Board Previous Year Questions (Board PYQs)

CBSE and State Board Previous Year Questions

Most important PYQs of Chapter 3 — Metals and Non-Metals. Each question with — full solution and board tips.

PYQ 1: CBSE 2023, 5 marks

(a) Why is Na stored in kerosene oil? (b) Write reaction of Mg + O₂. Nature of MgO. (c) Why doesn't Au react with dilute acids?

Solution:

(a) Na in kerosene

Na very reactive. Reacts immediately with O₂ and H₂O in air. Kerosene — protects from air.

Reactions (if exposed):

• $4Na + O_2 \rightarrow 2Na_2O$
• $2Na + 2H_2O \rightarrow 2NaOH + H_2\uparrow$ (fire)

Hence safe in kerosene.

(b) Mg + O₂

$$2Mg + O_2 \xrightarrow{\Delta} 2MgO$$

Nature of MgO: Basic.

Evidence:

• $MgO + H_2O \rightarrow Mg(OH)_2$ (alkali)
• $MgO + 2HCl \rightarrow MgCl_2 + H_2O$

(c) Au + dilute acid

Au — at the bottom of reactivity series. Way below H. To release $H_2$ from dilute acid, must be above H. Au — no reaction.

Only aqua regia can dissolve Au.

[CBSE 2023, very important]

PYQ 2: CBSE 2022, 3 marks

Difference between roasting and calcination — with chemical reactions.

Solution:

Difference

Property	Roasting	Calcination
Ore	Sulphide	Carbonate/Hydroxide
Air	Yes (more)	No (or limited)
Product	Oxide + SO₂	Oxide + CO₂ or H₂O

Example of Roasting (ZnS):

$$2ZnS + 3O_2 \xrightarrow{\Delta} 2ZnO + 2SO_2$$

Example of Calcination (ZnCO₃):

$$ZnCO_3 \xrightarrow{\Delta} ZnO + CO_2$$

Other Examples

Roasting:

• $2Cu_2S + 3O_2 \rightarrow 2Cu_2O + 2SO_2$
• $2HgS + 3O_2 \rightarrow 2HgO + 2SO_2$

Calcination:

• $CaCO_3 \rightarrow CaO + CO_2$
• $2Al(OH)_3 \rightarrow Al_2O_3 + 3H_2O$

[CBSE 2022, every year]

PYQ 3: CBSE 2021, 5 marks

Explain Fe extraction process in detail — main reactions in Blast Furnace.

Solution:

Blast Furnace

Setup: Tall, cylindrical furnace (~25-30 m). Charge (ore + coal + limestone) from top. Hot air from bottom.

Charge Contains:

1. Ore: Fe₂O₃ (Hematite) — concentrated.
2. Coke: pure carbon — fuel and reducing agent.
3. Limestone: CaCO₃ — flux.

Reactions in Three Zones

1. Lower zone (~1500-2000°C): $$C + O_2 \rightarrow CO_2 + \text{heat}$$ $$CO_2 + C \rightarrow 2CO$$

2. Middle zone (~1000°C) — main reaction: $$Fe_2O_3 + 3CO \rightarrow 2Fe + 3CO_2$$

3. Upper zone (~500°C): $$CaCO_3 \rightarrow CaO + CO_2$$ $$CaO + SiO_2 \rightarrow CaSiO_3 \text{ (slag)}$$

Final Products

• Bottom: molten Fe (Pig iron, 4% C, at 1500°C).
• Above: slag (CaSiO₃).

Both — released through different openings.

Further Use of Pig Iron

Bessemer process for steel.

[CBSE 2021 — every year question]

PYQ 4: CBSE 2020, 3 marks

Which of the following can displace Ag from AgNO₃ solution? Reasons?

(a) Cu + AgNO₃ (b) Au + AgNO₃ (c) Mg + AgNO₃

Solution:

Series: $Mg > Cu > Ag > Au$

(a) Cu + AgNO₃ → ?

Cu more reactive than Ag.

Reaction occurs: $$Cu + 2AgNO_3 \rightarrow Cu(NO_3)_2 + 2Ag$$

Shiny Ag layer on Cu.

(b) Au + AgNO₃ → ?

Au less reactive than Ag.

No reaction.

(c) Mg + AgNO₃ → ?

Mg more reactive than Ag.

Reaction occurs: $$Mg + 2AgNO_3 \rightarrow Mg(NO_3)_2 + 2Ag$$

Ag layer on Mg.

Summary

Metal	Displacement?
Cu	Yes
Au	No
Mg	Yes

Key Insight: Only metals more reactive than Ag can displace.

[CBSE 2020]

PYQ 5: CBSE 2019, 5 marks

(a) What is an ionic compound? Process of formation. (b) Formation of NaCl and MgCl₂ by electron transfer. (c) 3 properties of ionic compounds.

Solution:

(a) Ionic Compound

Compound formed between metal and non-metal — by electrostatic attraction between opposite ions.

Process:

1. Metal loses electrons (cation).
2. Non-metal gains electrons (anion).
3. Opposite charges attract.
4. Crystal lattice formed.

(b) Formation

NaCl:

• Na (2,8,1) → Na⁺ (2,8) + e⁻
• Cl (2,8,7) + e⁻ → Cl⁻ (2,8,8)
• $2Na + Cl_2 \rightarrow 2NaCl$

MgCl₂:

• Mg (2,8,2) → Mg²⁺ (2,8) + 2e⁻
• 2 × [Cl + e⁻ → Cl⁻]
• $Mg + Cl_2 \rightarrow MgCl_2$

(c) 3 Properties

1. High melting/boiling points (NaCl - 801°C, MgO - 2852°C).
2. Soluble in water (mostly). Reason: water is polar.
3. Electrical conductivity:

• Solid — No.
• Molten/aqueous — Yes.

[CBSE 2019]

PYQ 6: CBSE 2019, 3 marks

Explain elements required for rust. Prove with three test tubes experiment.

Solution:

Required Elements

Two elements required:

1. Water (H₂O / moisture).
2. Oxygen (O₂ / air).

Both required — not from one alone.

Three Test Tubes Experiment

Setup: Three test tubes. An Fe nail in each.

Tube A:

• Fe + boiled water (air removed).
• Layer of oil on top.

Tube B:

• Fe + dry air.
• CaCl₂ at bottom (absorbs water).

Tube C:

• Fe + water + air (both).

Observations (after a few days)

Tube	Result
A	No rust
B	No rust
C	Rust!

Conclusion

Only A — water, no air → no rust. Only B — air, no water → no rust. Only C — both → rust.

Hence both required.

Basic Reaction

$$4Fe + 3O_2 + xH_2O \rightarrow 2Fe_2O_3 \cdot xH_2O$$

[CBSE 2019 — every year]

PYQ 7: CBSE 2018, 5 marks

Divide metal extraction by reactivity — example of each category.

Solution:

Three Categories

1. Highly Reactive (K, Na, Ca, Mg, Al)

Method: Electrolysis.

Example: Al by Hall-Héroult:

• $2Al_2O_3 \xrightarrow{\text{electricity, cryolite}} 4Al + 3O_2$
• Cathode: $Al^{3+} + 3e^- \rightarrow Al$
• Anode: $2O^{2-} \rightarrow O_2 + 4e^-$

Example: Na by Down's cell:

• Electrolysis of molten NaCl.

2. Moderately Reactive (Zn, Fe, Pb)

Method: Roasting/Calcination + reduction by carbon.

Example: Zn:

• $2ZnS + 3O_2 \rightarrow 2ZnO + 2SO_2$ (roasting)
• $ZnO + C \rightarrow Zn + CO$ (reduction)

Example: Fe:

• $Fe_2O_3 + 3CO \rightarrow 2Fe + 3CO_2$ (Blast Furnace)

3. Less Reactive (Hg, Cu, Ag)

Method: Direct heating.

Example: Hg:

• $2HgS + 3O_2 \rightarrow 2HgO + 2SO_2$
• $2HgO \rightarrow 2Hg + O_2$

Example: Cu (self-reduction):

• $2Cu_2O + Cu_2S \rightarrow 6Cu + SO_2$

Summary

Reactivity	Method
High	Electrolysis
Moderate	Reduction by C
Low	Direct heating

[CBSE 2018]

PYQ 8: CBSE 2017, 3 marks

What is galvanisation? Why only Zn on Fe?

Solution:

Galvanisation

Process of coating Fe object with Zn.

Method: Dip Fe in molten Zn (~420°C). Cool. Now thin Zn layer on Fe.

Why Zn?

Principle: Cathodic Protection.

Reactivity: $Zn > Fe$

If layer scratches:

• Zn (more reactive) — corrodes first.
• Fe (less reactive) — protected.

That is, Zn — 'sacrificial' metal.

Why Not Cu?

Cu, less reactive than Fe. If Cu layer — on scratch, Fe corrodes first! Reverse — harmful.

Practical Use

'Galvanised Iron' (GI):

• Taps, pipes.
• Buckets.
• Roofs (with asbestos).
• Fences.

[CBSE 2017]

PYQ 9: ICSE 2022, 5 marks

(a) Nature of metal oxides. (b) What are amphoteric oxides? 2 examples. (c) Why are some metals very reactive?

Solution:

(a) Nature of Metal Oxides

Most: basic.

Examples:

• $Na_2O + H_2O \rightarrow 2NaOH$ (alkali)
• $CaO + H_2O \rightarrow Ca(OH)_2$
• $MgO + 2HCl \rightarrow MgCl_2 + H_2O$

Exceptions: $Al_2O_3$, $ZnO$ — amphoteric.

(b) Amphoteric Oxides

Those that react with both acids and bases.

1. Al₂O₃:

• With acid: $Al_2O_3 + 6HCl \rightarrow 2AlCl_3 + 3H_2O$
• With base: $Al_2O_3 + 2NaOH \rightarrow 2NaAlO_2 + H_2O$

2. ZnO:

• With acid: $ZnO + 2HCl \rightarrow ZnCl_2 + H_2O$
• With base: $ZnO + 2NaOH \rightarrow Na_2ZnO_2 + H_2O$

(c) Why Some Metals Reactive?

Principle: tendency to lose electrons.

Highly reactive (Na, K, Ca):

• 1-2 outer electrons.
• Lost easily.
• Highly reactive.

Less reactive (Cu, Au):

• Stable outer shell.
• Difficult to lose electrons.
• Less reactive.

[ICSE 2022]

PYQ 10: CBSE 2017, 3 marks

What is aqua regia? Formula, ratio, and uses.

Solution:

Definition

Aqua Regia = concentrated HCl + concentrated HNO₃.

Ratio: 3 : 1 (HCl : HNO₃)

Meaning

'Aqua' = water, 'Regia' = royal. 'Royal Water' — dissolves the 'king of metals' (gold).

Reaction with Gold

$$Au + 3HCl + HNO_3 \rightarrow AuCl_3 + NO\uparrow + 2H_2O$$

Why Both Together?

Alone HCl or HNO₃ — don't dissolve Au. HNO₃ liberates Cl₂ from HCl. Cl₂ reacts with Au to form AuCl₃.

Uses

1. Refining of gold.
2. Dissolving platinum (Pt).
3. Laboratory testing.

Properties

• Highly corrosive.
• Yellow-orange colour.
• Sharp smell.
• In glass containers.

[CBSE 2017]

PYQ 11: Bihar Board 2022, 5 marks

What is thermite reaction? Chemical reaction, principle, and 2 uses.

Solution:

Definition

Very vigorous reduction of Fe₂O₃ by Al powder — releases so much heat that Fe melts.

Reaction

$$2Al + Fe_2O_3 \xrightarrow{\text{heat}} Al_2O_3 + 2Fe + \text{lots of heat}$$

Principle

Reactivity: $Al > Fe$. Al more reactive than Fe — displaces Fe. Very fast reaction = lots of heat.

Temperature: ~3000°C. Fe is molten — like a volcano.

Uses

1. Joining railway tracks (Welding):

• Joining broken tracks.
• Molten Fe in crack solidifies, joining them.
• 'Thermite welding'.

2. Joining machine parts:

• Filling cracks in heavy parts.

3. Reduction of other metals:

• $2Al + Cr_2O_3 \rightarrow Al_2O_3 + 2Cr$ (Cr extraction)
• $8Al + 3Mn_3O_4 \rightarrow 4Al_2O_3 + 9Mn$

Precautions

• Safety goggles.
• Fire extinguisher ready.
• In open space.
• Start from a distance.

[Bihar Board 2022]

PYQ 12: UP Board 2021, 3 marks

Answer: (a) Most reactive metal? (b) Least reactive metal? (c) Which metal is liquid? (d) How to dissolve Au?

Solution:

(a) Most Reactive — K (Potassium)

At top of reactivity series. Reacts explosively with cold water. Violet flame.

(b) Least Reactive — Au (Gold)

At bottom of reactivity series. 'Noble metal'. In free form in nature. Lasts thousands of years.

(c) Liquid Metal — Hg (Mercury)

Only metal liquid at room temperature. Melting point: -39°C. Use: thermometers, barometers.

'Quick silver' — old name.

(d) How to Dissolve Au?

Only by aqua regia.

Conc. HNO₃ + Conc. HCl (1:3).

$$Au + 3HCl + HNO_3 \rightarrow AuCl_3 + NO + 2H_2O$$

Alone, no acid — dissolves Au.

[UP Board 2021]

PYQ 13: CBSE 2016, 5 marks

Explain electrolytic refining in detail — with Cu example.

Solution:

Principle

Impure metal — Anode. Thin strip of pure metal — Cathode. Salt solution of same metal. Pass electric current.

Setup (Cu Example)

• Anode (+): thick strip of impure Cu.
• Cathode (-): thin strip of pure Cu.
• Electrolyte: $CuSO_4$ + dilute $H_2SO_4$.
• Power source: DC.

Reactions

At Anode: $$Cu(\text{impure}) \rightarrow Cu^{2+} + 2e^-$$

At Cathode: $$Cu^{2+} + 2e^- \rightarrow Cu(\text{pure})$$

Fate of Impurities

1. Less reactive than Cu (Au, Ag, Pt):

• Don't leave the anode.
• Fall as 'anode mud' below the anode.

2. More reactive than Cu (Fe, Zn):

• Go into solution.
• Don't deposit at cathode.

Final Result

• Cathode: 99.99% pure Cu.
• Anode mud: Au, Ag, Pt (valuable!).

Importance of Anode Mud

Not 'unwanted waste' — highly valuable! Bonus of Cu industry.

Each year in India — significant gold/silver from Cu refining.

For Different Metals

Al, Zn, Ni — same method. For each — corresponding metal salt.

[CBSE 2016 — every year]

PYQ 14: CBSE 2015, 3 marks

Balance the following: (a) Zn + HCl → (b) Mg + O₂ → (c) Al + O₂ → (d) Fe + H₂O (steam) →

Solution:

(a) Zn + HCl

$$Zn + 2HCl \rightarrow ZnCl_2 + H_2$$

Check: Zn=1=1, H=2=2, Cl=2=2 ✓

(b) Mg + O₂

$$2Mg + O_2 \rightarrow 2MgO$$

Check: Mg=2=2, O=2=2 ✓

(c) Al + O₂

$$4Al + 3O_2 \rightarrow 2Al_2O_3$$

Check: Al=4=4, O=6=6 ✓

(d) Fe + H₂O (steam)

$$3Fe + 4H_2O \rightarrow Fe_3O_4 + 4H_2$$

Check: Fe=3=3, H=8=8, O=4=4 ✓

Tips

Balancing rules:

1. Start from most complex compound.
2. Change coefficients, not formulas.
3. Balance H, O at end.

[CBSE 2015]

PYQ 15: MP Board 2022, 5 marks

5 physical and 3 chemical properties of metals with examples.

Solution:

5 Physical Properties

1. Physical state:

• Mostly solid.
• Exception: Hg (liquid).

2. Metallic Lustre:

• All shiny.
• Au, Ag — extra shiny.
• Exception: some surface coating.

3. Malleability:

• Can be made into thin sheets.
• Au — most (1 g into 1 m² sheet).

4. Ductility:

• Can be drawn into thin wires.
• Au — most ductile (1 g into 2 km wire).

5. Electrical/thermal conductivity:

• Good.
• Ag > Cu > Au > Al.
• Electric wires: Cu (most common).

Others: hardness, sonority, high melting point.

3 Chemical Properties

1. Reaction with oxygen: Metal + O₂ → metal oxide (basic).

Example: $2Mg + O_2 \rightarrow 2MgO$

2. Reaction with acids: Metal + dilute acid → salt + H₂↑.

Example: $Zn + 2HCl \rightarrow ZnCl_2 + H_2$

3. Displacement from salt solution: More reactive → displaces less reactive.

Example: $Zn + CuSO_4 \rightarrow ZnSO_4 + Cu$

[MP Board 2022]

PYQ 16: CBSE 2014, 3 marks

(a) Cu strip + AgNO₃ — what happens? (b) Ag strip + CuSO₄ — what happens? (c) Reasoning from reactivity series.

Solution:

(a) Cu + AgNO₃

Series: $Cu > Ag$.

Cu displaces Ag.

Reaction: $$Cu + 2AgNO_3 \rightarrow Cu(NO_3)_2 + 2Ag$$

Observations:

1. Solution turns from colourless to blue.
2. Shiny Ag layer on Cu strip.
3. Cu slowly dissolves.

(b) Ag + CuSO₄

Series: $Cu > Ag$.

Ag cannot displace Cu.

Reaction: None.

Observations:

• Blue CuSO₄ — remains blue.
• Ag strip — unaffected.

(c) Reasoning

Principle: More reactive metal — displaces less reactive.

Case (a): Cu (more) → displaces Ag. ✓ Case (b): Ag (less) → cannot displace Cu. ✗

This — practical use of reactivity series.

[CBSE 2014]

PYQ 17: CBSE 2013, 5 marks

4 methods of corrosion prevention — in detail.

Solution:

1. Painting

Principle: Layer of paint on Fe.

Barrier from air and moisture.

Advantages: cheap, easy, various colours. Disadvantages: rust on scratches.

Use: cars, windows, garden items.

2. Oiling/Greasing

Principle: Thin layer of oil.

Advantages: immediate, simple. Disadvantages: dries up quickly.

Use: machine parts, locks.

3. Galvanisation

Principle: Layer of Zn on Fe (Cathodic protection).

Zn — more reactive than Fe. On scratch — Zn corrodes first. Fe protected.

Method: Dip Fe in molten Zn.

Use: taps, pipes, buckets, roofs.

4. Electroplating

Principle: Coating one metal with another by electrolytic method.

Example: Cr plating.

• Cathode: Fe object.
• Anode: Cr (or inert).
• Solution: $CrO_3$ + $H_2SO_4$.

Advantages: shiny, durable. Use: taps, car parts.

Additional — Alloys

Stainless steel (Fe + Cr + Ni) — permanent protection. $Cr_2O_3$ layer from Cr — 'self-healing'.

[CBSE 2013 — every year]

PYQ 18: ICSE 2021, 3 marks

What are alloys? Five examples and their uses.

Solution:

Definition

'Alloy' = homogeneous mixture of two or more metals — or one metal + one non-metal.

Purpose: better properties than pure metal.

5 Examples

1. Stainless Steel

Composition: Fe (74%) + Cr (18%) + Ni (8%) Use: kitchen utensils, knives, medical instruments. Special: corrosion-resistant ($Cr_2O_3$ layer from Cr).

2. Brass

Composition: Cu (70%) + Zn (30%) Use: decoration, utensils, musical instruments. Colour: yellow.

3. Bronze

Composition: Cu (88%) + Sn (12%) Use: statues, medals, bells. 'Bronze Age'.

4. Solder

Composition: Pb (50%) + Sn (50%) Use: electrical welding, circuit boards. Low melting point (~180°C).

5. Duralumin

Composition: Al (95%) + Cu (4%) + Mg (0.5%) + Mn (0.5%) Use: aircraft, cars. Light + strong.

Benefits of Alloys

1. Increased hardness.
2. Corrosion-resistance.
3. Specific properties.
4. Long life.

[ICSE 2021]

PYQ 19: CBSE 2012, 5 marks

(a) Why can't Na be reduced by carbon? (b) What is the correct method? (c) Difference between molten vs aqueous NaCl electrolysis.

Solution:

(a) Why Na + C Doesn't Work?

Principle: More reactive metal — displaces less reactive.

Reactivity: $Na > C$

Na, more strongly bonded with O than C is. C cannot free Na from Na₂O.

If we try: $Na_2O + C \rightarrow ?$ — no reaction.

(b) Correct Method — Down's Cell

Electrolysis of molten NaCl.

Setup:

• A steel box.
• Solution: molten NaCl + CaCl₂ (~600°C).
• Cathode: steel ring.
• Anode: carbon rod.

Reactions:

• Cathode: $Na^+ + e^- \rightarrow Na$
• Anode: $2Cl^- \rightarrow Cl_2 + 2e^-$

Overall: $2NaCl(l) \rightarrow 2Na(l) + Cl_2(g)$

(c) Molten vs Aqueous

Property	Molten NaCl	Aqueous NaCl
At Cathode	Na	H₂ (from water)
At Anode	Cl₂	Cl₂
Product	Na metal	NaOH
Use	Na extraction	Chlor-alkali

Why not Na in aqueous? At Cathode:

• Na (more reactive) vs H₂O (less reactive).
• Less reactive will be reduced.
• Hence H₂O — reduced at cathode (H₂).
• Na not formed.

[CBSE 2012]

PYQ 20: A Concluding Question — Mixed

(a) Most important reactions (5). (b) Reactivity series. (c) 3 steps of extraction. (d) 3 prevention methods.

Solution:

(a) 5 Most Important Reactions

1. Mg + O₂: $2Mg + O_2 \rightarrow 2MgO$ (white flame)

2. Na + H₂O: $2Na + 2H_2O \rightarrow 2NaOH + H_2$ (explosive)

3. Zn + HCl: $Zn + 2HCl \rightarrow ZnCl_2 + H_2$

4. Zn + CuSO₄: $Zn + CuSO_4 \rightarrow ZnSO_4 + Cu$ (displacement)

5. Fe + H₂O (steam): $3Fe + 4H_2O \rightarrow Fe_3O_4 + 4H_2$

(b) Reactivity Series

$$K > Na > Ca > Mg > Al > Zn > Fe > Pb > (H) > Cu > Hg > Ag > Au$$

Mnemonic: "Please Stop Calling Me A Zebra…"

(c) 3 Steps of Extraction

1. Concentration: removing gangue (hand picking, gravity, magnetic, froth flotation).

2. Reduction: ore → metal (by carbon, by electrolysis, self-reduction).

3. Refining: purification (electrolytic).

(d) 3 Prevention Methods

1. Painting. 2. Galvanisation (Zn coating — Cathodic protection). 3. Alloy (stainless steel = Fe + Cr + Ni).

Others: oil, Cr plating, anodising.

Final Formula

'Reactivity ↑ → extraction difficulty ↑ → corrosion also ↑.' Metal + Non-metal = Ionic compound. Metal + Metal = Alloy.

[Board — entire chapter summary]