Class 8 Science · Original EZT practice
Our Home: Earth, a Unique Life Sustaining Planet (Class 8 Science Chapter 13): summary and practice
Earth is the only planet we know that sustains life. This chapter explores why: its distance from the Sun, its size and gravity, its magnetic field, and its air, water and soil - and how reproduction keeps life going despite disasters. It closes with the threats to life, including the triple planetary crisis of climate change, biodiversity loss and pollution. All questions are original EZT tutor practice following the chapter's own section order.
6 original worked examples, 6 fresh self-check questions and 4 mixed questions, with explained answers. These are not copied textbook exercises, an official chapter index or a full answer key. No affiliation with NCERT is claimed.
Official sources for topic and edition context, checked 22 October 2026:
1. Why is Earth a unique planet?
Of the eight planets, Mercury, Venus, Earth and Mars are small and rocky. Venus is the hottest planet - hotter even than Mercury - because its thick carbon dioxide atmosphere traps heat by the greenhouse effect.
2. What makes Earth suitable for life?
Earth sits in the Sun's habitable zone (the Goldilocks zone) where temperatures allow liquid water; its near-circular orbit keeps sunlight nearly steady; its size gives enough gravity to hold an atmosphere; and its magnetic field, believed to arise from molten iron moving in the core, shields life from high-energy particles.
3. What allows life to be sustained?
Air, water and sunlight, together with soil, rocks and minerals, support plants, animals and microorganisms. Life persists because these components work in balance, and reproduction - asexual or sexual - lets life continue even when individuals die.
4. What are the threats to life on Earth?
Human actions are disturbing the balance that sustains life. Climate change, biodiversity loss and pollution together are called the triple planetary crisis, and even small changes in temperature, oxygen levels or the ozone layer can put life at risk.
Choose, solve, check, then try a fresh question.
Before calculating, read the method prompt and name the condition that makes it useful. Write your own attempt. Open the steps only when you need them, and use the check to catch a wrong sign, unit or interpretation. Finish with the fresh question before reading its answer.
Explain why Venus is hottest
Choose the method: It is the blanket, not the distance.
EZT practice question
Mercury is closest to the Sun, yet Venus is the hottest planet. Explain.
Show the worked solution
- Mercury has almost no atmosphere to hold heat.
- Venus has a dense atmosphere made almost entirely of carbon dioxide.
- The strong greenhouse effect traps heat, pushing Venus past Mercury.
Answer: Venus has a dense atmosphere made almost entirely of carbon dioxide. The resulting strong greenhouse effect traps heat, making Venus even hotter than Mercury, which has no such blanket of gas.
Check: Distance alone would make Mercury hottest - so the atmosphere must be the deciding factor, and it is.
Watch for this: Do not answer 'because Venus is closer' - it is not; Mercury is closest to the Sun.
Now try it yourself
List the planets in order from the Sun.
Check your answer
Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
Earth is the third planet from the Sun.
Compare Venus with Earth
Choose the method: Third from the Sun, near-twin in size.
EZT practice question
Which planet is third from the Sun, and roughly how big is Venus compared with Earth?
Show the worked solution
- Count outward from the Sun: Mercury, Venus, Earth.
- Earth is the third planet.
- Venus is nearly Earth's size - about 0.95 times.
Answer: Earth is third; Venus is nearly Earth's size, about 0.95 times.
Check: Similar size, very different conditions - which is why the atmosphere matters so much in the previous question.
Watch for this: Do not call Venus the second-largest planet - the comparison here is about its size relative to Earth.
Now try it yourself
Which four planets are small and rocky?
Check your answer
Mercury, Venus, Earth and Mars.
They are the inner planets of the solar system.
Define the habitable zone
Choose the method: Not too hot, not too cold.
EZT practice question
What is the habitable zone, and why is Mars of special interest to scientists looking for past life?
Show the worked solution
- The habitable zone is the region around a star where temperatures allow liquid water to exist.
- It is also called the Goldilocks zone - not too hot, not too cold.
- Mars lies at the edge of the Sun's habitable zone, and rovers have found signs it may once have had conditions suitable for life.
Answer: The habitable zone is the region around a star where temperatures allow liquid water to exist - also called the Goldilocks zone. Mars lies at the edge of the Sun's habitable zone, and rovers have found signs that it may once have had conditions suitable for life.
Check: Liquid water is the key test - that is why the zone is defined by temperature.
Watch for this: Do not say Mars is inside the comfortable middle of the zone - it sits at the edge, which is why its past interests scientists.
Now try it yourself
How does Earth's almost circular orbit help keep conditions liveable?
Check your answer
It keeps the amount of sunlight and heat nearly steady through the year, preventing extreme summers and winters at most places.
A steady orbit means steady heating.
Recall the magnetic shield
Choose the method: Molten iron on the move.
EZT practice question
What is believed to be the origin of Earth's magnetic field?
Show the worked solution
- Deep inside the Earth lies a core containing molten iron.
- The movement of this molten iron is believed to generate the magnetic field.
- The field shields life from high-energy particles coming from space.
Answer: The movement of molten iron in the Earth's core.
Check: The field's job - deflecting high-energy particles - is one more condition that keeps Earth liveable.
Watch for this: Do not credit the crust or the oceans - the chapter traces the field to the moving molten iron in the core.
Now try it yourself
What does Earth's magnetic field protect life from?
Check your answer
High-energy particles from space.
Without the shield, those particles would reach the surface.
Explain how life continues
Choose the method: New individuals replace the old.
EZT practice question
How does reproduction keep life from disappearing even though every organism dies?
Show the worked solution
- Every organism eventually dies.
- Before or as they die, organisms produce new individuals - asexually with one parent, or sexually with two.
- Each generation replaces the last, so life has continued for billions of years.
Answer: Organisms produce new individuals before or as they die - asexually (one parent, as in many microbes and plants) or sexually (two parents) - so each generation replaces the last and life continues.
Check: Without reproduction, one disaster could end a species; with it, life recovers and carries on.
Watch for this: Do not say individual organisms survive - it is the generations replacing each other that sustain life.
Now try it yourself
Name three non-living things on Earth that living things depend on.
Check your answer
Air, water and sunlight - also soil, rocks and minerals.
Living things need both the living and non-living components.
Explain the CO2 imbalance
Choose the method: Input outruns absorption.
EZT practice question
Why can the Earth not simply absorb the extra carbon dioxide from burning fossil fuels?
Show the worked solution
- Burning coal and oil releases carbon locked underground for millions of years.
- Trees, plants and ocean plankton do absorb CO2 as they grow.
- But they absorb far too slowly to match the extra input, so heat builds up - melting ice caps and raising sea levels.
Answer: Burning coal and oil releases carbon locked underground for millions of years. Trees, plants and ocean plankton absorb CO2 as they grow, but far too slowly to match this extra input, so heat builds up - melting ice caps and raising sea levels.
Check: The imbalance is a rate problem: the sinks still work, just not fast enough.
Watch for this: Do not say nothing absorbs CO2 - plenty does; the problem is that the extra input is too fast.
Now try it yourself
What is the triple planetary crisis?
Check your answer
Climate change, biodiversity loss and pollution, occurring together.
The three threats reinforce one another.
Try a mixed set
Choose a method or explanation before opening each answer.
1. Why is Venus hotter than Mercury?
Show answer and reason
Its thick carbon dioxide atmosphere traps heat by the greenhouse effect.
Mercury has almost no atmosphere to hold heat.
2. Goldilocks zone?
Show answer and reason
The habitable zone around a star where temperatures allow liquid water.
Earth lies within it.
3. Origin of Earth's magnetic field?
Show answer and reason
Movement of molten iron in the core.
The field shields life from high-energy particles.
4. Triple planetary crisis?
Show answer and reason
Climate change, biodiversity loss and pollution together.
Caused by human actions disturbing the balance.
Try it yourself
Work these on paper before checking any notes. Answers are deliberately not shown here.
- Which planet is third from the Sun, and roughly how big is Venus compared with Earth?
- How does Earth's almost circular orbit help keep conditions liveable?
- How could global warming threaten coastal cities?
Know the rule but lose track of the steps?
Bring your book edition, your school topic list and a question you attempted. Ask a tutor to explain the choice of method and test it with a different problem. Agree the lesson topics, schedule and fee before starting.
For cost context, see home tuition fees in Delhi NCR. Agree the topics, schedule and fee with your chosen tutor.
Questions about this practice page
- Why is Venus hotter than Mercury even though Mercury is closer to the Sun?
- Venus has a thick carbon dioxide atmosphere whose greenhouse effect traps heat; Mercury has almost no atmosphere to hold heat.
- What is the Goldilocks zone?
- Another name for the habitable zone - the region around a star where temperatures are just right for liquid water, which Earth lies within.
- What protects Earth from high-energy particles from space?
- Earth's magnetic field, believed to be produced by the movement of molten iron in its core.
- What is the triple planetary crisis?
- The combined threat of climate change, biodiversity loss and pollution caused by human actions.
- Are these the NCERT exercise questions for Class 8 Science Chapter 13?
- No. EZT pages are original practice following the chapter's section order - use them to prepare for the textbook exercises, not as a substitute for your book.