Q. Why are carbon and its compounds used as fuels for most applications?

Ans. Carbon and its compounds are used as fuels for most applications because most of the carbon compounds give a lot of heat and light when burnt in air. Saturated hydrocarbons burn with a clean flame and no smoke is produced. The carbon compounds, used as a fuel, have high calorific values.

Q. A mixture of oxygen and ethyne is burnt for welding. Can you tell why a mixture of ethyne and air is not
used?

Q. Why is the conversion of ethanol to ethanoic acid an oxidation reaction?

Ans. The conversion of ethanol to ethanoic acid involves the addition of oxygen to ethanol. Therefore, it is an oxidation reaction.

$$\underset{\text{Ethanol}}{CH_3CH_3OH} \xrightarrow{[O]} \underset{\text{Ethanoic acid}}{CH_3COOH}$$

Q. Why does micelle formation takes place when soap is added to water? Will a micelle is formed in other solvents such as ethanol also?

Ans. Soap may be represented by the formula RCOONa where R is an alkyl group which represents long chain of carbon with fifteen or more atoms. Oil drops containing dirt particles and water do not mix. Soap helps in their mixing by reducing interfacial tension or friction. Actually it forms a sort of bridge between oil drops and water in which the alkyl portion (hydrophobic end) point towards oil drop while other portion (hydrophilic end) is directed towards water. This is known as micelle formation. Thus, soap helps in the formation of a stable emulsion between oil and water. Ethanol and other similar solvents which are of organic nature do not help in micelle formation because soap is soluble in them.

(i) What are the two properties of carbon which lead to the huge number of carbon compounds we see around us?

(ii) Which element exhibit the property of catenation to maximum extent and why?

Ans. (i) The two features of carbon that give rise to a large number of compounds are as follows:
1. Catenation: The ability to form bonds with other atoms of carbon to form a long chain
2. Tetravalency: With the valency of four, carbon is capable of bonding with four other atoms of carbon or atoms of some other monovalent element such as oxygen, hydrogen, nitrogen, sulphur and chlorine.

(ii) Carbon exhibit the property of catenation to maximum extent. It is due to strong tetravalency of carbon.

(i) Give a test that can be used to differentiate chemically between butter and cooking oil.

(ii) How does graphite acts as a lubricant?

Ans. (i) Butter contains saturated fats. Therefore, it cannot be hydrogenated. On the other hand, oil has unsaturated fats. That is why it can be hydrogenated to saturated fatty solids.

(ii) Graphite is used as a lubricant in the form of graphite powder or mixed with petroleum jelly or with any lubricant oil to form graphite grease.

(i) What will be the formula and electron dot structure of cyclopentane?

(ii) Draw electron dot structure of carbon dioxide and write the nature of bonding between carbon and oxygen in its molecule.

Ans. (i) The formula for cyclopentane is C5H10. Its electron dot structure is given below.

Covalent bond (double bond) is present in between C and O.

(i)  What would be the electron dot structure of a molecule of sulphur which is made up of eight atoms of sulphur?

(ii) What would be the electron dot structure of carbon dioxide which has the formula CO2?

Ans. (i) Electron dot structure of a sulphur molecule.

(ii) Electron dot structure of CO2 is:
Q. Explain the nature of the covalent bond using the bond formation in CH3Cl.

Ans. Carbon has 4 valence electrons. It completes its octet by sharing its four electrons with other carbon atoms or with atoms of other elements as it can neither lose four of its electrons nor gain four electrons as both the processes require extra amount of energy and would make the system unstable. Such bonds that are formed by sharing electrons are known as covalent bonds. In covalent bonding, both the atoms share the valence electrons, i.e., the shared electrons belong to the valence shells of both the atoms.

In the formation of CH3Cl, carbon requires 4 electrons to complete its octet, while each hydrogen atom requires one electron to complete its duplet. Also, chlorine requires an electron to complete the octet. Therefore, all of these share the electrons and as a result, carbon forms 3 bonds with hydrogen and one with chlorine.

Q. What is a homologous series? Explain with an example.

Ans. A homologous series is a series of carbon compounds that have different numbers of carbon atoms but contain the same functional group. There is a difference of —CH2 unit between each successive member and mass differ by 14u.
For example, methane, ethane, propane, butane, etc., are all part of the alkane homologous series. The general formula of this series is CnH2n+2.

Q. What is hydrogenation? What is its industrial application?

Ans. Hydrogenation is the process of addition of hydrogen. Unsaturated hydrocarbons are added with hydrogen in the presence of palladium and nickel catalysts to give saturated hydrocarbons.

Industrial application: This reaction is used in the hydrogenation of vegetables oils, which contain long chains of unsaturated carbons.

(i) Explain the formation of scum when hard water is treated with soap.

(ii) People use a variety of methods to wash clothes. Usually after adding the soap, they ‘beat’ the clothes on a stone, or beat it with a paddle, scrub with a brush or the mixture is agitated in a washing machine. Why is agitation necessary to get clean clothes?

(i) Hard water often contains salts of calcium and magnesium. Soap molecules react with the salts of
calcium and magnesium and forms a precipitate. This precipitate begins floating as an off-white layer over water. This layer is called scum. Soaps lose their cleansing property in hard water because of formation of scum.

(ii) A soap molecule has two parts namely hydrophobic and hydrophilic. With the help of these, it attaches to the grease or dirt particle and forms a cluster called micelle. These micelles remain suspended as a colloid. To remove these micelles (entrapping the dirt), it is necessary to agitate clothes.

$$\underset{\text{Ethyne}}{\ce{2HC#CH}} + 5O_2 \longrightarrow 4CO_2 + 2H_2O + Heat$$

(i)  CH3—CH2—Br

(ii) Methanal

(iii) Hexyne

• (a) zinc is costlier than tin.
• (c) zinc is more reactive than tin.
• (d) zinc is less reactive than tin.

Explanation : Food cans are coated with tin and not with zinc because zinc is above the tin in the reactive series and can react with the food items and alter their taste. Hence, due to this reason food cans are coated with tin and not with zinc.

Q. Which one of the following types of medicines is used for treating indigestion?

• (a) Mg
• (b) Ag
• (c) Zn
• (d) Cu

Ans. (b) Ag

Explanation : Silver (Ag) metal would be displaced from the solution of its salts by other three metals because silver is less reactive than magnesium (Mg), zinc (Zn) and copper (Cu).

Q. Why is sodium kept immersed in kerosene oil?

Ans. Sodium is a very reactive metal and reacts vigorously with the oxygen, carbon dioxide and moisture present in the air such that it may even cause a fire. Therefore, to prevent these accidental fires, sodium is kept immersed in kerosene because it does not react with kerosene oil.

Q. Give reasons why copper is used to make hot water tanks and not steel (an alloy of iron).

Ans. Copper does not react with cold water, hot water, or steam. However, iron reacts with steam. If the hot water tanks are made of steel (an alloy of iron), then iron would react vigorously with the steam formed from hot water to corrode the tank due to the formation of iron oxide.

$$\underset{\text{Iron}}{3Fe} + \underset{\text{Steam}}{4H_2O} \longrightarrow \underset{\text{Iron oxide}}{Fe_3O_4} + \underset {\text{Hydrogen}}{4H_2}$$

Therefore, copper is used to make hot water tanks and not steel.

Q. Define the following terms.

• (i) Mineral
• (ii) Ore
• (iii) Gangue
• Ans. (i) Mineral: The elements or compounds, which occur naturally in the earth’s crust, are known as minerals.
• (ii) Ore: The mineral from which metal can be profitably extracted is called an ore.
• (iii) Gangue: Ores mined from the earth are usually contaminated with large amounts of impurities such as soil, sand, etc. These impurities are called as gangue.

Q. Explain the meanings of malleable and ductile.

Ans. Malleable: Substances that can be converted into thin sheets by beating are called malleable. Most of the metals are malleable. Gold and Silver are most malleable metals.
Ductile: Substances that can be drawn into thin wires are called ductile. Most of the metals are ductile. Gold is the most ductile metal.

Q. Give an example of a metal which:

• (i) Is a liquid at room temperature.
• (ii) Can be easily cut with a knife.
• (iii) Is the best conductor of heat.
• (iv) Is a poor conductor of heat.
• Ans. (i) Metal that exists in liquid state at room temperature : Mercury
• (ii) Metal that can be easily cut with a knife : Sodium and Potassium
• (iii) Metal that is the best conductor of heat : Silver and gold
• (iv) Metals that are poor conductors of heat : Mercury and lead

Explanation : Aqua regia is a mixture of hydrochloric acid and nitric acid in the ratio 3 : 1 and it can dissolve noble metals such as gold, palladium, and platinum, which however, not soluble in either of the acids alone.

Q. During the preparation of hydrogen chloride gas on a humid day, the gas is usually passed through the guard tube containing calcium chloride. The role of calcium chloride taken in the guard tube is to:

• (a) Absorb the evolved gas
• (b) Moisten the gas
• (c) Absorb moisture from the gas
• (d) Absorb Cl ions from the evolved gas
Q. Which gas is produced when dilute hydrochloric acid is added to a reactive metal? Write the chemical reaction when iron reacts with dilute H2SO4.
Ans. When reactive metals react with dilute hydrochloric acids, gives a salt and hydrogen gas. The reaction between iron and dil. H2SO4:

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

Ans. Zinc is more reactive than Iron. When Zn is added to Iron (II) Sulphate, Zinc displaces Iron from its solutions and zinc sulphate is formed.

Zn(s) + FeSO4(aq) → ZnSO4(aq) + Fe(s)

Q. In the electrolytic refining of a metal M, what would you take as the anode, the cathode and the electrolyte?

Ans. In the electrolytic refining of a metal M:
Anode → Impure metal M
Cathode → Thin strip of pure metal M
Electrolyte → Solution of salt of the metal M

Q. What type of oxides are formed when non-metals combine with oxygen?

Ans. Non-metals combine with oxygen to form acidic oxides.

$$For \space example: S(s) + O_2(g) \longrightarrow \underset{\text{Acidic in nature}}{SO_2(g)}$$

Q. State the following:

Explanation : Calcium chloride is used as an absorbent in the guard tube during the preparation of hydrogen chloride gas on a humid day as it absorbs the moisture present in the air and gives out white fumes on reacting with moisture.

Q. Why do HCl, HNO3, etc., show acidic characters in aqueous solutions while solutions of compounds like alcohol and glucose do not show acidic character?
Ans. Acids like HCl, HNO3, etc; dissociate to form hydrogen ions. These hydrogen ions (H+) combine with H2O to form hydronium ion(H3O+). This H+ or H3O+ is responsible for acidity. It can be represented as:
HCl + H2O → H+ + Cl
H+ + H2O → H3O+
• (i) A green layer is gradually formed on a copper plate left exposed to air for a week in a bathroom. What could this green substance be?
• (ii) Name the metal which has low melting point and can melt with heat of your palm?
• (iii) Which gas is liberated when a metal reacts with an acid? How will you test the presence of this gas?
• (iv) Name the metal which reacts with a very dilute HNO3 to evolve hydrogen gas?
• (v) Name two metals which are found in nature in the free state?
• (vi) Name one metal which has a low melting point?
• (vii) Name the metal which is the poorest conductor of heat?
• (viii) What name is given to those metal oxides which show basic as well as acidic behaviour?
• (ix) Name two metals which form amphoteric oxides.
• (x) Write the names and formulae of a metal hydride and a non-metal hydride.
• (xi) Which of the two metals is more reactive: copper or silver?
• (xii) Which metal foil is used for packing some of the medicine tablets?
• (xiii) Name two metals which are found in nature in the free State.
• (xiv) Name an alloy of lead and tin.
• (xv) What is an amalgam?
• (xvi) Name two metals which are highly resistant to corrosion.
• (xvii) Which metal becomes black in the presence of hydrogen sulphide gas in air?
• (xviii) Give an example of an elementary substance which is a good conductor of electricity but it is not a metal.
• (xix) Name a common metal which is highly resistant to corrosion.
• (xx) What chemical process is used for obtaining a metal from its oxide?
• (xxi) Name a non-metal which is lustrous and a metal which is non-lustrous?
• Ans. (i) It is due to the formation of basic copper carbonate.
• (ii) Gallium
• (iii) Hydrogen gas is liberated. When a burning matchstick is brought near to it, hydrogen gas will burn explosively with ‘pop’ sound.
• (iv) Magnesium
• (v) Gold and Silver
• (vi) Caesium
• (viii) Amphoteric oxides
• (ix) Aluminium and zinc
• (x) Sodium hydride, Hydrogen sulphide
• (xi) Copper
• (xii) Aluminium foil
• (xiii) Gold and silver, are found in the free state in nature.
• (xiv) Solder is an alloy of lead and tin.
• (xv) An alloy of mercury metal with one or more other metals is known as an amalgam.
• (xvi) Gold and platinum are highly resistant to corrosion.
• (xvii) Silver metal becomes black in the presence of hydrogen sulphide gas in air.
• (xviii) Graphite
• (xix) Aluminium
• (xx) Reduction.
• (xxi) Iodine is a non- metal which is lustrous, whereas lead is a non-lustrous metal.

Q. Samples of four metals A, B, C and D were taken and added to the following solution one by one. The results obtained have been tabulated as follows:

Metal UIron (II) Sulphate Copper (II) Sulphate Zinc Sulphate Silver Nitrate
A No reaction Displacement
B Displacement No reaction
C No reaction No reaction No reaction Diplsacement
D No reaction No reaction No reaction No reaction

Use the Table above to answer the following questions about metals A, B, C and D.

(i) Which is the most reactive metal?

(ii) What would you observe if B is added to a solution of Copper (II) sulphate?

Arrange the metals A, B, C and D in the order of decreasing reactivity.

Ans. As per reactivity series, Iron is most reactive metal among Iron, Silver and Copper. Since B can displace Iron from its sulphate, so B is the most reactive metal.

(i) As B is more reactive than Iron, it will displace Copper from its Copper Sulphate solution.

Q. Write equations for the reactions of:

(i) Iron with steam

(ii) Calcium and potassium with water

Ans. (i) Iron react with steam to form the metal oxide and hydrogen.

3Fe(s) + 4H2O(g) → Fe3O4(s) + 4H2(g)

(ii) The reaction of calcium with water is exothermic but the heat evolved is not sufficient for the hydrogen to catch fire. Calcium starts floating because the bubbles of hydrogen gas formed stick to the surface of the metal.

Ca(s) + 2H2O(l) → Ca(OH)2(aq) + H2(g)

Potassium react violently with cold water and its reaction is so violent and exothermic that the evolved hydrogen immediately catches fire.

2K(s) + 2H2O(l) → 2KOH(aq) + H2(g) + heat energy

(i)  Write the electron-dot structures for sodium, oxygen and magnesium.

(ii) Show the formation of Na2O and MgO by the transfer of electrons.

(iii) What are the ions present in these compounds?

Ans. (i) Electron – dot structure for Sodium (2, 8, 1):

Electron – dot structure for Oxygen (2, 8, 6):

(ii) Formation of Na2O by transfer of electron:

Formation of MgO by transfer of electron:

(iii) Ions present in these compounds are Mg2+, O2– and Na+.

Q. 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?

Ans. The man must have dipped the gold metal in the solution of aqua regia - a 3 : 1 mixture of conc. HCl and conc. HNO3. Aqua regia is a fuming, highly corrosive liquid. It dissolves gold in it. After dipping the gold ornaments in aqua regia, the outer layer of gold gets dissolved and the inner shiny layer appears. As a result, the weight of gold ornament reduced.

Q. What chemical process is used for obtaining a metal from its oxide?

Ans. The extraction of metal from its oxide depends on its position in reactivity series. For example, Metals low in the activity series are very unreactive. The oxides of these metals can be reduced to metals by heating alone.

$$2HgO(s) \xrightarrow{Heat} 2Hg(l) + O(g)$$

The metals in the middle of the activity series such as iron, zinc, lead, copper, etc., are moderately reactive. These metal oxides are reduced to the corresponding metals by using suitable reducing agent :

$$ZnO(s) +C(s) \longrightarrow Zn(s) + CO(g)$$

The metals high up in the reactivity series are very reactive. They are separated from their oxides by electrolysis process.

Q. Differentiate between metal and non-metal on the basis of their chemical properties.

Ans. Yes, basic solution also has H+ (aq) ions. But the concentration of H+ ions is less as compared to the concentration of OH ions that makes the solution basic.
Property Metals Non-metals
1. Nature of Oxides Metals form basic oxides. Ex. Na + O2 → Na2O Non-metals form acidic or neutral oxides. Ex. C +O2 → CO2 (acidic)
2. Nature of ions They form positive ions by iosing electrons.
Ex. Na → Na+ + e
They form negative ions by gaining electrons. Ex. Cl + e → Cl
3. Reaction with water Some metals lies Na, K, Ca, Al etc., displace hydrogen from water Non-metals except fluorine generally do not react with water.
4. Reaction with dilute acids Metals which lies above hydrogen in activity series displace hydrogen from dilute acids.
Ex. Zn + H2SO4 → ZnSO4 + H2
Non-metals do not react with dilute acids.
5. Nature Metals can lose electrons and hence behave as reducing agents. Non-metals can accept electrons and hence behave as oxidising agents.