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NCERT Solutions for Class 7th Science Chapter 4 End-of-chapter exercise — Let Us Enhance Our Learning

Book page 55 & 56 Updated on2026-09-19

Q1.
Which metal is commonly used to make food packaging materials as it is cheaper, and its thin sheets can be folded easily into any shape? (i) Aluminium (ii) Copper (iii) Iron (iv) Gold
Answer

(i) Aluminium

OptionWhy it is or is not the answer
(i) AluminiumCorrect — cheap, light, highly malleable, does not rust, safe with food
(ii) CopperCostlier, and its surface forms a green coating that is not safe for food packing
(iii) IronHeavy, and it rusts in moist air
(iv) GoldThe most malleable, but far too expensive for wrapping food
Why it happens: the question gives two clues — “cheaper” and “thin sheets folded easily”. Only a metal that is both inexpensive and very malleable fits, and that is aluminium. This is the foil you see around chocolates, medicines and packed food.
Q2.
Which of the following metal catches fire when it comes in contact with water? (i) Copper (ii) Aluminium (iii) Zinc (iv) Sodium
Answer

(iv) Sodium

Sodium + water → vigorous reaction + a lot of heat → the metal catches fire
That is why sodium is stored in kerosene.
Why it happens: sodium reacts with oxygen and water so violently that the heat produced is enough to set it alight. Kerosene keeps both moisture and air away from it. Copper, aluminium and zinc do not react with water in this way.
Tip: do not confuse this with phosphorus, which is a non-metal and is stored in water because it catches fire in air. Sodium: metal, kept in kerosene. Phosphorus: non-metal, kept in water.
Q3.
State with reason(s) whether the following statements are True [T] or False [F]. (i) Aluminium and copper are examples of non-metals used for making utensils and statues. [ ] (ii) Metals form oxides when combined with oxygen, the solution of which turns blue litmus paper to red. [ ] (iii) Oxygen is a non-metal essential for respiration. [ ] (iv) Copper vessels are used for boiling water because they are good conductors of electricity. [ ]
Answer
StatementT/FReason
(i) Aluminium and copper are examples of non-metals used for making utensils and statues.FalseAluminium and copper are metals, not non-metals. They are lustrous, hard, malleable, ductile and good conductors. The uses stated are correct, only the word “non-metals” is wrong.
(ii) Metals form oxides when combined with oxygen, the solution of which turns blue litmus paper to red.FalseMetal oxides are basic, so their solution turns red litmus blue. Turning blue litmus red is the behaviour of an acidic solution, which is what a non-metal oxide such as sulfur dioxide gives.
(iii) Oxygen is a non-metal essential for respiration.TrueOxygen is a non-metal, and we breathe it in. Without it we would not survive.
(iv) Copper vessels are used for boiling water because they are good conductors of electricity.FalseThe reason is wrong. Copper is chosen because it is a good conductor of heat, so heat passes quickly from the flame into the water. Its electrical conductivity has nothing to do with boiling water.
Tip: in a True/False question, always check the reason as carefully as the fact. Statement (iv) is a true fact joined to a wrong reason — and that makes the whole statement false.
Q4.
Why are only a few metals suitable for making jewellery?
Answer

Jewellery makes an unusual set of demands, and only a handful of metals — mainly gold, silver and platinum — meet all of them.

  1. They must not corrode. Jewellery is worn every day against sweat and in moist air. Gold and silver resist corrosion, so they keep their shine for generations. Iron would rust and copper would grow a green coating.
  2. They must be highly malleable and ductile. A necklace or an earring needs the metal to be drawn into fine wire and beaten into delicate shapes. Gold is the most malleable and most ductile of all metals — one gram can be drawn into a 2 kilometre-long wire.
  3. They must be soft enough to work, yet lasting. Gold and silver can be shaped by a goldsmith with simple tools, unlike iron which needs a furnace.
  4. They must be lustrous and attractive. The yellow shine of gold and the white shine of silver do not fade.
  5. They must be safe on the skin. These metals do not harm the body.
  6. They are rare. Rarity gives them value, which is why jewellery is also a family's savings.
Why it happens: most metals fail on the very first point. Ordinary metals react slowly with air and moisture, and a corroded ornament loses both its beauty and its worth.
Q5.
Match the uses of metals and non-metals given in Column I with the jumbled names of metals and non-metals given in Column II. Column I: (i) Used in electrical wiring (ii) Most malleable and ductile (iii) Living organisms cannot survive without it. (iv) Plants grow healthy when fertilisers containing it are added to the soil. (v) Used in water purification. Column II: (a) E N X Y G O (b) N E C O H I R L (c) P E P O R C (d) T E N G O I N R (e) O G D L
Answer

First unjumble Column II, then match.

Column II (jumbled)Unjumbled nameMetal or non-metal
(a) E N X Y G OOXYGENNon-metal
(b) N E C O H I R LCHLORINENon-metal
(c) P E P O R CCOPPERMetal
(d) T E N G O I N RNITROGENNon-metal
(e) O G D LGOLDMetal
Column IMatchesAnswerReason
(i) Used in electrical wiring(c)CopperAn excellent conductor of electricity and ductile enough to be drawn into wire
(ii) Most malleable and ductile(e)GoldThe most malleable and most ductile of all metals
(iii) Living organisms cannot survive without it.(a)OxygenNeeded for respiration
(iv) Plants grow healthy when fertilisers containing it are added to the soil.(d)NitrogenUsed in making fertilisers; an essential nutrient for plant growth
(v) Used in water purification(b)ChlorineCommonly used to purify drinking water
(i) – (c), (ii) – (e), (iii) – (a), (iv) – (d), (v) – (b)
Q6.
What happens when oxygen reacts with magnesium and sulfur. What are the main differences in the nature of products formed?
Answer

Both burn in oxygen and both form an oxide — but one oxide is basic and the other is acidic.

Magnesium + oxygen → magnesium oxide (a white powder)
Magnesium oxide + water → a solution that turns red litmus blue → basic

Sulfur + oxygen → sulfur dioxide (a gas)
Sulfur dioxide + water → sulfurous acid, which turns blue litmus red → acidic
Point of differenceMagnesium (a metal)Sulfur (a non-metal)
How it burnsWith a dazzling white flameWith a pale blue flame, giving off a choking gas
Product formedMagnesium oxide — a white solid powderSulfur dioxide — a gas
Nature of the solution in waterBasicAcidic
Effect on litmusRed litmus → blueBlue litmus → red
Why it matters: this is the general rule of the chapter — metal oxides are basic, non-metal oxides are acidic. Testing an element's oxide with litmus is therefore a reliable way of deciding whether the element is a metal or a non-metal.
Q7.
Complete the following flow chart:
?+Air+HeatAshWater?Add blue and red litmussolutions separatelyChange in bluelitmus solutionChange in redlitmus solution?Blue
The flow chart printed with question 7 on page 56.
Answer

Work backwards from the one box that is filled in. The red litmus turns blue, so the solution is basic, so the ash must be a metal oxide, so the burnt substance must be a metal. The metal of this chapter that burns in air to a white ash is magnesium.

Magnesium + Air + Heat Ash Water Magnesium oxide solution (basic) Add blue and red litmus solutions separately Change in blue litmus solution Change in red litmus solution No change (stays blue) Blue
The completed flow chart. The three purple boxes are the ones you have to fill in.
Box marked “?”AnswerReason
First box (burnt in air with heat)Magnesium (a metal)Burning magnesium ribbon in air gives a white ash
Ash + waterMagnesium oxide solution — a basic solutionThe ash is magnesium oxide; metal oxides give basic solutions
Change in blue litmus solutionNo change — it remains blueA basic solution does not change blue litmus
Tip: the filled-in “Blue” at the bottom right is the key to the whole chart. Red litmus turning blue can only mean a base, and only a metal oxide is basic.
Q8.
You are provided with the following materials. Discuss which material would be your choice to make a pan that is most suitable for boiling water and why? Iron, copper, sulfur, coal, plastic, wood, cardboard
Answer

Copper is the best choice, with iron a close and cheaper second. Everything else in the list is unusable.

MaterialSuitable?Reason
CopperBest choiceMetal — the best conductor of heat here, so water boils fast; malleable, so it can be beaten into a pan; hard, strong and does not melt or burn
IronAlso suitableA good conductor and very strong, but heavier and it rusts if left wet
SulfurNoNon-metal — brittle, cannot be shaped into a pan, poor conductor, and it burns
CoalNoBrittle, poor conductor — and it is a fuel, so it would burn
PlasticNoPoor conductor of heat and it melts on a flame
WoodNoPoor conductor and it catches fire
CardboardNoPoor conductor, soaks up water and burns at once
Why it happens: a pan for boiling water must do three things — carry heat quickly from the flame into the water, hold its shape when hot, and not catch fire or melt. Only a metal does all three, and among the metals offered copper conducts heat best.
Tip: the handle should not be copper. Fit a wooden or plastic handle, because a poor conductor there keeps your hand safe.
Q9.
You are provided with three iron nails, each dipped in oil, water and vinegar. Which iron nail will not rust, and why?
Answer

The nail dipped in oil will not rust.

Nail dipped inWill it rust?Why
OilNoOil does not contain water and does not let air dissolve into it, so the nail is cut off from both air and moisture. Oiling is in fact one of the standard ways of preventing rust.
WaterYesOrdinary water has air dissolved in it, so both air and water reach the nail
VinegarYes — fastest of the threeVinegar is an acid solution in water, and acid speeds up the eating away of the iron surface
Rusting needs air + water together
Oil supplies neither → no rusting
Why it happens: a film of oil acts as a physical barrier. This is exactly why cycle chains, tools and machine parts are oiled or greased, and why your grandmother rubs a little oil on the tawa before putting it away.
Q10.
How do the different properties of metals and non-metals determine their uses in everyday life?
Answer

A material is chosen for a job because of the property the job demands. Change the property and the use changes with it.

PropertyWho has itUse it leads to
MalleabilityMetalsAluminium foil, silver vark, tawas and buckets beaten out of sheets
DuctilityMetalsElectrical wires, sitar and veena strings, tea strainers, chains and ornaments
Good conductor of heatMetalsCooking vessels, pressure cookers, kadhais
Poor conductor of heatWood, plasticHandles of pans and ladles
Good conductor of electricityMetalsCopper and aluminium wiring, switches, plug pins
Poor conductor of electricityRubber, plastic (non-metallic materials)Wire insulation, screwdriver grips, electrician's gloves and shoes
SonorityMetalsSchool bells, temple bells, ghungroos, cymbals
Hardness and strengthMetalsFarm tools, girders, railway lines, suspension bridge cables
Lustre and resistance to corrosionGold, silverJewellery and coins
Chemical usefulnessNon-metalsOxygen for breathing, nitrogen for fertilisers, chlorine for water purification, iodine as an antiseptic, carbon as the building block of life
Why it matters: look at any single object and you can read the science in it. A screwdriver is metal where it must carry force and plastic where it must stop current; a kadhai is metal where it meets the flame and wood where it meets your hand. Nothing is chosen by accident.
Q11.
One of the methods of protecting iron from getting rusted is to put a thin coating of zinc metal over it. Since sulfur does not react with water, can it be used for this purpose? Justify your answer.
Answer

No, sulfur cannot be used. Not reacting with water is only one of the things a protective coating must do, and sulfur fails all the others.

  1. Sulfur is brittle, not malleable or ductile. A coating has to be spread as a thin, continuous, tightly stuck film. Sulfur is a soft, crumbly non-metal — it cannot be beaten or drawn into a layer, and any layer would crack and flake off.
  2. It will not stick to the iron. Zinc bonds firmly to the iron surface; loose sulfur powder simply rubs away, leaving the iron bare again.
  3. A cracked coating is worse than none. Once moist air reaches the iron through the gaps, rusting starts there — and being trapped under the coating, it may go unnoticed until the damage is done.
  4. Zinc does more than merely cover. Even if a galvanised sheet is scratched, the zinc keeps protecting the exposed iron. Sulfur offers no such protection at all.
  5. Sulfur burns. A sulfur-coated iron object near a flame would burn and give off harmful sulfur dioxide gas.
Why it happens: the argument in the question is a trap. “Sulfur does not react with water” is true, but the question a scientist must ask is “can it form and hold a protective layer?” — and there the answer is a clear no. The correct methods remain painting, oiling, greasing and galvanisation.
Q12.
An ironsmith heats iron before making tools. Why is heating necessary in this process?
Answer

Because hot iron is far more malleable than cold iron. Heating turns a stubborn block into something that will take a shape under the hammer.

  1. Cold iron is hard and stiff. Hammering it wastes effort — the blows bounce off, and a heavy blow may crack the piece instead of bending it.
  2. Red hot iron is soft. Each hammer blow now spreads and bends the metal. Sudarshan uncle heats the block until it is red hot and only then beats it into an axe.
  3. The shape can be changed many times. The smith can flatten, curve, punch a hole and draw out an edge, reheating whenever the piece cools and stiffens.
  4. The tool comes out stronger. Shaping by hammering, rather than by cutting metal away, leaves the iron dense and tough, so the edge of a kulhadi lasts.
Cold iron + hammer → resists, may crack
Heated (red hot) iron + hammer → flattens and bends into shape
Heating increases the malleability of iron.
Did you know? This craft is thousands of years old. The Iron Pillar of Delhi was shaped by smiths working iron in exactly this way, and it still stands almost free of rust after more than 1600 years.
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