CBSE Important Questions Class 8 Science Chapter 13 – Our Home: Earth, a Unique Life Sustaining Planet

Class 8 Science Chapter 13 Our Home: Earth, a Unique Life Sustaining Planet is part of the new NCERT Curiosity textbook. It explains why the Earth is the only planet we know of that supports life. Students learn about the habitable zone, the role of the atmosphere, the ozone layer, the greenhouse effect and the Earth's magnetic field, the four spheres of the Earth, and how nutrients such as carbon, oxygen and nitrogen keep cycling through nature.

These CBSE important questions for Class 8 Science Chapter 13 cover identifying a nutrient cycle from a diagram, growing a rose plant from a stem cutting, why a smaller planet loses its atmosphere, why Venus is hotter than Mercury, how the magnetic field protects us, the habitable zone, the four spheres of the Earth, and the many factors that together make the Earth suitable for life. Every question has a clear step-by-step answer in simple English with figures, so students can practise on their own, parents can check the answers, and teachers can use them for class tests.

Q1. The given diagram shows a nutrient cycle. Identify which nutrient cycle is depicted in the diagram.
(a) Water cycle
(b) Carbon cycle
(c) Oxygen cycle
(d) Nitrogen cycle

Answer: (b) Carbon cycle
The diagram shows photosynthesis, respiration, decomposition, the burning of fossil fuels and the movement of the gas back to the air. These are exactly the steps of the carbon cycle. Plants take carbon dioxide from the air during photosynthesis; animals and plants return it by respiration; decomposers release it from dead matter; and burning fossil fuels adds a large amount back to the air.

Q2. A girl wants to grow a rose plant in her garden from a cutting. She has cut four branches of the plant for propagation. Predict which stem cutting will give rise to a new plant.

Answer: The cutting that has healthy buds (nodes) on it will grow into a new plant.
In vegetative propagation, new roots and shoots grow from the buds present at the nodes of the stem. A cutting without any node or bud cannot produce a new plant, however fresh it looks. The lower end of the cutting is planted in moist soil, roots grow from the buried node, and a shoot grows from the bud above the soil. The new plant is exactly like the parent plant.

Q3. A scientist discovers a new planet at the same distance from its star as the Earth is from the Sun, but it is only half the size of the Earth with the same average density. What would happen to this planet's atmosphere over time?

Answer: The planet would slowly lose its atmosphere into space.
Being half the size with the same density, it has much less mass, and so its gravity is much weaker. Gravity is what holds the gases of an atmosphere close to a planet. With weak gravity, the fast-moving gas molecules escape into space one by one.
Without an atmosphere there would be no shield against harmful radiation, no air pressure to keep water in the liquid state, and no gases to breathe. So the planet could not support life even though it lies at the right distance from its star.

Q4. Venus is hotter than Mercury despite being farther from the Sun. A student argues: "If we could remove half of Venus's atmosphere, its temperature would become suitable for life." Evaluate whether this solution would work.

Answer: This solution would not work.
The surface of Venus is about 450 °C because its atmosphere is 96 per cent carbon dioxide, which traps heat through a runaway greenhouse effect. Removing half of it would still leave an extremely thick atmosphere made almost entirely of carbon dioxide, so it would go on trapping enormous amounts of heat.
Compare this with the Earth, which stays at a comfortable 15 °C with only traces of greenhouse gases. The problem on Venus is the composition of its atmosphere, not merely the quantity. Venus also has no magnetic field and lies closer to the Sun than the comfortable part of the habitable zone, so it would still be far too hostile for life.

Q5. Compare how the greenhouse effect operates on Earth and on Venus, including atmospheric composition, temperature outcomes and water availability.

The key point is that a balanced greenhouse effect keeps the Earth warm enough for life, while an excessive one has made Venus uninhabitable.

Q6. Earth's magnetic field protects life. Explain how it provides this protection by describing at least three specific mechanisms.

Answer: The Earth's magnetic field is produced by the movement of molten iron in the Earth's core, and it protects us in these ways:
1. It acts as a shield that pushes away the high-energy charged particles of the solar wind and cosmic rays coming from space.
2. By turning these particles away, it protects the atmosphere from being stripped off and keeps the ozone layer from being destroyed.
3. It blocks harmful radiation that would otherwise damage the cells of living things and make the surface unsafe for life.
Mars is thought to have lost most of its atmosphere because its magnetic field weakened long ago.

Q7. A space agency plans to set up a research station either on Mars or on a moon of Jupiter. Both are outside the habitable zone. Explain what critical challenge would exist at both locations.

Answer: The habitable zone, also called the Goldilocks zone, is the band of distances from a star where the temperature is just right for water to stay liquid.
Mars lies at the outer edge of this zone and Jupiter's moons lie far beyond it. So at both places the temperature is far too low for water to remain liquid on its own; it would stay frozen as ice.
Liquid water is essential for life as we know it, because all the chemical reactions in a living body take place in water. A station at either location would therefore need artificial systems to heat and hold liquid water, and natural life could not survive there on its own.

Q8. Fill in the blanks.
(a) The four spheres that interact to sustain life on Earth are atmosphere, _______, _______, and biosphere.
(b) Earth's core contains _____ whose movement creates Earth's _______.

Answer: (a) The four spheres are the atmosphere, the hydrosphere (all the water), the geosphere (the rocks and soil) and the biosphere (all living things).
(b) The Earth's core contains molten iron, whose movement creates the Earth's magnetic field.

Q9. State true or false with justification.
(a) "Mars once had liquid water, proving it was always in the habitable zone."
(b) "Earth's circular orbit alone makes it suitable for life."

Answer: (a) False. Mars lies at the very edge of the habitable zone today, and scientists believe it may have had liquid water long ago under different conditions, when it had a thicker atmosphere. This shows that conditions on Mars changed over time; it does not prove that Mars was always comfortably inside the zone.
(b) False. A nearly circular orbit does help, because it gives the Earth a steady supply of heat and prevents extreme seasons. But it is only one of many factors. The Earth also needs the right size so that gravity can hold the atmosphere, a magnetic field for protection, the right distance from the Sun, and an atmosphere with an ozone layer. All of these together make the Earth suitable for life.

Q10. Identify the odd one out and justify.
(a) Ozone layer, Magnetic field, Greenhouse effect, Asteroid belt
(b) Mercury's thin atmosphere, Venus's thick atmosphere, Earth's moderate atmosphere, Mars's thin atmosphere

Answer: (a) The odd one is the asteroid belt. The ozone layer, the magnetic field and the greenhouse effect are all things that protect the Earth or control its temperature so that life can survive. The asteroid belt is simply a ring of rocky bodies between Mars and Jupiter and plays no part in making the Earth habitable.
(b) The odd one is Earth's moderate atmosphere. Mercury and Mars have atmospheres that are far too thin, and Venus has one that is far too thick; none of the three can support life. Only the Earth has an atmosphere of just the right thickness and composition, which is why life exists here.

Q11. What is the habitable zone? Why is the Earth said to lie in it?

Answer: The habitable zone, or Goldilocks zone, is the range of distances from a star within which a planet is neither too hot nor too cold, so that water can stay in the liquid state on its surface.
The Earth lies in this zone around the Sun. It is close enough to receive plenty of warmth, but far enough that the water does not boil away. Because of this, the Earth has oceans, rivers and lakes of liquid water, which are essential for life.
A planet too close, like Venus, becomes scorching hot; one too far, like Mars, becomes freezing cold.

Q12. Name the four spheres of the Earth and write one line about each. How do they interact?

Answer: 1. Atmosphere: the blanket of gases around the Earth, which gives us air to breathe and shields us from harmful radiation.
2. Hydrosphere: all the water on the Earth, in oceans, rivers, lakes, ice and underground.
3. Geosphere (lithosphere): the solid part, made of rocks, soil and minerals.
4. Biosphere: all the living things, from bacteria to plants, animals and human beings.
How they interact: they are constantly exchanging matter and energy. Water evaporates from the hydrosphere into the atmosphere and falls back as rain onto the geosphere; plants of the biosphere draw water and minerals from the soil and give oxygen to the air. Life is possible only because all four spheres work together.

Related Study Material on Extramarks

• CBSE Important Questions Class 8 Science Chapter 1
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