CBSE Class 10 Science Metals and Non metals VBQs Set 05

This is Set 05 of value based and real-life application questions (VBQs) for CBSE Class 10 Science Chapter 3, Metals and Non-metals, with step-wise answers and a Teacher's Note for each of the 18 questions. The questions connect the properties of metals to everyday situations such as cooking vessels, electrical wiring, jewellery, welding and rust prevention, and bring out values like honesty, safety and care for resources.

Value Based Questions

Question 1. A man went door to door posing as a goldsmith. He promised to bring back the glitter of old and dull gold ornaments. An unsuspecting lady gave a set of gold bangles to him which he dipped in a particular solution. The bangles sparkled like new but their weight was reduced drastically. The lady was upset but after a futile argument the man beat a hasty retreat. Can you play the detective to find out the nature of the solution he had used?
Answer:
Step 1: Gold does not react with a single acid such as HCl, HNO₃ or H₂SO₄. Only a mixture that can dissolve gold would reduce its weight.
Step 2: The man used aqua regia (royal water), a freshly prepared mixture of concentrated hydrochloric acid and concentrated nitric acid in the ratio 3 : 1. It is a highly corrosive, fuming liquid.
Step 3: Aqua regia dissolves the outer dull layer of gold, so the bangles look shiny, but the gold that dissolved makes the bangles lighter.
Value: Be careful about handing valuables to strangers, and test claims with scientific knowledge.
In simple words: The man used aqua regia, which dissolves a thin layer of gold, so the bangles became shiny but lighter.

Teacher's Note:
1. Remember the ratio: 3 parts conc. HCl to 1 part conc. HNO₃.
2. Link the science answer to the value of honesty and alertness.

Question 2. A farmer was working with an agricultural machine when he observed a crack. He realized that if the crack is not mended it will lead to further damage of the machine. He reached out to the local mechanic who repaired the crack with some iron compound and aluminium. What is the technique used by the local mechanic? Write relevant chemical reaction equation for the same.
Answer:
Technique: The mechanic used the thermite reaction (aluminothermy).
Working: A mixture of iron(III) oxide and aluminium powder is ignited. Aluminium is more reactive than iron, so it displaces iron from its oxide. The reaction gives out so much heat that the iron is formed in the molten state and flows into the crack; on cooling it solidifies and joins the broken part.
Equation: Fe₂O₃(s) + 2Al(s) → 2Fe(l) + Al₂O₃(s) + heat
Value: Repairing instead of discarding saves money and resources.
In simple words: Aluminium melts iron out of iron oxide, and the molten iron fills the crack like a weld.

Teacher's Note:
1. Use the words ‘more reactive’ and ‘highly exothermic’ in the answer.
2. The same method is used to join railway tracks.

Question 3. Mr. Joe has a long-cherished wish of wearing a golden frame spectacle. He expressed his wish in front of his chemist daughter. His daughter was overwhelmed to hear her father’s wish and decided to set up the experiment herself to get the golden frame.
Suggest the rough set up of experiment with a diagram.
Answer:
Idea: Gold can be deposited on the frame by electroplating.
Set-up: Take the clean aluminium spectacle frame as the cathode (connected to the negative terminal of a battery) and a small bar of pure gold as the anode (positive terminal). Dip both in a suitable electrolyte, a solution of a gold salt, and complete the circuit through the outer wires.
Working: When current flows, gold from the anode dissolves as ions (Au → Au³⁺ + 3e⁻) and the gold ions are deposited on the frame at the cathode (Au³⁺ + 3e⁻ → Au). A shiny golden coating forms on the frame.
In simple words: Put the frame and a gold bar in a gold-salt solution and pass current; gold moves from the bar to the frame.

Teacher's Note:
1. Always state the cathode, the anode and the electrolyte.
2. The object to be plated is made the cathode.

Question 4. When Mohit took his laptop to the service centre for repair, he noticed that the engineer was soldering a wire to the motherboard of the device. He described this case to his science teacher. The teacher told Mohit that a fusible metal alloy is used to create a permanent bond between the wire and the motherboard. This alloy is resistant to several factors that can degrade the joint over time. Name the alloy that is used by the engineer for welding the electrical wires together and also, give its composition.
Answer:
The alloy is solder.
Composition: It is an alloy of lead and tin (Pb and Sn). It has a low melting point, so it melts easily and sets quickly, making a firm joint between the wire and the board.
In simple words: Solder is a lead–tin alloy that melts at a low temperature and joins wires.

Teacher's Note:
1. Remember: solder = lead + tin; brass = copper + zinc; bronze = copper + tin.
2. A low melting point protects delicate electronic parts from heat.

Question 5. A metal ‘X’ acquires a green colour coating on its surface on exposure to air. (i) Identify the metal ‘X’ and name the process responsible for this change. (ii) Name and write chemical formula of the green coating formed on the metal. (iii) List two important methods to prevent the process.
Answer:
(i) The metal is copper and the process is corrosion.
(ii) The green coating is basic copper carbonate, CuCO₃·Cu(OH)₂. It forms when copper reacts slowly with moist carbon dioxide and oxygen of the air.
(iii) Corrosion can be prevented by (1) painting, oiling or varnishing the surface so that air and moisture cannot reach it, and (2) making an alloy (for example brass) or electroplating with a less reactive metal.
In simple words: Copper slowly turns green in damp air because it forms basic copper carbonate; coating it keeps air and water away.

Teacher's Note:
1. Write the formula as CuCO₃·Cu(OH)₂, not only CuCO₃.
2. Preventing corrosion saves the money spent on replacing metal objects.

Real-Life Application Questions

Question 6. Give reasons why copper is used to make hot water tanks and not steel (an alloy of iron).
Answer:
Copper does not react with cold water, hot water or steam. Iron, the main constituent of steel, reacts with steam. If hot water tanks were made of steel, the iron would react with the steam formed from the hot water and the tank would corrode:
3Fe + 4H₂O → Fe₃O₄ + 4H₂
So copper is used for hot water tanks and not steel.
In simple words: Copper is not attacked by hot water or steam, but iron is.

Teacher's Note:
1. Link the use of a metal to its place in the reactivity series.
2. Write the iron and steam reaction to support your answer.

Question 7. Give reason for the following: (i) School bells are made up of metals. (ii) Electric wires are made up of copper.
Answer:
(i) Metals are sonorous, that is, they produce a ringing sound when struck with a hard object, so a school bell made of metal can be heard from far away.
(ii) Copper is a very good conductor of electricity, it is ductile (can be drawn into thin wires) and it is cheaper than silver, the best conductor.
In simple words: Metals ring when struck, so they make good bells; copper carries current well and can be drawn into wires.

Teacher's Note:
1. Name the property (sonority, ductility, conductivity) before explaining the use.
2. Silver conducts best but it is far too costly for wires.

Question 8. Give reason why : Electric wires are coated with plastic (PVC) ?
Answer:
Metal wires carry current, so touching a bare wire can give a severe electric shock. PVC (plastic) is a bad conductor (an insulator) of electricity, so a PVC coating stops the current from passing through our body, and it also prevents short circuits between wires.
In simple words: PVC does not let electricity pass, so the coating keeps us safe from shocks.

Teacher's Note:
1. Link the safety use to the poor conductivity of plastics.
2. Insulators and conductors are opposites in daily use.

Question 9. Why are cooking vessels and water boilers generally made of copper and aluminium ?
Answer:
Copper and aluminium are good conductors of heat, so the vessels and boilers get heated up quickly and the food or water inside is heated evenly. They are also malleable and have high melting points, so they can be shaped into vessels and do not melt on the stove. Aluminium also forms a protective oxide layer that keeps it from reacting with food.
In simple words: These metals pass heat quickly, so cooking and boiling is fast.

Teacher's Note:
1. Name the property: good thermal conductivity.
2. Mention one more property, such as a high melting point.

Question 10. You must have seen tarnished copper vessels being cleaned with lemon or tamarind juice. Explain why these sour substances are effective in cleaning the vessels.
Answer:
Copper vessels left in damp air get a dull coating of basic copper carbonate (or copper oxide). This coating is basic in nature.
Lemon and tamarind juice contain organic acids (citric acid and tartaric acid). These acids neutralise the basic coating and dissolve it, so the shiny red-brown surface of copper appears again.
In simple words: Sour juices are acids; they dissolve the basic green coating and make the vessel shine.

Teacher's Note:
1. An acid cleans a basic coating by neutralising it.
2. This is a safe, low-cost household use of a chemical reaction.

Question 11. Give reasons for the following: (i) Copper is used for making electrical circuits but brass is not. (ii) Steel is used for making utensil but pure iron is not.
Answer:
(i) Copper is a pure metal with very high electrical conductivity. Brass is an alloy of copper and zinc, and an alloy conducts electricity less well than its pure metals, so brass is not used for making circuits.
(ii) Pure iron is soft, stretches easily and rusts quickly. Steel is an alloy of iron with carbon (and sometimes nickel and chromium); it is hard, keeps its shape on heating and resists rusting, so steel is used for utensils.
In simple words: Alloys conduct less but are stronger; so wires use copper and utensils use steel.

Teacher's Note:
1. Alloys have properties different from their metals; note which property each use needs.
2. Stainless steel contains iron, chromium and nickel.

Question 12. Why are the heating elements of electric toasters and electric irons made of an alloy rather than a pure metal?
Answer:
The resistivity of an alloy is generally higher than that of its constituent metals, so it produces more heat when current flows through it. Alloys also do not oxidise (burn) readily at high temperatures, so the element lasts long. That is why heating elements are made of alloys (such as nichrome) and not pure metals.
In simple words: An alloy resists current more and does not burn away when red hot.

Teacher's Note:
1. Two reasons are needed: high resistivity and no oxidation on heating.
2. Nichrome is a common heating alloy.

Question 13. Silver articles become black when kept in open for some time, whereas copper vessels lose their shiny brown surfaces and gain a green coat when kept in open. Name the substances present in air with which these metals react and write the name of the products formed.
Answer:
Silver: It reacts with hydrogen sulphide (sulphur compounds) present in air and forms black silver sulphide (Ag₂S).
Copper: It reacts slowly with carbon dioxide and moisture (water vapour) of the air and forms a green coat of basic copper carbonate, CuCO₃·Cu(OH)₂.
In simple words: Silver reacts with sulphur gas in air and turns black; copper reacts with carbon dioxide and moisture and turns green.

Teacher's Note:
1. Match each metal with its coating colour and compound.
2. This slow eating away of a metal is called corrosion.

Question 14. Explain why the galvanised iron article is protected against rusting even if the zinc layer is broken.
Answer:
In galvanisation the iron article is coated with a layer of zinc. Zinc is more reactive than iron, so even if the layer is scratched and some iron is exposed, the zinc is oxidised (corroded) in preference to iron. The zinc acts as a sacrificial metal and protects the iron from rusting as long as some zinc is present.
In simple words: Zinc rusts instead of iron, so the iron stays safe even when the coating is scratched.

Teacher's Note:
1. The key words are ‘more reactive’ and ‘sacrificial’.
2. Paint protects only while it is unbroken, zinc keeps protecting.

Question 15. Give an advantage of corrosion of metals.
Answer:
Corrosion of some metals is an advantage. When aluminium (or zinc, magnesium) is exposed to air it forms a thin, tough layer of its oxide on the surface. This layer does not let air and moisture reach the metal underneath, so it protects the metal from further damage. In the same way, the green coating on copper protects the copper below it.
In simple words: A thin oxide layer on aluminium protects the metal from further corrosion.

Teacher's Note:
1. Give a named example, such as aluminium oxide on aluminium.
2. Corrosion of iron is harmful because rust is flaky and keeps exposing fresh iron.

Case Based Questions

Question 16. Metals have a great importance in our everyday life. They are used in making utensils, buildings, bridges, automobile parts, coins, jewellery, etc. Metals like Ti, Cr, Mn and Zr are called strategic metals as they are used in country’s defense. (A) A wire of about 2 km length can be drawn from one gram of gold. Assess the uses of this property. (B) (i) The wiring for transmission of electricity needs to be changed in Seema’s household. She was given a choice between copper and aluminium wires. Which wire do you think she would have chosen for transmission of electricity? (ii) Seema prefers to use anodised articles than un-anodised articles of aluminum. Explain. (C) Seema wants to get broken parts of her iron machinery welded. Someone suggested her that she should call a mechanic who should use thermite reaction. What is thermite reaction and what materials are used in this reaction?
Answer:
(A) The ability to be drawn into thin wires is ductility. Because gold is so ductile, very fine gold wires and thin gold threads can be made for jewellery and ornaments.
(B)(i) Seema would choose copper wires, because copper is a better conductor of electricity than aluminium, so less energy is lost as heat.
(B)(ii) Anodising coats aluminium with a thick, protective layer of aluminium oxide, so anodised articles resist corrosion and last longer than un-anodised ones.
(C) The reduction of a metal oxide with aluminium is called the thermite reaction (aluminothermy). Iron(III) oxide and aluminium powder are used: Fe₂O₃ + 2Al → 2Fe + Al₂O₃ + heat. The heat produced melts the iron, which joins the broken parts of the machinery.
In simple words: Gold can be drawn into very thin wire; copper wire carries current best; thermite uses aluminium to melt iron for welding.

Teacher's Note:
1. Name the property first (ductility), then give the use.
2. The thermite mixture is iron(III) oxide plus aluminium powder.

Teacher's Note:
1. Compare the surface area, not just the mass.
2. Iron needs both air and moisture to rust, and a coating keeps them away.

Question 18. Case Study-I. The huge annual loss due to corrosion is a national waste and should be minimized. Following are some process that are helpful in preventing corrosion. (1) Galvanisation of iron is a process in which a thin layer of zinc will form on the iron surface. As zinc is more reactive than iron so it will act as a sacrificial metal and is preferentially oxidised thus protecting oxidation of iron. (2) The iron surface is coated with paint, oil or grease protect the metal from coming in contact with the moist oxygen and thus effectively protects rusting of iron. (3) Electroplating with metals like tin, nickel or chromium also prevents rusting process. (4) Alloying (mixing of iron in its molten state with other metals) protects rusting. Stainless steel is an alloy of iron with constituents like Cr or Ni. Answer the following questions on the basis of above paragraph. 1. The metal plating which is most durable and is used on iron to protect against corrosion is (a) Nickel plating (b) Copper plating (c) Tin plating (d) Zinc plating. 2. The most common method that is used to protect the bottom of ship made of iron is: (a) Connecting it with Mg block (b) White tin plating (c) Coating it with red lead oxide (d) Connecting it with Pb block. 3. The rusting of iron is best prevented by which of the following method? (a) Making it cathode (b) Putting in saline water (c) Both of these (d) None of these
Answer:
1. The most durable metal plating on iron is (d) zinc plating (galvanising), because zinc keeps protecting iron even when scratched.
2. The bottom of an iron ship is protected by (a) connecting it with a Mg block. Magnesium is more reactive than iron, so it corrodes in place of the iron hull.
3. Rusting is best prevented by (a) making it cathode. When the iron is made the cathode of a cell, it is protected, because it cannot lose electrons and be oxidised.
In simple words: Zinc plating and a block of magnesium both protect iron because they are more reactive and corrode first.

Teacher's Note:
1. A more reactive metal in contact with iron acts as a sacrificial metal.
2. Prevention of corrosion saves a lot of national wealth.