Class 10 Science · Chapter 3

Metals and Non-metals

344 Class 10 Science questions on Metals and Non-metals from the NCERT textbook, NCERT Exemplar and CBSE board papers 2017–2026, each with its answer and a step-by-step solution.

CBSE Class 10 Science previous year papers, with answers →

Questions · page 15 of 18

1. Q281 CBSE 2020
   Draw a labelled ​diagram of the experimental set-up to study the action of steam on ​a ​metal. ​Name ​the ​gas ​produced in the reaction.
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   Steam passed over a heated metal in a boiling tube gives hydrogen, collected over water in an inverted test tube.

   Labelled diagram: a boiling tube clamped to a stand holds glass wool soaked in water at its closed end and the metal sample (iron filings or zinc granules) in the middle; a burner heats the metal strongly, so steam from the wet glass wool passes over it; a delivery tube passes through the cork at the mouth and leads under water in a trough, up through a beehive shelf, into an inverted test tube filled with water, where the gas collects by pushing the water out. The gas produced is hydrogen, for example $3\text{Fe}(s)+4{\text{H}}_2\text{O}(g)⟶{\text{Fe}}_3{\text{O}}_4(s)+4{\text{H}}_2(g)$.

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2. Q282 CBSE 2020
   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 from their ores? Take an example to explain the process of extraction along with chemical equations.
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   Na, Mg and Al hold oxygen more strongly than carbon, sit high in the series and are obtained by electrolysis of molten compounds.

   Carbon cannot reduce the oxides of sodium, magnesium and aluminium because these metals have more affinity for oxygen than carbon has, so carbon cannot take the oxygen away from their oxides. These metals are placed towards the top of the reactivity series, with potassium and calcium, above zinc and iron. They are obtained by electrolytic reduction of their molten compounds (molten chlorides, or the molten oxide for aluminium): electricity is passed through the molten compound and the metal is deposited at the cathode. Example: sodium is extracted by the electrolysis of molten sodium chloride. Molten $\text{NaCl}$ is taken in an electrolytic cell; on passing electricity, sodium is deposited at the cathode and chlorine is liberated at the anode. At the cathode: ${\text{Na}}^{+}+e^{-}⟶\text{Na}$ At the anode: $2{\text{Cl}}^{-}⟶{\text{Cl}}_2+2e^{-}$ Overall: $2\text{NaCl}(l)⟶2\text{Na}(l)+{\text{Cl}}_2(g)$ Any other correct example, such as magnesium from molten magnesium chloride, earns the same credit.

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3. Q283 CBSE 2020
   1. (a)What is thermit process ? Where ​is this process ​used? Write balanced chemical ​equation for the reaction ​involved.
   2. (b)Where does the metal aluminium, used in the process, occurs in the reactivity ​series of metals?
   3. (c)Name the substances that ​are getting oxidised and reduced in ​the process.
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   In the thermit process aluminium displaces molten iron from iron(III) oxide, joining rails; Al is oxidised and ${\text{Fe}}_2{\text{O}}_3$ is reduced.

   (a) The thermit process is the reaction of iron(III) oxide with aluminium powder, in which the more reactive aluminium displaces iron from its oxide; the reaction is so highly exothermic that the iron formed is molten. It is used to join railway tracks and to repair cracked machine parts (thermit welding). ${\text{Fe}}_2{\text{O}}_3(s)+2\text{Al}(s)⟶2\text{Fe}(l)+{\text{Al}}_2{\text{O}}_3(s)+\text{heat}$ (b) Aluminium is placed high in the reactivity series: above zinc and iron, just below magnesium. (c) Aluminium ($\text{Al}$) is oxidised to ${\text{Al}}_2{\text{O}}_3$; iron(III) oxide (${\text{Fe}}_2{\text{O}}_3$) is reduced to $\text{Fe}$.

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4. Q284 CBSE 2020
   Define an ​alloy. How is an alloy prepared ​? List two advantages of ​making ​alloys. ​Write the composition of stainless steel. Why is steel preferred over iron ​? List two reasons.
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   An alloy is a homogeneous mixture of metals, or a metal and a non-metal; steel is preferred because it is harder than iron.

   An alloy is a homogeneous mixture of two or more metals, or of a metal and a non-metal. It is prepared by melting the primary metal, dissolving the other elements in it in definite proportions and cooling to room temperature. Advantages: alloys are harder and stronger than the pure metal and resist corrosion better (other correct advantages, such as a lower melting point, also earn credit). Stainless steel is iron with chromium and nickel. Steel is preferred over iron because it is hard (pure iron is soft) and stainless steel does not rust (iron rusts easily).

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5. Q285 CBSE 2020
   1. (a)How is the method ​of extraction of ​metals high up in the reactivity ​series different from that ​for metals in the middle ? ​Why cannot the same process be applied for ​them ? Name and explain the ​process of extraction of sodium.
   2. (b)Draw a labelled ​diagram of electrolytic refining of copper.
   Show answer

   Carbon reduces oxides of middle metals but not highly reactive ones, which are electrolysed; sodium forms at the cathode, and copper is refined electrolytically.

   (a) Metals in the middle of the reactivity series (zinc, iron, lead, copper) usually occur as sulphides or carbonates; the ore is first converted into the oxide, by roasting a sulphide or calcining a carbonate, and the oxide is then reduced to the metal by heating it with carbon. Metals high up in the series (potassium, sodium, calcium, magnesium, aluminium) cannot be obtained this way: they are extracted by electrolytic reduction of their molten chlorides or oxides. The carbon process cannot be applied to them because these metals have a greater affinity for oxygen than carbon has, so carbon cannot reduce their oxides. Extraction of sodium is by electrolytic reduction: electricity is passed through molten sodium chloride; sodium metal is deposited at the cathode and chlorine gas is liberated at the anode. At the cathode: ${\text{Na}}^{+}+e^{-}⟶\text{Na}$ At the anode: $2{\text{Cl}}^{-}⟶{\text{Cl}}_2+2e^{-}$ (b) Labelled diagram (see the figure): a tank of acidified copper sulphate solution as the electrolyte, a block of impure copper as the anode (+), a thin strip of pure copper as the cathode (−), and the anode mud of insoluble impurities below the anode; copper dissolves from the anode and pure copper is deposited on the cathode.

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6. Q286 CBSE 2020
   Write all ​the steps in proper sequence of obtaining ​pure copper ​from the ore in ​which it ​exists ​in ​nature.
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   Concentrate the copper glance ore, roast it, let ${\text{Cu}}_2\text{O}$ and ${\text{Cu}}_2\text{S}$ react to give copper, then refine the copper electrolytically.

   Copper occurs in nature as sulphide ores such as copper glance, copper(I) sulphide (${\text{Cu}}_2\text{S}$). Pure copper is obtained from copper glance in this sequence: Step 1: Concentration (enrichment) of the ore: the gangue, such as soil and sand, is removed (for a sulphide ore, by froth flotation). Step 2: Roasting: the concentrated ore is heated strongly in air, and part of the copper(I) sulphide is converted into copper(I) oxide: $2{\text{Cu}}_2\text{S}+3{\text{O}}_2\stackrel{\text{heat}}{\to }2{\text{Cu}}_2\text{O}+2{\text{SO}}_2$ Step 3: Self-reduction: on continued heating, the copper(I) oxide reacts with the remaining copper(I) sulphide to give copper: $2{\text{Cu}}_2\text{O}+{\text{Cu}}_2\text{S}\stackrel{\text{heat}}{\to }6\text{Cu}+{\text{SO}}_2$ Step 4: Electrolytic refining: the copper so obtained is impure. It is refined in an electrolytic cell with a thick block of impure copper as the anode, a thin strip of pure copper as the cathode, and a solution of copper sulphate acidified with dilute sulphuric acid as the electrolyte. On passing current, copper from the anode dissolves into the electrolyte, $\text{Cu}⟶{\text{Cu}}^{2+}+2e^{-}$, and an equivalent amount of pure copper from the electrolyte is deposited on the cathode, ${\text{Cu}}^{2+}+2e^{-}⟶\text{Cu}$. Soluble impurities go into the solution, and insoluble impurities settle at the bottom below the anode as anode mud.

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7. Q287 CBSE 2020
   Write balanced ​chemical equations to explain what happens, ​when

   1. (i)​Mercuric oxide ​is heated.
   2. (ii)Mixture of cuprous oxide and cuprous sulphide is heated.
   3. (iii)Aluminium is ​reacted with manganese dioxide.
   4. (iv)Ferric ​oxide is reduced with aluminium.
   5. (v)Zinc carbonate undergoes ​calcination.
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   HgO gives mercury, ${\text{Cu}}_2\text{O}$ with ${\text{Cu}}_2\text{S}$ gives copper, Al displaces Mn and Fe from their oxides, and ${\text{ZnCO}}_3$ gives ZnO.

   (i) Mercuric oxide decomposes into mercury and oxygen: $2\text{HgO}(s)\stackrel{\text{heat}}{\to }2\text{Hg}(l)+{\text{O}}_2(g)$ (ii) The mixture reacts to give copper and sulphur dioxide (self-reduction): $2{\text{Cu}}_2\text{O}+{\text{Cu}}_2\text{S}\stackrel{\text{heat}}{\to }6\text{Cu}+{\text{SO}}_2$ (iii) Aluminium displaces manganese, with the release of a lot of heat: $3{\text{MnO}}_2(s)+4\text{Al}(s)⟶3\text{Mn}(l)+2{\text{Al}}_2{\text{O}}_3(s)+\text{heat}$ (iv) Aluminium displaces iron (thermit reaction), giving molten iron: ${\text{Fe}}_2{\text{O}}_3(s)+2\text{Al}(s)⟶2\text{Fe}(l)+{\text{Al}}_2{\text{O}}_3(s)+\text{heat}$ (v) Zinc carbonate is converted into zinc oxide and carbon dioxide: ${\text{ZnCO}}_3(s)\stackrel{\text{heat}}{\to }\text{ZnO}(s)+{\text{CO}}_2(g)$

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8. Q288 CBSE 2020
   Two ores X and Y were taken. On heating these ores ​it ​was observed that

   1. (a)​ore ​X gives ${\text{CO}}_2$ gas, and
   2. (b)ore Y ​gives ${\text{SO}}_2$ gas. Write steps to convert these ores into metals, giving chemical equations ​of the reactions that take ​place.
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   X is a carbonate ore and Y a sulphide ore; calcination or roasting gives the oxide, which carbon reduces to the metal.

   The gases show that X is a carbonate ore (for example ${\text{ZnCO}}_3$) and Y is a sulphide ore (for example $\text{ZnS}$). Ore X, calcination: heat strongly in a limited supply of air, ${\text{ZnCO}}_3(s)\stackrel{\text{heat}}{\to }\text{ZnO}(s)+{\text{CO}}_2(g)$. Ore Y, roasting: heat strongly in excess air, $2\text{ZnS}(s)+3{\text{O}}_2(g)\stackrel{\text{heat}}{\to }2\text{ZnO}(s)+2{\text{SO}}_2(g)$. Reduction: heat the metal oxide with a reducing agent such as carbon, $\text{ZnO}(s)+\text{C}(s)⟶\text{Zn}(s)+\text{CO}(g)$. The carbonate and sulphide ores of another suitable metal, with correct equations, earn the same credit.

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9. Q289 CBSE 2020
   What happens when (Write the balanced equation involved) ​–

   1. (i)​Copper is ​heated in ​air ?
   2. (ii)Aluminium oxide is reacted ​with hydrochloric acid ?
   3. (iii)​Potassium reacts with water ?
   4. (iv)Cinnabar is heated in ​air ​?
   5. (v)Aluminium oxide reacts with sodium hydroxide ?
   Show answer

   Copper forms black $\text{CuO}$; ${\text{Al}}_2{\text{O}}_3$ reacts with both $\text{HCl}$ and $\text{NaOH}$; potassium reacts violently with water; cinnabar gives mercury.

   (i) Copper combines with oxygen and a black coating of copper(II) oxide forms: $2\text{Cu}+{\text{O}}_2\stackrel{\text{heat}}{\to }2\text{CuO}$ (ii) Aluminium oxide reacts, as a basic oxide, to form aluminium chloride and water: ${\text{Al}}_2{\text{O}}_3+6\text{HCl}⟶2{\text{AlCl}}_3+3{\text{H}}_2\text{O}$ (iii) Potassium reacts violently with cold water, forming potassium hydroxide and hydrogen; the heat released sets the hydrogen on fire: $2\text{K}+2{\text{H}}_2\text{O}⟶2\text{KOH}+{\text{H}}_2+\text{heat energy}$ (iv) Cinnabar (mercury(II) sulphide) is converted into mercury(II) oxide and sulphur dioxide is given off: $2\text{HgS}+3{\text{O}}_2\stackrel{\text{heat}}{\to }2\text{HgO}+2{\text{SO}}_2$; on further heating the oxide gives mercury, $2\text{HgO}\stackrel{\text{heat}}{\to }2\text{Hg}+{\text{O}}_2$ (v) Aluminium oxide reacts, as an acidic oxide, to form sodium aluminate and water: ${\text{Al}}_2{\text{O}}_3+2\text{NaOH}⟶2{\text{NaAlO}}_2+{\text{H}}_2\text{O}$

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10. Q290 CBSE 2020
    1. (a)​Show the formation of aluminium ​ion and chloride ion by transfer of ​electrons.
    2. (b)State the formula ​and the nature of compound formed if ​electron transfer takes place between the two .
    3. (c)State with reason any three characteristics of ​the compounds formed by ​transfer of electrons.
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    Al loses three electrons and Cl gains one to form ionic ${\text{AlCl}}_3$; ionic compounds are hard, melt at high temperatures and conduct when molten.

    (a) Aluminium (atomic number $13$, electronic configuration $2,8,3$) loses its three valence electrons to form the aluminium ion, and chlorine (atomic number $17$, electronic configuration $2,8,7$) gains one electron to form the chloride ion; one aluminium atom transfers one electron to each of three chlorine atoms (see the electron-dot diagram): $\text{Al}(2,8,3)⟶{\text{Al}}^{3+}(2,8)+3e^{-}$ $\text{Cl}(2,8,7)+e^{-}⟶{\text{Cl}}^{-}(2,8,8)$ $\text{Al}+3\text{Cl}⟶{\text{Al}}^{3+}+3{\text{Cl}}^{-}({\text{AlCl}}_3)$ Both ions then have a completely filled outermost shell. (b) Formula: ${\text{AlCl}}_3$ (aluminium chloride). Nature: an ionic (electrovalent) compound, because it is formed by the transfer of electrons. (c) Any three characteristics of compounds formed by transfer of electrons (ionic compounds), with reasons: (i) They are solids and somewhat hard, because of the strong force of attraction between the positive and negative ions. (ii) They have high melting and boiling points, because a considerable amount of energy is needed to break the strong inter-ionic attraction. (iii) They conduct electricity in the molten state (and in aqueous solution), because the ions are then free to move; they do not conduct in the solid state, where the ions are held in fixed positions. (They are also generally soluble in water and insoluble in kerosene and petrol; other correct characteristics with reasons also earn credit.)

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11. Q291 CBSE 2020
    1. (i)By the transfer of electrons, illustrate the ​formation of ​bond in magnesium chloride and identify the ions present in this compound.
    2. (ii)Ionic compounds are ​solids. Give ​reasons.
    3. (iii)​With the help of a labelled diagram show ​the experimental set up of action of steam ​on a metal.
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    Magnesium transfers two electrons to two chlorine atoms, forming ${\text{Mg}}^{2+}$ and ${\text{Cl}}^{-}$ ions; strong attraction between the ions makes ionic compounds solid.

    (i) Magnesium ($2,8,2$) loses its two outermost electrons, one to each of two chlorine atoms ($2,8,7$): $\text{Mg}⟶{\text{Mg}}^{2+}+2e^{-}$ and $\text{Cl}+e^{-}⟶{\text{Cl}}^{-}$, so $\text{Mg}+2\text{Cl}⟶{\text{Mg}}^{2+}+2{\text{Cl}}^{-}$, that is, ${\text{MgCl}}_2$. The ions present are the magnesium ion ${\text{Mg}}^{2+}$ (cation) and the chloride ion ${\text{Cl}}^{-}$ (anion). (ii) In ionic compounds very strong forces of attraction exist between the positive and negative ions; they hold the ions firmly in place, so ionic compounds are solids (and somewhat hard, with high melting points). (iii) Labelled diagram: glass wool soaked in water and the metal sample in a boiling tube clamped on a stand and heated with a burner; a delivery tube through the cork leads under water in a trough into an inverted test tube, in which the hydrogen gas formed collects (for example, $3\text{Fe}+4{\text{H}}_2\text{O}⟶{\text{Fe}}_3{\text{O}}_4+4{\text{H}}_2$).

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12. Q292 CBSE 2020
    1. (a)​With ​the help of a ​diagram ​explain the method of refining of copper by electrolysis.
    2. (b)How ​are broken railway tracks joined ? Give ​the name of the process and the chemical equation ​of the reaction involved.
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    In acidified copper sulphate, the impure copper anode dissolves and pure copper deposits on the cathode; the thermit process joins rails with molten iron.

    (a) Electrolytic refining of copper (labelled diagram as shown): the anode is a thick block of impure copper, joined to the positive terminal; the cathode is a thin strip of pure copper, joined to the negative terminal; the electrolyte is acidified copper sulphate solution. On passing the current, copper from the impure anode dissolves into the electrolyte, and an equivalent amount of pure copper is deposited on the cathode. Soluble impurities go into the solution, and insoluble impurities settle below the anode as anode mud. (b) The gap between the broken ends is filled with molten iron, formed by the reaction of iron(III) oxide with aluminium powder. The process is the thermit process (thermit reaction): ${\text{Fe}}_2{\text{O}}_3(s)+2\text{Al}(s)⟶2\text{Fe}(l)+{\text{Al}}_2{\text{O}}_3(s)+\text{heat}$.

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13. Q293 CBSE 2020
    (a)

    1. (i)​Write two properties of gold which make it ​the most ​suitable metal ​for ​ornaments.
    2. (ii)Name two metals which are the best conductors of heat.
    3. (iii)Name two metals which melt when you keep them on your palm. (b) Explain the ​formation ​of ionic compound $\text{CaO}$ with electron-dot structure. Atomic numbers of calcium and oxygen are 20 and 8 respectively.
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    Gold is lustrous, malleable and ductile; silver and copper conduct heat best; gallium and caesium melt on the palm; $\text{CaO}$ forms by electron transfer.

    (a) (i) Gold has a bright metallic lustre, and it is highly malleable and ductile, so it can be worked into fine ornaments (it is also very unreactive, so it does not tarnish easily). (ii) Silver and copper. (iii) Gallium and caesium. (b) Calcium $(2,8,8,2)$ loses its two valence electrons to form ${\text{Ca}}^{2+}$ $(2,8,8)$; oxygen $(2,6)$ gains these two electrons to form ${\text{O}}^{2-}$ $(2,8)$. In the electron-dot structure the two dots of calcium move to oxygen (six crosses), giving $[\text{Ca}{]}^{2+}$ and $[\text{O}{]}^{2-}$ with a complete octet. The oppositely charged ions are held together by strong electrostatic attraction, forming the ionic compound $\text{CaO}$.

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14. Q294 CBSE 2020
    (a) Define the ​terms ​‘alloy’ and ‘amalgam’. Name the alloy used for ​welding electric wires together and ​write its constituents. (b) ​Name the constituents of the following alloys :

    1. (i)​Brass
    2. (ii)​Stainless steel
    3. (iii)Bronze State one property in each ​of these alloys which is different from its constituents.
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    Solder (lead and tin) welds wires; an amalgam is an alloy containing mercury; brass, stainless steel and bronze have properties different from their constituents.

    (a) An alloy is a homogeneous mixture of two or more metals, or of a metal and a non-metal, in definite proportions. An amalgam is an alloy in which one of the metals is mercury. The alloy used for welding electric wires together is solder, whose constituents are lead ($\text{Pb}$) and tin ($\text{Sn}$); it has a low melting point. (b) (i) Brass: copper ($\text{Cu}$) and zinc ($\text{Zn}$). It has a much lower electrical conductivity than copper. (ii) Stainless steel: iron ($\text{Fe}$), chromium ($\text{Cr}$) and nickel ($\text{Ni}$). It does not rust (corrode) easily, whereas iron rusts. (iii) Bronze: copper ($\text{Cu}$) and tin ($\text{Sn}$). It has a much lower electrical conductivity than copper. Another correct difference in each case is greater hardness: brass and bronze are harder than copper, and stainless steel is harder than pure iron.

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15. Q295 CBSE 2020
    1. (a)​What is a ​cinnabar ? How is ​metal extracted from ​it ? Explain ​briefly.
    2. (b)Define the term ​alloy. List two advantages of ​making alloys.
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    Cinnabar (HgS) is roasted to HgO, which on heating gives mercury; an alloy is a homogeneous mixture, harder and stronger than the pure metal.

    (a) Cinnabar is an ore of mercury; it is mercury sulphide, $\text{HgS}$. Mercury is low in the activity series, so it is extracted by heating alone. The enriched ore is heated in air (roasting), which converts it into mercuric oxide: $2\text{HgS}+3{\text{O}}_2\stackrel{\text{heat}}{\to }2\text{HgO}+2{\text{SO}}_2$. On further heating, the mercuric oxide is reduced to mercury: $2\text{HgO}\stackrel{\text{heat}}{\to }2\text{Hg}+{\text{O}}_2$. (b) An alloy is a homogeneous mixture of two or more metals, or of a metal and a non-metal. Advantages (any two): an alloy is harder and stronger than the pure metal (iron with a little carbon gives hard, strong steel); an alloy can have a lower melting point (solder, an alloy of lead and tin, is used for welding electrical wires); an alloy can resist corrosion better (stainless steel does not rust).

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16. Q296 CBSE 2020
    (a) Complete and ​balance the following ​chemical equations :

    1. (i)${\text{Al}}_2{\text{O}}_3+\text{HCl}⟶$
    2. (ii)${\text{K}}_2\text{O}+{\text{H}}_2\text{O}⟶$
    3. (iii)$\text{Fe}+{\text{H}}_2\text{O}⟶$ (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.
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    X displaces Cu, Zn and Ag from their salts; increasing order of reactivity is $\text{Ag}<\text{Cu}<\text{Zn}<\text{X}$.

    (a) (i) ${\text{Al}}_2{\text{O}}_3+6\text{HCl}⟶2{\text{AlCl}}_3+3{\text{H}}_2\text{O}$ (ii) ${\text{K}}_2\text{O}+{\text{H}}_2\text{O}⟶2\text{KOH}$ (iii) $3\text{Fe}+4{\text{H}}_2\text{O}⟶{\text{Fe}}_3{\text{O}}_4+4{\text{H}}_2$ (iron reacts with steam) (b) X is more reactive than iron and, being a metal other than zinc, lies above zinc in the textbook activity series, so it is more reactive than zinc, copper and silver and displaces each of them from its salt solution. With copper sulphate the blue colour fades and a reddish-brown coating of copper forms on X; with zinc sulphate a grey coating of zinc forms on X; with silver nitrate a shiny greyish-white coating of silver forms on X. Increasing order of reactivity: $\text{Ag}<\text{Cu}<\text{Zn}<\text{X}$.

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17. Q297 CBSE 2020
    A metal ‘M’ is stored under kerosene. It vigorously ​catches fire, if a small piece of this metal is ​kept open in air. ​Dissolution of this metal in water releases great amount of ​energy and the metal catches fire. The solution so formed turns red litmus blue.

    1. (a)Name the ​metal ‘M’.
    2. (b)Write formula of the compound formed when this metal is exposed to air.
    3. (c)Why is metal ‘M’ stored under kerosene ?
    4. (d)If ​oxide of ​this metal is treated with hydrochloric acid, what would be the products ?
    5. (e)Write balanced ​equations for : (i) Reaction of ‘M’ with air. (ii) Reaction of ‘M’ with water. (iii) Reaction of metal oxide with hydrochloric acid.
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    Metal M is sodium; it forms ${\text{Na}}_2\text{O}$ in air, is kept under kerosene to exclude air and moisture, and gives NaOH and hydrogen with water.

    (a) The metal M is sodium ($\text{Na}$). (b) ${\text{Na}}_2\text{O}$ (sodium oxide). (c) Sodium reacts so vigorously with the oxygen and moisture of air that it catches fire if kept in the open. Kerosene does not react with it and keeps air and moisture away, so accidental fires are prevented. (d) Sodium chloride and water ($\text{NaCl}$ and ${\text{H}}_2\text{O}$). (e) (i) $4\text{Na}(s)+{\text{O}}_2(g)⟶2{\text{Na}}_2\text{O}(s)$ (ii) $2\text{Na}(s)+2{\text{H}}_2\text{O}(l)⟶2\text{NaOH}(aq)+{\text{H}}_2(g)+\text{heat energy}$ (iii) ${\text{Na}}_2\text{O}(s)+2\text{HCl}(aq)⟶2\text{NaCl}(aq)+{\text{H}}_2\text{O}(l)$

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18. Q298 CBSE 2020
    1. (a)What is an ​amalgam ?
    2. (b)Why is ​solder ​used ​for making electrical fuse ?
    3. (c)What is galvanisation ? State its significance.
    4. (d)In the electrolytic refining of copper, ​name the anode and cathode ​used and the electrolyte taken in ​the cell.
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    Amalgam contains mercury, solder's low melting point suits fuses, zinc coating (galvanisation) stops rusting, and copper is refined with impure anode and pure cathode.

    (a) An amalgam is an alloy in which one of the constituent metals is mercury. (b) Solder (an alloy of lead and tin) has a low melting point, so a fuse wire made of it melts quickly and breaks the circuit when an excessive current flows. (c) Galvanisation is the process of coating iron or steel articles with a thin layer of zinc. Significance: it protects the articles from rusting, and they stay protected even if the zinc coating is broken. (d) Anode: impure copper (a thick block); cathode: a thin strip of pure copper; electrolyte: acidified copper sulphate solution.

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19. Q299 CBSE 2020
    (a) Name the following :

    1. (i)Metal that ​can be cut by knife
    2. (ii)Lustrous non-metal
    3. (iii)Metal that exists ​in liquid state ​at room temperature
    4. (iv)Most malleable and ductile metal
    5. (v)Metal that is best conductor of electricity
    6. (vi)Non-metal that can ​exist in different forms (b) How are ​alloys ​better than ​metals ? Give composition ​of solder and amalgam.
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    Sodium, iodine, mercury, gold, silver and carbon; alloys are harder, stronger and resist corrosion; solder is lead and tin, and amalgam contains mercury.

    (a) (i) Sodium (potassium and lithium can also be cut with a knife) (ii) Iodine (iii) Mercury (iv) Gold (v) Silver (copper, the next best conductor, is also accepted) (vi) Carbon, as diamond, graphite and fullerene (sulphur and phosphorus also exist in different forms) (b) Alloys are generally harder and stronger than the pure metal, resist corrosion better, and have a higher electrical resistivity and a lower melting point, which suit them to uses the pure metal cannot serve (any two of these). Solder is an alloy of lead and tin ($\text{Pb}$ and $\text{Sn}$); an amalgam is an alloy in which one of the metals is mercury ($\text{Hg}$).

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20. Q300 CBSE 2020
    1. (a)Write electron dot structures of Ca (At. No. 20) ​and O(At. No. ​8).
    2. (b)Show the formation of calcium oxide ​by transfer of electrons.
    3. (c)Name the ions ​present in this compound.
    4. (d)​List ​four ​important characteristics ​of this compound.
    Show answer

    Ca transfers two electrons to O, forming ${\text{Ca}}^{2+}$ and ${\text{O}}^{2-}$ ions in CaO, a hard solid with a high melting point.

    (a) Ca (2, 8, 8, 2) has $2$ valence electrons, shown as two dots beside the symbol Ca; O (2, 6) has $6$ valence electrons, shown as six crosses around the symbol O (see the figure). (b) Calcium transfers its $2$ valence electrons to oxygen: $\text{Ca}⟶{\text{Ca}}^{2+}+2e^{-}$ and $\text{O}+2e^{-}⟶{\text{O}}^{2-}$, giving ${\text{Ca}}^{2+}+{\text{O}}^{2-}⟶\text{CaO}$. In dot structures, Ca with two dots and O with six crosses give $[\text{Ca}{]}^{2+}[\text{O}{]}^{2-}$, the oxide ion now carrying eight electrons (see the figure). (c) Calcium ion (${\text{Ca}}^{2+}$) and oxide ion (${\text{O}}^{2-}$). (d) (1) It is a hard, crystalline solid. (2) It has very high melting and boiling points. (3) It is soluble in water, dissolving by reacting with it to form calcium hydroxide, but not in kerosene or petrol. (4) It conducts electricity in the molten state, but not as a solid.

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