CBSE Class 10 Science Metals and Non metals VBQs Set 06

This CBSE Class 10 Science value based and competency based questions page for Chapter 3, Metals and Non-metals, has 21 questions with detailed answers. Case studies on electrolytic refining, ores, the reactivity series and thermite reactions are followed by real-life questions on jewellery, cooking vessels, electric wires and the protection of iron.

Case Based Questions

Question 1. The metals produced by various reduction processes are not very pure. They contain impurities, which must be removed to obtain pure metals. The most widely used method for refining impure metals is electrolytic refining.
(i) What are the cathode and anode made of in the refining of copper by this process?
(ii) Name the solution used in the process and write its formula.
(iii) How does copper get refined when an electric current is passed through the cell?
(iv) Two beakers A and B contain copper sulphate solution. What will you observe after about 2 hours if you dip a strip of zinc in A and a strip of silver in B? Give reasons.
Answer:
(i) The anode is a block of impure copper and the cathode is a thin strip of pure copper.
(ii) Acidified copper sulphate solution, CuSO₄.
(iii) Copper from the anode dissolves in the solution and an equal amount of pure copper is deposited on the cathode. Soluble impurities stay in solution and insoluble impurities settle below the anode as anode mud.
(iv) In beaker A the blue colour fades and reddish-brown copper is deposited on the zinc strip, because zinc is more reactive than copper: Zn + CuSO₄ → ZnSO₄ + Cu. In beaker B nothing happens, because silver is less reactive than copper.
In simple words: Pure copper moves from the anode to the cathode, and zinc can displace copper but silver cannot.

Teacher's Note:
1. Write the answers part by part as numbered in the question.
2. A reactive metal displaces copper from copper sulphate, giving a colour change.

Question 2. A metal M reacts vigorously with water to form a solution S and a gas G. The solution S turns red litmus blue, whereas gas G, which is lighter than air, burns with a pop sound. Metal M has a low melting point and is used as a coolant in nuclear reactors.
(i) What is metal M?
(ii) What is solution S?
(iii) What is gas G?
Answer:
(i) M is sodium.
(ii) S is sodium hydroxide solution, NaOH, which is basic.
(iii) G is hydrogen, H₂.
2Na + 2H₂O → 2NaOH + H₂
In simple words: Sodium reacts with water to give an alkali and hydrogen gas.

Teacher's Note:
1. Hydrogen burns with a pop sound.
2. Sodium is kept under kerosene.

Question 3. Refining is the process of purification of metals. One of the important methods of refining is electrolysis. In electrolysis, an electric current is passed through a liquid containing ions, called an electrolyte. The solid conductors inserted into the liquid are called electrodes; the positive one is the anode and the negative one is the cathode. A diagram of the electrolysis of copper is shown below.
(i) Name the electrolyte used in refining of copper. (ii) (a) Cu -> Cu²⁺ + 2e⁻; (b) Cu²⁺ + 2e⁻ -> Cu. At which electrode does each reaction occur? (iii) What is anode mud? (iv) Name two metals that can be refined by the electrolytic method.
Answer:
(i) Acidified copper sulphate solution.
(ii) Reaction (a), Cu → Cu²⁺ + 2e⁻, occurs at the anode. Reaction (b), Cu²⁺ + 2e⁻ → Cu, occurs at the cathode.
(iii) Anode mud is the insoluble impurity that settles at the bottom below the anode.
(iv) Nickel and zinc (silver and gold can also be refined this way).
In simple words: At the anode copper goes into solution and at the cathode it is deposited again.

Teacher's Note:
1. Anode: oxidation; cathode: reduction.
2. Anode mud often contains valuable metals such as gold and silver.

Question 4. The reactivity series is a list of metals arranged in the order of their decreasing activity. The metal at the top is the most reactive and the metal at the bottom is the least reactive. A more reactive metal displaces a less reactive metal from its salt solution. (Series: K, Na, Ca, Mg, Al, Zn, Fe, Pb, [H], Cu, Hg, Ag, Au.)
(i) Name the metals which react with steam but not with hot water.
(ii) What happens when calcium reacts with nitric acid?
(iii) Which method is used to extract the metal at the top of the series?
(iv) Which of the following exist in their native state: Cu, Au, Zn, Ag?
Answer:
(i) Aluminium, zinc and iron.
(ii) Calcium forms calcium nitrate. Hydrogen gas is not evolved, because nitric acid is a strong oxidising agent that oxidises the hydrogen to water and is itself reduced to oxides of nitrogen.
(iii) Electrolytic reduction (electrolysis of the molten compound).
(iv) Gold (Au) and silver (Ag).
In simple words: Metals low in the series are found free, and the one at the top needs electrolysis.

Teacher's Note:
1. Know which metals react with cold water, hot water and steam.
2. Nitric acid behaves differently from other dilute acids.

Question 5. Metals occur in nature in the free as well as in the combined state. The less reactive metals are generally found in the free state. Most metals are found in the combined form as minerals. The minerals from which metals can be extracted profitably are called ores. Bauxite (Al₂O₃.2H₂O) and clay are minerals of aluminium, but only bauxite is used to obtain aluminium commercially, so bauxite and not clay is an ore of aluminium.
(i) Name a metal that occurs in the native state.
(ii) Name a sulphide ore.
(iii) Name a halide ore.
(iv) Which is the most abundant metal in the earth's crust?
Answer:
(i) Gold (or silver).
(ii) Galena (PbS) or cinnabar (HgS).
(iii) Horn silver (AgCl).
(iv) Aluminium.
In simple words: Ores are minerals from which a metal is extracted at a profit.

Teacher's Note:
1. Oxygen is the most abundant element and aluminium the most abundant metal in the crust.
2. Learn one ore each of Al, Fe, Zn, Hg and Cu.

Question 6. The earth's crust is the major source of metals. The elements or compounds which occur naturally in the earth's crust are known as minerals. At some places minerals contain a very high percentage of a particular metal and the metal can be profitably extracted from them. These minerals are called ores.
(i) Name the chief ore of mercury and of zinc.
(ii) Write the equations for the extraction of copper from its sulphide ore.
(iii) Name the process used to convert carbonate ores into metal oxide and define it.
(iv) Name two metals that exist in their native state.
Answer:
(i) Mercury: cinnabar, HgS. Zinc: zinc blende, ZnS.
(ii) 2Cu₂S + 3O₂ → 2Cu₂O + 2SO₂; then 2Cu₂O + Cu₂S → 6Cu + SO₂.
(iii) Calcination: heating a carbonate ore strongly in a limited supply of air to form the metal oxide.
(iv) Gold and silver.
In simple words: Know the ores, and remember that carbonate ores are calcined.

Teacher's Note:
1. Calcination is for carbonates and roasting is for sulphides.
2. Copper is obtained from Cu₂S without any added reducing agent.

Question 7. The metals high up in the reactivity series are very reactive and cannot be obtained from their compounds by heating with carbon. These metals are obtained by electrolytic reduction.
(i) Why can the oxide of sodium not be reduced by carbon?
(ii) Write the steps involved in the extraction of sodium from molten sodium chloride.
(iii) Write one reaction in which carbon acts as a reducing agent.
(iv) Why is molten sodium chloride used and not its aqueous solution?
Answer:
(i) Sodium has a greater affinity for oxygen than carbon has, so carbon cannot take oxygen from sodium oxide.
(ii) At the cathode Na⁺ + e⁻ → Na (reduction); at the anode 2Cl⁻ → Cl₂ + 2e⁻ (oxidation).
(iii) ZnO + C → Zn + CO.
(iv) In aqueous solution hydrogen, which is less reactive than sodium, is liberated at the cathode instead of sodium.
In simple words: Sodium is obtained by electrolysing molten NaCl because carbon is too weak and water would give hydrogen.

Teacher's Note:
1. Cathode: reduction; anode: oxidation.
2. Use molten, not aqueous, electrolyte for very reactive metals.

Question 8. Besides using carbon to reduce metal oxides to metals, displacement reactions can also be used. Highly reactive metals such as sodium, calcium and aluminium are used as reducing agents.
(i) What happens when manganese dioxide is heated with aluminium powder?
(ii) Identify the substances that are oxidised and reduced in this reaction.
(iii) Complete the equation: Fe₂O₃(s) + 2Al(s) →
(iv) What is the reaction of Fe₂O₃ with Al known as?
Answer:
(i) 3MnO₂ + 4Al → 3Mn + 2Al₂O₃ + heat. Manganese metal is obtained.
(ii) Aluminium is oxidised and manganese dioxide is reduced.
(iii) Fe₂O₃ + 2Al → 2Fe + Al₂O₃ + heat
(iv) The thermite reaction.
In simple words: Aluminium takes oxygen from the oxides of less reactive metals and gives heat.

Teacher's Note:
1. The substance that gains oxygen is oxidised and the one that loses oxygen is reduced.
2. Thermite reaction is used to weld railway tracks.

Question 9. Alloying is a very good method of improving the properties of a metal. Galvanisation is a method of protecting steel and iron from rusting.
(i) What is the chemical formula and chemical name of rust?
(ii) Define galvanisation.
(iii) Define an alloy.
(iv) Name two important alloys containing copper.
Answer:
(i) Rust is hydrated iron(III) oxide, Fe₂O₃.xH₂O.
(ii) Galvanisation is the process of coating iron or steel with a thin layer of zinc to protect it from rusting.
(iii) An alloy is a homogeneous mixture of two or more metals, or of a metal and a non-metal.
(iv) Brass (copper and zinc) and bronze (copper and tin).
In simple words: Rust is hydrated iron oxide, zinc coating prevents it and alloys improve properties.

Teacher's Note:
1. Rust is not a single compound; it is a hydrated oxide of iron.
2. Remember the constituents of brass and bronze.

Question 10. According to the octet rule, an atom with 8 electrons in its outermost shell is stable. The noble gases have complete outer electron shells, which makes them very stable and least reactive. Other elements seek stability by transferring, gaining or sharing electrons. This rule was proposed by G.N. Lewis and is also known as the Rule of Eight.
(i) How many valence electrons are present in neon?
(ii) How many electrons are present in helium?
Answer:
(i) Neon (2, 8) has 8 electrons in its outermost shell, so it has 8 valence electrons and a complete octet. Its valency is zero.
(ii) Helium has 2 electrons; its single shell is complete (a duet), so it is also stable.
In simple words: Neon has 8 valence electrons and helium has 2 electrons.

Teacher's Note:
1. Valency of neon is zero, but its number of valence electrons is eight.
2. Sodium attains the neon structure by losing one electron.

Real Life and Competency Based Questions

Question 11. Give an example to show that corrosion of some metals is an advantage.
Answer:
Aluminium is quickly covered by a thin layer of aluminium oxide when it is exposed to air. This layer is hard and does not react further, so it protects the metal underneath from further corrosion. This is why aluminium utensils and window frames last long.
In simple words: The oxide layer on aluminium protects it from further damage.

Teacher's Note:
1. Corrosion is usually harmful, but this protective oxide layer is useful.
2. Anodising makes this layer thicker.

Question 12. Write some uses of metals which are based on the properties of malleability and ductility.
Answer:
1. Silver foils are used for decorating sweets (malleability).
2. Aluminium foils are used for wrapping chocolates and food items (malleability).
3. Thin copper wires are used in electric circuits (ductility).
4. Gold and silver are made into ornaments of different designs (malleability and ductility).
In simple words: Malleable metals become sheets and ductile metals become wires.

Teacher's Note:
1. Malleability: beaten into sheets. Ductility: drawn into wires.
2. Gold is the most ductile metal and gold and silver are the most malleable.

Question 13. Why do articles made of aluminium not corrode?
Answer:
When aluminium is exposed to air, its surface is coated with a thin layer of aluminium oxide. This layer is non-reactive and protects the metal underneath from further damage, so aluminium articles do not corrode.
In simple words: A tough oxide film shields the metal.

Teacher's Note:
1. Aluminium is quite reactive but this film makes it resistant.
2. The film also makes aluminium look dull.

Question 14. Why is sodium kept immersed in kerosene oil?
Answer:
Sodium reacts so vigorously with the oxygen of air and with moisture that it catches fire if left in the open. Kerosene keeps air and moisture away, so the metal is protected and accidental fires are prevented.
In simple words: Kerosene keeps sodium away from air and water.

Teacher's Note:
1. Potassium and lithium are also stored under oil.
2. Never touch sodium with wet hands.

Question 15. Give reasons: (i) Gold and platinum are used in jewellery. (ii) Copper cannot displace hydrogen from dilute acids. (iii) Stainless steel does not rust easily. (iv) Metals can be given different shapes. (v) Zinc does not give hydrogen gas with nitric acid.
Answer:
(i) Gold and platinum are shiny and very unreactive, so they do not tarnish.
(ii) Copper is less reactive than hydrogen and lies below it in the reactivity series.
(iii) Stainless steel is an alloy of iron with chromium and nickel, which resist rusting.
(iv) Metals are malleable and ductile, so they can be shaped as needed.
(v) Nitric acid is a strong oxidising agent. It oxidises the hydrogen formed to water and is itself reduced to oxides of nitrogen.
In simple words: Each property of a metal decides its use.

Teacher's Note:
1. Give one short reason for each part.
2. Magnesium and manganese give hydrogen only with very dilute nitric acid.

Question 16. Give reasons: (a) Platinum, gold and silver are used to make jewellery. (b) Sodium, potassium and lithium are stored under oil. (c) Aluminium is a highly reactive metal, yet it is used to make cooking utensils. (d) Carbonate and sulphide ores are usually converted into oxides during the extraction of metals.
Answer:
(a) These metals are lustrous, malleable, ductile and very unreactive, so they are not affected by air or moisture.
(b) They react so vigorously with oxygen and moisture that they catch fire in the open; oil keeps air and water away.
(c) Aluminium forms a thin, non-reactive layer of aluminium oxide on its surface, which prevents further reaction.
(d) It is easier to obtain a metal by reducing its oxide than from its carbonate or sulphide.
In simple words: Use depends on reactivity and on properties such as lustre and conductivity.

Teacher's Note:
1. Write each reason in one or two lines.
2. Roasting and calcination convert ores to oxides.

Question 17. Give reasons: (i) School bells are made of metals. (ii) Electric wires are made of copper.
Answer:
(i) Metals are sonorous, i.e. they produce a ringing sound when struck.
(ii) Copper is a very good conductor of electricity and is ductile, so it can be drawn into thin wires.
In simple words: Metals ring when struck, and copper conducts well.

Teacher's Note:
1. Sonorous means producing a sound when hit.
2. Silver is the best conductor, but copper is cheaper.

Question 18. Why do we use copper and aluminium wires for the transmission of electric current?
Answer:
Copper and aluminium are good conductors of electricity with low resistance, and they are ductile, so they can be drawn into thin wires.
In simple words: They conduct well and can be drawn into wires.

Teacher's Note:
1. Aluminium is also light, so it is used for overhead cables.
2. Both metals are ductile.

Question 19. Why do we apply paint on iron articles?
Answer:
Paint forms a protective coating on the surface of iron. Air and moisture cannot touch the metal directly, so the iron is protected from rusting.
In simple words: Paint keeps air and water away from iron.

Teacher's Note:
1. Rusting needs both oxygen and moisture.
2. Grease and oil work in the same way.

Question 20. How does a metal conduct heat?
Answer:
When a metal is heated, its atoms gain energy and vibrate more vigorously. This energy is passed on to the free electrons that move through the metal. These electrons transfer their energy to other electrons and atoms, so heat is conducted. This is why cooking vessels are made of metals.
In simple words: Heat travels through a metal by moving electrons and vibrating atoms.

Teacher's Note:
1. Silver and copper are the best conductors of heat; lead is poor.
2. Handles of cooking vessels are made of plastic or wood, which are poor conductors.

Question 21. In one method of rust prevention, the iron is not coated with anything. Name the method and define it.
Answer:
The method is cathodic (sacrificial) protection. The iron article, such as an underground pipe, is connected to a more reactive metal such as magnesium or zinc. The more reactive metal corrodes instead and the iron is protected from rusting.
In simple words: A more reactive metal rusts in place of the iron.

Teacher's Note:
1. Ships and underground pipes are protected in this way.
2. Zinc and magnesium blocks are replaced when used up.