ICSE Class 8 Chemistry Sample Paper with Solutions Set 04

Question 1
Choose the correct answer out of the four available choices given under each question. [15]

1. The valency of carbon is 4 and that of oxygen is 2. What is the molecular formula of carbon dioxide? [1 Mark]
(a) C₂O
(b) C₂O₂
(c) CO
(d) CO₂

Answer: (d) CO₂

By criss-crossing the valencies of carbon (4) and oxygen (2), we get C₂O₄, which simplifies to the empirical formula CO₂.

Teacher's Note:
a) To find the chemical formula, divide the combining valencies by their highest common factor to get the simplest ratio.
b) Students often make errors in formula writing by not simplifying subscripts to their lowest whole number ratio.

2. Which of the following pictures represents the arrangement of particles in liquids? [1 Mark]
(a) Picture II
(b) Picture I and II
(c) Picture I
(d) Picture II and III

[Figure: Three pictures showing particle arrangements. Picture I shows tightly packed regular particles (solid), Picture II shows closely packed but random particles (liquid), and Picture III shows widely spaced random particles (gas).]

Answer: (a) Picture II

In liquids, particles are close together but lack the rigid, ordered lattice structure of solids, moving about randomly within the volume.

Teacher's Note:
a) Solids have a tightly packed regular arrangement, liquids have a close but random arrangement, and gases have large empty spaces between particles.
b) Pay close attention to intermolecular spacing and order when identifying states of matter from particulate diagrams.

3. The maximum number of electrons which can be accommodated in an orbit is given by the formula ________, where n is the number of orbit. [1 Mark]
(a) 2(n + 2)
(b) 2n + 2
(c) 2n²
(d) 2/n²

Answer: (c) 2n²

Bohr and Bury scheme states that the maximum capacity of any energy shell is given by 2n².

Teacher's Note:
a) Here n represents the principal quantum number or the shell number (1, 2, 3, etc.).
b) Do not confuse the electron capacity formula with formula for maximum orbitals or other quantum rules.

4. Which of the following is incorrect about a heterogeneous mixture? [1 Mark]
(a) Constituents can be distinctly seen
(b) Constituents are uniformly mixed
(c) Different composition throughout its mass
(d) Different composition in different parts of its mass

Answer: (b) Constituents are uniformly mixed

Heterogeneous mixtures do not have a uniform composition throughout; their constituents are visibly separate and unevenly distributed.

Teacher's Note:
a) Uniform mixing is a characteristic feature of homogeneous mixtures, not heterogeneous ones.
b) Read the question carefully to spot negative terms like "incorrect".

5. If atomic number of an atom is 17 and mass number is 35 then number of neutrons will be [1 Mark]
(a) 35
(b) 17
(c) 18
(d) 52

Answer: (c) 18

Number of neutrons = Mass number - Atomic number = 35 - 17 = 18.

Teacher's Note:
a) Mass number (A) is the sum of protons and neutrons, while atomic number (Z) is equal to the number of protons.
b) A common mistake is adding the atomic number and mass number instead of subtracting them.

6. The phenomenon of existence of a substance in various physical forms but the same chemical form is known as _________. [1 Mark]
(a) isomerism
(b) enantiomerism
(c) allotropy
(d) anisotropy

Answer: (c) allotropy

Allotropy is the property by which an element exists in two or more different physical forms in the same physical state.

Teacher's Note:
a) Carbon, sulfur, and phosphorus are classic examples of elements showing allotropy.
b) Remember that allotropes have different physical properties but identical chemical properties.

7. The metal reacts with cold water to produce hydrogen is . [1 Mark]
(a) Magnesium
(b) Aluminium
(c) Calcium
(d) Iron

Answer: (c) Calcium

Calcium reacts vigorously with cold water to form calcium hydroxide and hydrogen gas.

Teacher's Note:
a) Highly reactive metals like potassium, sodium, and calcium react with cold water.
b) Less reactive metals like iron and zinc react only with steam.

8. Which inert gas is used in the advertisement hoarding shown in the given picture? [1 Mark]
(a) Radon
(b) Argon
(c) Krypton
(d) Neon

[Figure: An advertisement hoarding with glowing letters reading "Espresso Coffee" and an arrow pointing to the glowing gas labelled "Inert gas".]

Answer: (d) Neon

Neon glow signs are widely used in advertising hoardings because neon emits a bright reddish-orange glow when an electric current passes through it.

Teacher's Note:
a) Different noble gases emit different colors when subjected to high voltage in discharge tubes.
b) Neon is particularly famous for bright red advertising signs.

9. In metal reactivity series the most reactive metal are at [1 Mark]
(a) Top
(b) Bottom
(c) Middle
(d) None

Answer: (a) Top

The activity series of metals arranges metals in order of decreasing reactivity, placing the most reactive metals at the top.

Teacher's Note:
a) Potassium and sodium are at the very top of the series, while gold and platinum are at the bottom.
b) Metals at the top displace those below them from their salt solutions.

10. When the temperature of water increases above 0°C up to 4°C, its density _________. [1 Mark]
(a) decreases
(b) increases
(c) becomes zero
(d) remains unchanged

Answer: (b) increases

Water exhibits anomalous expansion; when heated from 0°C to 4°C, it contracts, causing its density to increase to a maximum at 4°C.

Teacher's Note:
a) Water has maximum density and minimum volume at 4°C.
b) Do not confuse volume behavior with density behavior; they are inversely proportional.

11. Hydrogen burns in oxygen to form __________. [1 Mark]
(a) hydrochloric acid
(b) water
(c) hydrogen sulphide
(d) ammonia

Answer: (b) water

Hydrogen is a combustible gas that burns in oxygen or air to produce water vapor.

Teacher's Note:
a) The chemical equation is 2H₂ + O₂ → 2H₂O.
b) This reaction is highly exothermic and is the reason hydrogen is termed water-former.

12. The credit for the discovery of hydrogen goes to __________ .. [1 Mark]
(a) Rutherford
(b) James Chadwick
(c) Henry Cavendish
(d) Satyendra Nath Bose

Answer: (c) Henry Cavendish

Henry Cavendish discovered hydrogen in 1766 by reacting metals with acids, calling it "inflammable air".

Teacher's Note:
a) Lavoisier later named the gas hydrogen, which means "water-producer".
b) Note that the OCR text contained an artifact ("James ChadwickAcids, bases"), but the correct intended option is Henry Cavendish.

13. The process of removing oxygen from their compounds is called __________. [1 Mark]
(a) Reduction
(b) Combination
(c) Synthesis
(d) Oxidation

Answer: (a) Reduction

Reduction is classically defined as the removal of oxygen or the addition of hydrogen.

Teacher's Note:
a) Oxidation and reduction always occur simultaneously in redox reactions.
b) Memorize the classical definitions alongside electronic concepts.

14. Which of the following shells represent the orbit number (n) = 2? [1 Mark]
(a) K shell
(b) L shell
(c) M shell
(d) N shell

[Figure: A diagram of an atomic structure showing concentric circles labelled K, L, M, and N from the inside out around a central nucleus.]

Answer: (b) L shell

Shells are designated outwards from the nucleus as K (n=1), L (n=2), M (n=3), and N (n=4).

Teacher's Note:
a) The second shell from the nucleus corresponds to n = 2 and is named the L shell.
b) Always count shells starting from the innermost shell nearest to the nucleus.

15. The ___________ of water enables aquatic animals to survive in a reservoir where the surface layer of water has frozen. [1 Mark]
(a) anomalous expansion
(b) anomalous contraction
(c) volume
(d) Temperature

Answer: (a) anomalous expansion

Anomalous expansion causes water at 4°C to sink to the bottom of ponds, keeping the bottom liquid and allowing aquatic life to survive under ice.

Teacher's Note:
a) This unique property protects aquatic ecosystems during harsh winters.
b) Note that option (b) in the original scan had a typographical artifact ("anomalous contractionlead"), but anomalous expansion is the standard scientific term.

Question 2

(A) Define Matter. State the main postulates of kinetic theory of matter. [5 Marks]

Answer:
Definition of Matter:
Anything which occupies space or volume, has mass and can be perceived by our senses is called matter. All materials in the universe are made up of matter.
Postulates of the Kinetic Theory of Matter:
1. All matter is made up of tiny particles called atoms, molecules, or ions.
2. The particles of matter have spaces between them, known as intermolecular spaces.
3. The particles attract each other with a force of attraction known as intermolecular force.
4. The particles are in a continuous, random motion and possess kinetic energy.

Teacher's Note:
a) Ensure all three core properties of particles (spaces, forces, motion) are mentioned to secure full marks.
b) Students should clearly distinguish between interparticle space and interparticle force.

(B) Fill in the blanks and rewrite the sentences: [5 Marks]
1. The density of water is maximum at _________.
2. Galvanising is a process in which iron and steel are coated with a thin layer of _________ to protect them from corrosion.
3. The process of removing oxygen from its compounds is called __________.
4. The process in which a solid directly changes into a gas is called____________.
5. A change which alters the composition of a substance is known as a __________ change.

Answer:
1. The density of water is maximum at 4°C.
2. Galvanising is a process in which iron and steel are coated with a thin layer of zinc to protect them from corrosion.
3. The process of removing oxygen from its compounds is called reduction.
4. The process in which a solid directly changes into a gas is called sublimation.
5. A change which alters the composition of a substance is known as a chemical change.

Teacher's Note:
a) Direct factual recall is tested in fill-in-the-blank questions.
b) Spelling accuracy is vital for scientific terms like sublimation and galvanising.

Question 3

(A) State whether the following statements are true or false. Rewrite the false statement. [5 Marks]
1. Distilled water is used as an electrolyte in the electrolysis of water.
2. Graphite is the purest form of carbon.
3. Charcoal is good adsorbent.
4. Carbon monoxide is a poisonous gas.
5. The rays emitted from the cathode towards the anode in the discharge tube are called cathode rays.

Answer:
1. False. Acidified water (or water containing a small amount of acid/base) is used as an electrolyte in the electrolysis of water.
2. False. Diamond is the purest crystalline form of carbon.
3. True.
4. True.
5. True.

Teacher's Note:
a) Pure distilled water is a poor conductor of electricity, hence electrolytes must be added.
b) Diamond is the purest allotrope of carbon, while graphite is thermodynamically most stable.

(B) Explain efflorescence and deliquescence with the help of an example. [5 Marks]

Answer:
1. Efflorescence:
Efflorescence is the property of certain crystalline hydrated substances to lose their water of crystallisation, either partly or completely, when exposed to dry air even for a short period, turning into an amorphous powder.
Example: Washing soda (sodium carbonate decahydrate, Na₂CO₃·10H₂O) loses 9 molecules of water of crystallisation on exposure to dry air to form monohydrate sodium carbonate.
Na₂CO₃·10H₂O → Na₂CO₃·H₂O + 9H₂O
2. Deliquescence:
Deliquescence is the property of certain water-soluble substances to absorb moisture from atmospheric air at ordinary temperatures, become moist, and eventually dissolve in the absorbed water to form a saturated solution.
Example: Caustic soda (NaOH), calcium chloride (CaCl₂), and magnesium chloride (MgCl₂).

Teacher's Note:
a) Emphasize that efflorescent substances give up moisture to the air, whereas deliquescent substances absorb moisture from the air.
b) Chemical formulas and balanced equations enhance answers for efflorescence.

Question 4

(A) Explain the Greenhouse Effect. How can the effect be controlled? [5 Marks]

Answer:
Greenhouse Effect:
1. The greenhouse effect is the process of heating of the Earth's atmosphere due to the trapping of infrared radiations (reflected from the Earth's surface) by greenhouse gases such as carbon dioxide, water vapor, methane, and ozone.
2. These gases act as a thermal blanket, preventing heat energy from escaping directly into space, keeping the Earth warm enough to sustain life.
3. However, excessive accumulation of these gases due to human activities leads to enhanced global warming.
[Figure: Diagram showing sunlight passing through the atmosphere, warming the Earth, and infrared heat being trapped by greenhouse gases.]
Control Measures:
1. Reducing the emission of carbon dioxide by cutting down fossil fuel usage.
2. Planting more trees (afforestation) to absorb carbon dioxide via photosynthesis.
3. Adopting cleaner, renewable sources of energy such as solar, wind, and hydro power.

Teacher's Note:
a) Students should differentiate between the natural greenhouse effect (essential for life) and global warming (harmful excess).
b) Listing practical preventive measures is crucial for full marks.

(B) State physical properties of water. How does anomalous expansion of water helps aquatic organism in cold climates? [5 Marks]

Answer:
Physical Properties of Water:
1. Water is a clear, colorless, odorless, and tasteless liquid at room temperature.
2. Pure water boils at 100°C and freezes at 0°C under normal atmospheric pressure.
3. Water can exist in three states of matter: solid (ice), liquid (water), and gas (steam).
4. Water has a high specific heat capacity, allowing it to moderate temperatures effectively.
Anomalous Expansion and Aquatic Life:
1. Unlike other liquids that contract on cooling, water contracts when cooled down to 4°C, but on further cooling from 4°C to 0°C, it expands.
2. Consequently, water has maximum density at 4°C. Ice formed at 0°C is less dense than water and floats on top.
3. In cold climates, when the surface layer of a pond freezes into ice, the warmer, denser water at 4°C stays at the bottom, enabling aquatic organisms to survive safely beneath the frozen surface.

Teacher's Note:
a) Highlight the temperature of maximum density (4°C) explicitly.
b) Explain clearly why ice floats (lower density than liquid water) and how it insulates the water underneath.

Question 5

(A) Describe Rutherford’s scattering experiment. [5 Marks]

Answer:
1. Setup and Materials:
Rutherford bombarded a very thin gold foil (about 1000 atoms thick) with a stream of high-speed alpha particles (positively charged helium ions, He²⁺) emitted from a radioactive source.
[Figure: Diagram of Rutherford scattering experiment showing an alpha particle source, a thin gold foil, undeflected alpha particles, and deflected alpha particles striking a circular fluorescent zinc sulphide screen.]
2. Observations:
a. Most of the alpha particles passed straight through the gold foil without any deflection.
b. A few alpha particles were deflected through small or large angles.
c. A very small fraction (about 1 in 20,000) bounced back completely by 180°.
3. Conclusions:
a. Since most particles passed undeflected, most of the space inside the atom is empty.
b. The deflection of a few particles proved that the positive charge in an atom occupies very little space.
c. The rebounding of particles indicated that the entire mass and positive charge of the atom are concentrated in a tiny central core called the nucleus.

Teacher's Note:
a) Break the answer down into experimental setup, observations, and conclusions for clear presentation.
b) Emphasize alpha particles as doubly charged helium ions.

(B) Draw a neat labelled diagram for the laboratory preparation of hydrogen. Give balanced equations for reaction and how is hydrogen gas is collected? Why? [5 Marks]

Answer:
Laboratory Preparation of Hydrogen:
Hydrogen is prepared by the action of dilute hydrochloric acid or dilute sulphuric acid on granulated zinc.
[Figure: Labelled diagram of laboratory preparation of hydrogen showing a round-bottom flask with granulated zinc and dilute hydrochloric acid, a thistle funnel, a delivery tube, a glass trough filled with water, and a gas jar collecting hydrogen by downward displacement of water.]
Use of Granulated Zinc:
Granulated zinc contains copper impurities which act as a positive catalyst to increase the rate of the reaction, making the process smooth and steady.
Balanced Equations:
With hydrochloric acid:
Zn + 2HCl → ZnCl₂ + H₂↑
With sulphuric acid:
Zn + H₂SO₄ → ZnSO₄ + H₂↑
Collection of Gas:
Hydrogen gas is collected by the downward displacement of water.
Reason: Hydrogen is extremely sparingly soluble in water and is much lighter than air (highly insoluble and lighter than air).

Teacher's Note:
a) Ensure all parts of the apparatus (thistle funnel, delivery tube, gas jar, trough) are clearly labelled in diagrams.
b) Always write state symbols or upward arrows (↑) for gaseous products in chemical equations.

Question 6

(A) Explain with example: [5 Marks]
1. Combination reaction
2. Decomposition reaction
3. Displacement reaction
4. Double displacement reaction
5. Neutralisation reaction.

Answer:
1. Combination reaction: A reaction in which two or more elements or compounds combine to form a single new compound.
Example: Heating iron and sulfur together.
Fe + S → FeS
2. Decomposition reaction: A reaction in which a single compound breaks down into two or more simpler substances upon heating.
Example: Thermal decomposition of mercuric oxide.
2HgO → 2Hg + O₂↑
3. Displacement reaction: A reaction in which a more reactive element displaces a less reactive element from its aqueous salt solution.
Example: Zinc displacing copper from copper sulfate solution.
Zn + CuSO₄ → ZnSO₄ + Cu
4. Double displacement reaction: A reaction in which two compounds in aqueous solution react by exchanging their ions to form two new compounds.
Example: AB + CD → CB + AD
5. Neutralisation reaction: A reaction in which an acid reacts with a base to form salt and water.
Example: Reaction between sodium hydroxide and hydrochloric acid.
NaOH + HCl → NaCl + H₂O

Teacher's Note:
a) Provide balanced chemical equations for each reaction type to ensure full credit.
b) Clearly define the general mechanism alongside the specific example.

(B) State the atomic number, mass number and electronic configuration for each of the following elements: [5 Marks]
1. Carbon [p = 6, n = 6]
2. Helium [p = 2, n = 2]
3. Magnesium [p = 12, n = 12]
4. Boron [p = 5, n = 6]
5. Sodium [p = 11, n = 12]

Answer:
1. Carbon:
- Atomic number (Z) = p = 6
- Mass number (A) = p + n = 6 + 6 = 12
- Electronic configuration = 2, 4
2. Helium:
- Atomic number (Z) = p = 2
- Mass number (A) = p + n = 2 + 2 = 4
- Electronic configuration = 2
3. Magnesium:
- Atomic number (Z) = p = 12
- Mass number (A) = p + n = 12 + 12 = 24
- Electronic configuration = 2, 8, 2
4. Boron:
- Atomic number (Z) = p = 5
- Mass number (A) = p + n = 5 + 6 = 11
- Electronic configuration = 2, 3
5. Sodium:
- Atomic number (Z) = p = 11
- Mass number (A) = p + n = 11 + 12 = 23
- Electronic configuration = 2, 8, 1

Teacher's Note:
a) Atomic number equals the number of protons, and mass number is the sum of protons and neutrons.
b) Follow the 2, 8, 18 shell filling rules when writing electronic configurations.

Question 7

(A)

1. Differentiate between physical change and chemical change [3 Marks]

Answer:

Teacher's Note:
a) Tabular format is best for differentiation questions.
b) Ensure key differences like reversibility and formation of new substances are highlighted.

2. State the formula of the following compounds: [2 Marks]
a. Ammonium bicarbonate
b. Aluminium oxide

Answer:
a. Ammonium bicarbonate:
Symbol / Radical: NH₄⁺¹ and HCO₃^-1
Formula: NH₄HCO₃
b. Aluminium oxide:
Symbol / Radical: Al³⁺ and O^2-
Formula: Al₂O₃

Teacher's Note:
a) Cross-valency method helps in deriving correct chemical formulas easily.
b) Always write radicals with correct electrical charges.

(B)

1. Explain the term compound with suitable examples. [2 Marks]

Answer:
1. A compound is a pure substance composed of two or more elements chemically combined in a fixed proportion by mass.
2. The properties of a compound are entirely different from the properties of its constituent elements.
Examples: Water (H₂O) and Carbon dioxide (CO₂).

Teacher's Note:
a) Emphasize that constituents in a compound are combined chemically, not physically.
b) Mention that compounds have fixed melting and boiling points.

2. What is destructive distillation? What are the products formed due to the destructive distillation of coal? [3 Marks]

Answer:
Destructive Distillation:
The process of heating a solid substance strongly in the absence of air to decompose it into volatile and non-volatile products is called destructive distillation.
Products Formed from Coal:
When coal undergoes destructive distillation, the main products formed are:
1. Coke: A solid carbon-rich residue.
2. Coal tar: A thick, dark brown liquid mixture.
3. Coal gas: A combustible gaseous mixture used as fuel.
4. Ammoniacal liquor: A watery solution of ammonia.
[Figure: Flowchart showing Coal undergoing destructive distillation yielding Coke + Coal tar + Coal gas + Ammoniacal liquor.]

Teacher's Note:
a) The key condition for destructive distillation is the strict absence of air.
b) List all four primary byproducts of coal processing clearly to earn full marks.