CBSE Class 10 Science HOTs Metals and Non Metals Set 08

This CBSE Class 10 Science HOTS page for Chapter 3, Metals and Non-metals, has 24 higher-order thinking questions with complete step-by-step answers. The questions ask you to reason about displacement reactions, the reactivity series, amphoteric oxides, ionic compounds and the protection of iron, and to identify unknown metals from clues.

Reasoning Based HOTS Questions

Question 1. A copper plate was dipped into a solution of silver nitrate. After some time a black layer was deposited on the copper plate. State the reason for it and write the chemical equation of the reaction involved.
Answer:
1. Copper is more reactive than silver, so it displaces silver from silver nitrate solution.
2. The silver metal is deposited on the copper plate as a thin dark layer, and the solution slowly turns blue because copper(II) nitrate is formed.
Cu + 2AgNO₃ → Cu(NO₃)₂ + 2Ag
In simple words: Copper pushes silver out of its solution, and the silver sticks to the plate.

Teacher's Note:
1. A more reactive metal always displaces a less reactive metal from its salt solution.
2. Mention the colour change of the solution to show good observation.

Question 2. The oxide X₂O₃ of a metal X is unaffected by water and cannot be reduced by carbon. Outline a method by which a sample of metal X can be obtained from its ore. Give one reason for choosing this method.
Answer:
1. Such a stable oxide belongs to a metal high in the reactivity series; X is aluminium and the oxide is Al₂O₃.
2. The metal is obtained by electrolytic reduction: molten aluminium oxide is electrolysed and aluminium collects at the cathode.
3. Reason: aluminium has a greater affinity for oxygen than carbon, so chemical reducing agents such as carbon cannot reduce its oxide.
In simple words: A very stable oxide can only be broken down by electricity.

Teacher's Note:
1. Metals above carbon in the reactivity series cannot be reduced by carbon.
2. The ore of aluminium is bauxite.

Question 3. Give reasons: (i) Silicon is considered a metalloid. (ii) Carbon is not used for the extraction of aluminium from aluminium oxide. (iii) For producing hydrogen gas from hydrochloric acid, granulated zinc is preferred over a block of zinc.
Answer:
(i) Silicon has some properties of metals and some of non-metals, so it is a metalloid.
(ii) Aluminium is a stronger reducing agent than carbon (it has more affinity for oxygen), so carbon cannot reduce Al₂O₃.
(iii) Granulated zinc has a larger surface area in contact with the acid, so the reaction is faster and hydrogen is produced at a steady rate.
In simple words: Silicon is in between metals and non-metals; carbon is too weak for aluminium; granules react faster.

Teacher's Note:
1. More surface area means a faster reaction.
2. Other metalloids are germanium and arsenic.

Question 4. Give reasons: (i) Sodium, potassium and calcium form hydrides by combining with hydrogen gas, but most other metals do not. (ii) Zinc oxide is called an amphoteric oxide.
Answer:
(i) Na, K and Ca are highly electropositive and readily lose electrons to hydrogen, which accepts them to form the hydride ion. Most other metals do not lose electrons so easily.
(ii) ZnO reacts with acids as well as with bases to form salt and water, so it shows both basic and acidic behaviour.
ZnO + 2HCl → ZnCl₂ + H₂O
ZnO + 2NaOH → Na₂ZnO₂ + H₂O
In simple words: Very reactive metals hand over electrons to hydrogen, and zinc oxide reacts with both acids and bases.

Teacher's Note:
1. Amphoteric oxides give salt and water with both an acid and a base; Al₂O₃ is another example.
2. Write both reactions to prove the amphoteric nature.

Question 5. (a) What is corrosion of metals? Name one metal that does not corrode and one that corrodes when kept in the open. (b) How will you show that the rusting of iron needs both oxygen and moisture?
Answer:
(a) Corrosion is the slow eating away of the surface of a metal by the action of air, moisture or chemicals around it. Gold does not corrode; iron corrodes.
(b) Place clean iron nails in three test tubes: (1) water and air, (2) boiled water with a layer of oil (no air), (3) anhydrous calcium chloride (dry air). After a few days only the nail in test tube (1) rusts. So both oxygen and moisture are needed.
In simple words: A nail rusts only where it gets both air and water.

Teacher's Note:
1. Boiling the water removes dissolved air and the oil layer keeps fresh air out.
2. Calcium chloride absorbs moisture, so the nail in dry air stays bright.

Question 6. Metals replace hydrogen from acids, whereas non-metals do not. Why?
Answer:
1. Metals are electropositive; they lose electrons easily and so can give electrons to the hydrogen ions of an acid, which become hydrogen gas.
2. Non-metals tend to gain electrons and are not able to give electrons to hydrogen ions, so they cannot displace hydrogen from acids.
In simple words: Metals can give electrons to H⁺ ions; non-metals cannot.

Teacher's Note:
1. This idea links reactivity with the tendency to lose electrons.
2. Example: Zn + 2HCl → ZnCl₂ + H₂ shows a metal displacing hydrogen.

Question 7. Explain why a galvanised iron article remains protected against rusting even if the zinc layer is broken.
Answer:
1. Zinc is more reactive than iron and is more easily oxidised.
2. When the zinc layer is scratched and iron is exposed, zinc corrodes first and iron stays protected.
3. This is called sacrificial protection.
In simple words: Zinc sacrifices itself, so the iron underneath does not rust.

Teacher's Note:
1. Compare the positions of zinc and iron in the reactivity series.
2. Tin plating does not behave like this, because tin is less reactive than iron.

Question 8. Hydrogen is not a metal, but it has been given a place in the reactivity series of metals. Why?
Answer:
Hydrogen loses its electron to form positive ions, H⁺, just as metals do. It is included in the series only for comparison, to decide which metals can displace hydrogen from acids: metals above hydrogen can, metals below it cannot.
In simple words: Hydrogen is placed in the series because it also forms a positive ion, and it works as a reference point.

Teacher's Note:
1. Metals above hydrogen react with dilute acids; those below do not.
2. Copper, silver and gold lie below hydrogen.

Question 9. When calcium metal is added to water the gas evolved does not catch fire, but the gas evolved when sodium is added to water does. Why?
Answer:
1. Both metals release hydrogen gas: Ca + 2H₂O → Ca(OH)₂ + H₂; 2Na + 2H₂O → 2NaOH + H₂.
2. Sodium reacts violently and the reaction is highly exothermic, so the heat produced is enough to ignite the hydrogen.
3. With calcium the heat is not enough to burn hydrogen. The bubbles of hydrogen sticking to calcium make it float.
In simple words: Sodium makes much more heat, which sets the hydrogen on fire.

Teacher's Note:
1. Calcium is less reactive than sodium, so its reaction is slower.
2. Sodium is stored under kerosene for the same reason.

Question 10. Why does a solution of cane sugar not conduct electricity, while a solution of common salt is a good conductor?
Answer:
1. Common salt is an ionic compound. In water its crystal breaks into free ions, which carry current.
2. Cane sugar is a covalent compound. It dissolves as molecules and gives no ions, so its solution does not conduct electricity.
In simple words: Salt gives ions in water, sugar gives only molecules.

Teacher's Note:
1. Free ions or free electrons are needed for conduction.
2. Ionic compounds also conduct in the molten state.

Question 11. Why does aluminium not react with water under ordinary conditions?
Answer:
Aluminium is covered by a thin but strong layer of aluminium oxide, Al₂O₃, formed when it meets air. This layer does not allow water to reach the metal, so no reaction takes place at ordinary temperature. Aluminium reacts only with steam: 2Al + 3H₂O → Al₂O₃ + 3H₂.
In simple words: A protective oxide film keeps water away from aluminium.

Teacher's Note:
1. The same oxide layer protects aluminium utensils from corrosion.
2. Aluminium is above iron in the reactivity series, yet this layer makes it resistant.

Identification and Application HOTS Questions

Question 12. An element A forms two oxides, AO and AO₂. The oxide AO is neutral whereas AO₂ is acidic in nature. Would you call A a metal or a non-metal? Give a reason.
Answer:
Element A is a non-metal.
Metals form basic (or amphoteric) oxides. Only non-metals form neutral and acidic oxides. For example, carbon forms neutral CO and acidic CO₂.
In simple words: Neutral and acidic oxides come from non-metals.

Teacher's Note:
1. Know the oxide type of metals: basic or amphoteric.
2. Carbon is a good example to remember.

Question 13. A substance X, which is an oxide of a metal, is used intensively in the cement industry. The element is present in bones also. On treatment with water it forms a solution which turns red litmus blue. Identify X and write the chemical reactions involved.
Answer:
1. X is calcium oxide (quicklime), CaO. Calcium is a part of bones and CaO is used in making cement.
2. CaO reacts with water to form calcium hydroxide, which is basic and turns red litmus blue.
CaO(s) + H₂O(l) → Ca(OH)₂(aq)
In simple words: X is quicklime, which gives a basic solution with water.

Teacher's Note:
1. A basic solution turns red litmus blue.
2. The reaction releases a lot of heat and is called slaking of lime.

Question 14. When a metal X is treated with cold water, it gives a base Y with molecular formula XOH (molecular mass 40) and liberates a gas Z which easily catches fire. Identify X, Y and Z and write the reaction involved.
Answer:
1. Mass of XOH is 40 and OH is 17, so the atomic mass of X is 40 − 17 = 23. This is sodium.
2. X is sodium (Na), Y is sodium hydroxide (NaOH) and Z is hydrogen (H₂).
2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g) + heat
In simple words: Subtract the OH mass from 40 to find the metal, which is sodium.

Teacher's Note:
1. Use the atomic masses H = 1 and O = 16.
2. The hydrogen catches fire because the reaction is highly exothermic.

Question 15. State the reason for the following behaviour of zinc metal: when a piece of zinc is placed in mercuric chloride solution it acquires a shining silvery surface, but when it is placed in magnesium sulphate solution no change is observed.
Answer:
1. Zinc is more reactive than mercury. It displaces mercury from mercuric chloride and the silvery mercury forms a coating on it: Zn + HgCl₂ → ZnCl₂ + Hg.
2. Zinc is less reactive than magnesium, so it cannot displace magnesium from magnesium sulphate and no reaction takes place.
In simple words: Zinc beats mercury but not magnesium in the reactivity series.

Teacher's Note:
1. Write the order Mg > Zn > Hg to explain both observations.
2. A more reactive metal displaces a less reactive one from its salt.

Question 16. (a) How will you prove with a simple experiment that magnesium is placed above zinc in the reactivity series? (b) Why can copper not liberate hydrogen when it reacts with dilute HCl?
Answer:
(a) Dip a clean magnesium strip in zinc sulphate solution. Magnesium displaces zinc, which is deposited on the strip: Mg + ZnSO₄ → MgSO₄ + Zn. This shows that magnesium is more reactive than zinc.
(b) Copper is below hydrogen in the reactivity series, so it is less reactive than hydrogen and cannot displace it from dilute HCl.
In simple words: A metal that displaces another from its salt is more reactive, and copper lies below hydrogen.

Teacher's Note:
1. Use displacement from a salt solution as the proof of reactivity.
2. Metals below hydrogen: copper, mercury, silver and gold.

Question 17. Write the balanced equation for the reaction between manganese dioxide and aluminium powder. What happens if manganese powder is heated with aluminium oxide?
Answer:
1. 3MnO₂ + 4Al → 3Mn + 2Al₂O₃ + heat. Aluminium reduces manganese dioxide.
2. If manganese is heated with aluminium oxide no reaction takes place, because manganese is less reactive than aluminium and cannot displace it.
In simple words: Aluminium can take oxygen from manganese oxide, but manganese cannot take it from aluminium oxide.

Teacher's Note:
1. Check that the numbers of Mn, O and Al atoms are equal on both sides.
2. The reaction is a displacement reaction and it gives out heat.

Question 18. State which of the following reactions will take place and which will not, and why: (i) Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s); (ii) Fe(s) + ZnSO₄(aq) → FeSO₄(aq) + Zn(s); (iii) Zn(s) + FeSO₄(aq) → ZnSO₄(aq) + Fe(s).
Answer:
(i) Takes place, because zinc is more reactive than copper and displaces it.
(ii) Does not take place, because iron is less reactive than zinc and cannot displace it.
(iii) Takes place, because zinc is more reactive than iron and displaces it.
In simple words: A reaction occurs only when the free metal is more reactive than the metal in the salt.

Teacher's Note:
1. Order to remember: Zn > Fe > Cu.
2. Write 'no reaction' clearly for case (ii).

Question 19. Explain why sodium hydroxide solution cannot be kept in an aluminium container. Write the equation for the reaction that may take place.
Answer:
Aluminium is an amphoteric metal and reacts with a base. It reacts with sodium hydroxide to form sodium aluminate and hydrogen, so the container would be eaten away.
2Al + 2NaOH + 2H₂O → 2NaAlO₂ + 3H₂
In simple words: Aluminium reacts with alkalis, so the container would be damaged.

Teacher's Note:
1. Aluminium oxide is also amphoteric.
2. Zinc also reacts with sodium hydroxide.

Question 20. A metal E is stored under kerosene. When a small piece of it is left open in air it catches fire. The product formed dissolves in water and turns red litmus blue. (i) Name the metal E. (ii) Write the equations for the reaction in air and for the product dissolving in water. (iii) How is the metal obtained from its molten chloride?
Answer:
(i) E is sodium.
(ii) 4Na + O₂ → 2Na₂O; Na₂O + H₂O → 2NaOH.
(iii) By electrolysis of molten sodium chloride. At the cathode Na⁺ + e⁻ → Na and sodium is deposited; at the anode 2Cl⁻ → Cl₂ + 2e⁻.
In simple words: Sodium catches fire in air, forms a basic oxide, and is obtained by electrolysis of molten NaCl.

Teacher's Note:
1. Highly reactive metals can only be obtained by electrolysis.
2. Sodium is kept in kerosene to keep out air and moisture.

Question 21. An element reacts with oxygen to form an oxide. When dissolved in water the solution turns red litmus blue. Is the element a metal or a non-metal? Justify your answer.
Answer:
The element is a metal. The oxide gives a basic solution that turns red litmus blue, and basic oxides are formed by metals. For example, Na₂O + H₂O → 2NaOH and MgO + H₂O → Mg(OH)₂.
In simple words: A basic oxide that turns red litmus blue comes from a metal.

Teacher's Note:
1. Non-metal oxides are acidic and turn blue litmus red.
2. Soluble metal hydroxides are called alkalis.

Question 22. A metal X combines with a non-metal Y by the transfer of electrons to form a compound Z. (i) State the type of bond in Z. (ii) What can you say about its melting and boiling points? (iii) Will it dissolve in kerosene or petrol? (iv) Will it conduct electricity?
Answer:
(i) An ionic bond.
(ii) The melting and boiling points are high because of the strong attraction between the oppositely charged ions.
(iii) No. Ionic compounds dissolve in water but not in kerosene or petrol.
(iv) Not in the solid state, as the ions cannot move. It conducts electricity in the molten state or in aqueous solution.
In simple words: Z is an ionic compound: hard, high melting, soluble in water only, and conducting only when molten or dissolved.

Teacher's Note:
1. Link every property to the strong electrostatic force between ions.
2. Ions are free to move only in the molten or dissolved state.

Question 23. Out of three metals P, Q and R, P is less reactive than Q and R is more reactive than both P and Q. Suggest an activity to arrange P, Q and R in the order of decreasing reactivity.
Answer:
1. Take the salt solutions of P, Q and R in separate test tubes and dip a strip of each metal in the other two solutions.
2. R displaces both P and Q from their salt solutions, so R is the most reactive.
3. Q displaces P from the solution of P but does not displace R.
4. P displaces neither, so P is the least reactive.
The decreasing order of reactivity is R > Q > P.
In simple words: Check which metal can displace which, and rank them.

Teacher's Note:
1. Make a small table of observations: displacement or no reaction.
2. The metal that displaces most others is the most reactive.

Question 24. (a) An element X reacts with oxygen to form an oxide X₂O. The oxide dissolves in water and turns blue litmus red. Predict whether X is a metal or a non-metal. (b) A solution of copper sulphate was kept in an iron pot. After a few days the pot developed holes. How will you account for this?
Answer:
(a) X is a non-metal, since its oxide is acidic and turns blue litmus red.
(b) Iron is more reactive than copper, so iron displaces copper from copper sulphate solution: Fe + CuSO₄ → FeSO₄ + Cu. The iron of the pot is used up, so holes form.
In simple words: Non-metal oxides are acidic, and iron displaces copper, which eats the pot.

Teacher's Note:
1. Never store copper sulphate solution in an iron vessel.
2. The blue colour of the solution fades to light green.