CBSE Class 10 Science Metals and Non Metals Assignment Set 05

This is Assignment Set 05 for CBSE Class 10 Science Chapter 3, Metals and Non-metals, with 21 questions grouped by marks (1, 2, 3 and 5 marks) and two case based questions, each with a step-wise answer and a Teacher's Note. It practises properties of metals and non-metals, the reactivity series, the formation of oxides, alloys and the occurrence of metals.

Very Short Answer Questions (1 Mark)

Question 1. What length of wire can be drawn from one gram of gold ?
Answer:
A wire about 2 kilometres long can be drawn from one gram of gold. This shows that gold is extremely ductile.
In simple words: One gram of gold can be stretched into a wire two kilometres long.

Teacher's Note:
1. Ductility means the ability to be drawn into wires.
2. Gold is the most ductile metal.

Question 2. Name two metals which have very low melting point.
Answer:
Gallium and caesium have very low melting points; they melt if kept on the palm of the hand.
In simple words: Gallium and caesium melt at body temperature.

Teacher's Note:
1. Most metals have high melting points; these two are exceptions.
2. Mercury is liquid at room temperature.

Question 3. What is the general nature of metal oxides ?
Answer:
Metal oxides are generally basic in nature. Some (such as Al₂O₃ and ZnO) are amphoteric, and oxides of sodium and potassium dissolve in water to form alkalis.
In simple words: Metal oxides are mostly basic.

Teacher's Note:
1. Basic oxides turn red litmus blue after dissolving in water.
2. Non-metal oxides are acidic or neutral.

Question 4. Name the compounds formed by metals when they react with hydrogen.
Answer:
The compounds are hydrides. Only very reactive metals such as sodium, potassium and calcium combine with hydrogen, for example 2Na + H₂ → 2NaH.
In simple words: Metal and hydrogen compounds are called hydrides.

Teacher's Note:
1. Most metals do not form hydrides.
2. Non-metal hydrides such as NH₃ are covalent.

Question 5. Name the poorest conductors of heat among metals.
Answer:
Lead (and mercury) are the poorest conductors of heat among metals.
In simple words: Lead and mercury conduct heat badly.

Teacher's Note:
1. Silver and copper are the best conductors of heat.
2. Choose metals for cooking vessels by their conductivity.

Question 6. How are metals towards the top of the activity series extracted ?
Answer:
They are extracted by electrolysis (electrolytic reduction) of their molten chlorides or oxides, because they cannot be reduced by carbon.
In simple words: Very reactive metals are obtained by passing electricity through their molten salts.

Teacher's Note:
1. The metal is deposited at the cathode.
2. Examples are Na, K, Ca and Al.

Short Answer Questions I (2 Marks)

Question 7. Sodium or potassium pieces should not be thrown into a sink in the laboratory. Why ?
Answer:
Sodium and potassium react vigorously with water. The reaction is highly exothermic and the hydrogen gas evolved may catch fire or explode, and the caustic alkali formed can damage the sink: 2Na + 2H₂O → 2NaOH + H₂ + heat.
In simple words: These metals react violently with the water in the sink and can start a fire.

Teacher's Note:
1. Safe disposal of reactive metals is a lab safety value.
2. They are stored under kerosene for the same reason.

Question 8. Hydrogen is a non-metal, still it is given a place in the reactivity series. Why ?
Answer:
The reactivity of metals depends on how easily they lose electrons. Like metals, hydrogen also loses an electron to form a positive ion (H → H⁺ + e⁻). It is placed in the series for comparison: metals above hydrogen displace it from dilute acids, and those below it do not.
In simple words: Hydrogen loses electrons like a metal, so it is used as a reference point in the series.

Teacher's Note:
1. Use the series to predict which metals give hydrogen with acids.
2. Hydrogen sits between lead and copper.

Question 9. Aluminium occurs in combined state whereas gold is found in free state. Why ?
Answer:
Aluminium is a very reactive metal; it reacts with air, water and acids and forms compounds, so it occurs in the combined state (for example bauxite, Al₂O₃·2H₂O). Gold is the least reactive metal; it does not react with air, water or acids, so it is found in the free state.
In simple words: Reactive metals form compounds in nature; unreactive gold stays free.

Teacher's Note:
1. Link where a metal occurs to its place in the reactivity series.
2. Silver and platinum are also found free.

Question 10. (a) Fe + CuSO₄ → FeSO₄ + Cu (b) Cu + FeSO₄ → CuSO₄ + Fe. Which of the above two reactions will take place and why ?
Answer:
Reaction (a) will take place. Iron is more reactive than copper, so it can displace copper from copper sulphate solution.
Reaction (b) will not take place because copper is less reactive than iron and cannot displace it from iron(II) sulphate.
In simple words: Only the more reactive metal can replace the less reactive metal.

Teacher's Note:
1. Check which metal is higher in the reactivity series.
2. A blue solution turning green shows reaction (a).

Question 11. Give reasons for the following : (a) We can store copper sulphate solution in silver vessel but not silver nitrate solution in a copper vessel. (b) Food cans are coated with tin rather than zinc.
Answer:
(a) Silver is less reactive than copper, so it does not displace copper from copper sulphate, and the solution can be stored in a silver vessel. Copper is more reactive than silver, so a copper vessel would displace silver from silver nitrate: Cu + 2AgNO₃ → Cu(NO₃)₂ + 2Ag, and the vessel would be eaten away.
(b) Tin is less reactive than zinc, so it is not easily attacked by the acids and other components of food. Zinc is more reactive and would react with the food.
In simple words: A vessel must be less reactive than the solution it holds; tin is safer than zinc for food.

Teacher's Note:
1. Always compare the vessel metal with the metal of the salt.
2. Tin plating of cans is a food-safety measure.

Short Answer Questions II (3 Marks)

Question 12. Explain the following : (i) Sodium chloride is an ionic compound which does not conduct electricity in solid state where as it does conduct electricity in molten state as well as in aqueous solution. (ii) Reactivity of aluminium decrease if it is dipped in nitric acid. (iii) Metals like calcium and magnesium are never found in their free state in nature.
Answer:
(i) In solid NaCl the Na⁺ and Cl⁻ ions are held in fixed positions and cannot move. On melting or dissolving in water the ions become free to move and carry current.
(ii) Nitric acid is a strong oxidising agent. It forms a thin, stable layer of aluminium oxide on the surface, which prevents further reaction, so the reactivity of aluminium decreases.
(iii) Calcium and magnesium are near the top of the reactivity series. They are so reactive that they combine with oxygen, water and other substances of the surroundings and form compounds, so they are not found free.
In simple words: Ions must be free to move to carry current; nitric acid makes aluminium inert; very reactive metals always occur as compounds.

Teacher's Note:
1. Use ‘free ions’ in every conduction answer.
2. Passivation means a protective oxide layer.

Question 13. (a) Arrange the metals Zn, Mg, Al, Cu and Fe in decreasing order of reactivity. (b) What would you observe when you put (i) Some zinc pieces into blue copper sulphate solution ? (ii) Some copper pieces into green ferrous sulphate solution. (c) Name a metal which combines with hydrogen gas. Name the compound formed.
Answer:
(a) Mg > Al > Zn > Fe > Cu.
(b)(i) Zinc is more reactive than copper, so the blue colour fades to colourless and reddish-brown copper is deposited: Zn + CuSO₄ → ZnSO₄ + Cu.
(b)(ii) No change, because copper is less reactive than iron.
(c) Sodium combines with hydrogen to form sodium hydride: 2Na + H₂ → 2NaH.
In simple words: Mg is the most reactive and Cu the least; zinc displaces copper but copper cannot displace iron.

Teacher's Note:
1. Write the series in order before answering part (b).
2. Name the compound and write its formula in part (c).

Question 14. Name a metal of medium reactivity and write three main steps in the extraction of this metal from its sulphide ore.
Answer:
The metal is zinc (ore: zinc blende, ZnS).
Step 1 (roasting): The sulphide ore is heated strongly in excess air to convert it into the oxide: 2ZnS + 3O₂ → 2ZnO + 2SO₂.
Step 2 (reduction): The oxide is reduced with carbon to give the metal: ZnO + C → Zn + CO.
Step 3 (refining): The impure zinc is refined, usually by electrolytic refining, to get pure metal.
In simple words: Zinc ore is roasted to the oxide, reduced with carbon and then refined.

Teacher's Note:
1. Sulphide ores are roasted, carbonate ores are calcined.
2. Moderately reactive metals are reduced with carbon.

Question 15. (i) Define the term alloy and amalgam. Name the alloy used for welding electric wires together. What are its constituents? (ii) Name the constituents of the following alloys: (a) Brass (b) Stainless steel (c) Bronze. State one property in each of these alloys, which is different from its main constituents.
Answer:
(i) An alloy is a homogeneous mixture of a metal with another metal or a non-metal. An amalgam is an alloy of mercury with another metal. The alloy used for welding (soldering) electric wires is solder; its constituents are lead and tin.
(ii)(a) Brass: copper and zinc. It is harder and more resistant to corrosion than copper, but conducts electricity less well.
(b) Stainless steel: iron with chromium and nickel (and a little carbon). Unlike iron, it does not rust.
(c) Bronze: copper and tin. It is stronger and harder than copper, and it is a poorer conductor of electricity.
In simple words: Alloys are mixtures of metals; solder joins wires, brass, bronze and stainless steel each resist wear or rust better than their metals.

Teacher's Note:
1. Memorise the three alloys with their metals.
2. An alloy has a lower melting point and lower conductivity than its pure metals.

Question 16. (i) What type of oxides are formed when non-metals react with oxygen? Explain with an example. (ii) What type of oxides are formed when metals combine with oxygen? Explain with the help of an example.
Answer:
(i) Non-metals form acidic or neutral oxides. Example: carbon burns in oxygen to form carbon dioxide, an acidic oxide: C + O₂ → CO₂. Its solution in water (carbonic acid) turns blue litmus red: CO₂ + H₂O → H₂CO₃. (Carbon monoxide, CO, is neutral.)
(ii) Metals form basic oxides. Example: sodium burns in oxygen to form sodium oxide: 4Na + O₂ → 2Na₂O. The oxide dissolves in water to form sodium hydroxide, which turns red litmus blue: Na₂O + H₂O → 2NaOH.
In simple words: Non-metal oxides are acidic (or neutral); metal oxides are basic.

Teacher's Note:
1. Give a test with litmus for each type of oxide.
2. Amphoteric oxides are the exception among metal oxides.

Long Answer Questions (5 Marks)

Question 17. (i) What are metals? Name five metals. (ii) Name a metal which is so soft that it can be cut with a knife. (iii) Name the metal which is the best conductor of heat and electricity. (iv) What happens when a metal reacts with dilute hydrochloric acid? Explain with the help of an example.
Answer:
(i) Metals are elements that are lustrous, malleable, ductile and good conductors of heat and electricity; their atoms lose electrons to form positive ions. Examples: iron, aluminium, copper, gold and silver.
(ii) Sodium (or potassium) is so soft that it can be cut with a knife.
(iii) Silver is the best conductor of heat and electricity.
(iv) A metal above hydrogen in the reactivity series reacts with dilute hydrochloric acid to form a metal chloride and hydrogen gas. Example: Mg + 2HCl → MgCl₂ + H₂.
In simple words: Metals shine, bend and conduct; sodium is soft, silver conducts best and active metals give hydrogen with acids.

Teacher's Note:
1. Write one equation in part (iv).
2. Copper, silver and gold do not react with dilute HCl.

Question 18. (i) Define non-metals. Give five examples of non-metals. (ii) Name a non-metal which conducts electricity. (iii) Name a non-metal having lustre (shining surface). (iv) Name a non-metal which is extremely hard. (v) How do non-metals react with oxygen? Explain with an example. Give equation of the reaction involved. What is the nature of the product formed? How will you demonstrate it?
Answer:
(i) Non-metals are elements that are neither malleable nor ductile and are bad conductors of heat and electricity; their atoms gain electrons to form negative ions. Examples: carbon, sulphur, phosphorus, nitrogen and oxygen.
(ii) Graphite (a form of carbon). (iii) Iodine. (iv) Diamond (a form of carbon).
(v) Non-metals react with oxygen to form acidic or neutral oxides. Carbon burns in air to form carbon dioxide: C + O₂ → CO₂. The product is acidic: when it is dissolved in water, it forms carbonic acid (CO₂ + H₂O → H₂CO₃), which turns blue litmus red.
In simple words: Non-metals are dull and poor conductors, with graphite, iodine and diamond as exceptions; their oxides are acidic.

Teacher's Note:
1. Learn the exceptions: graphite, iodine and diamond.
2. Demonstrate acidic nature with blue litmus.

Question 19. (i) What is meant by the reactivity series of metals? Arrange the following metals in an increasing order of their reactivities towards water: zinc, iron, magnesium, sodium. (ii) Hydrogen is not a metal but still it has been assigned a place in the reactivity series of metals. Why? (iii) Name one metal more reactive and another less reactive than hydrogen. (iv) Name one metal which displaces copper from copper sulphate solution and one which does not. (v) Name one metal which displaces silver from silver nitrate solution and one which does not.
Answer:
(i) The arrangement of metals in the decreasing order of their reactivities is called the reactivity series. Increasing order towards water: iron < zinc < magnesium < sodium.
(ii) Hydrogen can lose an electron and form H⁺ ions like metals, so it is placed in the series for comparison.
(iii) Zinc (or lead) is more reactive than hydrogen; copper is less reactive.
(iv) Zinc displaces copper from copper sulphate solution; mercury (or silver) does not.
(v) Copper displaces silver from silver nitrate solution; gold does not.
In simple words: The series ranks metals; higher metals push lower metals out of their salt solutions.

Teacher's Note:
1. Use the series: Na, Mg, Zn, Fe, Pb, H, Cu, Hg, Ag, Au.
2. Apply: a metal can displace those below it.

Case Based Questions (4 Marks)

Teacher's Note:
1. Read every column of the table row by row.
2. A container metal must be less reactive than every metal in its solution.

Question 21. Activity series : Relative reactivities of metals
Potassium (most reactive), Sodium, Calcium, Magnesium, Aluminium, Zinc, Iron, Lead, Hydrogen, Copper, Mercury, Silver, Gold (least reactive)(i) What happens when iron nail is added to copper sulphate solution? What is the colour change? (ii) Identify the metal which reacts with very dilute nitric acid to evolve hydrogen gas. Name one more metal not given in the above series which reacts in the same way with dilute nitric acid. (iii) Name one important ore of copper with its chemical formula. (iv) Which method is used to extract sodium from molten sodium chloride? (v) Which metal is used in the galvanization of iron?
Answer:
(i) Iron is more reactive than copper, so the iron nail displaces copper. The blue colour of the solution changes to pale green and a reddish-brown coating of copper forms on the nail: Fe + CuSO₄ → FeSO₄ + Cu.
(ii) Magnesium reacts with very dilute nitric acid to evolve hydrogen; manganese does the same and is not given in the series.
(iii) An important ore of copper is copper glance, Cu₂S (copper pyrites, CuFeS₂, is another).
(iv) Sodium is extracted by the electrolysis of molten sodium chloride (electrolytic reduction).
(v) Zinc is used in the galvanisation of iron.
In simple words: Use the series to answer: iron displaces copper, and very reactive sodium is obtained by electrolysis.

Teacher's Note:
1. The colour change is blue to pale green, not blue to brown.
2. Galvanisation means coating iron with zinc.