemoglobin (contains Fe).

Q13: Describe the method to write CO₂ by cross-multiplication if valencies are C=4 and O=2.

Write C(4) and O(2), cross-multiply to get C₂O₄, divide by common factor 2 → CO₂.

Q14: Why is sea water a mixture and not a compound?

Sea water contains water, dissolved salts and other substances in varying proportions — no fixed chemical composition, so it's a mixture (solution).

Q15: What is Tyndall effect and which mixtures show it?

Tyndall effect: scattering of light by particles in colloids; colloids like milk, fog show Tyndall effect, solutions do not.

Q16: Explain why milk cannot be separated by ordinary filtration.

Milk is a colloid with tiny dispersed particles smaller than filter paper pores, so ordinary filtration does not separate its components.

Q17: Define solvent & solute and give example with quantities.

Solvent: major component (e.g., 100 mL water); Solute: minor (e.g., 10 g salt). Together they form a solution where salt dissolves in water.

Q18: Why do some compounds leave black residue on strong heating?

Organic compounds containing carbon may undergo incomplete combustion leaving carbonaceous (black) residue; inorganic salts usually decompose differently or not at all.

Q19: What character of elements is tested by hammering and rubbing with sandpaper?

Malleability (flatten on hammering) and lustre (shiny surface) — metals are often malleable and lustrous; non-metals may be brittle and dull.

Q20: Explain why proportion of substances in a mixture can vary but in a compound it's fixed.

In mixtures components are physically mixed — proportions can change. In compounds atoms bond chemically in definite ratios determined by valencies, so proportion is fixed.

All Textbook Exercises — Questions & Perfect Answers

Exercise 1: Choose the appropriate option

1(a) The intermolecular force is ........ in the particles of solid.

(iii) maximum — In solids intermolecular forces are strongest, holding particles closely in fixed positions.

1(b) Solids retain their volume even when external pressure is applied. This property is called ........

(ii) incompressibility — Solids resist compression and thus retain volume under normal pressure.

1(c) Matter is classified into the types mixture, compound and element, by applying the criterion ........

(iii) chemical composition of matter — Classification by whether substances are chemically pure (element/compound) or mixtures.

1(d) Matter that contain two or more constituent substances is called ........

(i) mixture — A material made of two or more substances not chemically combined.

1(e) Milk is an example of type of matter called ........

(iii) heterogeneous mixture — Specifically a colloid; components are not in single uniform molecular form.

1(f) Water, mercury and bromine are similar to each other, because three are ........

(i) liquids — They are found in liquid state at room temperature.

1(g) Valency of carbon is 4 and that of oxygen is 2. From this, we understand that there are ...... chemical bond/bonds between the carbon atom and one oxygen atom in the compound-carbon dioxide.

(ii) 2 — In CO₂ each carbon-oxygen bond is a double bond (overall carbon forms two bonds with each oxygen in structures, formula shows CO₂).

Exercise 2: Identify the odd term out and explain

2(a) Gold, silver, copper, brass

Odd: brass — because gold, silver, copper are elements (metals); brass is an alloy (mixture of metals).

2(b) Hydrogen, hydrogen peroxide, carbon dioxide, water vapour

Odd: hydrogen — it is an element (H₂); others (H₂O₂, CO₂, H₂O vapour) are compounds.

2(c) Milk, lemon juice, carbon, steel

Odd: carbon — milk and lemon juice are mixtures (colloid and solution respectively) and steel is an alloy; carbon is a pure element.

2(d) Water, mercury, bromine, petrol

Odd: water — water is a compound; mercury & bromine are elements, petrol is a mixture (hydrocarbon mixture).

2(e) Sugar, salt, baking soda, blue vitriol

Odd: sugar — sugar is an organic compound (C-based); salt (NaCl), baking soda (NaHCO₃) and blue vitriol (CuSO₄) are inorganic.

2(f) Hydrogen, sodium, potassium, carbon

Odd: carbon — hydrogen, sodium, potassium are all reactive elements in group/with metallic character (two are alkali metals and H is a gas); carbon is a non-metal with different properties.

Exercise 3: Answer the following

3(a) Plants synthesize glucose in sunlight with the help of chlorophyll from carbon dioxide and water and give away oxygen. Identify the four compounds in this process and name their types.

Compounds involved: Glucose (C₆H₁₂O₆) — organic compound; Carbon dioxide (CO₂) — inorganic compound; Water (H₂O) — inorganic compound; Oxygen (O₂) — elemental gas. Photosynthesis converts inorganic (CO₂, H₂O) to organic (glucose) releasing O₂.

3(b) In one sample of brass, the following ingredients were found: copper (70%) and zinc (30%). Identify the solvent, solute and solution from these.

Solution: brass (a solid solution). Solvent (major): copper (70%). Solute (minor): zinc (30%).

3(c) Sea water tastes salty due to the dissolved salt. The salinity of some water bodies given: Lonar lake 7.9%, Pacific Ocean 3.5%, Mediterranean sea 3.8%, Dead sea 33.7%. Explain two characteristics of mixtures from the above information.

1) Variable composition: Proportions (salinity) vary between water bodies — mixture composition is not fixed. 2) Properties retained: The dissolved salts retain chemical identity (can be separated by evaporation), showing mixtures keep constituent properties.

Exercise 4: Give two examples each

4(a) Liquid element

Mercury (Hg), Bromine (Br₂).

4(b) Gaseous element

Oxygen (O₂), Nitrogen (N₂).

4(c) Solid element

Iron (Fe), Carbon (C - as graphite/diamond).

4(d) Homogeneous mixture

Salt solution (NaCl in water), vinegar (acetic acid in water).

4(e) Colloid

Milk (liquid in liquid/solid in liquid colloid), fog (liquid in gas).

4(f) Organic compound

Glucose (C₆H₁₂O₆), Urea (CH₄N₂O).

4(g) Complex compound

Chlorophyll (contains Mg), Hemoglobin (contains Fe).

4(h) Inorganic compound

Sodium chloride (NaCl), Copper sulfate (CuSO₄).

4(i) Metalloid

Silicon (Si), Boron (B).

4(j) Element with valency 1

Hydrogen (H, valency 1), Chlorine (Cl in many compounds behaves as 1).

4(k) Element with valency 2

Oxygen (O - usually valency 2), Calcium (Ca commonly 2).

Exercise 5: Write names, symbols & valencies from molecular formulae

5. For: KCl, HBr, MgBr₂, K₂O, NaH, CaCl₂, CCl₄, HI, H₂S, Na₂S, FeS, BaCl₂

Formula	Constituent elements & symbols	Valencies (deduced)
KCl	Potassium (K) and Chlorine (Cl)	K:1, Cl:1
HBr	Hydrogen (H) and Bromine (Br)	H:1, Br:1
MgBr₂	Magnesium (Mg) and Bromine (Br)	Mg:2, Br:1 (each)
K₂O	Potassium (K) and Oxygen (O)	K:1, O:2
NaH	Sodium (Na) and Hydrogen (H)	Na:1, H:1 (hydride)
CaCl₂	Calcium (Ca) and Chlorine (Cl)	Ca:2, Cl:1
CCl₄	Carbon (C) and Chlorine (Cl)	C:4, Cl:1 (each)
HI	Hydrogen (H) and Iodine (I)	H:1, I:1
H₂S	Hydrogen (H) and Sulphur (S)	H:1, S:2 (valency 2)
Na₂S	Sodium (Na) and Sulphur (S)	Na:1, S:2
FeS	Iron (Fe) and Sulphur (S)	Fe:2 (in FeS), S:2
BaCl₂	Barium (Ba) and Chlorine (Cl)	Ba:2, Cl:1

Exercise 6: Chemical composition table — identify main type

6. Identify main type of matter

Name	Chemical composition	Main type of matter
Sea water	H₂O + NaCl + MgCl₂ + ...	Mixture (homogeneous solution)
Distilled water	H₂O	Compound (pure water)
Hydrogen gas filled in a balloon	H₂	Element (diatomic hydrogen)
Gas in LPG cylinder	Mixture of hydrocarbons e.g., C₄H₁₀ + C₃H₈	Mixture
Baking soda	NaHCO₃	Compound (inorganic)
Pure gold	Au	Element
Gas in oxygen cylinder	O₂	Element (diatomic oxygen)
Bronze	Cu + Sn	Alloy (mixture of metals)
Diamond	Carbon (C)	Element (solid carbon allotrope)
Heated white powder of blue vitriol	CuSO₄ heated → gives CuO + SO₃/H₂O etc. (decomposition)	Compound (blue vitriol is a compound; decomposition product is compound/oxide)
Limestone	CaCO₃	Compound (calcium carbonate)
Dilute hydrochloric acid	HCl in water	Solution (acidic solution — homogeneous mixture)

Exercise 7: Write scientific reason

7(a) Hydrogen is combustible, oxygen helps combustion, but water helps to extinguish fire. Why?

Water (H₂O) is a chemical compound whose properties differ from constituent gases. In water H and O are chemically combined; it is stable, nonflammable and absorbs heat, thus extinguishing flames.

7(b) Constituent substances of a colloid cannot be separated by ordinary filtration. Why?

Particles in a colloid are too small (around 10⁻⁵ m) to be trapped by ordinary filter paper, so they pass through and remain dispersed; hence ordinary filtration fails to separate them.

7(c) Lemon sherbat has sweet, sour and salty taste and it can be poured in a glass. Explain.

Lemon sherbat is a mixture (solution) of water with sugar, salt, lemon juice etc.; its components retain individual tastes and it flows (liquid) so it can be poured in a glass.

7(d) A solid matter has the properties of definite shape and volume. Why?

Because particles in solids are tightly packed and fixed in positions; they only vibrate, so solids keep both shape and volume.

Exercise 8: Deduce molecular formulae by cross-multiplication

8(a) C (valency 4) & Cl (valency 1)

Cross-multiply: C₁Cl₄ → formula: \(CCl_4\) (carbon tetrachloride).

8(b) N (valency 3) & H (valency 1)

Cross-multiply: N₁H₃ → formula: \(NH_3\) (ammonia).

8(c) C (valency 4) & O (valency 2)

Cross-multiply: C₂O₄ → divide by 2 → \(CO_2\) (carbon dioxide).

8(d) Ca (valency 2) & O (valency 2)

Cross-multiply: Ca₂O₂ → divide by 2 → \(CaO\) (calcium oxide / quicklime).

End of Textbook Exercises — Summary

All exercise questions from the chapter are answered above with clear reasoning, formulas using MathJax for correct rendering, and practical examples where required.

6. Composition of Matter