Where Mindful Learning Begins
Where Mindful Learning Begins
Class 8 Science Curiosity NCERT Solutions Chapter 13 — every Probe and Ponder prompt, all 3 Activities, all 12 "Keep the Curiosity Alive" exercise questions, and all 5 "Discover, Design, and Debate" project prompts, solved and explained on one page.
This final chapter of the book pulls together the whole year's science to explain why Earth — and no other known planet — sustains life: its position in the Sun's habitable zone, its size, its magnetic field, and its four interacting systems (atmosphere, hydrosphere, geosphere, biosphere). It then covers how reproduction keeps life going and changing, and closes with the real threats life on Earth faces today.
Chapter 13 is the final chapter of the Grade 8 Curiosity book, and it ties together nearly everything learnt across the year to answer one big question: why is Earth able to sustain life when, as far as we know, no other planet can? It builds the case through Earth's position in the Sun's habitable (Goldilocks) zone, its right size for holding an atmosphere without crushing gravity, its protective magnetic field and ozone layer, and the way its four systems — atmosphere, hydrosphere, geosphere, and biosphere — interact. It then explains how reproduction (asexual and sexual) keeps life continuing and slowly changing across generations, and closes with the real threats — climate change, biodiversity loss, and pollution — that are now testing that balance. Every Activity and exercise is solved here exactly as the textbook presents it.
The right distance from the Sun, the right size, a magnetic field, and an ozone layer all combine to make Earth suitable for life.
Asexual and sexual reproduction keep species going — and sexual reproduction lets new features appear across generations.
Climate change, biodiversity loss, and pollution are today's biggest threats to the delicate balance that sustains life.
Why Earth is a unique planet
All of life exists only on Earth's thin crust — if Earth were the size of an apple, the crust would be as thin as the apple's skin. Four factors together make this crust habitable:
| Factor | Why it matters |
|---|---|
| Position (habitable/Goldilocks zone) | Earth orbits the Sun at just the right distance for water to stay liquid — not so close that it evaporates, not so far that it freezes |
| Size | Earth is large enough for its gravity to hold onto an atmosphere, but not so large that gravity would crush life |
| Magnetic field | Shields Earth from harmful cosmic rays and solar wind that could strip away the atmosphere and ozone layer |
| Ozone layer | Blocks harmful ultraviolet (UV) rays from the Sun that would otherwise damage living cells |
Venus, not Mercury, is the hottest planet — even though Mercury is closer to the Sun — because Venus's thick carbon dioxide atmosphere traps heat via the greenhouse effect. On Earth, a milder greenhouse effect (from gases like carbon dioxide) keeps the planet just warm enough for liquid water, rather than letting heat escape entirely into space. A plant greenhouse traps warmth differently — simply by physically enclosing warmed air behind glass walls, not by absorbing radiation the way atmospheric gases do.
Earth's four interacting systems
| System | Made of |
|---|---|
| Atmosphere | The layer of air/gases surrounding the Earth |
| Hydrosphere | All the water on Earth — oceans, rivers, lakes, groundwater (covers ~70% of the surface) |
| Geosphere | The solid parts of the Earth — rocks, soil, and minerals |
| Biosphere | All living beings, together with the land, water, and air where they live |
Geodiversity is the variety of landforms, rocks, and soils on Earth, along with the processes that shape and alter them — it helps create the many different habitats where different forms of life can thrive.
Reproduction — keeping life going and changing
| Type | How it works | Result |
|---|---|---|
| Asexual reproduction | A single parent produces new individuals (e.g. vegetative propagation from a stem, root, or leaf) | Offspring are exact genetic copies of the parent |
| Sexual reproduction | Two parents each contribute a gamete (sperm/egg in animals, pollen/ovule in plants) carrying half their genetic material | Offspring get a unique mix of genes — resembling but never identical to either parent |
Genes are the instructions stored in every cell that guide how an organism develops. In plants, pollination (pollen moving from anther to another flower's ovule) is followed by fertilisation (forming a zygote that becomes a seed); the fleshy part around the ovule becomes the fruit. In animals, fertilisation may happen outside the body in water (fish, frogs) or inside the female's body (birds, mammals) — after which birds lay a nutrient-packed egg to develop outside, while most mammals develop the embryo inside the mother's body.
Threats to life on Earth — the triple planetary crisis
| Threat | Cause | Effect |
|---|---|---|
| Climate change | Burning fossil fuels releases greenhouse gases (CO₂, methane) faster than Earth can absorb them | Global warming, melting ice caps, rising sea levels, extreme weather |
| Biodiversity loss | Destruction of natural habitats (deforestation, land-use change) | Species disappear, disrupting food chains and ecosystem balance |
| Pollution | Factory, vehicle, farm, and plastic waste entering air, water, and soil | Breathing problems, smog, acid rain, unsafe water, reduced crop yields |
Since the atmosphere, hydrosphere, geosphere, and biosphere are all connected, damage to one threatens the others — which is why protecting clean air, water, soil, and biodiversity together is essential for keeping Earth habitable.
Sample answer: without any life, Earth would likely look far more like Mars or the Moon — a barren, rocky landscape of mountains, craters, and perhaps ice or liquid water, but with no green vegetation, no visible biodiversity, and possibly a very different atmosphere (since plants play a huge role in producing the oxygen we currently see in Earth's air).
Life has survived for billions of years mainly because of reproduction (which keeps each kind of organism continuing) combined with small genetic variations passed on through sexual reproduction (which let some organisms adapt to changing conditions over time) — plus Earth's own systems (atmosphere, hydrosphere, geosphere, biosphere) working together to keep conditions within a survivable range even through major disruptions.
Dogs don't lay eggs and hens don't give birth to live chicks because each species' reproductive strategy is fixed by its genetic instructions passed down through countless generations — mammals like dogs evolved to develop their embryo inside the mother's body (live birth), while birds like hens evolved to lay a nutrient-rich egg that completes development outside the body. These are simply two different, well-established survival strategies, not something an individual animal can switch between.
If a spaceship carried Earth-like soil and water to Mars, plants might have a better chance than in Martian regolith alone, since Earth soil already contains organic nutrients and beneficial microbes that plant roots depend on — but success would still depend heavily on other Martian conditions like extremely low atmospheric pressure, intense radiation (due to the lack of a strong magnetic field and thick atmosphere), and very cold average temperatures, meaning the soil and water alone likely wouldn't be enough without additional protection like a controlled greenhouse environment.
This is an open reflection prompt meant to set up the chapter's central ideas — Earth's uniqueness, reproduction, and life's resilience — all explained in full through the Activities below.
Given examples: the air we breathe doesn't fly off into space (even though gas particles move freely and don't have a fixed volume), and we can stand on the ground held by gravity while our heart still pumps blood up to our head.
Additional features to add (Table 13.1, rows 3–4):
| S.No. | Interesting feature of the Earth |
|---|---|
| 3. | Water on Earth stays mostly in liquid form, rather than boiling away or freezing solid everywhere, because Earth sits in the Sun's "habitable zone." |
| 4. | We can't see or feel it, but the ozone layer is constantly blocking harmful ultraviolet rays from the Sun that would otherwise damage our skin and cells. |
Conclusion: Earth provides the air we breathe, the water we drink, the soil that grows our crops, and materials like rock and timber for building — all quietly essential, taken-for-granted features that make Earth a genuinely unique and life-sustaining planet.
Sample completed table (approximate values — exact figures can vary slightly by source):
| S.No. | Planet | Average temperature (°C) | Radius (× Earth) | Has an atmosphere? |
|---|---|---|---|---|
| 1 | Mercury | 170 | ~0.38 | No |
| 2 | Venus | 450 | 0.95 | Yes |
| 3 | Earth | 15 | 1 | Yes |
| 4 | Mars | ~ -60 | ~0.53 | Yes (very thin) |
| 5 | Jupiter | ~ -110 | ~11 | Yes |
| 6 | Saturn | ~ -140 | ~9.1 | Yes |
| 7 | Uranus | ~ -195 | ~4.0 | Yes |
| 8 | Neptune | -200 | 4 | Yes |
Is this the pattern expected? Generally yes — planets get colder as they move farther from the Sun, since they all get their energy from it. The one clear exception is Venus, which is hotter than even Mercury (which is closer to the Sun) because of its extremely thick carbon-dioxide atmosphere trapping heat through the greenhouse effect.
Typical expected observations (actual timing will vary with conditions):
| Plant part used | Roots typically appear | First new leaf typically appears |
|---|---|---|
| Money plant stem cutting (in water/moist soil) | ~1–2 weeks | ~2–3 weeks |
| Potato "eyes" | ~1–2 weeks | ~2–4 weeks |
| Ginger piece | ~2–3 weeks | ~3–5 weeks |
Conclusion: all three grow into complete new plants from just a plant part (stem, "eye"/bud, or rhizome piece), without needing a seed — this is called vegetative propagation, an example of asexual reproduction. Other plants that grow this same way (without visible seeds) include bamboo and sugarcane, which grow from underground stems or stem cuttings.
Exact timing depends on temperature, moisture, and light conditions where you conduct the activity — the values above are typical ranges to compare your own daily observations against, not a fixed answer key.
Launched in 2013 by the Indian Space Research Organisation (ISRO), Mangalyaan (the Mars Orbiter Mission) was a landmark step in exploring the Red Planet. It carried instruments to study Mars's atmosphere, surface, and signs of past water — helping scientists investigate the big question of whether Mars was ever suitable for life. Beyond its scientific findings, Mangalyaan demonstrated that India could carry out advanced space science using smart, low-cost technology, and it played a major role in bringing Mars exploration closer to the Indian public.
The greenhouse effect that heats up Venus and Earth works differently from the greenhouse gardeners use for plants. On Venus or Earth, atmospheric gases like carbon dioxide trap heat by absorbing radiation given off by the warmed planet. A plant greenhouse, on the other hand, simply traps already-warmed air inside a closed, usually glass-walled space — both keep things warm, but through very different mechanisms.
Mars sits at the edge of the Sun's habitable zone. Rovers have landed and explored its surface, but no proof of life has been found so far. Scientists believe Mars may once have had liquid water — possibly even lakes — and conditions that could have supported simple life forms, which is exactly why Mars continues to fascinate researchers and reminds us that science stays open to new discoveries as we explore further.
Plants aren't the only ones — single-celled organisms like bacteria and amoeba simply divide into two identical individuals, some multicellular organisms like algae can regrow from small cut fragments, and Hydra grows tiny buds on its body that break off into new individuals. Planaria, a type of flatworm, can regenerate an entire new individual from just a small fragment of its body — a phenomenon scientists study closely to understand regeneration in animals.
The Montreal Protocol (1987) helped reduce ozone-damaging chemicals like CFCs, letting the ozone layer slowly recover. The Earth Summit (1992) launched international efforts on climate change and biodiversity. The Kyoto Protocol and the Paris Agreement (2015) committed countries to reduce greenhouse gas emissions, with the Paris Agreement aiming to limit global warming to below 1.5°C — a goal the world is not yet fully on track to meet, meaning continued action is still needed.
Answer: (iii) It lacks a thick atmosphere and liquid water.
Reasoning: Mars's atmosphere is about 100 times thinner than Earth's, and while it may have had liquid water in the past, no confirmed liquid water exists on its surface today — both are essential conditions this chapter identifies as necessary for life as we know it to exist and thrive.
Answer: (ii) Different landforms like mountains, valleys, and deserts.
Reasoning: geodiversity specifically refers to the variety of landforms, rocks, and soils on Earth, along with the processes that shape and alter them — not variety in living organisms (which would relate to biodiversity) or weather patterns.
Answer: (ii) It would escape into space due to weaker gravity.
Reasoning: a smaller Earth with the same average density would have weaker gravity, and weaker gravity means the planet couldn't hold onto its atmospheric gases as strongly — they would gradually escape into space, similar to why Mars (smaller than Earth) has a much thinner atmosphere and Mercury has essentially none at all.
Answer: (iv) They get mixed instructions (genes) from both parents.
Reasoning: in sexual reproduction, each parent contributes a gamete carrying only half its genetic material; when these combine, the offspring receives a unique mixture of genetic instructions from both parents — which is exactly why offspring resemble their parents without being identical copies of either one.
Where the seeds likely came from: most probably carried there by birds or animals that ate fruit elsewhere and later dropped or excreted the seeds near the wall, or the seeds may have been blown there by wind and settled into the crack — this is exactly the same mechanism the chapter describes for how a banyan seed might sprout in a wall crack after a bird excretes it there.
Conditions that helped them grow: monsoon rains provided the water the seeds needed to begin germinating and using their stored nutrients to grow roots and shoots; a small amount of dust, debris, or soil likely accumulated in the crack over time, giving the seed something to anchor into; and enough sunlight reaching that spot allowed the seedling to begin photosynthesising once it sprouted.
Effect on local climate: without tree cover providing shade and releasing water vapour through transpiration, the local area is likely to become noticeably hotter and drier, and with fewer plants absorbing carbon dioxide, the loss could contribute somewhat to local warming.
Effect on biodiversity: removing forest habitat displaces or endangers the plants, animals, and microorganisms that depended on it — some may die out locally, and animals like those seen in Chapter 12 (e.g. elephants) may be forced into human settlements looking for food and shelter, increasing human-wildlife conflict.
Effect on water availability/quality: tree roots normally help rainwater soak into the ground (recharging groundwater) and bind soil in place; without them, more rain runs off the surface rapidly (raising flood risk) rather than replenishing groundwater, and exposed soil is more easily washed into water bodies, reducing water quality through increased silt, debris, and potential pollutants.
Response: it's true that Earth's climate has changed naturally over its long history, but what makes today's global warming different is both its cause and its speed. Normally, Earth keeps carbon dioxide levels balanced because plants, trees, and ocean plankton absorb it as they grow. But burning fossil fuels releases extra carbon that was locked underground for millions of years, adding it to the atmosphere far faster than natural processes can absorb it back — trapping much more heat than usual through the greenhouse effect.
Why the speed matters: because this human-driven warming is happening over just decades rather than thousands of years, plants, animals, and ecosystems have far less time to adapt compared to slower, natural climate shifts in Earth's past — making today's rapid changes a genuinely urgent concern rather than "nothing new."
Loss of protection from harmful particles: without its magnetic field, Earth would lose its shield against cosmic rays (from across the universe) and the solar wind (from the Sun) — these high-energy particles could then strike the atmosphere directly, gradually damaging it and thinning the ozone layer.
More harmful UV radiation: a weakened ozone layer would let more harmful ultraviolet rays reach the surface, increasing the risk of damage to living cells for humans, animals, and plants alike.
Longer-term atmospheric loss: over a much longer timescale, unprotected atmospheric gases could gradually be stripped away by the solar wind — a process scientists believe happened to Mars, which lacks a strong global magnetic field today and has a much thinner atmosphere as a result.
Three things to recreate: (1) a breathable atmosphere with the right balance of oxygen and stable air pressure; (2) a reliable supply of liquid water; and (3) protection from harmful radiation, equivalent to what Earth's magnetic field and ozone layer provide.
Hardest to replicate: radiation shielding at a planet-wide scale is arguably the hardest — Mars lacks a strong global magnetic field of its own, and there is currently no realistic technology to generate an artificial magnetic shield covering an entire settlement or planet. This means any Mars settlement would likely need to rely on physical shielding (like thick walls, underground habitats, or Martian regolith cover) rather than truly replicating Earth's natural, planet-wide protection.
Likely causes: this pattern is consistent with broader climate change driven by rising greenhouse gas levels globally, and could be made worse locally by deforestation or land-use changes in or around the village that disrupt natural temperature and rainfall regulation.
Adaptation suggestion 1: switch to more drought-resistant or climate-resilient crop varieties, and invest in water storage and rainwater harvesting systems to better cope with unpredictable rainfall patterns.
Adaptation suggestion 2: restore local vegetation and tree cover (which helps regulate local temperature and supports more consistent rainfall) and build small-scale water conservation structures like check dams to improve groundwater recharge.
Effect on life: without an atmosphere, there would be no oxygen available for most life forms to breathe, and no protective ozone layer, exposing any surviving life to harmful UV radiation — making survival for life as we know it essentially impossible.
Effect on temperature: without atmospheric gases to trap heat through the greenhouse effect, Earth would lose heat rapidly into space, leading to extreme temperature swings between day and night (much like the Moon or Mercury, which have little to no atmosphere).
Effect on water: without sufficient atmospheric pressure, liquid water could not remain stable on the surface — it would either evaporate/boil away or freeze, depending on temperature, rather than staying in the liquid form that life depends on.
| Plant | Part used to grow a new plant |
|---|---|
| Potato | The "eyes" (buds) on a sprouted potato tuber |
| Ginger | A piece of the underground rhizome (stem) |
| Money plant | A stem cutting rooted in water or moist soil |
| Sugarcane | Stem cuttings planted directly in soil |
| Bamboo | Underground rhizomes/culm cuttings, rather than seeds |
Conclusion: in each case, a single parent plant's part (stem, root, or bud) grows directly into a new, genetically identical plant — this is vegetative propagation, a form of asexual reproduction.
These five prompts are design challenges and open-ended reasoning tasks rather than fixed-answer questions. Here's guidance on how to approach each one.
Guidance: base the kit on Earth's four interacting systems, each represented in miniature — a breathable atmosphere (right balance of oxygen and pressure, plus a mild greenhouse effect to hold in warmth); a hydrosphere supply (a source of liquid water, possibly recycled); a geosphere component (nutrient-rich soil to grow food and anchor plants); and a way to represent the biosphere's self-sustaining cycle (plants for oxygen/food, and decomposers to recycle waste, as covered in Chapter 12). Add radiation shielding (mimicking the magnetic field and ozone layer) to protect against harmful cosmic rays and UV radiation, since without it, none of the other components would stay safe for long.
Reasoning: lunar soil (regolith) likely lacks the organic matter, nutrients, and beneficial microorganisms that Earth soil builds up over time from decomposed plants and animals, so even with water present, plants might struggle to get the nutrition they need — though it's not necessarily impossible, especially with some nutrient supplementation.
Suggested experiment: grow identical seeds under controlled conditions (same light, temperature, and watering schedule) in three separate groups — one in real or simulated lunar regolith with just water added, one in the same lunar regolith enriched with compost/organic nutrients, and one control group in regular Earth potting soil. Measure and compare germination rate, growth rate, and overall plant health across the three groups over several weeks to determine whether lunar soil alone, or only nutrient-enriched lunar soil, can support plant growth.
Guidance: bright colours and pleasant scents act as advertising signals that attract pollinators such as insects, birds, and bats towards the flower. Drawn in by these visual and chemical cues, pollinators visit the flower (often to feed on its nectar), and in doing so, brush against the pollen-bearing anther and unintentionally carry pollen grains to the ovule of the same or another flower. This directly increases the chances of successful pollination and fertilisation, which are essential for the plant to produce seeds and fruit and thus reproduce.
Why so many eggs: fish and frogs typically fertilise their eggs externally in open water, leaving them highly exposed to predators, currents, and changing environmental conditions with little to no parental protection. Laying huge numbers of eggs is a strategy to offset this very high mortality rate — even if only a small fraction survive to adulthood, that small fraction can still be enough to maintain the population.
Advantages: a much higher overall chance that at least some offspring survive despite heavy losses, even without any real parental investment in individual eggs.
Disadvantages: producing so many eggs costs the parent a great deal of energy and resources, and since there's little to no protection or care for each individual egg, the survival rate per egg remains very low — a "quantity over individual protection" strategy, in contrast to animals that invest more care in far fewer offspring.
Birds (high parental care): building a nest, incubating eggs to keep them warm, and feeding chicks after hatching all significantly increase the survival chances of each individual egg and chick — which is exactly why birds can typically get away with laying relatively few eggs per clutch, since a larger share of them are likely to survive to adulthood.
Reptiles like snakes (little to no parental care): eggs left unprotected are far more vulnerable to predators and environmental extremes, so the survival rate per individual egg tends to be much lower — meaning species relying on this strategy generally need to lay a larger number of eggs overall to ensure that enough survive.
Overall pattern: this reflects a broader trade-off in nature (also seen in Q4 with fish and frogs) — more parental investment per offspring generally means fewer offspring are needed, while less parental investment generally means many more offspring are needed to achieve the same result.
This completes all 13 chapters of Class 8 Science (Curiosity) NCERT Solutions. Revisit Chapter 12 on ecosystems, browse the full chapter hub, or book a free demo class for personalised CBSE coaching.
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