Iit-foundation-olympiad-explorer_class_8_chemistry.pdf

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Integrated Syllabus

FOUNDATION & OLYMPIAD

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Cross word Puzzles

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Graded Exercise n

Basic Practice

n

Further Practice

n

Brain Nurtures

Numerical Problems Conceptual Questions

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Multiple Answer Questions

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Paragraph Questions

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Assertion & Reason Type Questions

CLASS

- VIII

Solutions

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75

Detailed solutions for all problems of IIT Foundation & Olympiad Explorer are available in this book ` 200

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CLASS - VIII

CHEMISTRY

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IIT Foundation & Olympiad Explorer - Chemistry Class - VIII

UNIQUE ATTRACTIONS

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FOUNDATION & OLYMPIAD E:

X P L D R E: R

CHEMISTRY CLASS - 8

BRAIN MAPPING

ACADEMY

MapptngYour Future

www.bmatalent.com

Published by:

Brain Mapping Academy

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Publication Team

Content Team: E.V.S.S. Lakshmi & Sk. Jamila Begum Design & Typing: V. Prasanna Kumari & M. Manjula

ISBN: 978-81-907285-6-0

Disclaimer

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Every care has been taken by the compilers and publishers to give correct, complete and updated information. In case there is any omission, printing mistake or any other error which might have crept in inadvertently, neither the compiler / publisher nor any of the distributors take any legal responsibility. In case of any dispute, all matters are subject to the exclusive jurisdiction of the courts in Hyderabad only.

Preface Speed and accuracy play an important role in climbing the competitive ladder. Students

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have to integrate the habit of being able to calculate and function quickly as well as efficiently in order to excel in the learning culture. They need to think on their feet, understand basic requirements, identify appropriate information sources and use that to their best advantage. The preparation required for the tough competitive examinations is fundamentally different from that of qualifying ones like the board examinations. A student can emerge successful in a qualifying examination by merely scoring the minimum percentage of marks, whereas in a competitive examination, he has to score high and perform better than the others taking the examination.

This book provides all types of questions that a student would be required to tackle at the foundation level. The questions in the exercises are sequenced as Basic Practice, Further Practice, Multiple Answer Questions, Paragraph Questions, Numerical Problems, Conceptual Questions and Brain Nurtures. Simple questions involving a direct application of the concepts are given in Basic Practice. More challenging questions on direct application are given in Further Practice. Questions involving higher order thinking or an open-ended approach to problems are given in Brain Nurtures. These questions encourage students to think analytically , to be creative

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and to come up with solutions of their own. Constant practice and familiarity with these questions will not only make him/her conceptually sound, but will also give the student the confidence to face any entrance examination with ease. Valuable suggestions as well as criticism from the teacher and student community are most welcome and will be incorporated in the ensuing edition. Publisher

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CONTENTS ` 1. States of Matter ..........................................

01

2. Symbols, Formulae & Equations ...............

29

3. Transformation of substances ...................

51

4. Atomic Structure ........................................

79

5. Water ...........................................................

101

6. Hydrogen & Oxygen ..................................

125

7. Carbon and its Compounds .......................

154

8. Metals and Non-metals ..............................

181

9. Synthetic fibres and Plastics ......................

199

Answers .......................................................

213

Answers to Crossword Puzzles ...................

230

IIT Foundation & Olympiad Explorer

Chemistry / Class -VIII

Chapter

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1 3

Mathematical Induction Transformation of Substances Force and Pressure

Common misconception

Fact

1. Physical changes are reversible while chemical 1. changes are not.

A very common misconception. Chemical changes are also reversible. In equilibrium reactions, products can be reconverted to reactions leading to reversibility of chemical change. On the other hand, some physical changes are also hard to reverse for eg: crushing a rock.

2. Chemical reactions will continue until all the reactants are exhausted.

2.

In case of equilibrium reactions before exhaustion of reactants, products get reconverted to reactants.

3. (i) Energy is used up in the chemical reactions. 3. (ii) Energy is created in chemical reaction.

Energy is not ”used up” or created in chemical reaction instead, they are released or stored in the form of chemical bonds between atoms.

4. A candle burning is endothermic since heat is used to burn wax.

Heat is needed at the beginning to initiate orneeded to initiate the reaction or activate the reaction. Once activated, the reaction proceeds without further energy input, and releases energy in the form of light. Therefore, it is an exothermic reaction. Another example is heating a piece of magnesium metal on bunsen burner.

4.

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SYNOPSIS

3.1 INTRODUCTION

From the dawn of creation, the world has been subjected to the escapable phenomenon called change. In fact, the only thing which is permanent perhaps, is change. Change is inexorable law of nature. It has manifested itself in every branch of human activity. Chemistry is that branch of science which deals with the reactions of various substances and the resulting changes. They range from very simple ones like change in state, colour, volume etc, to the more complex interconnecting reactions that keep our bodies alive.

3. Transformation of substances

51

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Types of Changes (i) Physical change

(ii) Chemical change

3.2 Physical change

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The process where a change is occurring only in physical properties, without affecting the identity of the molecules is called a physical change. In a physical change, the chemical composition remains the same and the original substances can be regenerated by reversing the conditions. Hence, physical change is a temporary change.

Examples

Melting of ice, Melting of Wax

3.3 Chemical change

The process in which the change is affecting the identity of a molecule by a change in chemical composition is called chemical change. In a chemical change, the original substance cannot be regenerated. Hence, it is a permanent change.

Examples

curdling of milk, burning of wood

3.4 Chemical reactions

Physical state

Homogeneous Reaction

If all the reactants and products in a chemical reaction are in the same physical state, then the reaction is said to be a homogenous reaction. Same physical state means that all of them are in gaseous state or in liquid state or in solid state.

Example

H2(g) + Cl2(g) Hydrogen chlorine

→

2HCl (g) Hydrogen chloride

Heterogeneous Reaction

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If all the reactant and products in a chemical reaction are not in the same physical state, then the reaction is said to be a heterogeneous reaction. Example

CaCO3(s) Calcium carbonate

3. Transformation of substances

→

CaO(s) + CO2 (g) calcium oxide Carbondioxide

52

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Difference between exothermic and endothermic reactions: Endothermic reaction

1. Heat is given out during the reaction. 2. Heat liberated is indicated by a positive sign on the product side. 3. An exothermic reaction is represented Reactants Product + Heat 4. Energy of the reactants is greater than the products.

1. Heat is absorbed during the reaction 2. Heat absorbed is indicated by a negative sign on the product side. 3. An endothermic reaction is represented as Reactants Product – Heat 4. Energy of the reactants is lesser than the products.

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Exothermic reaction

Irreversible Reaction

A reaction that occurs only in one direction, i.e., from reactants to products, is called an irreversible reaction.

Example

The reaction of quicklime with water to form hydrated lime is an irreversible reactions. Ca(OH)2 + Heat CaO (s) + H2O (l) → Calcium oxide Water Calcium hydroxide (quick lime) (hydrated lime) Similarly, burning of natural gas is an irreversible reaction. CH4 (g) + 2O2 (g) CO2 (g) + 2H2O (g) → methane oxygen carbon dioxide water vapour

+ Heat

Reversible Reaction

A reaction that occurs in both directions is called a reversible reaction. The reactant change to products and simultaneously products change to reactants. The reactants and products achieve a state of equilibrium with one another.

A reversible reaction is represented by two half – arrows between the reactants and products. One half – arrow points from reactants towards products and the other half arrow points in the opposite direction. Example

3H2 (g) ƒ Hydrogen

2NH3 (g) Ammonia

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N2 (g) + Nitrogen

Decomposition Reaction

A reaction in which a compound decomposes into two or more elements or compounds is known as a decomposition reactions. When limestone is strongly heated, it decomposes into quick lime (calcium oxide) and carbon dioxide.

Example

CaCO3 (s) → Calcium carbonate

3. Transformation of substances

CaO (s) + calcium oxide

CO2 (g) carbon dioxide

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Precipitation Reaction A reaction in which a precipitate is formed is called a precipitation reaction. A precipitate is an insoluble product that separates out as solid particles at the bottom of the solution. If we add a few drops of dilute hydrochloric acid to a solution of silver nitrate, a white precipitate of silver chloride is formed.

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Example

HCl (aq) + Hydrochloric acid

AgNO3 (aq) → Silver nitrate

AgCl (s) + HNO3 (aq) Silver chloride (ppt.) Nitric acid

Oxidation Reaction

A chemical reaction in which there is gain of oxygen or loss of hydrogen is called an oxidation reaction.

Example

When magnesium ribbon burns in oxygen, it forms magnesium oxide. Since oxygen has been added to magnesium, the reaction is said to represent oxidation of magnesium. 2Mg (s) Magnesium

+

O2 (g) Oxygen

→

2MgO (s) Magnesium oxide

Reduction Reaction

A chemical reaction in which there is gain of hydrogen or loss of oxygen is called a reduction reaction.

Example

Consider the reaction between hydrogen and chlorine gases. If a mixture of H2 and Cl2 gases are kept in sunlight, it reacts instantaneously to form hydrogen chloride gas. Since chlorine gains hydrogen, the reaction represents reduction of chlorine. H2 (g) + Hydrogen

Cl2 (g) Chlorine

2HCl (g) Hydrogen chloride

→

Redox Reaction

In a chemical reaction, whenever one substance is oxidised, another substance is reduced. This means that oxidation and reduction reactions always occur simultaneously.

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Example

2H2 (g) Hydrogen

+

O2 (g) Oxygen

2H2 O (l) Water

→

Consider the reaction between hydrogen and oxygen. Here hydrogen gains oxygen, so it is oxidation reaction. And oxygen gains hydrogen, so it is reduction reaction. Similarly, consider the reaction between iron oxide and carbon monoxide to form iron and carbon dioxide. Since iron oxide loses oxygen, it is undergoing reduction while carbon monoxide gains oxygen, it is undergoing oxidation reaction. Fe2O3 (s) Iron oxide

+ 3CO (g) → Carbon monoxide

3. Transformation of substances

2Fe (s) Iron 55

+

3CO2 (g) Carbon dioxide

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We see that every oxidation reaction is also a reduction reaction and vice versa. Hence, the chemical reactions involving reduction and oxidation are called redox reactions. Oxidizing agent Substances that help in oxidation of other substances are known as oxidising agents For example, in the reaction ZnO + C → Zn + CO

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‘C’ undergoes oxidation by gaining ‘O’ from ZnO. That is, ZnO helps C to get oxidised by giving oxygen. Hence, Zn is the oxidising agent. Similarly, oxidising agents also gain electrons or hydrogen and help the other substances to get oxidised.

Reducing agent

Substances that help in reduction of other substances are known as reducing agents. For example, in the reaction. H2S + Cl2 → S + 2HCl

‘Cl’ undergoes reduction by gaining ‘H’ from H2S. That is, H2S helps in reduction of ‘Cl’ by giving hydrogen. Hence, H2S is the reducing agent. Similarly, reducing agents also lose electrons or oxygen and help the other substances to get reduced.

SOLVED EXAMPLES Example 1:

Why is hydrogen placed in the metal reactivity series, though it is not a metal? Answer:

Hydrogen is placed in the metal reactivity series, since it shows the following properties of metals: Hydrogen atom has one electron in its valence shell and it readily loses its electron to form a positive ion like metals. Hence, hydrogen is placed in metal reactivity series. Example 2:

When a piece of copper is dipped in silver nitrate solution, formation of a white silvery compound is observed. Explain.

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Answer:

When a piece of ‘Cu’ is dipped in AgNO3 solution, ‘Cu’ disappears and a white silvery compound settles down. This process can be represented as,

Cu + 2AgNO3 → 2Ag ↓ +Cu (NO 3 )2 As copper (Cu) is present above silver (Ag) in reactivity series, it is more reactive than Ag. Therefore, ‘Cu’ loses electrons more easily than ‘Ag’ and displaces ‘Ag’ from silver nitrate (AgNO3) solution, forming copper nitrate (Cu(NO3)2) and metallic silver (Ag) which settles down as a white compound. 3. Transformation of substances

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Example 3: Which of the following reactions are not possible and why? 2. ZnSO4 + Ag → AgSO4 + Zn

3. Pb (NO3 )2 + Ca → Ca (NO3 )2 + Pb

4. H2SO4 + Cu → CuSO4 + H2

5. HgCl2 + Mg → MgCl 2 + Hg

6. 2MgO → 2Mg + O 2

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1. FeSO4 + Mg → MgSO4 + Fe

Answer:

We know that a more reactive metal replaces a less reactive metal from its salt solution. The reactivity series is as follows:

More reactive than Hydrogen

Less reactive than Hydrogen

Potassium(K) Sodium (Na) Calcium (Ca) Magnesium(Mg) Aluminium (Al) Zinc (Zn) Iron (Fe) Tin (Sn) Lead (Pb) [Hydrogen] Copper (Cu) Mercury (Hg) Silver (Ag) Gold (Au)

(More reactive)

Descending order of chemical reactivity.

(less reactive)

Using the above reactivity series, let us identify the reactions that are not possible in the given reactions. 1. FeSO4 + Mg → MgSO4 + Fe

This reaction is possible as ‘Mg’ is more reactive than ‘Fe’ (as evident from the reactivity series). Hence, it can displace ‘Fe’ from its solution (FeSO4).

2. ZnSO4 + Ag → AgSO4 + Zn

This reaction is not possible as ‘Ag’ is less reactive than ‘Zn’. As per reactivity series, a less reactive metal cannot displace a more reactive metal from its salt solution. Hence, this reaction is not possible.

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3. Pb (NO3 )2 + Ca → Ca (NO3 )2 + Pb

This reaction is possible as ‘Ca’ is more reactive than ‘Pb’ (reactivity series). As a result, ‘Ca’ can displace ‘Pb’ from its solution [Pb(NO3)2].

4. H2SO4 + Cu → CuSO4 + H2 This reaction is not possible as ‘Cu’ is less reactive than Hydrogen as per reactivity series, as a result it cannot displace Hydrogen from its solution. 5. HgCl2 + Mg → MgCl 2 + Hg This reaction is possible as ‘Mg’ is more reactive than ‘Hg’ as per reactivity series, as a result ‘Mg’ can displace ‘Hg’ from its solution (HgCl2). 3. Transformation of substances

57

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6. 2MgO → 2Mg + O 2 This reaction is not possible as formation of MgO is an example for chemical change and in a chemical change reverse reaction cannot take place (decomposition of MgO). Example 4: Identify the type of chemical reactions. (A) H2 + Cl2 → 2HCl (C) CaCO3 → CaO + CO2

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(B) CaO + H2O → Ca(OH)2 (D) FeSO4 + Mg → MgSO4 + Fe

(E) 2KI + Pb(NO3)2 → 2KNO3 + PbI2 ↓

Answer:

Reaction

Explanation

(A) H2 + Cl2 → 2HCl

Two elements H2 and Cl2 combine to form a single compound, HCl. Hence, it is a combination reaction. As two elements combine to form a compound, it is an Element– Element combination

(B) CaO + H2O → Ca (OH)2

Two compound CaO and H2O combine to form a single compound, Ca(OH) 2 . Hence, it is a combination reaction. As two compounds combine to form a new compound, it is a compound – compound combination.

(C) CaCO3 → CaO + CO2

The compound CaCO3 splits (decomposes) into two compounds, CaO and CO2. Hence, it is a compound – compound decomposition. Here, magnesium (Mg) is displacing, iron (Fe) from its salt solution (FeSO 4 ). Hence, it is a displacement reaction.

(D) FeSO4 + Mg → MgSO4 + Fe

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(E) 2KI + Pb(NO3)2 → 2KNO3 + PbI2 ↓ The reactants potassium iodide and lead nitrate react to form potassium nitrate and lead iodide. From this it is clear that, radicals are interchanged by the decomposition of reactants. Hence, it is a double decomposition reaction. Example 5: Compound A reacts with compound B to form compounds C and D. ‘C’ can easily dissolve in ‘D’ and ‘D’ on electrolysis gives hydrogen and oxygen. What type of compounds are A, B, C and D? Answer:

Compound D on electrolysis gives H2 and O2 and hence it is identified as H2O (water). ∴ A+B →C+D (water) Compound C easily dissolves in water, hence its a salt. ∴A + B → C + D (salt) (water) Salt and water are the products of a neutralization reaction. Therefore, compounds A and B are identified as Acid and Base. 3. Transformation of substances

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Example 6: Match the following equations with the type of reactions. (i) Al(OH)3 → Al2O3 + H2O (p) Neutralisation reaction. (ii) Zn + 2HCl → ZnCl2 + H2 (q) Decomposition reaction. (iii) CO2 + C → 2CO (r) Displacement reaction. (iv) 2NaOH + H2SO4 → Na2SO4 + 2H2O (s) Combination reaction. Answer:

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(i) Al(OH)3 → Al2O3 + H2O In this reaction, we have one reactant which decomposed to form more than one product. Hence, the reaction is a decomposition reaction. (ii) Zn + 2HCl → ZnCl2 + H2 In this reaction, we have two reactants one of which is an element, that it is zinc (Zn). Zinc is displacing H2 from HCl to form ZnCl2. Hence this reaction is a displacement reaction (iii) CO2 + C → 2CO In this reaction, two substances combine to form a single product. Hence, the reaction is a combination reaction. Here a compound and an element combine so it is an Element – Compound combination reaction. (iv) 2NaOH + H2SO4 → Na2SO4 + 2H2O In this reaction, two reactants react to form two products. Both the reactants and the products are compounds and it can be seen that the radicals got interchanged after the reaction. Hence, the reaction is a double decomposition reaction. The exchange of radicals can be seen as follows:

2[Na+ + OH– ] + [2H+ + SO4 –2]

[2Na+ + SO4 –2] + 2[H+ + OH–]

On the other side, a base reacts with an acid to form salt and water. Hence it is a neutralisation reaction. Therefore, the correct matches are i – q, ii – r, iii – s, iv – p. Example 7: Double decomposition reactions take place in which new compounds are formed by interchange of radicals. Complete the table given below representing double decomposition reactions. (A)

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→

+

[salt]

[water]

(B) Cu(OH)2 + –––––––––– → CuCl 2 +  salt   

[water]

(C) CuO + H2SO4 →

+ [salt] [water] The above reactions are examples of _____ [Precipitation/neutralisation] reactions.

(D)

3. Transformation of substances

→

[salt]

+

[Pr ecipitate] 59

↓

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(E) __________ + NH4OH → ( NH 4 )2 SO4 + Cu(OH)2 ↓ [Precipatate]

[salt ]

(F) Pb(NO3)2 + ________ → NaNO3 + ________ ↓ [salt] [Pr ecipitate ]

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Solution:

(A)

→

(B) Cu(OH)2 + 2HCl → (C) CuO + H2SO4 →

NaCl

+ H 2O

CuCl 2

+ 2H 2O

[Sodium chloride ]

[Cupric chloride]

CuSO 4

[Copper sulphate ]

[water ]

[water ]

+ H 2O

[ w ater ]

The above reactions are examples of Neutralisation reactions.

(D)

2NaCl + → [Sodium chloride ]

(E) CuSO4 + 2 NH4OH →

(F) Pb(NO3)2 + 2NaOH →

Fe (OH )2

[Ferrous hydroxide ]

↓

(NH 4 )2 SO4

+

[Copper hydroxide ]

2NaNO3

+

[Lead hydroxide ]

[ Ammonium sulphate]

[ Ammonium nitrate ]

Cu (OH )2

↓

Pb (OH )2

Example 8: A substance X undergoes a chemical change involving the loss of Hydrogen and this results in the formation of a product Y. What type of agents are X and Y? (Reducing agent or Oxidising agent)

Answer:

We know that, oxidising agent undergoes reduction and reducing agent undergoes oxidation. Hence, we need to know whether the substance has undergone oxidation or reduction. As X undergoes a chemical change by losing hydrogen atom, it is undergoes oxidation. Hence, it is a reducing agent. To the product Y, hydrogen is added, it undergoes reduction. Hence, Y acts as an oxidising agent.

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Example 9: In the formation of a compound AB, atoms of element A lost two electrons each while atoms of element B gained two electrons each. Which of the elements A or B is oxidised in the formation of AB?

Answer:

It is to be noted that the substance that undergoes oxidation is said to be oxidised and the substance that undergoes reduction is said to be reduced. As A loses 2 electrons, it undergoes oxidation and hence is oxidised. B gained 2 electrons, it undergoes reduction and hence is reduced. Therefore, the element A is oxidized in the formation of AB. 3. Transformation of substances

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utilizes the light energy of the Sun trapped by chlorophyll, the green pigment in the leaves. In this process, as energy is absorbed in the form of light, the reaction can be called an endothermic reaction. Within the plants, glucose first gets converted into starch. Then the carbon atoms are used to build proteins and the other complex compounds needed for life.

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Example 12: The equation below shows the reaction between nitrogen and oxygen.

N2 (g ) + 2O2 (g ) + 66 kJ → 2NO 2 (g )

(a) Is this an exothermic or endothermic reaction? Explain your answer. Solution:

(a) Endothermic reaction. 66 kJ of heat energy is absorbed in this reaction.

Example 13: In an experiment, the weight of carbon in carbon dioxide was found to be 25%. In another experiment, 1.2 g of carbon on heating produces 4.8 g of carbondioxide. Show that the results prove the law of constant proportions. Solution:

Case – I (I experiment) % of carbon in CO2 = 25%

Case – II (II experiment) Weight of carbon = 1.2 g

⇒ % of oxygen = 75% (100% – 25%) ∴ the ratio of weights of carbon and

Weight of carbon dioxide = 4.8 g

% of carbon in carbon dioxide

oxygen is 25% : 75% = 1 : 3

=

weight of carbon × 100 weight of carbon dioxide

=

1.2 1 × 100 = × 100 = 25% 4.8 4

st

st

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⇒ % of oxygen = 100% – 25% = 75% ∴ The ratio of weights of carbon and oxygen is 25% : 75% = 1 : 3 From both the above cases, it is clear that the ratio of weights of carbon and oxygen is 1 : 3. Thus, the results prove the law of constant proportions.

3. Transformation of substances

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Example 14: 1.375 g of cupric oxide was reduced by heating in a current of hydrogen, and the weight of copper that remained was 1.098 g. In another experiment, 1.179 g of copper was dissolved in nitric acid and the resulting copper nitrate was converted into cupric oxide by ignition. The weight of cupric oxide formed was 1.476 g. Show that these results illustrate the law of constant proportions. Solution:

Case – I Weight of cupric oxide taken = 1.375 g Weight of copper left = 1.098 g

⇒ Weight of oxygen present in cupric oxide = weight of cupric oxide – wieght of copper = 1.375 gm – 1.098 gm = 0.277 g

Case – II Weight of cupric oxide formed = 1.476 g Weight of copper taken = 1.179 g

⇒ Weight of oxygen present in cupric oxide = weight of cupric oxide – weight of copper = 1.476 gm – 1.179 gm = 0.297 g

We know that % of oxygen in cupric oxide =

weight of oxygen in cupric oxide × 100 weight of cupric oxide taken

Applying the above formula in both the cases, we get % of oxygen in cupric oxide =

0.277 × 100 = 20.15 1.375

% of oxygen in cupric oxide =

0.297 × 100 = 20.12 1.476

In both the cases, percentage of oxygen in cupric oxide is 20%. Hence, it proves the law of constant proportions.

Example 15: Carbon reacts with oxygen to produce CO (Carbon monoxide) and CO2 (Carbon dioxide). The atomic weights of Carbon and Oxygen are 12 and 16 respectively. Show that the results illustrate law of multiple proportions.

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Solution:

For compound CO2

For compound CO

Elements

Carbon

Oxygen

Elements

Carbon

Oxygen

No. of atoms 1 2 1 No. of atoms Atomic weight 12 16 12 Atomic weight Weight of each 12 16 12 Weight of each element element Ratio of weights of carbon and oxygen in the compound CO is 12 : 16 = 3 : 4

1 16 32

Ratio of weights of carbon and oxygen in the compound CO2 is 12 : 32 = 3 : 8 The ratio of weights of the element oxygen, combining with a part of carbon in the two compounds, is 4 : 8 = 1 : 2. 3. Transformation of substances

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3. Transformation of substances

65

Laws that govern chemical reactions

Irreversible reaction

Reversible reaction

Endothermic reaction

Heterogeneous reaction

Based on the direction of the reaction

Exothermic reaction

Based on the change in heat during a chemical reaction

Types of chemical changes

Homogeneous reaction

Based on the physical state of the reactants and products

Chemical change

Physical change

Types of changes

Transformation of substances

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Oxidation- reduction reaction

Precipitation reaction

Neutralisation reaction

Double displacement reaction

Displacement reaction

Combination reaction

Decomposition reaction

Based on reaction mechanism

Law of constant proportion

Law of multiple proportion

Law of conservation of mass

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CONCEPT MAP

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Summative Exercise SECTION – A

BASIC PRACTICE

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FILL IN THE BLANKS 1.

Decomposition is reverse of __________ reaction.

2.

In double decomposition reaction __________ are exchanged.

3.

A decomposition reaction taking place due to passage of electric current is called __________.

4.

Formation of SO3 from SO2 and O2 is a __________ reaction.

5.

Heat is released in _________ reactions.

6.

Zn displaces __________ from CuSO4 solution.

7.

Presence of soluble impurities __________ the boiling points of liquid.

8.

In oxidation reaction either __________ is added or __________ is removed.

9.

Oxidising agent undergoes __________.

10.

A reaction in which there is transfer of electrons from one reactant to another is called a __________.

11.

Addition of silver nitrate to sodium chloride solution causes preciptation of silver chloride. This is an example of ____________ reaction.

12.

Burning magnesium ribbon in air to form magnesium oxide is an example of ________ reaction.

13.

Digestion of food is an example of _________ change.

14.

Respiratory reaction is an example of __________ change.

15.

Dissolution of salt in water is a __________ change.

TRUE OR FALSE

Endothermic reactions are accompanied by absorption of heat.

17.

In redox reactions, oxidation and reduction take place simultaneously.

18.

Combination is the reverse of decomposition reaction.

19.

Neutralisation is a double displacement reaction.

20.

Two elements combine to form one compound in a displacement reaction.

21.

Formation of calcium oxide and carbon dioxide by heating limestone is an example of double decomposition reaction.

22.

Reaction of hydrochloric acid with zinc oxide to form zinc chloride and water is a neutralization reaction.

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16.

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Chemistry / Class -VIII

23.

Heating of sugar solution is a chemical change.

24.

Reaction of magnesium ribbon with dilute sulphuric acid to form magnesium sulphate and hydrogen is a combination reaction.

25.

Melting of butter is a physical change.

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MATCH THE FOLLOWING 26.

Column - I

Column - II

(A) Expansion or contraction of metals on heating or cooling

(p) Physical change

(B) Action of heat on zinc hydroxide

(q) Chemical change

(C) A stone place under sunlight

(r) Combination of physical and chemical change

(D) Shaping of glass by heating

(s) Neither a physical nor a chemical change.

27.

Column - I

Column - II

(A) Absorption of energy

(p) Physical change

(B) Curdling of milk

(q) Exothermic reaction

(C) Glowing of an electric bulb

(r) Chemical change

(D) Releasing of energy

(s) Endothermic reaction

28.

Column - I

Column - II

p) Decomposition of a compound into two elements.

(B) CaO + H2O → Ca(OH)2

(q) Combination of two elements to form a new compound.

heat (C) 2HgO  → 2Hg + O2

(r) Decomposition of a compound into a compound and an element.

heat (D) 2KClO3  → 2KCl + 3O2

(s) Combination of two compounds to give new compound.

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(A) H2+Cl2 → 2HCl

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Chemistry / Class -VIII

FURTHER PRACTICE MULTIPLE CHOICE QUESTIONS Which of the following statements is correct? (A) Physical changes are irreversible (B) All chemical changes are reversible (C) New substances are formed during a physical change. (D) Chemical change produces new substances. Identify the physical change in the following: (A) Reaction of zinc metal with dilute sulphuric acid. (B) Formation of solutions of soluble substances in water. (C) Burning of coal (D) Photosynthesis

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1.

2.

3.

4.

5.

6.

(A) Physical change (B) Chemical change (C) Both physical and chemical changes (D) None Chemical reaction does not involves: (A) Absorption (or) release of energy (B) Forming of new substances (or) products (C) Two (or) more reactants (D) None of the above In which type of change, we observe alteration only in specific property of matter but not composition? (A) Chemical change (B) Physical change (C) Both physical and chemical change (D) None of the above The formation of nitrous oxide from nitrogen and oxygen is the example for: (A) Decomposition reaction (B) Chemical combination of one element and one compound (C) Chemical combination of two compounds (D) Chemical combination of two elements Chemical combination involves: (A) Combination of two atoms. (B) Combination of only same type of elements. (C) Combination of only same type of compounds. (D) Combination of any two or more elements or compounds.

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7.

heating heating ˆˆˆˆˆˆˆ † water ‡ˆˆˆˆˆˆ ˆˆˆˆˆˆˆ † steam . Identify the type of change. Ice ‡ˆˆˆˆˆˆ cooling ˆ cooling ˆ

8.

A +B – + C+D – → A +D – + C+B – . Identify the above generalised type of reaction. (A) Chemical replacement reaction (B) Chemical double decomposition reaction (C) Chemical displacement reaction (D) Chemical combination reaction

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9.

Chemistry / Class -VIII

Identify the following reactions as endothermic or exothermic. (i) 2NaHCO3 + Heat energy → Na2CO3+ H2O + CO2 (ii) 2Mg + O2 → 2MgO + energy

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(i) (ii) (A) Exothermic Endothermic (B) Endothermic Exothermic (C) Exothermic Exothermic (D) Endothermic Endothermic Which type of chemical decomposition reaction is the following reaction?

10.

heat Cu(OH)2  → CuO + H2O

11.

12.

(A) Decomposition of one element into one compound and one element. (B) Decomposition of a compound into two new compounds. (C) Decomposition of one compound into two elements. (D) Decomposition of one compound into one compound and one element. (i) Reaction between calcium oxide and water is a _____ reaction. (ii) Reaction of mercuric oxide with strong heat is a _____ reaction. (i) (ii) (A) Decomposition Combination (B) Combination Combination (C) Combination Decomposition (D) Decomposition Decomposition The reaction Pb(NO3)2 + 2 NaI → 2 NaNO3 + PbI2 ↓ is classified as

(A) combination reaction (C) single replacement reaction

13.

4 Al + 3O2 → 2Al2O3

In the above equation, identify the oxidising agent. (A) Al (B) O2 (C) Al2O3 (D) Both ‘a’ and ‘c’ “Nitric oxide reacts with oxygen to form reddish brown nitrogen dioxide gas”. Choose the correct equation for the above reaction.

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14.

(B) decomposition reaction (D) double replacement reaction

15.

(A) 2NO + O2 → 2NO2

(B) N2O + O2 → NO2 + NO

heat (C) NaNO3  → 2 NaNO2+ O2

(D) 2NO3 + O2 → 2NO2 + 2 O2

Choose the incorrect statement: (A) Combination of two different elements is possible. (B) Decomposition of single compound is not possible. (C) Decomposition of a chemical compound into two new compounds is possible. (D) Combination of an element with a compound to form a new product is possible.

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25.

Chemistry / Class -VIII

(A) Law of multiple proportions (B) Law of reciprocal proportions (C) Law of conservation of mass (D) Law of constant proportions Which one of the following pairs of compounds illustrates the law of multiple proportions? (A) H2O, Na2O (B) MgO, Na2O (C) Na2O, BaO (D) SnCl2, SnCl4

MULTIPLE ANSWER QUESTIONS Chemical change involves: (A) No change in weight of the reacting substance. (B) Change in the composition. (C) Absorption (or) the release of energy. (D) Formation of precipitate. Choose the incorrect statement(s). (A) Chemical reactions are characterised by the change in colour. (B) Chemical reactions are not alone characterised by change in state. (C) Physical changes are characterised by change in colour of reacting substances. (D) Physical changes are not reversible.

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1.

2.

3.

The reaction, AB → A + B is a:

4.

(A) Decombination reaction (B) Combination reaction (C) Chemical composition (D) Decomposition reaction Identify the combination reaction in the following:

5.

6.

(B) NaCl + KOH → NaOH + KCl

(C) 2CO + O2 → 2 CO2

heat (D) Cu(OH)2  → CuO + H2O

Identify the correct combination reaction of two compounds to form a new product.

(A) H2 + Cl2 → 2HCl

(B) 2CO + O2 → 2 CO2

(C) CaO + H2O → Ca(OH)2

(D) NH3 + HCl → NH4Cl

“Reaction of potassium iodide solution with lead nitrate solution” is the example of: (A) Chemical replacement reaction (B) Neutralisation reaction (C) Precipitation reaction (D) Chemical double decomposition reaction Chemical compounds are formed to have constant composition irrespective of their (A) Method of formation (or) Source (B) Conditions for formation (C) Substances taken for formation (D) Time of formation

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7.

(A) NH3 + HCl → NH4Cl

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Chemistry / Class -VIII

ASSERTION AND REASON TYPE QUESTIONS

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The questions given below consists of statement of an Assertion and a Reason. Use the following key to choose the appropriate answer. (A) If both assertion and reason are CORRECT and reason is the CORRECT explanation of the assertion. (B) If both assertion and reason are CORRECT, but reason is NOT THE CORRECT explanation of the assertion. (C) If assertion is CORRECT, but reason is INCORRECT. (D) If assertion is INCORRECT, but reason is CORRECT. (E) If both assertion and reason are INCORRECT.

1.

2.

3.

4.

Physical change of a substance involves change in all properties and composition of its molecules.

Reason:

The change of a substance with alteration of composition of its molecules without any change in its specific property is called physical change.

Assertion:

Burning of coal is a physical change.

Reason:

When coal is burnt, the chief component, carbon is changed to carbon dioxide. This process cannot be reversed.

Assertion:

“A golden yellow precipitate of lead iodide is formed when colourless lead nitrate solution is mixed with colourless solution of potassium iodide.” So, it is a chemical change.

Reason:

Formation of new products in the reaction with the precipitate indicates a chemical change.

Assertion:

Beating of metals into sheets is a physical change.

Reason:

The conversion of metals into sheets involves no chemical reaction and sheets can be easily reconverted into metals with some external conditions. So, it is reversible.

Assertion:

Catalyst is a substance which takes part in a chemical reaction, undergoes chemical change and alters the rate of a reaction.

Reason:

Catalyst is a substance which does not take part in a chemical reaction, but changes the rate of a reaction.

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5.

Assertion:

6.

7.

Assertion:

When an electric current is passed through an aqueous solution of sodium chloride, sodium ions move towards cathode and chloride ions move towards anode.

Reason:

Cathode is a negatively charged electrode and therefore attracts Na+ ions. Anode is a positively charged electrode and therefore attracts Cl– ions

Assertion:

The process of depositing a thin and compact layer of a superior metal over an inferior metal by the process of electrolysis is known as electroplating.

Reason:

Electroplating is a process to improve the appearance of the articles.

3. Transformation of substances

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8.

Chemistry / Class -VIII

Assertion:

The laws of chemical combination ultimately led to the idea of ‘atoms’ being the “smallest unit of” matter.

Reason:

The laws of chemical combination played a significant role in the development of Dalton’s atomic theory of matter.

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Paragraph Questions

Passage - I

Limestone is held with tongs and heated on the flame directly. It crumbled to give white powder of calcium oxide. After cooling, when water is added to it a hissing sound is heard and water started boiling. If it is allowed to stay over night, a clear solution (top layer) is taken and for this experiment below as lime water. When carbon dioxide is bubbled into testtube a clear solution of lime water, turbidity appears first and later a white precipitate of calcium carbonate is formed. Carbon dioxide is continued to passed into the same solution and the solution again became clear because of formation of calcium bicarbonate. On heating the contents of the tube, white precipitate of calcium carbonate reappears. (i) On heating limestone, the following are formed. (A) calcium hydroxide, carbon dioxide

(B) calcium bicarbonate

(C) calcium oxide and carbon dioxide

(D) calcium oxide and water

(ii) Passing of carbon dioxide to lime water gave the following initially (A) calcium bicarbonate and carbon dioxide

(B) calcium carbonate and water

(C) calcium carbonate and carbon dioxide

(D) calcium oxide and water

(iii) Heating of calcium bicarbonate gave the following:

(B) calcium carbonate, water and CO2

(C) calcium bicarbonate and CO2

(D) calcium hydroxide and carbon dioxide

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(A) calcium oxide and carbon dioxide

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Chemistry / Class -VIII

SECTION – B Conceptual Questions What type of reaction is the reaction of quick lime with water?

2.

4 Al + 3O2 → 2Al2O3 In the above equation, identify the oxidising agent.

3.

Fe + S → FeS. In the above reaction,` how many number of electrons are lost by the iron atom and how it acts? Calcium carbonate decomposes on heating to form calcium oxide and carbon dioxide. According to law of conservation of mass, if 5.6 gms of calcium oxide and 4.4 gms of carbon dioxide are formed, how much calcium carbonate was taken?

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1.

4.

5.

We have seen combination or synthesis, decomposition, double decomposition and displacement reactions. Identify the nature of the following reactions: Write against each. (A) CaO + H2O → Ca(OH)2

heat (B) 2NaHCO3  → Na2CO3 + H2O + CO2 (C) 4Fe + 3O2 + 3H2O → 2Fe2O3 + 3H2O

(D) CaCl2 + 2AgNO3 → 2AgCl ↓ + Ca(NO3)2 heat (E) CaCO3  → CaO + CO2 (F) 2KI + Cl2 → 2KCl + I2

What happens when (A) a copper rod is placed in FeSO 4 solution? (B) an iron rod is placed in CuSO4 solution?

7.

Classify the following combination reactions into different types. (A) Na2O + H2O → 2NaOH (B) 4Fe + 3O2 → 2Fe2O3 (C) MgCl2 + 3O2 → Mg(ClO3)2

8.

Arrange the following in the increasing order of their reactivity: Ca, Cu, Hg, Al, Ag, K, Zn.

9.

Name of the oxidising agent in the following reactions. (A) ZnO + C → Zn + CO (B) Cl2 + H2S → 2HCl + S (C) 4HCl + MnO2 → MnCl2 + 2H2O + Cl2

10.

Hydrogen and Oxygen combine in the ratio of 1 : 8 by mass to form water. What mass of Oxygen gas would be required to react completely with 3 grams of hydrogen gas? In two different experiments, copper oxide was obtained from copper. The ratio of mass of copper and mass of oxygen found in the two experiments are same. They are in the ratio of 4 : 1. What do you infer about the experiments? In an experiment 1.288 g of copper oxide was obtained from 1.03 g of copper and 0.258 g of oxygen. Calculate the ratio of copper and oxygen in the sample? Certain non-metal X forms two oxides I and II. The mass percentage of oxygen in I(X4O6) is 43.7 which is same as that of X in the 2nd oxide. Find the formula of 2nd oxide.

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6.

11.

12. 13.

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IIT Foundation & Olympiad Explorer

Chemistry / Class -VIII

SECTION – C Previous Contest Questions 1. The strongest reducing agent is: (A) HNO2

(B) H2S

(C) H2SO3

((D) SnCl2

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2. Which substance is serving as a reducing agent in the following reaction? FeSO4 + Mg → MgSO4 + Fe

(A) Mg

(B) SO4–2

(C) Fe

((D) MgSO4

3. In the reaction, 4Fe + 3O 2 ƒ 4Fe3 + + 6O 2 − Which of the following statements is incorrect? (A) It is a redox reaction (C) Fe3+ is an oxidising agent

(B) Metallic iron is a reducing agent (D) Metallic iron is reduced to Fe3+

4. In the reaction, 2Ag + 2H 2SO 4 → Ag 2SO 4 + SO 2 + 2H 2O , sulphuric acid acts as: (A) oxidising agent (B) reducing agent (C) catalyst (D) acid and oxidant

5. When iron is added to CuSO4 solution, copper is precipitated. It is due to _________. (A) oxidation of Cu2+ (B) reduction of Cu2+ (C) hydrolysis of CuSO4 (D) ionisation of CuSO4

6. An oxidising agent is a substance which can _____. (A) accept electrons (B) donate electrons (C) accept protons (D) donate protons 7. The reaction, Zn 2 + (aq ) + 2e − → Zn (s ) is: (A) oxidation

(B) reduction

(C) redox reduction

(D) none

8. The reaction, 2K 2 MnO4 + Cl 2 → 2KMnO 4 + 2KCl , is an example of ________. (A) oxidation (B) reduction (C) neutralisation (D) chlorination

9. In the reaction, Cu (s ) + 2Ag + (aq ) → Cu 2 + + 2Ag (s ) (A) Cu(s) is oxidised to Cu2+ (C) Ag (aq) is oxidised to Ag (s)

(B) Cu(s) reduced to Cu2+ (aq) (D) both Cu(s) and Ag+(aq) are oxidised.

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10. Which of the following reactions involves oxidation and reduction? (A) NaBr + HCl → NaCl + HBr

(B) HBr + AgNO3 → AgBr + HNO3

(C) H2 + Br2 → 2HBr

(D) Na 2O + H2SO4 → Na 2SO4 + H2O

11. In the reaction between zinc and iodine, zinc iodide is formed. What is being oxidised. (A) zinc ions (B) iodide ioins (C) zinc atom (D) iodine 12. In the reaction, Cl 2 + OH − → Cl − + ClO3− + H 2O chlorine is ________. (A) oxidised (B) reduced (C) oxidised as well as reduced (D) neither oxidised nor reduced 3. Transformation of substances

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Chemistry / Class -VIII

13. Oxidation is defined as __________. (A) gain of electrons (C) gain of electrons

(B) loss of electrons (D) loss of neutrons

14. In the rusting of iron, 4Fe + 3O2 → 2Fe 2O 3 , iron is: (A) reduced (B) oxidised (C) precipitated

(D) hydrolysed

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15. Which of the following reactions indicates the oxidising behaviour of H2SO4? (A) 2PCl 5 + H 2SO 4 → 2POCl 3 + 2HCl + SO 2Cl 2

(B) NaCl + H 2SO4 → NaHSO4 + HCl

(C) 2NaOH + H 2SO 4 → Na 2SO 4 + 2H 2O (D) 2HI + H 2SO4 → I 2 + SO2 + 2H 2O

Brain Nurtures

1.

When a zinc strip is placed in blue copper sulphate solution, the colour fades away and the solution turns colourless. Why?

2.

Ramu has a copper coin. But he wants a silver coin. As a chemist, can you help him get the silver coin by giving him the right solution from those given below. (A) Cu(NO3)2

3.

(B) AgNO3

(C) both

We often see beautiful hydrogen gas balloons flying in the sky. Hydrogen gas is filled into these balloons from a cylinder. Can you guess how the hydrogen gas is produced in this cylinder? Which of the following reactions produces hydrogen in the cylinder. (A) Zn + HCl Explain?

(B) Ag + HCl

(C) Cu + HCl

(D) AgCl + HCl.

Copper sulphate solution is placed in an iron pot and left over night. The next morning, a number of holes are formed in the iron pot. But, when iron sulphate solution is placed in a copper pot and left over night, the copper pot remained as it was. Why?

5.

An element ‘A’ forms a sulphide A2S. In terms of loss or gain of electrons, find out which atom is oxidised and which atom is reduced?

6.

In a solution of lead acetate, a strip of metal ‘M’ is dipped. After some time, a white coating of lead is observed on the metal strip. Which is more reactive ‘M’ or lead?

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4.

7.

We have seen metal activity series. Similarly we also have halogen activity series, Cl, Br, I. Applying the same principle of metal activity series, can you answer the following questions: When chlorine is passed into KBr solution, a deep red coloured liquid is formed. When chlorine is passed into KI solution, a violet coloured solution is formed. Explain?

8.

When 0.28 g of iron filings were heated in a current of dry air, 0.36 g of iron oxide is formed. Find the weight of oxygen that combines with this weight of metal, with the help of law of conservation of mass.

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Chemistry / Class -VIII

CROSS WORD PUZZLE

1

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2 3

4

5

6

7

8

9

10

DOWN

2

The gas displaced by Zn on reaction with HCl

1

Type of change, magnetisation of iron

4

A reaction that occurs in both directions

3

Loss of hydrogen

6

Decomposition of lime stone gives quick lime

5

Heat is absorbed during a chemical reaction

8

Heat is evolved during a chemical reaction

7

Type of change, curdling of milk

9

Reaction of an acid and a base

10

Loss of oxygen

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3. Transformation of substances

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Brain Mapping Academy

l

l

l

Cross word Puzzles

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Graded Exercise n

Basic Practice

n

Further Practice

n

Brain Nurtures

Numerical Problems

Conceptual Questions

l

Multiple Answer Questions

l

Paragraph Questions

l

Assertion & Reason Type Questions

CLASS

- VIII

Solutions

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