ICSE Class 8 Chemistry Sample Paper with Solutions Set 02

Question 1

Choose the correct answer out of the four available choices given under each question. [15]

1. Which of the following pictures represent the Bohr's model of an atom? [1 Mark]
(a) I
(b) II
(c) III
(d) IV

[Figure: Four diagrams representing atomic models: I shows a plum pudding or Thomson model with embedded charges, II shows a planetary model with circular orbits crossing a central nucleus, III shows concentric circular orbits with electrons around a central nucleus representing Bohr's model, IV shows a nuclear atom with dotted paths.]

Answer: (c) III

According to Bohr's model of an atom, electrons revolve around the nucleus in certain definite circular paths called orbits.

Teacher's Note:
a) Bohr's model specifically introduces well-defined circular orbits or shells for electron revolution.
b) Students must carefully distinguish between Thomson's plum pudding model and Bohr's planetary model.

2. Sodium forms sodium cation by loss of? [1 Mark]
(a) One electron
(b) Two electrons
(c) Three electrons
(d) Four electrons

Answer: (a) One electron

Sodium has the electronic configuration (2, 8, 1) and loses its single valence electron to form a sodium cation (Na⁺).

Teacher's Note:
a) Metals lose valence electrons to achieve a stable octet configuration.
b) Sodium belongs to Group 1 and hence forms a univalent positive ion.

3. Hydrogen frees metals from their [1 Mark]
(a) Sulphates
(b) Oxides
(c) Nitrates
(d) Chlorides

Answer: (b) Oxides

Hydrogen acts as a reducing agent and removes oxygen from heated metal oxides to liberate free metals.

Teacher's Note:
a) Hydrogen reacts with less reactive metal oxides like copper oxide or lead oxide upon heating.
b) Remember that hydrogen reduces only metal oxides placed below it in the reactivity series.

4. Which of the following metals starts floating in water when allowed to react with it? [1 Mark]
(a) Sodium
(b) Calcium
(c) Potassium
(d) Iron

Answer: (b) Calcium

Calcium reacts less vigorously with water, and the hydrogen gas bubbles evolved stick to the metal surface, making it float.

Teacher's Note:
a) Sodium and potassium react so violently that the evolved hydrogen catches fire, whereas calcium floats.
b) Iron does not react with cold or hot water under normal conditions.

5. In which case will the iron nail not corrode? [1 Mark]
(a) Test tube A
(b) Test tube B
(c) Test tube C
(d) Both test tubes A and B

[Figure: Three test tubes showing rusting of iron: Test tube A contains an iron nail, anhydrous CaCl₂ and air; Test tube B contains an iron nail, boiled water and an oil layer; Test tube C contains an iron nail and tap water.]

Answer: (d) Both test tubes A and B

Rusting requires both moisture and oxygen. Test tube A lacks moisture due to anhydrous CaCl₂, and Test tube B lacks dissolved oxygen due to boiling and oil layer protection.

Teacher's Note:
a) Both oxygen and water are essential prerequisites for the corrosion of iron.
b) Anhydrous calcium chloride is a drying agent used to absorb moisture from the air.

6. Which of the following is not a deliquescent substance? [1 Mark]
(a) Concentrated sulphuric acid
(b) Sugar
(c) Common salt
(d) Copper sulphate

Answer: (a) Concentrated sulphuric acid

Concentrated sulphuric acid is a hygroscopic substance that absorbs moisture from the air without dissolving into a solution like a deliquescent substance.

Teacher's Note:
a) Hygroscopic substances absorb moisture without forming a solution, unlike deliquescent substances.
b) Common salt, caustic soda, and ferric chloride are typical examples of deliquescent substances.

7. The process of sterilisation of water by the addition of chlorine which acts as a treatment against bacterial infection is [1 Mark]
(a) Chlorination
(b) Precipitation
(c) Sedimentation
(d) Decantation

Answer: (a) Chlorination

Chlorination involves adding chlorine to water to kill harmful bacteria and microorganisms, ensuring it is safe for drinking.

Teacher's Note:
a) Sedimentation and decantation remove suspended impurities, whereas chlorination specifically kills bacteria.
b) This is a standard final purification step in municipal water treatment plants.

8. Which of the following is a solid fuel? [1 Mark]
(a) Petrol
(b) Kerosene
(c) Coke
(d) Methanol

Answer: (c) Coke

Coke is a solid carbonaceous residue derived from destructive distillation of coal, making it a solid fuel.

Teacher's Note:
a) Petrol and kerosene are liquid fuels, while methanol is a liquid alcohol fuel.
b) Solid fuels include coal, coke, wood, and charcoal.

9. Allotropes have [1 Mark]
(a) Similar chemical properties
(b) Similar physical properties
(c) Different chemical properties
(d) Similar physical and chemical properties

Answer: (a) Similar chemical properties

Allotropes of an element exhibit different physical properties due to structural variations, but they share similar chemical properties because they consist of the same atoms.

Teacher's Note:
a) Diamond and graphite are classic allotropes of carbon with identical chemical combustion products.
b) Always remember that allotropy arises from different structural arrangements of the same element.

10. Allotropes have __________ properties. [1 Mark]
(a) similar chemical
(b) similar physical
(c) different chemical
(d) similar physical and chemical

Answer: (a) similar chemical

Allotropes possess similar chemical properties since they are composed of the same type of atoms.

Teacher's Note:
a) This is a variation of the preceding conceptual question tested directly.
b) Physical properties like density, melting point, and hardness vary significantly among allotropes.

11. Substances used to determine whether a particular substance is an acid or a base are called [1 Mark]
(a) Salts
(b) Indicators
(c) Reagents
(d) Catalysts

Answer: (b) Indicators

Indicators are chemical substances that exhibit different colours in acidic and basic mediums, helping to identify them.

Teacher's Note:
a) Common indicators include litmus, phenolphthalein, and methyl orange.
b) Natural indicators like turmeric and red cabbage extract are also widely used.

12. What will be the colour change when phenolphthalein is added to ammonium hydroxide? [1 Mark]
(a) Pink
(b) Orange
(c) Colourless
(d) Green

Answer: (a) Pink

Ammonium hydroxide is a basic solution, and phenolphthalein turns pink in alkaline mediums.

Teacher's Note:
a) Phenolphthalein remains colourless in neutral and acidic solutions.
b) It turns pink or magenta in the presence of strong or weak bases like sodium hydroxide or ammonium hydroxide.

13. Which is the correct balanced equation for the reaction between aluminium and hydrochloric acid? [1 Mark]
(a) Al + 3HCl → AlCl₃ + 3H
(b) 2Al + 3HCl → Al₂Cl₃ + 3H
(c) 2Al + 6HCl → 2AlCl₃ + 3H₂
(d) Al + 6HCl → Al₂Cl₃ + 3H₂

Answer: (c) 2Al + 6HCl → 2AlCl₃ + 3H₂

Aluminium reacts with hydrochloric acid to form aluminium chloride and hydrogen gas, satisfying mass conservation.

Teacher's Note:
a) Hydrogen gas is always written as a diatomic molecule (H₂).
b) Balancing requires checking both atom counts and valency formulas.

14. What is the charge on an electron? [1 Mark]
(a) -1
(b) +1
(c) -2
(d) +2

Answer: (a) -1

An electron carries a unit negative relative charge, denoted as -1.

Teacher's Note:
a) Protons carry a relative charge of +1, while neutrons are neutral (0).
b) The absolute charge of an electron is 1.6 × 10^-19 Coulombs.

15. Conversion of a solid to a liquid on heating is [1 Mark]
(a) Melting
(b) Condensation
(c) Condensation
(d) Freezing

Answer: (a) Melting

The phase transition from solid to liquid upon heating is termed melting or fusion.

Teacher's Note:
a) Condensation is gas to liquid, and freezing is liquid to solid.
b) Melting occurs at a constant temperature known as the melting point for pure substances.

Question 2

(A) State the electronic configuration of the following atoms: [5 Marks]
1. Atom 'A' (Atomic number = 12)
2. Atom 'B' (Atomic number = 19)
3. Atom 'C' (Atomic number = 7)
4. Atom 'D' (Atomic number = 26)
5. Atom 'E' (Atomic number = 10)

Answer:
1. Electronic configuration of atom 'A' (Z = 12, Magnesium) = (2, 8, 2)
2. Electronic configuration of atom 'B' (Z = 19, Potassium) = (2, 8, 8, 1)
3. Electronic configuration of atom 'C' (Z = 7, Nitrogen) = (2, 5)
4. Electronic configuration of atom 'D' (Z = 26, Iron) = (2, 8, 14, 2)
5. Electronic configuration of atom 'E' (Z = 10, Neon) = (2, 8)

Teacher's Note:
a) Shell capacities are given by the formula 2n² (K=2, L=8, M=18, N=32).
b) Transition elements like iron follow specific Aufbau filling rules where outer shells may fill before inner d-orbitals complete.

(B) Fill in the blanks and rewrite the sentences: [5 Marks]
1. When few drops of phenolphthalein are added to sodium hydroxide, the solution turns _________.
2. Separation of components of air such as liquid nitrogen and liquid oxygen is possible by __________.
3. The three states of matter are classified on the basis of differences in certain _________ properties.
4. A mixture which has different composition and properties in different parts of their mass is called a ________ mixture.
5. ___________ is used in the laboratory for the preparation of hydrogen using dilute hydrochloric acid.

Answer:
1. Pink
2. fractional distillation
3. physical
4. heterogeneous
5. Granulated zinc

Teacher's Note:
a) Fractional distillation relies on differences in boiling points of liquefied gases.
b) Granulated zinc is preferred because impurities in zinc catalyse and speed up the rate of hydrogen evolution.

Question 3

(A) State whether the following statements are true or false. Rewrite the false statement. [5 Marks]
1. A compound is a pure substance composed only of one kind of element combined chemically in a fixed proportion by mass.
2. A chemical reaction in which two or more substances combine to form a single product is called a combination reaction or synthesis.
3. In the formation of ammonia from hydrogen and nitrogen iron is used as positive catalyst.
4. Magnesium has atomic number 12 and atomic mass number 24.
5. The insoluble solid settels down in a beaker is called as sediment.

Answer:
1. False. A compound is a pure substance composed of two or more elements combined chemically in a fixed proportion by mass.
2. True.
3. True. Finely divided iron is used as a positive catalyst in the manufacture of ammonia from hydrogen and nitrogen.
4. True.
5. True.

Teacher's Note:
a) Compounds always consist of multiple distinct elements chemically bonded in definite proportions.
b) Catalysts alter the rate of a chemical reaction without themselves undergoing permanent chemical change.

(B) Name the following: [5 Marks]
1. Conversion of a solid to a liquid on heating
2. Conversion of a liquid to a vapour (or gas)
3. Conversion of a vapour (or gas) to a liquid
4. Conversion of a liquid to a solid
5. The outermost shell or orbit of an atom.

Answer:
1. Melting (or Fusion)
2. Vaporisation (or Boiling / Evaporation)
3. Condensation
4. Freezing (or Solidification)
5. Valence shell

Teacher's Note:
a) Phase change terminology must be precise and accurate.
b) The valence shell determines the chemical reactivity and valency of an atom.

Question 4

(A) Give distinguishing explanation between alpha rays, beta rays and gamma rays. [5 Marks]

Answer:

Teacher's Note:
a) Tabular comparisons are best for radioactive radiations covering nature, charge, mass, and penetration.
b) Students should remember that alpha particles are identical to doubly ionized helium nuclei.

(B) What is a metal reactivity series? What are its important features? [5 Marks]

Answer:
Metal Reactivity Series:
A list in which metals are arranged in the decreasing order of their chemical reactivity is called the metal reactivity series. The most active metal (potassium) is placed at the top, and the least active metal (platinum or gold) is placed at the bottom.

Important Features:
1. The ease with which a metal loses electrons and forms a positive ion decreases down the series, from potassium to gold.
2. Hydrogen is included in the activity series because, like metals, it also loses an electron and forms a positive ion (H⁺) in certain chemical reactions.
3. The series facilitates the comparative study of metals in terms of their reactivity.
4. Reactions of metals with water, acids, and displacement reactions can be easily predicted using this series.

Teacher's Note:
a) A metal higher in the reactivity series can displace a lower metal from its salt solution.
b) Hydrogen acts as the reference baseline separating active metals from less active ones.

Question 5

(A) Define the following: [5 Marks]
1. Lamp black
2. Sugar charcoal
3. Chemical equation
4. Orbit
5. Bone charcoal

Answer:
1. Lamp black: It is the black smoke collected in the form of a fine black powder when carbon-rich substances are burned in a limited supply of air over damp blankets.
2. Sugar charcoal: It is the purest form of amorphous carbon prepared by heating cane sugar or glucose in the absence of air.
3. Chemical equation: It is the symbolic representation of a chemical reaction using the chemical formulae and symbols of the reactants and products.
4. Orbit: It is a definite circular path around the nucleus of an atom in which electrons revolve.
5. Bone charcoal: It is obtained as a residue by the destructive distillation of bones, containing about 10 percent carbon mixed with calcium phosphate.

Teacher's Note:
a) Ensure precise definitions for all allotropic forms of carbon and atomic terms.
b) Mentioning preparation conditions like 'absence of air' or 'limited air' is crucial for charcoal definitions.

(B) Match the name of the radical given in Column A with its formula given in Column B. [5 Marks]

Teacher's Note:
a) Radicals must be matched with correct valencies and formulas.
b) Students should carefully distinguish between chlorate (ClO₃^-) and chloride (Cl^-).

Question 6

(A) Write the formula of the given compounds: [5 Marks]
1. Acetic acid
2. Sodium hydroxide
3. Sulphuric acid
4. Hydrochloric acid
5. Ammonium hydroxide

Answer:
1. Acetic acid - CH₃COOH
2. Sodium hydroxide - NaOH
3. Sulphuric acid - H₂SO₄
4. Hydrochloric acid - HCl
5. Ammonium hydroxide - NH₄OH

Teacher's Note:
a) Chemical formulas must have correct subscripts representing combining ratios.
b) Acetic acid is an organic acid containing the carboxyl functional group.

(B) Write the electronic configuration of the following elements: [5 Marks]
1. Sodium
2. Chlorine
3. Hydrogen
4. Nitrogen
5. Oxygen

Answer:
1. Sodium (Na₁₁) - (2, 8, 1)
2. Chlorine (Cl₁₇) - (2, 8, 7)
3. Hydrogen (H₁) - (1)
4. Nitrogen (N₇) - (2, 5)
5. Oxygen (O₈) - (2, 6)

Teacher's Note:
a) Electronic configuration is written shell by shell from the innermost K-shell outwards.
b) Total number of electrons equals the atomic number of the neutral atom.

Question 7

(A)

1. Differentiate between compound and mixture. [3 Marks]

Answer:

Teacher's Note:
a) Emphasize that compounds involve chemical bonding whereas mixtures involve no new chemical bonds.
b) Give tabular comparison highlighting composition, properties, and separation methods.

2. What is atomicity? Give one example each of mono atomic and diatomic molecule. [2 Marks]

Answer:
Atomicity is defined as the total number of atoms present in a molecule of an element.
- Monoatomic molecule: Helium (He)
- Diatomic molecule: Oxygen (O₂)

Teacher's Note:
a) Noble gases exist as single independent atoms, hence their atomicity is one.
b) Common atmospheric gases like oxygen, hydrogen, and nitrogen are diatomic.

(B)

1. Give the chemical equations for a reaction of potassium with [2 Marks]
(a) Oxygen
(b) Water

Answer:
(a) Reaction with oxygen:
4K + O₂ → 2K₂O
(b) Reaction with water:
2K + 2H₂O → 2KOH + H₂ + Heat

Teacher's Note:
a) Potassium reacts extremely vigorously with both oxygen and cold water.
b) Equations must be balanced properly with correct physical state indicators or heat terms.

2. Write the main features of Rutherford’s atomic model. [3 Marks]

Answer:
1. There is a positively charged centre in the atom called the nucleus, in which nearly all the mass of the atom is concentrated.
2. Negatively charged particles called electrons revolve around the nucleus in circular paths called orbits.
3. The size of the nucleus is extremely small compared to the total size of the atom.
4. The atom is mostly empty space.

Teacher's Note:
a) Rutherford's model is often referred to as the planetary model of the atom.
b) Mentioning the nuclear center and empty space is vital for full marks.