Metals and Non-metals

• About 118 elements are known today. There are more than 90 metals, 22 non metals and a few metalloids.

• Sodium(Na), potassium(K), magnesium(Mg), aluminium(Al), calcium(Ca), Iron(Fe), Barium(Ba) are some metals.

• Oxygen(O), hydrogen(H), nitrogen(N), sulphur(S), phosphorus(P), fluorine(F), chlorine(Cl), bromine(Br), iodine(l) are some non-metals

Physical properties of metals:

• Solid at room temperature except mercury

• Ductile(drawn in to wires)

• Malleable(beaten in to thin sheets)

• Sonorous(produce sound)

• Lustrous(natural shine)

• Have high melting point.Cesium and gallium have very low melting point.

• Generally good conductor of heat and electricity, except lead and mercury which are comparatively poor conductors. Silver and copper are best conductors.

• Have high density.Sodium and potassium can be cut with knife,they have low density.

Physical properties of non-metals:

• Occur as solid or gas. Bromine is liquid.

• Generally bad conductors of heat and electricity. Graphite a natural form of carbon is a good conductor.

• Non-sonorous.

• Non-lustrous, only iodine has lustre.

• Metals form basic oxides like Magnesium oxide(MgO), while non-metals form acidic oxides (as in acid rain).

Chemical properties of metals:

1. Reaction with air

Metals can burn in air, react or don’t react with air.

Metal + oxygen → Metal Oxide

• Some metals like Na and K are kept immersed in kerosene oil as they react vigorously with air and catch fire.

• Some metals like Mg, Al, Zn, Pb react slowly with air and form a protective layer.

• Mg can also burn in air with a white dazzling light to form its oxide

• Fe and Cu don’t burn in air but combine with oxygen to form oxide.When heated iron filings burn when sprinkled over flame.

• Metals like silver, platinum and gold don’t burn or react with air.

2Na + O2 → Na2O

2Mg + O2 → 2MgO

2Cu + O2 → 2CuO

4Al + 302 → 2Al2O3

Amphoteric Oxides : metal oxides which react with both acids as well as bases to form salt and water e.g. Al2O3,

ZnO. Al2O3 + HCl → AlCl3 + H2O

Al2O3 + NaOH → NaAlO2 + H2O

2. Reaction with water

Metal + Water → Metal Hydroxide + Hydrogen gas

Metal + Steam → Metal oxide + Hydrogen gas

Na + H2O → NaOH + H2

K + H2O → KOH + H2

Ca + H2O → Ca(OH)2 + H2

Mg + H2O → Mg(OH)2 + H2

In case of Ca and Mg, the metal starts floating due to bubbles of hydrogen gas sticking to its surface.

Al + H2O → Al2O3 + H2

Fe + H2O → Fe3O4 + H2

Try Balancing these Chemical equations yourself

3. Reaction with dilute acids

Metal + dilute acid → Salt + Hydrogen gas

Metals react with dilute hydrochloricacid and dilute sulphuric acid to form salt and hydrogen gas.

Fe + 2HCl → FeCl2 + H2

Mg + 2HCl → MgCl2 + H2

Zn + 2HCl → ZnCl2 + H2

2Al + 6HCl → 2AlCl3 + 3H2

Copper, mercury and silver don’t react with dilute acids.

Hydrogen gas produced is oxidised to water when metals react with nitric acid. But Mg and Mn, react with very dilute nitric acid to evolve hydrogen gas.

Mg + 2HNO3 → Mg(NO3)2 + H2

4. Reaction of metals with other metal salts

Salt Salt Metal A + solution → solution + Metal B of B of A

All metals are note equally reactive.Reactive metals can displace less reactive metals from their compounds in solution. This forms the basis of reactivity series of metals.

Reactivity series is a list of metals arranged in order of their decreasing activities.

K, Na, Ca, Mg, Al, Zn, Fe, Sn, Pb, H, Cu, Hg, Ag, Au

Fe + CuSO4 → FeSO4 + Cu

Zn + CuSO4 → ZnSO4 + Cu

Reaction between Metals and Non-Metals

Reactivity of elements can be understood as a tendency to attain a completely filled valence shell.

• Atom of metals can lose electrons from valence shells to form cations (+ve ions).

• Atom of non-metals gain electrons in valence shell to form an ions(–ve ions).

• Oppositely charged ions attract each other and are held by strong electrostatic forces of attraction forming ionic compounds.

Formation of MgCl2

Mg → Mg2+ + 2e–

2,8,2 2,8(Magnesium ion)

Cl2 + 2e– → 2Cl– 2,8,7 2,8,8 (Chloride ion)

Properties of Ionic Compounds

• Are solid and mostly brittle.

• Have high melting and boiling points. More energy is required to break the strong inter-ionic attraction.

• Generally soluble in water and insoluble in kerosene,petrol.

• Conduct electricity in solution and in molten state.In both cases,free ions are formed and conduct electricity.

Occurrence of Metals Minerals

elements of compounds occurring naturally are minerals.

ORES

mineral from which metal can be profitably extracted is an ore.

For example, sulphide ore, oxide ore, carbonate ore.

• Metals at the bottom of activity series like gold, platinum, silver, copper generally occur in free state.But copper and silver also occur in sulphide and oxide ores.

• Metals of medium reactivity(Zn,Fe,Pb etc.)occur mainly as oxides, sulphides or carbonates.

• Metals of high reactivity(K, Na, Ca, MgandAl) are very reactive and thus found in combined state.

GANGUE

Ores are naturally found mixed impurities like soil, sand, etc. called gangue.
The gangue is removed from the ore. METALLURGY: step-wise process of obtaining metal from its ore.

• Enrichment of ore

• Obtaining metal from enriched ore.

• Refining of impure metal to obtain pure metal.

Extracting Metals Low in the Activity Series

By heating the ores in air at high temperature.

Mercury from cinnabar

2HgS + 3O2 →Heat 2HgO + 2SO2
2HgO →Heat 2Hg + O2

Copper from copper sulphide

Cu2S + 3O2 →Heat 2Cu2O _ 2SO2
2Cu2O + Cu2S →Heat 6Cu + SO2

Extracting Metals in the Middle of Activity Series

• Metals are easier to obtain from oxide ores,thus, sulphide and carbonate ores are converted into oxides.

• Metal ore heated stronglyin excess of air (Roasting)
2ZnS + 3O2 →Heat 2ZnO + 2SO2

Metal ore heated strongly in limited or no supply of air (Calcination)

ZnCO3 →Heat ZnO + CO2

Reduction of Metal Oxide

1.Using Coke : Coke as a reducing agent.

ZnO + C →Heat Zn + CO

2. Using displacement method : highly reactive metal like Na,Ca and Al are used to displace metals of lower reactivity from their compounds.

MnO2 + 4Al →Heat 3Mn + 2Al2O3 + heat
Fe2O3 + 2Al →Heat 2Fe + Al2O3 + heat

• In the above reaction molten iron is formed and is used to join railway tracks. This is called thermit reaction.

Extracting Metals at the Top of Activity Series

These metals

• have more affinity for oxygen than carbon.

• are obtained by electrolytic reduction.

Sodium is obtained by electrolysis of its molten chloride

NaCl → Na+ + Cl–

As electricity is passed through the solution metal gets deposited at cathode and non-metal at anode.

• At cathode :

Na+ + e– → Na –

• At anode :

2Cl– → Cl2 + 2e–

Refining of Metals : –

Impurities present in the obtained metal can be removed by electrolytic refining. Copper is obtained using this method.

Following are present inside the electrolytic tank.

• Anode

• slab of impure copper

• Cathode

• slab of pure copper

• Solution

• aqueous solution of copper sulphate with some dilute sulphuric acid

• From anode copper ions are released in the solution and equivalent amount of copper from solution is deposited at cathode.

• Impurities containing silver and gold gets deposited at the bottom of anode as anode mud.

Corrosion

• Metals are attacked by substances in surroundings like moisture and acids.
• Silver- it reacts with sulphur in air to form silver sulphide and articles become black.
• Copper – reacts with moist carbon dioxide in air and gains a green coat of copper carbonate.
• Iron-acquires a coating of a brown flaky substance called rust. Both air and moisture are necessary for rusting of iron.

Prevention of corrosion

Rusting of iron is prevented by painting,oiling,greasing,galvanizing,chrome plating, anodizing and making alloys.

• In galvanization, iron or steel is coated with a layer of zinc because zinc is preferably oxidized than iron.

Alloys

These are mixture of metals with metals or non-metals

• Adding small amount of carbon makes iron hard and strong.

• Stainless steel is obtained by mixing iron with nickel and chromium. It is hard and doesn’t rust.

• Mercury is added to other metals to make amalgam.

Brass

alloy of copper and zinc.
Bronze : alloy of copper and tin.

• In brass and bronze, melting point and electrical conductivity is lower than that of pure metal.

Solder : alloy of lead and tin has low melting point and is used for welding electrical wires.

NCERT Solution

Question 1:

Define amphoteric oxides. Give two examples.

Answer:

Oxides that react with both acids and bases to form salt and water are known as amphoteric oxides.

Examples:  PbO and Al2O3.

Question 2:

Write two metals that

  1. Displaces hydrogen from dilute acids.
  2. Does not displaces hydrogen from dilute acids.

Answer:

  1. Mg and Zn as they are very reactive.
  2. Ag, Cu, etc. as they are less reactive.

Question 3:

In the process of electrolytic refining of a metal called ‘M’, what will be considered as?

  1. The anode
  2. The cathode
  3. The electrolyte

Answer:

  1. Impure and thick block of metal M.
  2. Thin strip or wire of pure metal M.
  3. Suitable salt solution of metal M.

Question 4:

How to prevent iron from the process of rusting? Give two ways.

Answer:

  1. Rust proof paints can be used as a coat on the surface of iron.
  2. Oil/grease can be applied on the surface of iron objects as it will prevent the iron surface to get in contact with air consisting of moisture.



Question 5:

When non-metals combine with oxygen, what types of oxides are formed?

Answer:

It forms either acidic or neutral oxides. N2O5 or N2O3 is an acidic oxide; CO is a neutral oxide.

Question 6:

Give reasons for the following statements below:

  1. Metals replace hydrogen from dilute acids and non-metals do not.
  2. During the process of extraction, ores of sulphide and carbonate are converted into oxides.

Answer:

  1. As metals readily lose electrons they are electropositive in nature. These electrons reduce the number of protons liberated from the acid to further liberate hydrogen gas, whereas the non-metals gain electrons and they do not allow liberation of electrons to protons from acids. Therefore, H2 gas cannot be liberated.
  2. Before reduction takes place metal sulphides and carbonates has to be converted to oxides because it is easier to reduce metal oxides to metal.

Question 7:                                                                                                                                     On the basis of chemical properties, write the difference between metals and non-metals.

Answer:

MetalsNon-metals
When metals are heated with oxygen, they form ionic oxides which are basic in nature and dissolve in water to form of bases, turning red litmus to blue.When non-Metals are heated with oxygen, they form covalent oxides which are acidic in nature and dissolve in water to form acids, turning blue litmus to red.
They are electro positive, lose electrons readily and become a positive ion.They are electro negative, gain electrons and become negative ions.
Metals are lustrous.Non-metals are non-lustrous; exception is graphite.
Reducing agents.Good oxidizing agents.
Metals are conductor of electricity and heat.Non-metals are non-conductor of electricity and heat; exception is graphite.
All metals are solid except mercury.Non-metals are solid-liquid gaseous

Question 8:

Why do the surface of few metals have a dull appearance when they are exposed to air for a long period of time ?

Answer:

This is because metals undergo surface oxidation when exposed to moist air. E.g. Copper (Cu) turns green on its surface because of the formation of copper carbonate (Cu(OH)2.CuCO3). Silver turns black on its surface because of the formation of black Ag2S. Aluminum turns white on its surface because of Al2O3.

Question 9:

In the following metals which will give hydrogen when it is added to dilute hydrochloric acid.

  1. Iron
  2. Copper
  3. Magnesium

Answer:

Copper (Cu) does not react with dilute hydrochloric acid which means that copper is less reactive than iron.

Fe + 2HCl -> FeCl2 + H2

Mg + 2HCl -> MgCl2 + H2

Cu + HCl -> No reaction

Question 10:

Write down a non-metallic element that conducts electricity.

Answer:

Carbon in the form of graphite conducts electricity because of the free electron in each of the carbon atom, as it moves freely in between the hexagonal layers.

Question: 11

Write down the name of metals that do not corrode easily.

Answer:

Noble metals like gold, platinum, etc. do not corrode in air.

Question 12:

Define alloys.

Answer:

Homogenous mixtures of two or more metals, or a metal and a non-metal are known as alloys. E.g. brass, steel, bronze and etc.

 Question 13:

What do you mean by following terms ?

  1. Minerals
  2. Ores
  3. Gangue

Answer:

  1. Minerals are compounds also known as elements, which are naturally in the earth’s crust. E.g. Alums, K2SO.Al2(SO4)3 . 24H2O, etc.
  2. Ores are minerals from which metal can be extracted and as a result they are called ores. Bauxite Al2O3.2H2O is the ore of Al, copper pyrite CuFeS2.All minerals are not considered as ores but all ores are also minerals.
  3. Ores mined from the earth are naturally contaminated with sand rocky materials. There are impurities present in the ore which is known as gangue.



Question 14:

Name two metals that are found in nature which are in the free state.

Answer:

In nature, Platinum and Gold are found to be in the free state.

Question 15:

While obtaining a metal from its oxide, what is the chemical process used ?

Answer:

ZnO + C -> Zn + CO

PbO +C -> Pb + CO

Question 16:

Name two metals which can form hydrides with metals.

Answer:

Two metals that can form stable hydrides on reacting with hydrogen are sodium and calcium.

Question 17:

Explain that “Every mineral have a definite and a fixed composition”.

Answer:

Mineral can be widely distributed in the earth’s crust in the form of carbonates, oxides, sulphates, sulphides, nitrates, etc. These minerals can be formed as a result of chemical changes taking place during the formation of the earth. Therefore we can say that all minerals have a definite and fixed composition.

Question 18:

Define the terms:

Answer:

  1. Malleable is being able to be beaten/hammered into thin sheets.
  2. Ductile is being able to be drawn into thin wires.

Question: 19

  1. Write the electron dot structure or sodium, oxygen and magnesium.
  2. Show the formation of MgO and Na2O by the transfer of electrons.
  3. What are the ions present in these compounds ?

Answer:

1.Sodium:

3.Magnesium:

2. Formation of Magnesium oxide:

When magnesium reacts with oxygen, the magnesium atom transfers its two outermost electrons to an oxygen atom. By losing two electrons, the magnesium atoms form a magnesium ion (Mg2+) and by gaining two electrons, the oxygen atom forms an oxide ion (O2-).

Mg:      + O2        -> MgO

Formation of Sodium oxide:

Two sodium atoms transfer their 2 outermost electrons to an oxygen atom. By losing two electrons, the two sodium atoms form tow sodium ions ( 2Na+). And by gaining two electrons, the oxygen atom forms an oxide ion (O2-).

3.The ions present in sodium oxide compound (Na2O) are sodium ions (2Na+) and oxide ions (O2-).

The ions present in Magnesium oxide compound (MgO) are magnesium ions Mg2+ and oxide ions (O2-).

Question 20:

Tarnished copper vessels are being cleaned with lemon or tamarind juice. Explain why these sour substances are effective in cleaning the vessels.

Answer:

These sour substances contain acids. These acids dissolve the coating of copper oxide or basic copper carbonate present on the surface or tarnished copper vessels and make them shining red-brown again.

Question 21:

Give an example of a metal which –

  1. Is a liquid at room temperature.
  2. Can be easily cut with a knife.
  3. Is the best conductor of heat.
  4. Is a poor conductor of heat.

Answer:

  1. Mercury.
  2. Sodium and potassium.
  3. Silver
  4. Mercury



Question 22:

Why is sodium, kept immersed in kerosene ?

Answer:

Sodium metal is kept immersed in kerosene to prevent their reaction with oxygen, moisture and carbon dioxide of air.

Question 23:

Ionic compounds have high melting points. Why ?

Answer:

Ionic compounds are made up of positive and negative ions. There is a strong force of attraction between the oppositely charged ions, so a lot of heat energy is required to break this force of attraction and melt the ionic compounds. This is why ionic compounds have high melting points.

Question 24:

A man went door to door posing as a goldsmith. He promised to bring back the glitter of old and dull gold ornaments. An unsuspecting lady gave a set of gold bangles to him which he dipped in a particular solution. The bangles sparkled like new but their weight was reduced drastically. The lady was upset but after a futile argument the man beat a hasty retreat. Can you play the detective to find out the nature of the solution he had used?

Answer:

Aqua regia (By volume, this contains three parts of concentrated hydrochloric acid and one part of concentrated nitric acid) is the solution, which is used to sparkle the bangles like new but their weight will be reduced drastically.

Question 25:

Write equations for the reactions of

  1. Iron with water
  2. Calcium and potassium with water.

Answer:

  1. Iron reacts with steam to form magnetic oxide of Fe with the liberation of H2.

3Fe(s) + 4H2O(g) -> Fe3O4(s) + 4H2(g)

2.Calcium reacts with water to form calcium hydroxide and hydrogen.

Ca(s) + 2H2O(I) -> Ca(OH)2(aq) + H2(g)

Potassium reacts with cold water violently immediately with evolution of H2 which catches fire.

2K(s) + 2H2O(I) -> 2KOH(aq) + 2H2(g)

Question 26:

What would you observe when zinc is added to a sodium of iron(II) sulphate? Write the chemical reaction that takes place?

Answer:

Zinc is more reactive (more electro positive) than iron. Therefor it displaces iron from its salt solution. The colour of ferrous sulphate is pale green, which becomes colorless.

FeSO4 + Zn -> ZnSO4 + Fe(s)

Light green     Zinc sulphate

Question 27:

Pratyush took Sulphur powder on a spatula and heated it. He collected the gas evolved by inverting a test tube over the burning Sulphur. What will be the action of this gas on:

  1. Dry litmus paper?
  2. Moist litmus paper?

Write a balanced chemical equation for the reaction taking place.

Answer:

  1. When Sulphur is burnt in air then Sulphur dioxide gas is formed.
  2. Sulphur dioxide gas has no action on dry litmus paper.
  3. Sulphur dioxide gas turns moist blue litmus paper to red.
  4. S(s) + O2(g) -> SO2(g)

Important Questions

1. An element A forms two oxides AO and AO2. The oxide AO is neutral wheras the oxide AO2 is acidic in nature. would you call element A a metal or a non-metal ?

Ans. Element A is a non-metal. Only non-metals form neutral and acidic oxides.

2.  A non-metal X exists in two different forms Y and Z. Y is the hardest natural substance, whereas Z is a good conductor of electricity. Identify X, Y and Z.

Ans. X = Carbon, Y = Diamond, Z = Graphite

3. Give an example of a metal which is a liquid at room temperature.

Ans. Mercury(Hg).

4. Why do silver ornaments lose their shine when kept for sometime ?

Ans. They get tarnished by reacting with atmospheric gases to produce silver sulphide.

5.  Name the metal which has very low melting point and can melt with the heat of you palm.

Ans. Gallium/Caesium.

6. Choose the amphoteric oxides amongst the following :

Na2O, ZnO, Al2O3, CO2, H2O

Ans. ZnO and Al2O3

7. Name one metal which react with very dilute HNO3 to evolve hydrogen gas.

Ans. Magnesium and manganese react with very dilute HNO3 to evolve hydrogen gas.

8. How do we know whether a given element is a non-metal if we know its electronic configuration ?

Ans. If there are four or more than four electrons in the valence cell of the given element, it is a non-metal.

9. What kind of compounds are called ionic compound ?

Ans. Compounds formed by the transfer of electrons from one atom to the other, are called ionic compounds.

10. Define metallurgy.

Ans. The process of extraction of a metal form its ore is called metallurgy.

11. Differentiate between roasting and calcination processes giving one example of each.

Ans. Roasting : Sulphide ore is strongly heated in the presence of air into metal oxide.   2ZnS + 3O2 – 2ZnO + 2SO2

Calcination : Carbonate ore is strongly heated in the absence of air into metal oxide.  ZnCO3 – ZnO + CO2

12. A substance X which is an oxide of a metal is used intensively in the cement industry. This element is present in bones also. On treatment with water it forms a solution which turns red litmus blue. Identify X and also write the chemical reactions involved.

Ans. The substance X is calcium oxide, CaO.

The element calcium is present in bones On treatment with water, it produces calcium hydroxide as under :

CaO + H2O – Ca(OH)2

Calcium hydroxide is a base. It turns red litmus blue.

13. A metal A which is used in thermite process, when heated with oxygen gives an oxide B, which is amphoteric in nature. Identify A and B. Write down the reactions of oxide B with HCl and NaOH.

Ans. The metal A is aluminium Al.

The substance B is Al2O3 which is amphoteric because it reacts with both acid and base.

Al2O3 + 6HCl – 2Al2O3 + 3H2O

Al2O3 + 2NaOH – 2NaAlO2 + H2O

14. Why is sodium kept immersed in kerosene oil ?

Ans. Sodium reacts with ari so vigorously that it catches fire if kept in the open. Hence to protect it and prevent accidental fires, it is kept immersed in kerosene oil. Sodium also reacts with water to evolve hydrogen gas.

15. What are amphoteric oxides ? Give two examples of amphoteric oxide with balanced chemical equation.

Ans. An oxide which reacts with acids as well as bases to produce salt and water is called amphoteric oxide.

Al2O3 + 6HCl – 2AlCl3 +3H2O

Al2O3 + 2NaOH – 2NaAlO2 + H2O

16. Zinc does not evolve hydrogen gas on reacting with HNO3.

Ans. Nitric acid is a strong oxidising agent. It oxidises hydrogen produced to form water and is itself reduced to oxide of nitrogen.

17. Explain why calcium metal after reacting with water starts floating on its surface. Write the chemical equation for the reaction.

Ans. Calcium reacts with water to form hydrogen gas. The bubbles of hydrogen formed stick to the surface of calcium. Hydrogen gas being lighter, calcium starts floating on water.

Ca + 2H2O – Ca(OH)2 + H2

18. Write four important purposes of making alloys.

Ans. To increase hardness, To increase resistance towards corrosion, To modify reactivity, to lower melting point

19. Why do silver articles turn black and copper items turn green after sometime ?

Ans Silver articles turn black after sometime due to the formation of silver sulphide. Ag2S layer on it.

Copper items turn green after sometime due to the formation of copper carbonate layer on it.

20. Metals are said to be shiny. Why do metals generally appear to be dull ? How can their brightness be restored ?

Ans. Metals like copper and silver appear to be dull because of corrosion due to sulphur, carbon dioxide and moisture present in the atmoshphere.

Brightness of the metals can be restored by rubbing with a cloth moistened with a dilute acid.

21. Explain how, mercury is extracted from its sulphide ore cinnabar. Give chemical equations of the reactions involved.

Ans. When cinnabar is heated in air, It is first converted into mercury oxide (HgO). Mercury oxide on further heating is reduced to mercury.

HgS + 3O2 -(Heat) 2HgO + 2SO2

2HgO -(Heat) 2Hg + O2

22. Explain The following terms :

(i) Mineral (ii) Ore (iii) Gangue.

Ans. (i) Mineral : Elements or compounds which occur in their natural form in the earth’s crust are called minerals.

(ii) Ores : Minerals from which a metal can be extracted profitably are called ores.

(iii) Gangue : Impurities such as soil and sand which are present in the minerals or ores are called gangue.

23. What is concentration of an ore ? Why is it necessary to concentrate an ore before processing ? Name an ore of mercury and copper .

Ans. The process of removing unwanted impurities like sand, soil, etc. , present in an ore is called concentration of the ore.

We need to concentrate the ore so that the impurities do not interfere in the extraction of the metal.

Ore of mercury : cinnabar (HgS)

Ore of copper : copper pyrite(Cu2S)

24. Give reasons :

(i) Silver metal does not easily combine with oxygen but silver jewellary tarnishes after sometime.

(ii) Iron grills are frequently painted.

(iii) Gold ornaments retain their luster even after several years of use.

Ans. (i) Silver metal reacts with the H2S gas present in traces in the atmosphere to form silver sulphide which tarnishes the metal.

(ii) Iron forms a layer of rust (Fe2O3.xH2O) on combination with oxygen and moisture present in the atmosphere. This weakens the metal. To prevent this loss iron grills are frequently painted.

(iii) Gold is an unreactive metal. It does not react with atmospheric oxygen and acids. So the lustre of gold jewellery is retained even after several years.

25. A gas is produced when conc. H2SO4 is added to solid sodium chloride taken in a test tube. The gas coming out of the delivery tube is passed over a dry blue litmus paper and then over a moist blue litmus paper. what would you observe ? Explain reason with the help of a chemical equation.

Ans. HCl gas is produced when conc.H2SO4 is added to solid sodium chloride.

NaCl + H2SO4 -NaHSO4 + HCl

HCl gas when passed over dry blue litmus paper shows no effect.

HCl gas when passed over moist blue litmus paper turns it red.

This is because dry HCl gas does not produce H+ ions, therefore there is no action on dry blue litmus paper.

Moist blue litmus paper contains water. HCl gas when passed over moist blue litmus paper reacts with water to form H+ ions. An acid turns blue litmus red. Therefore, a red colour will be obtained.

26. Give reasons for the following :

(i) Carbonate and sulphide ores are usually converted into oxides during the process of extraction of metals.

(ii) Ionic compounds have generally high melting points.

(iii) Hydrogen is not a metal, but it has been assigned a place in the reactivity series of metals.

(iv) The galvanised iron article is protected against rusting even if the zinc layer is broken.

(v) The wires carrying current in homes have a coating of PVC.

Ans. (i) It is easier to convert metal oxides to metals as compared to carbonates and sulphides. Therefore carbonates are calcinated and sulphides are roasted to oxides.

(ii) There are electrostatic forces of attraction between the cations and anions in ionic compounds which are difficult to break. Therefore ionic compounds have high melting points.

(iii) Metals above hydrogen evolve hydrogen gas and metals below hydrogen do not evolve hydrogen when treated with an acid. That is when hydrogen has been assigned a place in the reactivity series of metals.

(iv) Even if zinc layer is broken, it will preferentially oxidised because it is more reactive than iron.

(v) The wires carrying current in homes have a coating of PVC. This is because PVC is an insulating substance and protects from electric shock.