CBSE Class 8 Science — NCERT Solutions

Given: Labels to be placed on a cell diagram.

Concept: A typical plant cell has a cell wall (outermost), cell membrane (just inside the cell wall), cytoplasm (fluid filling the cell), nucleus (control centre, surrounded by nuclear membrane), and chloroplasts (green plastids for photosynthesis). A prokaryotic cell (like a bacterium) has a nucleoid instead of a true nucleus and lacks a membrane-bound nucleus.

Placement of labels:

Key distinction: The nucleus (with a membrane) is found in plant and animal cells (eukaryotes), while the nucleoid (without a membrane) is found in bacteria (prokaryotes). Chloroplasts and cell wall are features of plant cells only.

Correct Option: (c) Yeast produced a gas inside the test tube B which inflated the balloon.

Justification: Yeast is a microorganism (fungus) that breaks down sugar in the presence of warmth through a process called fermentation. During fermentation, yeast converts sugar into alcohol and carbon dioxide (CO₂) gas. This CO₂ gas accumulates inside the test tube and inflates the balloon attached to test tube B. Test tube A has no yeast, so no gas is produced and its balloon remains uninflated. Options (a), (b), and (d) are incorrect because water evaporation, atmospheric warmth, and sugar alone cannot produce enough gas to inflate the balloon under these conditions.

Given: Five diseases — Cold and flu, Typhoid, Diabetes, Asthma, Chickenpox.

Concept: Communicable diseases are caused by pathogens (viruses, bacteria, etc.) and can spread from one person to another. Non-communicable diseases are not caused by pathogens and cannot spread from person to person.

Classification:

Answer:

• Communicable: Cold and flu, Typhoid, Chickenpox
• Non-communicable: Diabetes, Asthma

Correct Option: (b) (ii) and (iii)

Justification:

• Typhoid is caused by the bacterium Salmonella typhi — it is a communicable disease.
• Measles is caused by a virus — it is a communicable disease.
• Asthma is a chronic respiratory condition triggered by environmental and genetic factors — it is non-communicable.
• Diabetes is caused by hormonal imbalance, unhealthy lifestyle, and genetic factors — it is non-communicable.

Therefore, the non-communicable diseases are Asthma (ii) and Diabetes (iii). The correct answer is (b).

Given: A statement about the Voltaic cell.

Concept: In a Voltaic cell, a conducting liquid (solution) is used through which ions move to complete the circuit and allow chemical reactions to generate electricity.

Answer: The solution used in a Voltaic cell is called electrolyte.

The electrolyte (e.g., lemon juice, salt solution, dilute sulphuric acid) conducts electricity between the two metal electrodes and enables the chemical reactions that produce electric current.

Given: A statement about a current-carrying coil.

Concept: When electric current flows through a coil of wire, it produces a magnetic field around it, giving it properties similar to a bar magnet (with a North pole and a South pole).

Answer: A current carrying coil behaves like a magnet (electromagnet).

It can attract magnetic materials and deflect a magnetic compass needle, just like a bar magnet does.

Given: Five types of forces are to be matched with their examples.

Concept used: Each type of force is identified by its nature — muscular force involves body muscles, magnetic force involves magnets, frictional force opposes motion, gravitational force is Earth's pull, and electrostatic force involves charged objects.

Matching:

• (i) Muscular force → (b) A child lifting a school bag

A child uses the muscles of the body to lift the bag, which is an example of muscular force.

• (ii) Magnetic force → (e) A compass needle pointing North

The Earth acts like a giant magnet and exerts magnetic force on the compass needle, making it point North.

• (iii) Frictional force → (a) A cricket ball stopping on its own just before touching the boundary line

The ball slows down and stops due to the force of friction acting between the ball and the ground, opposing its motion.

• (iv) Gravitational force → (c) A fruit falling from a tree

The Earth exerts gravitational force (pull) on the fruit, causing it to fall downward.

• (v) Electrostatic force → (d) Balloon rubbed on woollen cloth attracting hair strands

Rubbing the balloon on woollen cloth creates static charges on it; the charged balloon then attracts hair strands through electrostatic force.

Final Answer:
(i) → (b), (ii) → (e), (iii) → (a), (iv) → (c), (v) → (d)

Correct option: (d) equal in all three vessels

Justification: Liquids find their own level when connected to each other. Since all three vessels P, Q, and R are connected at the bottom (communicating vessels), the water level will equalise and become the same in all three vessels, regardless of their shapes or sizes. This is a direct consequence of the principle that liquid pressure depends only on the height of the liquid column, not on the shape or volume of the container.

Correct Option: (iv) closely packed in solids and move past each other in liquids.

Justification:
In solids, the constituent particles are tightly packed and held in fixed positions by very strong interparticle forces of attraction; they can only vibrate about their mean positions but cannot move past each other.

In liquids, the particles are still relatively close together but the interparticle forces are slightly weaker, allowing the particles to move past each other freely within a limited space. This is why liquids flow and take the shape of their container.

Options (i), (ii), and (iii) are incorrect because:

• Particles in liquids are NOT stationary — they move freely.
• Particles in solids are NOT far apart — they are closely packed.
• Particles in solids do NOT always move freely — they only vibrate about fixed positions.

Correct Option: (iv) A and B are elements, C is a compound, and has a fixed composition.

Justification:

• Since A and B cannot be broken down into simpler substances by chemical reactions, they are elements (elements are pure substances that cannot be split into simpler substances by any chemical method).
• When two or more elements combine chemically in a fixed ratio, they form a compound. Here, A and B combine to form C, so C is a compound.
• Compounds always have a fixed composition (fixed ratio of constituent elements).
• Options (i), (ii), and (iii) are incorrect because A and B are elements, not compounds, and both elements and the compound C have fixed compositions. However, the most complete and accurate statement is option (iv).

(i) False [F]
Correction: Oxygen gas is less soluble in hot water than in cold water. Generally, the solubility of gases decreases with an increase in temperature.

(ii) False [F]
Correction: A mixture of sand and water is not a solution. It is a heterogeneous (non-uniform) mixture because sand does not dissolve in water and the particles are visible and settle down.

(iii) False [F]
Correction: The amount of space occupied by any object is called its volume, not mass. Mass is the amount of matter present in an object.

(iv) False [F]
Correction: A saturated solution has the maximum amount of solute dissolved at a given temperature. An unsaturated solution has less solute dissolved than a saturated solution (i.e., more solute can still be dissolved in it).

(v) True [T]
The atmosphere is a uniform mixture of different gases such as nitrogen, oxygen, carbon dioxide, argon, etc., so it qualifies as a solution (gaseous solution).

Given: Angle of incidence (angle between incident ray and normal) = 40°

Concept used: By the first law of reflection, the angle of incidence equals the angle of reflection. The angle made by a ray with the mirror = 90° − (angle made with the normal).

Working:

• Angle of reflection = angle of incidence = 40°
• Angle made by the reflected ray with the mirror = 90° − 40° = 50°

Answer: Option (ii) 50°

Correct answer: (iii) An ecosystem is part of a community.

Justification:
The correct hierarchy from smallest to largest is: Individual → Population → Community → Ecosystem.

• Statement (i) is correct: A community includes many populations, so it is larger than a single population.
• Statement (ii) is correct: A community is made up of biotic components only, whereas an ecosystem includes both biotic and abiotic components, making the ecosystem larger.
• Statement (iii) is wrong: It is the other way around — a community is a part of an ecosystem, not the ecosystem a part of the community. An ecosystem = community (biotic) + abiotic components.
• Statement (iv) is correct: A population (group of one species) is indeed a part of a community (group of different populations).

Therefore, the wrong statement is (iii).

Given: We need to identify features of Earth that make it special for life.

Additional features (rows 3 and 4) that can be filled in:

Row 3: The Earth has liquid water on its surface — oceans, rivers, and lakes — which is essential for all known life. Most other planets have water only as ice or vapour.

Row 4: The Earth has a moderate average temperature (about 15°C) that allows water to remain liquid and supports the survival of living organisms. This is maintained by the right distance from the Sun and the greenhouse effect of the atmosphere.

Other possible entries:

• The Earth has a magnetic field that protects us from harmful solar radiation.
• The Earth's atmosphere contains the right mix of gases (oxygen, nitrogen, carbon dioxide) needed for life.

Conclusion: Earth is unique because it has the right combination of gravity, atmosphere, temperature, water, and magnetic field to support and sustain life.