CBSE Class 10 Science Metals and Non Metals Assignment Set 04

This is Assignment Set 04 for CBSE Class 10 Science Chapter 3, Metals and Non-metals, with 21 questions arranged by marks (1, 2, 3 and 5 marks) and two case based questions, all with step-wise answers and a Teacher's Note. It covers reaction of metals with water, acids and salt solutions, displacement, ionic compounds, extraction of metals and corrosion.

Very Short Answer Questions (1 Mark)

Question 1. What changes in the colour of iron nails and copper sulphate solution do you observe after keeping the iron nails dipped in copper sulphate solution for about 30 minutes?
Answer:
Iron is more reactive than copper, so it displaces copper from copper sulphate. The blue colour of the copper sulphate solution fades and changes to pale green (iron(II) sulphate), and the iron nails get a reddish-brown coating of copper.
Fe + CuSO₄ → FeSO₄ + Cu
In simple words: The blue solution turns pale green and the nails turn reddish brown.

Teacher's Note:
1. Write the colour before and after for both the solution and the nail.
2. This is a displacement reaction.

Question 2. Which gas is generally liberated when a dilute solution of hydrochloric acid reacts with an active metal?
Answer:
Hydrogen gas is liberated, for example Mg + 2HCl → MgCl₂ + H₂. It burns with a ‘pop’ sound when a burning matchstick is brought near.
In simple words: Active metals push hydrogen out of dilute acids.

Teacher's Note:
1. Metal + dilute acid gives salt + hydrogen.
2. The pop test confirms hydrogen.

Question 3. Arrange the following metals in a decreasing order of activity : Na, K, Cu, Ag
Answer:
In the reactivity series the order is K, Na, Cu, Ag, so the decreasing order of activity is K > Na > Cu > Ag.
In simple words: Potassium is the most active and silver is the least active of the four.

Teacher's Note:
1. Learn the reactivity series as a ladder from K down to Au.
2. Read the question carefully: decreasing means most reactive first.

Question 4. From amongst the metals sodium, calcium, aluminium, copper and magnesium, name the metal (i) which reacts with water only on boiling and (ii) another which does not react even with steam.
Answer:
(i) Magnesium: it does not react with cold water but reacts with boiling (hot) water giving magnesium hydroxide and hydrogen.
(ii) Copper: it does not react with cold water, hot water or steam.
In simple words: Magnesium reacts only with hot water; copper does not react at all.

Teacher's Note:
1. Aluminium reacts only with steam, not with boiling water.
2. Na and Ca react with cold water.

Question 5. Name a reducing agent that may be used to obtain manganese from manganese dioxide.
Answer:
Aluminium powder. 3MnO₂ + 4Al → 3Mn + 2Al₂O₃ + heat. Carbon cannot be used satisfactorily.
In simple words: Aluminium pulls oxygen away from manganese dioxide and leaves manganese.

Teacher's Note:
1. The reaction is a thermite-type displacement.
2. It is highly exothermic.

Question 6. Give reason for the following observation : highly reactive metals cannot be obtained from their oxides by heating them with carbon.
Answer:
Highly reactive metals such as sodium, magnesium and aluminium have a greater affinity for oxygen than carbon has. So carbon cannot take oxygen away from their oxides. They are obtained by electrolytic reduction instead.
In simple words: These metals hold on to oxygen more strongly than carbon does.

Teacher's Note:
1. Compare the reactivity of the metal with carbon.
2. Name the alternative method: electrolysis of the molten compound.

Short Answer Questions I (2 Marks)

Question 7. Reverse of the following chemical reaction is not possible : Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s). Justify this statement with reason.
Answer:
Zinc is more reactive than copper, so it displaces copper from copper sulphate solution. The reverse reaction would need copper to displace zinc from zinc sulphate solution, but copper is less reactive than zinc and cannot do so.
Cu(s) + ZnSO₄(aq) → no reaction
In simple words: A less reactive metal cannot push a more reactive metal out of its salt.

Teacher's Note:
1. Displacement goes only from the more reactive metal to the less reactive metal.
2. Write ‘no reaction’ for the reverse.

Question 8. Name a metal which : (a) is the best conductor of heat. (b) has a very low melting point. (c) does not react with oxygen even at high temperature. (d) is most ductile.
Answer:
(a) Silver is the best conductor of heat.
(b) Gallium (or caesium) has a very low melting point; it melts on the palm.
(c) Gold (or silver) does not react with oxygen even at high temperature.
(d) Gold is the most ductile metal.
In simple words: Silver conducts best, gallium melts easily, gold does not burn and is the most ductile.

Teacher's Note:
1. Learn the physical-property exceptions in a list.
2. Gold and silver are both unreactive towards oxygen.

Question 9. Complete the following equation with balancing : (i) Al + HCl → (ii) Mg + HNO₃ →
Answer:
(i) 2Al + 6HCl → 2AlCl₃ + 3H₂
(ii) Mg + 2HNO₃ (very dilute) → Mg(NO₃)₂ + H₂. Magnesium (and manganese) are the only metals that give hydrogen with very dilute nitric acid.
In simple words: Aluminium gives hydrogen with hydrochloric acid; magnesium gives hydrogen even with very dilute nitric acid.

Teacher's Note:
1. Balance the atoms on both sides after writing the products.
2. Nitric acid normally does not give hydrogen because it is an oxidising agent.

Question 10. Name two metals which react violently with cold water. Write any three observations you would make when such a metal is dropped into water. How would you identify the gas evolved, if any, during the reaction?
Answer:
Metals: Sodium and potassium.
Observations: (1) Bubbles of a gas come out of the water. (2) The gas catches fire immediately. (3) The reaction is highly exothermic, so the beaker becomes hot and the metal melts into a silvery ball that darts about.
Test for the gas: It is hydrogen. A burning matchstick brought near it makes the gas burn with a ‘pop’ sound. 2Na + 2H₂O → 2NaOH + H₂
In simple words: Sodium and potassium fizz, heat up and set the hydrogen on fire.

Teacher's Note:
1. Always store such metals under kerosene.
2. State the pop test for hydrogen.

Question 11. Name first discovered alloy. Give its composition also.
Answer:
Bronze is the first alloy discovered by man. It is made of copper and tin (about 90 per cent copper and 10 per cent tin).
In simple words: Bronze, an alloy of copper and tin, was the first alloy.

Teacher's Note:
1. An alloy is a homogeneous mixture of a metal with other elements.
2. Brass is copper and zinc.

Short Answer Questions II (3 Marks)

Question 12. (a) Show the formation of Na₂O by the transfer of electrons between the combining atoms. (b) Why are ionic compounds usually hard? (c) How is it that ionic compounds in the solid state do not conduct electricity and they do so when in molten state?
Answer:
(a) A sodium atom (2, 8, 1) loses one electron to become Na⁺ (2, 8); an oxygen atom (2, 6) gains two electrons to become O²⁻ (2, 8). Two sodium atoms are needed to give two electrons to one oxygen atom:
Na → Na⁺ + e⁻ and O + 2e⁻ → O²⁻

(b) Ionic compounds are hard because of the strong electrostatic force of attraction between the oppositely charged ions in the crystal.
(c) In the solid state the ions are held in fixed positions in the rigid crystal, so they cannot move and carry current. In the molten state the heat overcomes the attraction between ions, so the ions become free to move and conduct electricity.
In simple words: Sodium gives electrons to oxygen; the oppositely charged ions stick strongly in a hard solid and conduct only when free to move.

Teacher's Note:
1. Show the electron count of every atom and ion.
2. Ions free to move means conduction.

Question 13. Give reason : (a) Aluminium is a reactive metal but is still used for packing food articles. (b) Calcium starts floating when water is added to it.
Answer:
(a) When aluminium is exposed to moist air its surface gets covered with a thin, tough, impervious layer of aluminium oxide (Al₂O₃). This layer does not allow air and moisture to reach the metal below, so aluminium resists corrosion and does not react with food. It is also light, malleable (can be made into foil) and a good conductor of heat.
(b) Calcium reacts with water to form calcium hydroxide and hydrogen gas. The bubbles of hydrogen stick to the surface of the metal and make it lighter, so it floats: Ca + 2H₂O → Ca(OH)₂ + H₂.
In simple words: Aluminium protects itself with an oxide layer; calcium floats because hydrogen bubbles stick to it.

Teacher's Note:
1. Protective oxide layer is the key phrase for aluminium.
2. The same reason explains floating of magnesium in hot water.

Question 14. State what would happen if : (i) some zinc pieces are placed in blue copper sulphate solution. (ii) some copper pieces are placed in green ferrous sulphate solution. (iii) an iron nail is dipped in a solution of copper sulphate for some time.
Answer:
(i) Zinc is more reactive than copper. The blue colour of the solution fades and becomes colourless, and reddish-brown copper is deposited: Zn + CuSO₄ → ZnSO₄ + Cu.
(ii) No change takes place, because copper is less reactive than iron: Cu + FeSO₄ → no reaction.
(iii) The blue colour fades gradually to pale green and a reddish-brown coating of copper forms on the nail: Fe + CuSO₄ → FeSO₄ + Cu.
In simple words: A more reactive metal replaces a less reactive one; a less reactive metal does nothing.

Teacher's Note:
1. Give the colour change and the deposit in each part.
2. Write ‘no reaction’ when the metal is less reactive.

Question 15. (i) Carbonate of metal ‘X’ is abundant in earth crust and its hydroxide is used in ‘white washing’. Identify metal ‘X’. (ii) How will you convert this carbonate into its oxide? Name the process and write its equation.
Answer:
(i) Calcium carbonate is abundant in the earth’s crust (limestone) and calcium hydroxide (slaked lime) is used in white washing, so metal X is calcium (Ca).
(ii) The carbonate is heated strongly in the absence (or limited supply) of air. The process is calcination.
CaCO₃(s) → CaO(s) + CO₂(g) (on heating)
In simple words: Heating limestone in little air gives quicklime; this process is calcination.

Teacher's Note:
1. Carbonate ores are calcined, sulphide ores are roasted.
2. Calcium oxide with water gives slaked lime.

Question 16. Why some metal surfaces acquire a dull appearance when they are exposed to moist air? Write colour acquired by the surfaces of copper and silver in such situation and also write the chemical names of the substances due to which it happens.
Answer:
Metals lose their shine when moist air, carbon dioxide or sulphur compounds slowly attack the surface and form a thin layer of oxide, carbonate or sulphide. This is corrosion.
Copper becomes green because of basic copper carbonate, CuCO₃·Cu(OH)₂.
Silver becomes black because of silver sulphide, Ag₂S.
In simple words: Air and moisture form a coating on the metal; copper turns green and silver turns black.

Teacher's Note:
1. Give both the colour and the chemical name.
2. Silver does not react with oxygen, it reacts with sulphur compounds.

Long Answer Questions (5 Marks)

Question 17. (a) Complete and balance the following chemical equations : (i) Al₂O₃ + HCl (ii) K₂O + H₂O (iii) Fe + H₂O (b) An element ‘X’ displaces iron from the aqueous solution of iron sulphate. List your observations if the element ‘X’ is treated with the aqueous solutions of copper sulphate, zinc sulphate and silver nitrate. Based on the observations arrange X, Zn, Cu and Ag in increasing order of their reactivities.
Answer:
(a)(i) Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O
(ii) K₂O + H₂O → 2KOH
(iii) 3Fe + 4H₂O(g) → Fe₃O₄ + 4H₂ (iron reacts only with steam)
(b) X displaces iron, so X is more reactive than iron. Therefore X also displaces copper from copper sulphate (the blue colour fades and copper is deposited) and silver from silver nitrate (silver is deposited). The result with zinc sulphate is not certain: it depends on whether X is above or below zinc, which only an experiment can show.
Order of reactivity: Ag < Cu < Fe < X < Zn if X cannot displace zinc, or Ag < Cu < Fe < Zn < X if it can.
In simple words: X is more active than iron, copper and silver; its place relative to zinc needs a test.

Teacher's Note:
1. Use the given clue (X displaces iron) to place X above iron.
2. Silver and copper are always below iron.

Question 18. (i) By the transfer of electrons, illustrate the formation of bond in magnesium chloride and identify the ions present in this compound. (ii) Ionic compounds are solids. Give reasons. (iii) With the help of a labelled diagram show the experimental set up of action of steam on a metal.
Answer:
(i) Magnesium (2, 8, 2) loses its two valence electrons to form Mg²⁺ (2, 8). Each chlorine atom (2, 8, 7) gains one electron to form Cl⁻ (2, 8, 8), so two chlorine atoms are needed. The ions present in the compound are Mg²⁺ and Cl⁻.
Mg → Mg²⁺ + 2e⁻ and 2Cl + 2e⁻ → 2Cl⁻

(ii) Ionic compounds are solids because the oppositely charged ions are held together by strong electrostatic forces of attraction in a closely packed crystal lattice.
(iii) Activity (action of steam on a metal):

Heat the metal sample in a hard glass tube. The glass wool soaked in water gives steam, which passes over the hot metal. A metal such as iron, aluminium or zinc reacts to form the metal oxide and hydrogen. Hydrogen is collected over water in an inverted test tube: 3Fe + 4H₂O(g) → Fe₃O₄ + 4H₂.
In simple words: Magnesium gives two electrons to two chlorine atoms; the ions pack tightly into a solid; steam over hot metal gives hydrogen.

Teacher's Note:
1. Draw the electron transfer and write the charge on each ion.
2. Label the diagram: glass wool, metal sample, burner, delivery tube, hydrogen gas.

Question 19. Carbon cannot reduce the oxides of sodium, magnesium and aluminium to their respective metals. Why? Where are these metals placed in the reactivity series? How are these metals obtained form their ores? Take an example to explain the process of extraction along with chemical equations.
Answer:
Why carbon cannot reduce them: Sodium, magnesium and aluminium have a greater affinity for oxygen than carbon has, so their oxides are very stable and carbon cannot remove the oxygen.
Position: These metals are at the top of the reactivity series (K, Na, Ca, Mg, Al).
Extraction: They are obtained by electrolytic reduction of their molten chlorides or oxides. Example: sodium from molten sodium chloride.
NaCl(molten) → Na⁺ + Cl⁻
At cathode: Na⁺ + e⁻ → Na (sodium is deposited)
At anode: 2Cl⁻ → Cl₂ + 2e⁻ (chlorine gas is released)
Aluminium is obtained in the same way by the electrolysis of molten aluminium oxide.
In simple words: These metals hold oxygen too tightly for carbon, so electricity is used to get them from their molten salts.

Teacher's Note:
1. Write the reaction at each electrode.
2. Metal is always deposited at the cathode.

Case Based Questions (4 Marks)

Question 20. Case Study-II. Some of the metals are chemically very reactive, while others are less reactive. On the basis of reactivity of different metals with oxygen, water, and acids, to show displacement reactions, the metals have been arranged in series according to their chemical reactivity known as activity series of metals. In reactivity series, the most reactive metal is placed at the top of series while the least reactive metal is placed at the bottom of series. As we go down in the series, the chemical reactivity of metals decreases. The metals placed at the bottom of the reactivity series like silver and gold are least reactive. Thus, they are usually found in native state in nature. 1. When metal A is added to dilute HCl solution, there is no liberation of gas. Metal is: (a) Ca (b) Zn (c) Ag (d) Na. 2. CuSO₄ solution can be safely kept in a container that is made of: (a) Aluminium (b) Tin (c) Silver (d) Zinc. 3. Metal that is always found in free state is: (a) Gold (b) Silver (c) Copper (d) Potassium.
Answer:
1. Silver (Ag) does not react with dilute HCl, so no gas is liberated. The answer is (c) Ag; Ca, Zn and Na all give hydrogen.
2. CuSO₄ solution can be safely kept in a container made of (c) silver. Aluminium, tin and zinc are more reactive than copper and would displace copper from the solution.
3. The metal that is always found in the free state is (a) gold. It is at the bottom of the series and is very unreactive.
In simple words: Silver and gold sit at the bottom of the series: they do not react with acids and are found free.

Teacher's Note:
1. A metal can be stored in a salt solution only if it is less reactive than the metal of the salt.
2. Cu and Ag can occur free, but gold is always free.

Question 21. Case Study-III. Reactive metals like Na and K react with cold water to liberate H₂ gas and form corresponding hydroxides while the metal like Mg is lesser in reactivity to react with cold water but metals like Zn and Al react only with steam when steam is passed over the red hot metal and forms corresponding oxides. 1. Which of the following metal will form corresponding hydroxide on reaction with water? (a) Cu (b) Na (c) Zn (d) Fe. 2. Which of the following metal do not react with cold water? (a) Na (b) K (c) Mg (d) Ca. 3. Which of the following metal react with steam only? (a) Na (b) Mg (c) Al (d) Pt.
Answer:
1. Sodium forms its hydroxide with water, so the answer is (b) Na. Copper does not react, and zinc and iron react only with steam.
2. Magnesium does not react with cold water (it reacts with hot water), so the answer is (c) Mg. Na, K and Ca react with cold water.
3. Aluminium reacts with steam only (not with cold or hot water), so the answer is (c) Al. Sodium reacts with cold water, magnesium with hot water and platinum does not react at all.
In simple words: Na reacts with cold water, Mg with hot water, Al only with steam.

Teacher's Note:
1. Arrange the metals by the temperature at which they start reacting.
2. Metal + steam gives metal oxide + hydrogen.