CBSE Class 10 Science HOTs Metals and Non Metals Set 06

Corrosion

Question. Assertion (A) : The metals and alloys are good conductors of electricity.
Reason (R) : Bronze is an alloy of copper and tin and it is not a good conductor of electricity.
(a) Both (A) and (R) are true and (R) is the correct explanation of the assertion (A).
(b) Both (A) and (R) are true, but (R) is not the correct explanation of the assertion (A).
(c) (A) is true, but (R) is false.
(d) (A) is false, but (R) is true.
Answer: (b) Both (A) and (R) are true, but (R) is not the correct explanation of the assertion (A).

Question. Silver articles become black when kept in open for some time, whereas copper vessels lose their shiny brown surfaces and gain a green coat when kept in open. Name the substances present in air with which these metals react and write the name of the products formed.
Answer: Silver reacts with Hydrogen sulphide (\( H_2S \)) in the air to form a black coating of Silver sulphide (\( Ag_2S \)). Copper reacts with moist carbon dioxide (\( CO_2 \)) in the air to form a green coating of basic Copper carbonate (\( CuCO_3 \cdot Cu(OH)_2 \)).

Question. Name first discovered alloy. Give its composition also.
Answer: Bronze was the first discovered alloy. Its composition is Copper (approx. 88%) and Tin (approx. 12%).

Question. List three differentiating features between the processes of galvanisation and alloying.
Answer:
1. Galvanisation is a surface coating of zinc; Alloying is a homogeneous mixture of two or more metals/non-metals.
2. Galvanisation protects only the surface; Alloying changes the properties of the entire bulk material.
3. Galvanisation is specifically for iron/steel protection; Alloying is used to enhance various properties like strength, hardness, and corrosion resistance of many metals.

Question. Describe an activity to find out the conditions under which iron rusts.
Answer: Take three test tubes A, B, and C with iron nails. (A) Contains water and air. (B) Contains boiled distilled water covered with oil (no air). (C) Contains anhydrous calcium chloride (dry air, no moisture). After a few days, only nails in tube A rust, showing that both air and moisture are necessary for rusting.

Question. 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: Metal surfaces acquire a dull appearance because they react with gases and moisture in the air to form a layer of oxide, carbonate, or sulphide. Copper acquires a green color due to basic Copper carbonate (\( CuCO_3 \cdot Cu(OH)_2 \)). Silver acquires a black color due to Silver sulphide (\( Ag_2S \)).

Question. (a) Name the following :
(i) Metal that can be cut by knife
(ii) Lustrous non-metal
(iii) Metal that exists in liquid state at room temperature
(iv) Most malleable and ductile metal
(v) Metal that is best conductor of electricity
(vi) Non-metal that can exist in different forms
(b) How are alloys better than metals? Give composition of solder and amalgam.
Answer: (a) (i) Sodium/Potassium, (ii) Iodine, (iii) Mercury, (iv) Gold, (v) Silver, (vi) Carbon (allotropes like Diamond, Graphite).
(b) Alloys are better because they are stronger, more resistant to corrosion, and can be tailored for specific uses. Solder: Lead and Tin (Pb and Sn). Amalgam: Mercury and any other metal (usually silver/tin/zinc for dental work).

Question. (a) Define corrosion.
(b) What is corrosion of iron called?
(c) How will you recognise the corrosion of silver?
(d) Why corrosion of iron is a serious problem?
(e) How can we prevent corrosion of iron?
Answer: (a) Corrosion is the gradual destruction of metals by chemical reaction with their environment (air, moisture).
(b) Rusting.
(c) Formation of a black coating on the silver surface.
(d) Iron is used extensively in infrastructure (bridges, ships, cars); rust weakens the structure, leading to huge economic losses and safety risks.
(e) Painting, oiling/greasing, galvanising, chrome plating, or alloying.

Question. Give reason for the following :
(a) Ionic compounds have higher melting point and higher boiling point.
(b) Sodium is kept immersed in kerosene.
(c) Reaction of calcium with water is less violent.
(d) Silver articles become black after some time when exposed to air.
(e) Prior to reduction the metal sulphides and carbonates must be converted into metal oxides for extracting metals.
Answer: (a) Due to strong electrostatic forces of attraction between oppositely charged ions.
(b) To prevent reaction with oxygen and moisture in air, as sodium is highly reactive.
(c) The heat evolved is not sufficient for the hydrogen gas to catch fire.
(d) Silver reacts with sulfur compounds (\( H_2S \)) in the air to form black silver sulphide.
(e) It is easier to reduce a metal from its oxide than from its sulphide or carbonate ore.

Physical Properties

Question. A cable manufacturing unit tested few elements on the basis of their physical properties.
Properties W X Y Z
Malleable Yes No No Yes
Ductile Yes No No Yes
Electrical conductivity Yes Yes Yes No
Melting Point High Low Low High
Which of the above elements were discarded for usage by the company?
(a) W, X, Y
(b) X, Y, Z
(c) W, X, Z
(d) W, Y, Z
Answer: (b) X, Y, Z

Chemical Properties of Metals

Question. On placing a copper coin in a test tube containing green ferrous sulphate solution, it will be observed that the ferrous sulphate solution
(a) turns blue, and a grey substance is deposited on the copper coin.
(b) turns colourless and a grey substance is deposited on the copper coin.
(c) turns colourless and a reddish–brown substance is deposited on the copper coin.
(d) remains green with no change in the copper coin.
Answer: (d) remains green with no change in the copper coin.

Question. Metal + Dil. HCl \(\rightarrow\) Metal Salt + Gas
Metal | Gas Evolved
(i) Copper | Yes
(ii) Iron | Yes
(iii) Magnesium | No
(iv) Zinc | Yes
Which of the following two combinations are correct?
(a) (i) and (iii)
(b) (i) and (iv)
(c) (ii) and (iii)
(d) (ii) and (iv)
Answer: (d) (ii) and (iv)

Question. The table given below shows the reaction of a few elements with acids and bases to evolve hydrogen gas.
Element | Acid | Base
A | \(\times\) | \(\times\)
B | \(\checkmark\) | \(\checkmark\)
C | \(\checkmark\) | \(\times\)
D | \(\checkmark\) | \(\checkmark\)
Which of these elements form amphoteric oxides?
(a) A and D
(b) B and D
(c) A and C
(d) B and A
Answer: (b) B and D

SA I 

Question. A clear solution of slaked lime is made by dissolving \( Ca(OH)_2 \) in an excess of water. This solution is left exposed to air. The solution slowly goes milky as a faint white precipitate forms. Explain why a faint white precipitate forms, support your response with the help of a chemical equation.
Answer: When slaked lime solution [\( Ca(OH)_2 \)] is left exposed to air, it reacts with the carbon dioxide present in the air. This reaction produces Calcium Carbonate (\( CaCO_3 \)), which is a white precipitate and is insoluble in water, making the solution appear milky.
Equation: \( Ca(OH)_{2(aq)} + CO_{2(g)} \rightarrow CaCO_{3(s)} + H_2O_{(l)} \)

Question. OR
Keerti added dilute Hydrochloric acid to four metals and recorded her observations as shown in the table given below:
Metal | Gas Evolved
Copper | Yes
Iron | Yes
Magnesium | No
Zinc | Yes
Select the correct observation(s) and give chemical equation(s) of the reaction involved.
Answer: The correct observations are for Iron and Zinc. Copper does not react with dilute acids to evolve hydrogen, and Magnesium reacts very vigorously to evolve hydrogen.
Reactions:
\( Fe_{(s)} + 2HCl_{(aq)} \rightarrow FeCl_{2(aq)} + H_{2(g)} \)
\( Zn_{(s)} + 2HCl_{(aq)} \rightarrow ZnCl_{2(aq)} + H_{2(g)} \)

How do Metals and Non-metals React?

Question. The table shown below gives information about four substances: A, B, C and D.
Substance | Melting point (K) | Electrical conductivity (Solid) | Electrical conductivity (Liquid/aqueous)
A | 295 | Good | Good
B | 1210 | Poor | Good
C | 1890 | Poor | Good
D | 1160 | Poor | Poor
Identify ionic compounds from the above given substances.
(a) A, B
(b) B, C
(c) A, B, D
(d) A, C, D
Answer: (b) B, C

SA II 

Question. (a) Explain the formation of calcium chloride with the help of electron dot structure. (At. numbers: Ca = 20; Cl = 17)
(b) Why do ionic compounds not conduct electricity in solid state but conduct electricity in molten and aqueous state?
Answer: (a) Calcium (20) electronic configuration: 2, 8, 8, 2. It loses 2 electrons to form \( Ca^{2+} \). Chlorine (17) electronic configuration: 2, 8, 7. It gains 1 electron to form \( Cl^{-} \). Two chlorine atoms accept one electron each from one calcium atom.
\( Ca \cdot \cdot + 2 \cdot \ddot{Cl} : \rightarrow [Ca]^{2+} 2[\cdot \ddot{Cl} :]^{-} \rightarrow CaCl_2 \)
(b) In solid state, ions are held in fixed positions by strong electrostatic forces and are not free to move. In molten or aqueous state, these forces are weakened or broken, allowing ions to move freely and conduct electricity.

Corrosion

Two students decided to investigate the effect of water and air on iron object under identical experimental conditions. They measured the mass of each object before placing it partially immersed in 10 mL of water. After a few days, the objects were removed, dried and their masses were measured. The table shows their results.
Student | Object | Mass of Object before Rusting in g | Mass of the coated object in g
A | Nail | 3.0 | 3.15
B | Thin plate | 6.0 | 6.33

Question. What might be the reason for the varied observations of the two students?
Answer: The thin plate has a larger surface area exposed to air and water compared to the nail. Rusting is a surface phenomenon, so a larger surface area allows for more extensive reaction, leading to a greater proportional increase in mass.

Question. In another set up the students coated iron nails with zinc metal and noted that, iron nails coated with zinc prevents rusting. They also observed that zinc initially acts as a physical barrier, but an extra advantage of using zinc is that it continues to prevent rusting even if the layer of zinc is damaged. Name this process of rust prevention and give any two other methods to prevent rusting.
Answer: This process is called Galvanisation. Two other methods are painting and alloying (e.g., making stainless steel).

Case Based Questions

Read the passage given below and answer the following questions :
The chemical properties of metals are mostly linked with the electron releasing tendency of their atoms. Greater the tendency, more will be the reactivity of the metal. They react with oxygen, water, hydrogen, acids, etc. Since they can lose electrons, they act as reducing agents. Some reactions of metals are given as :
Metal + Oxygen \(\rightarrow\) Metal oxide
Metal + Water \(\rightarrow\) Metal hydroxide + Hydrogen
Metal + Acid(dilute) \(\rightarrow\) Metal salt + Hydrogen
Metal X + Salt solution of metal Y \(\rightarrow\) Salt solution of X + Y (Displacement reaction)

Question. Name two metals that react so vigorously that they catch fire when kept in open.
Answer: Sodium (Na) and potassium (K).

Question. Name a metal that does not react with dilute HCl.
Answer: Copper (Cu)

Question. What will happen when silver and zinc are added to two separate test tubes each containing \( FeSO_4 \) solution?
Answer: When Ag is added to \( FeSO_4 \) solution, no reaction will take place since Fe is more reactive than Ag.
When Zn is added to \( FeSO_4 \) solution, the green solution of \( FeSO_4 \) becomes colourless due to formation of \( ZnSO_4 \) since Zn is more reactive than Fe.

Question. OR
(a) Give reason why are food cans coated with tin and not zinc?
(b) Why does an iron nail gain a brown coating when dipped in \( CuSO_4 \) solution?
Answer: (a) Zinc is more reactive as compared to tin and can react with the food items inside the can.
(b) Fe, being more reactive than Cu, displaces Cu from \( CuSO_4 \) solution. Cu gets deposited over the iron nail in the form of reddish-brown coating.

Read the passage given below and answer the following questions :
Non-metals are highly electronegative in nature. They have a tendency to gain electrons in their valence shell to achieve nearest noble gas configuration. Thus, they form anions and act as good oxidising agents.
\( X + ne^{-} \rightarrow X^{n-} \)
(non-metal atom) (anion)
They react with air or oxygen on heating to form oxides which react with water to form acids. Thus, non-metal oxides are acidic in nature. Non-metals do not react with dilute acids at all. This is because they are electronegative and therefore, cannot displace hydrogen from acids but they form covalent hydrides when heated with hydrogen.

Question. The acid formed when sulphur trioxide reacts with water is
(a) sulphurous acid
(b) sulphuric acid
(c) both (a) and (b)
(d) none of the options.
Answer: (b) sulphuric acid

Question. An element ‘X’ forms an oxide \( XO_2 \), which is a very useful gas used in the process of photosynthesis. The element ‘X’ is
(a) sulphur
(b) nitrogen
(c) carbon
(d) phosphorus.
Answer: (c) carbon

Question. Non-metals generally act as
(a) oxidising agents
(b) reducing agents
(c) both (a) and (b)
(d) none of the options.
Answer: (a) oxidising agents

Question. Which of the following elements produces basic oxide on reacting with oxygen?
(a) Chlorine
(b) Sulphur
(c) Phosphorus
(d) Magnesium
Answer: (d) Magnesium

Question. Which of the following is a covalent hydride?
(a) \( CH_4 \)
(b) \( NH_3 \)
(c) \( H_2S \)
(d) All of the options
Answer: (d) All of the options

A & R Questions

Question. Assertion : Elements Cu, Ag, Au, Pt, noble gases, etc. occur in native state in nature.
Reason : Elements which are not attacked by moisture, oxygen and \( CO_2 \) of air occur in native state.
(a) Assertion and reason both are correct statements and reason is correct explanation for assertion.
(b) Assertion and reason both are correct statements but reason is not correct explanation for assertion.
(c) Assertion is correct statement but reason is wrong statement.
(d) Assertion is wrong statement but reason is correct statement.
Answer: (a) Assertion and reason both are correct statements and reason is correct explanation for assertion.

Question. Assertion : Zinc is used in the galvanization of iron.
Reason : Its coating on iron articles increases their life by protecting iron from rusting.
(a) Assertion and reason both are correct statements and reason is correct explanation for assertion.
(b) Assertion and reason both are correct statements but reason is not correct explanation for assertion.
(c) Assertion is correct statement but reason is wrong statement.
(d) Assertion is wrong statement but reason is correct statement.
Answer: (a) Assertion and reason both are correct statements and reason is correct explanation for assertion.

Question. Assertion : Different metals have different reactivities with water and dilute acids.
Reason : Reactivity of a metal depends on its position in the reactivity series.
(a) Assertion and reason both are correct statements and reason is correct explanation for assertion.
(b) Assertion and reason both are correct statements but reason is not correct explanation for assertion.
(c) Assertion is correct statement but reason is wrong statement.
(d) Assertion is wrong statement but reason is correct statement.
Answer: (a) Assertion and reason both are correct statements and reason is correct explanation for assertion.

Multiple Choice Questions

Question. Identify the incorrect statements.
(i) In their pure state, metals have a shining surface.
(ii) Metals are generally soft.
(iii) Some metals can be beaten into thin sheets.
(iv) The ability of metals to be drawn into thin wires is called malleability.
(a) (i) and (iii)
(b) (i) and (iv)
(c) (ii) and (iv)
(d) (ii) and (iii)
Answer: (c) (ii) and (iv)

Question. State True (T) or False (F) for the given statements.
(i) Metals are good conductors of heat and have low melting points.
(ii) The best conductors of heat are silver and copper.
(iii) Lead and mercury are comparatively poor conductors of heat.
(iv) Metals are used for making cooking vessels.
(a) (i)-F, (ii)-T, (iii)-F, (iv)-T
(b) (i)-T, (ii)-F, (iii)-T, (iv)-F
(c) (i)-F, (ii)-T, (iii)-T, (iv)-F
(d) (i)-F, (ii)-T, (iii)-T, (iv)-T
Answer: (d) (i)-F, (ii)-T, (iii)-T, (iv)-T

Question. In an experiment each of the metals Al, Zn, Cu and Fe are reacted with \( ZnSO_4 \), \( CuSO_4 \), \( Al_2(SO_4)_3 \) and \( FeSO_4 \) solutions. The results are tabulated as follows where ‘\(\checkmark\)’ means that the reaction occurs and ‘\(\times\)’ means no reaction occurs.
[A table showing reactivity data: Al reacts with Zn, Cu, and Fe salts; Zn reacts with Cu and Fe salts; Fe reacts only with Cu salt; Cu reacts with none.]
On the basis of the above results, the order of the reactivity of the metals may be concluded as
(a) Zn > Fe > Al > Cu
(b) Al > Fe > Zn > Cu
(c) Zn > Al > Fe > Cu
(d) Al > Zn > Fe > Cu
Answer: (d) Al > Zn > Fe > Cu

VSA Type Questions

Question. Which of the following elements is a metal: \( ^{23}_{11}X \) or \( ^{19}_{9}Y \)?
Answer: Based on the atomic number, the electronic configurations of the two elements X and Y are:
\( ^{23}_{11}X \) (Z = 11) : K = 2, L = 8, M = 1
\( ^{19}_{9}Y \) (Z = 9) : K = 2, L = 7
Metals usually have 1 to 3 electrons in their outermost shell (when all inner shells are complete). Therefore, element \( ^{23}_{11}X \) is a metal.

Question. Define ores.
OR
Define gangue.
Answer: The minerals from which metals can be conveniently and profitably extracted are called ores.
OR
Gangue represents the earthy impurities such as mud, sand and clay associated with the mineral.

SA I Type Questions

Question. Give reasons :
(i) Metals displace hydrogen from acids whereas non-metals do not.
(ii) Carbonate ores are usually converted into oxide.
Answer: (i) Non-metals cannot supply electrons to convert \( H^{+} \) to \( H_2 \) gas whereas metals can give electrons to \( H^{+} \) to convert it into \( H_2 \).
\( 2H^{+} + 2e^{-} \rightarrow H_2 \)
(ii) It is easier to reduce oxides to metals as compared to carbonates.

Question. What are the main differences between calcination and roasting?
Answer:

• Calcination: It is carried out in the absence of oxygen. As a result of calcination, carbonate ore is converted to the oxide. Example: \( ZnCO_3 \xrightarrow{\text{Heat}} ZnO + CO_2 \)


• Roasting: It is carried out in the presence of excess oxygen. As a result of roasting, the sulphide ore is converted to the oxide. Example: \( 2ZnS + 3O_2 \rightarrow 2ZnO + 2SO_2 \)

Question. What are strategic metals? Give one example also.
Answer: The metals which are essential for the country’s economy or its defence are called strategic metals. These metals and their alloys are used in atomic energy, space science projects, jet engines, high grade steel, etc. For example, titanium.

Question. State the reason for the following behaviour of zinc metal :
On placing a piece of zinc metal in a solution of mercuric chloride, it acquires a shining silvery surface but when it is placed in a solution of magnesium sulphate no change is observed.
Answer: When piece of zinc metal is placed in a mercuric chloride solution, it displaces mercury from its solution. This is because zinc is more reactive than mercury.
\( Zn_{(s)} + HgCl_{2(aq)} \rightarrow ZnCl_{2(aq)} + Hg_{(l)} \)
The displaced mercury gives shining silvery surface. However, when zinc is placed in a solution of magnesium sulphate, no reaction takes place. This is because magnesium is more reactive than zinc.

Question. (a) Name a liquid metal which is good conductor of electricity.
(b) If pure water is used, no electrolysis takes place. Why?
Answer: (a) Hg (Mercury) is a liquid metal which is a good conductor of electricity.
(b) Pure water does not dissociate into ions on passing electric current.

Question. OR
An element E combines with oxygen to form an oxide \( E_2O \) which is a good conductor of electricity. Give the following information :
(a) How many electrons will be present in the valence shell of element E ?
(b) Write the formula of the compound formed when the element E combines with chlorine.
Answer: (a) From the formula \( E_2O \), it is clear that the valency of element E is one. This means that it has only one electron in the valence shell.
(b) Chlorine is a monovalent element. Since the valency of the element E is also one, thus the formula of the chloride of the element is ECl.

SA II Type Questions

Question. How does the term ore differ from mineral? Give an example.
Answer: Minerals are naturally occurring chemical compounds of a metal, which may be associated with more impurities. Ore is a chosen mineral of metal, from which metal is extracted profitably on a large scale, in pure form e.g., the ore of iron is haematite from which iron is extracted while the minerals having iron may be magnetite, iron pyrites, etc.

Question. State reasons for the following :
(i) Aluminium oxide is called an amphoteric oxide.
(ii) Hydrogen gas is not evolved when most metals react with nitric acid.
(iii) Nitrogen is used to preserve food.
Answer: (i) Aluminium reacts with acids as well as bases, therefore, it is called amphoteric oxide.
(ii) Nitric acid is a strong oxidising agent. Hence, it oxidises the hydrogen formed during the reaction to water.
(iii) Nitrogen prevents food from getting oxidised due to its inert nature hence it is used as a preservative.

Question. What is aqua regia? Why are gold and platinum soluble in it?
Answer: Aqua regia, also known as royal water is a mixture of concentrated hydrochloric acid and concentrated nitric acid mixed in the ratio of 3 : 1 by volume. The acids combine to evolve nascent chlorine (Cl) which reacts with both platinum and gold. It is used in cleaning the surface of gold. It removes the dull layer from the surface of gold giving it a shining look.
\( 3HCl + HNO_3 \rightarrow NOCl + 2H_2O + 2Cl \)
(Aqua regia) (Nitrosyl chloride) (Nascent chlorine)
\( Au + 3Cl \rightarrow AuCl_3 \)
\( Pt + 4Cl \rightarrow PtCl_4 \)

Question. A, B and C are three elements which undergo chemical changes according to following equations :
(i) \( A_2O_3 + 2B \rightarrow B_2O_3 + 2A \)
(ii) \( 3CSO_4 + 2B \rightarrow B_2(SO_4)_3 + 3C \)
(iii) \( 3CO + 2A \rightarrow A_2O_3 + 3C \)
(a) Which of these is most reactive?
(b) Which of these is least reactive?
(c) Arrange these elements in order of increasing reactivity.
Answer: We know that a more reactive metal can displace a less reactive metal from its salt solution or oxide. On this basis, we can say:
(i) Metal B displaces metal A from its oxide \( A_2O_3 \), therefore, B is more reactive than A.
(ii) Metal B displaces metal C from its salt \( CSO_4 \), therefore, B is more reactive than C.
(iii) Metal A displaces metal C from its oxide, therefore, A is more reactive than C.
Thus, it may be concluded.
(a) B is most reactive metal.
(b) C is least reactive metal.
(c) Order of increasing reactivity is C < A < B.

Question. OR
Give reasons for the following :
(a) Zinc can displace copper from copper sulphate solution.
(b) Silver articles become black after sometime when exposed to air.
(c) A metal sulphide is converted to its oxide to extract the metal from a sulphide ore.
Answer: (a) Zinc, being more reactive, displaces copper from its salt solution.
\( Zn_{(s)} + CuSO_{4(aq)} \rightarrow ZnSO_{4(aq)} + Cu_{(s)} \)
(b) The silver metal reacts with \( H_2S \) present in atmosphere and forms silver sulphide. Because of this layer of silver sulphide they appear black.
(c) This is so, because it is easier to extract a metal from its oxide than its sulphide form.

LA Type Questions

Question. Name the following :
(i) A molten metal that catches fire in chlorine gas and gives off white fumes.
(ii) A metal that forms two types of oxides and rusts in moisture, write the formulae of oxides.
(iii) A metal used in hot water apparatus.
(iv) A metal used in long distance cable wires.
(v) A metal added to gold to harden it.
Answer: (i) Sodium in molten form catches fire in chlorine to give white fumes.
(ii) Iron forms two oxides: iron (II) oxide and iron (III) oxide. It rusts in moisture. The formulae of oxides are FeO and \( Fe_2O_3 \).
(iii) Copper is used in hot water apparatus since it is a good conductor of heat.
(iv) Aluminium is used for long distance cable wires since it is a light metal.
(v) Copper is added to gold to harden it.

Question. (a) Explain the corrosion of iron.
(b) Why is it that aluminium which is more reactive than iron does not corrode like iron?
(c) How is corrosion of iron prevented by coating it with a layer of oil?
Answer: (a) Corrosion of iron means that when iron is exposed to moist air for a long period of time, its surface acquires a coating of brown flaky substance called rust. Rust is mainly hydrated iron (III) oxide (\( Fe_2O_3 \cdot xH_2O \)).
(b) Although aluminium is more reactive than iron, yet it does not corrode like iron. The reason being that when aluminium reacts with moist air, its surface is coated with a thin impervious layer of aluminium oxide which protects the metal underneath from further damage. In contrast, iron when exposed to moist air, is covered with a brown flaky substance called rust. Rust slowly falls from the surface exposing fresh surface for rusting. Thus, corrosion of iron is a continuous process which ultimately eats up the whole iron object.
(c) Corrosion of iron is prevented by coating it with a layer of oil. The reason being that the layer of oil does not allow air and water to reach the surface of iron.

Question. Why is galvanisation considered better than tinning? Name and briefly explain two other methods that can be used for prevention of rusting.
Answer: Galvanisation and tinning are very common for preventing rusting of iron. However, galvanisation is better than tinning. If we look at the reactivity series, the order in which these metals appear is : Zinc (Zn), Iron (Fe) and Tin (Sn).
Now, if some scratches appear on the surface of iron article, coating of zinc will be still effective because zinc will be corroded in preference to iron since it is placed above it. However, if same happens in case of tinning, then iron will be corroded in preference to tin because it is more reactive than tin. This means that corrosion will again start. Thus, galvanisation is always better than tinning.
Two other methods used to prevent rusting can be described as follows :
(i) By applying paint : Materials like railings, iron gates, iron bridges, bodies of cars, buses and trucks, etc. are all painted to protect them from rusting. Painting the metal surface does not allow them to come in contact with the moist air and thus, prevents rusting.
(ii) Greasing and oiling : When some grease or oil is applied on the surface of an iron object, then moisture and air cannot come in contact with it and hence, rusting is prevented.

Question. Give reasons for the following :
(a) Silicon counts among metalloids.
(b) Carbon is not used for making aluminium from aluminium oxide.
(c) Metals conduct electricity.
(d) For making gold ornaments, 22-carat gold is preferred our 24-carat gold.
(e) Na, K and Ca metals form hydrides by combination with hydrogen gas, but most of the other metals do not.
Answer: (a) Silicon shows characteristics of both the metals and the non-metals. Therefore, it is a metalloid.
(b) Carbon cannot reduce aluminium oxide because aluminium has more affinity for oxygen than carbon.
(c) Metals have electrons which are free to move.
(d) Pure gold or 24-carat gold is very soft, therefore, it is not suitable for making jewellery.
(e) Active metals like Na, K and Ca can force the hydrogen atom to accept electrons to form salts called hydrides.

Question. OR
(i) What happens when
(a) Calcium reacts with water
(b) Iron reacts with steam
(c) Magnesium reacts with hot water
(d) Sodium reacts with water
(ii) Arrange the metals given in (i) in order of decreasing activity.
Answer: (i) (a) Calcium reacts with water to form calcium hydroxide and hydrogen gas.
\( Ca_{(s)} + 2H_2O_{(l)} \rightarrow Ca(OH)_{2(aq)} + H_{2(g)} \)
(b) Iron reacts with steam to form iron (II, III) oxide and hydrogen.
\( 3Fe_{(s)} + 4H_2O_{(g)} \rightarrow Fe_3O_{4(s)} + 4H_{2(g)} \)
(c) Magnesium reacts with hot water to form magnesium hydroxide and hydrogen.
\( Mg_{(s)} + 2H_2O_{(l)} \rightarrow Mg(OH)_{2(aq)} + H_{2(g)} \)
(d) Sodium reacts with water to form sodium hydroxide and hydrogen gas. The reaction is so violent that the evolved hydrogen immediately catches fire.
\( Na_{(s)} + H_2O_{(l)} \rightarrow NaOH_{(aq)} + 1/2 H_{2(g)} \)
(ii) The arrangement of metals in decreasing order of activity is : Na > Ca > Mg > Fe