NCERT Solutions for Class 8 Science Chapter 13 Our Home Earth a Unique Life Sustaining Planet (PDF) – 2027

It would still be a blue planet, but a bare one — rock, sand and water, with no green anywhere.

• No forests, grasslands or crops — only bare rock and dust, like the pictures of Mars.
• Almost no oxygen in the air, because oxygen is released by plants and ocean plankton; so there would be no ozone layer either.
• No fertile soil — only broken rock, with no remains of plants and animals in it.
• Oceans, clouds and rain would still be there, because they depend on Earth’s distance from the Sun and its atmosphere, not on life.

Two things work together: the planet keeps its conditions steady, and life keeps changing enough to fit them.

• Steady conditions: Earth stays in the habitable zone in a nearly circular orbit; its gravity holds the atmosphere, with its mild greenhouse effect and ozone layer; its magnetic field turns away cosmic rays and the solar wind.
• Reproduction and variation: parents pass on genes, so each kind continues. Small changes in the genes, and their mixing in sexual reproduction, make individuals slightly different — so when the surroundings change, some of them survive and life carries on (for example, camels developed humps to survive in deserts).

Because a dog and a hen use two different strategies for feeding the developing embryo, and each animal inherits only its own kind’s set of instructions.

A hen’s body is built to make big, food-filled, shelled eggs; a dog’s body is not, and vice versa. Reproduction can only pass on the instructions the parents already carry — it cannot swap one whole body plan for another.

Not out in the open. Soil and water are necessary, but they are only two items on the list — and Mars fails most of the others.

Inside a sealed, heated and pressurised chamber, yes. If you also carry the air, the pressure, the warmth and a light source — that is, if you carry a small piece of Earth’s atmosphere with you — plants can grow in Martian conditions.

Frame questions of your own about what makes Earth special and how life keeps going, then look for the answers in the chapter, in the library or with your teacher. Good questions often begin with why, how and what if.

Sample questions:

• If Venus is so hot because of carbon dioxide, could the same thing happen to Earth?
• Why does the Moon have no atmosphere?
• How do scientists know that Mars once had lakes?
• Why do some plants grow from seeds and others from pieces of stem?
• Why do children of the same parents look different from each other?
• What would happen if all the decomposers on Earth disappeared?

All of it lives in an extremely thin layer at the surface — the crust, together with the water on it and the lower part of the air above it.

That is why the book compares it to an apple: if the Earth were an apple, the crust would be no thicker than the apple’s skin (Fig. 13.1).

Here is Table 13.1 completed. The two filled rows are the book’s; the rest are features you can add, each with the reason it is remarkable.

Table 13.2 completed. The values already printed in your book are shown in normal type; the ones you have to fill in are in orange.

Because of its atmosphere, not its distance. Venus’s air is almost entirely carbon dioxide, and carbon dioxide does not let heat escape.

Follow the energy in three steps:

1. Sunlight passes through the atmosphere and warms the surface of the planet.
2. The warmed surface gives off its own radiation, back towards space.
3. Carbon dioxide in the thick atmosphere absorbs that outgoing radiation instead of letting it through. The heat stays in.

This is the greenhouse effect. On Venus it is so strong that the planet reaches about 450 °C — hotter than Mercury, even though Mercury is closer to the Sun and receives more sunlight.

No proof of life has been found — but the question is still open, and that is the honest answer.

What we actually know:

• Mars lies at the edge of the Sun’s habitable zone, so it is right at the boundary of where water can stay liquid.
• Several spacecraft have been sent, and rovers have landed and explored the surface. None has found proof of life.
• Scientists do think that in the past Mars may have had liquid water — perhaps even lakes — and conditions that could support simple life forms.

No. Distance from the Sun is the most important reason, because it is what allows water to stay liquid — but at least four other conditions must hold at the same time.

Either way we would lose the balance we depend on — a small Earth would lose its air, a large Earth would crush what lives on it.

If the Earth were much smaller (with the same average density), its gravity would be too weak to hold the atmosphere. The gases would escape into space, and with them the greenhouse warmth, the oxygen and the ozone layer. Mars, which is smaller, has an atmosphere 100 times thinner; Mercury, smaller still, has none.

If the Earth were much larger, gravity would be much stronger and would pull us down with such force that our bones could get crushed.

Yes — it acts as a protective shield around the whole planet.

1. Earth is constantly hit by tiny, high-energy particles — cosmic rays from far across the universe and the solar wind from the Sun.
2. These particles can damage the atmosphere and reduce the ozone layer, letting in more harmful ultraviolet rays.
3. Earth’s magnetic field (believed to arise from molten iron moving in its core) pushes many of these particles away, so the atmosphere — and hence life — stays safe.

Not by any single thing. Life is sustained by the constant exchange between four systems — the atmosphere, the hydrosphere, the geosphere and the biosphere.

• Air (atmosphere): oxygen for respiration, carbon dioxide for photosynthesis, and a mild greenhouse effect that keeps water liquid.
• Water (hydrosphere): rain and snow refill rivers, lakes and groundwater; water carries nutrients and is home to aquatic life.
• Soil, rocks and minerals (geosphere): nutrients such as nitrogen and potassium for plants, and materials like salt, coal, oil and metals.
• Living things (biosphere): plants make food, animals eat plants or other animals, and decomposers return nutrients to the soil.

They stay in balance because they are not independent. Land, air, water and living things support and affect one another, so a push in one place is answered by the rest of the system.

Take the chapter’s own example and follow it through:

Because reproduction passes on a complete set of instructions — but not always the same complete set.

Similarity: every offspring receives a complete set of instructions (genes) for its own kind, so a calf grows into a cow and a kitten into a cat.

Variation: in sexual reproduction each gamete carries only half of a parent’s instructions — a different half each time — so every offspring gets a new mixture of its two parents’ instructions. Small changes in the instructions also appear. That is why brothers and sisters look different from each other.

They multiply by vegetative propagation — a piece of the parent plant itself grows into a new plant, so no seed is needed.

• Sugarcane is grown from short pieces of stem called setts. Each piece carries a node with a bud (an ‘eye’). Laid in moist soil, the bud sends up a shoot and the node sends down roots.
• Bamboo spreads mainly through underground stems (rhizomes) that creep sideways and send up new culms; a cut piece of culm with a node can also root.

Many of the plants in a kitchen, a garden or a field are grown from parts, not seeds. Look for the part that is planted — that tells you which type of vegetative propagation it is.

No — and the reason is that each parent does not pass on its whole set. Each makes specialised reproductive cells called gametes, and a gamete carries only half the parent’s genetic material.

Set-up: Put a money plant cutting with a node in a glass of water, with the node under water. Plant a sprouted potato ‘eye’ and a piece of ginger with a bud in moist soil, just covered. Keep them where they get air and indirect sunlight, and keep the soil damp.

What you should observe. Record your own dates — these are the ranges you can expect in ordinary Indian room conditions.

Both are solutions to one problem — how to feed and protect a developing embryo. Each buys safety at a different price.

No, they could not. Laying eggs is not a choice an animal makes; it is the way its whole body is built, and that is fixed by the instructions it inherits.

• A hen’s body makes a large egg packed with food for the whole development of the embryo, and puts a shell around it. A dog, a cow or a human makes a tiny egg cell with almost no stored food.
• In mammals the embryo develops inside the mother, whose body supplies food and oxygen until birth. An egg laid outside would get no such supply.
• Offspring inherit their parents’ instructions, so one generation cannot switch to a completely different way of reproducing.

(iii) It lacks a thick atmosphere and liquid water.

Mars’s atmosphere is about 100 times thinner than Earth’s, so water cannot stay liquid on its surface — and liquid water is essential for life.

(ii) Different landforms like mountains, valleys, and deserts.

Geodiversity is the variety of landforms, rocks and soils; mountains, valleys and deserts are different landforms.

(ii) It would escape into space due to weaker gravity.

A smaller planet of the same density has less mass, so its gravity is weaker and cannot hold on to the gases of the atmosphere.

(iv) They get mixed instructions (genes) from both parents.

Each parent’s gamete carries half of its instructions, and these mix in a new way in every offspring.

The seeds were almost certainly carried there — most likely by birds, and some by the wind.

Birds eat fruits and drop the seeds far away in their droppings — the book’s example is a banyan seed sprouting in a crack in a wall after the rains. Light seeds and spores can also be blown in by the wind.

Conditions that helped: rainwater collected in the crack; air reached the seed; dust and bits of leaves formed a little soil with nutrients; the narrow crack held the seed in place; sunlight reached the first leaves; and the monsoon kept everything moist.

Removing a forest does not remove one thing — it breaks several links at once, and the effects reach the climate, the living community and the water.

• Local climate: fewer trees release less water vapour, so local rainfall tends to drop; roads and buildings absorb heat, so the area gets hotter; the carbon dioxide the forest used to absorb now stays in the air.
• Biodiversity: plants and animals lose their habitat and may disappear; herbivores lose their food, and so do the predators that eat them.
• Water: rain runs off concrete instead of soaking in, so less groundwater is refilled and wells and springs get less water; soil washed into streams, and oil and waste from roads, make the water muddy and polluted.

Your friend is right about the first half and wrong about the conclusion. The Earth’s climate has changed before — but what is happening now differs in its cause and in its speed.

• The cause is different: today’s warming comes from burning fossil fuels like coal and oil, which releases carbon dioxide and methane — carbon that was locked underground for millions of years. Trees and ocean plankton cannot absorb it fast enough, so heat builds up.
• The speed is different: past changes took very long periods; today’s change is so fast that many plants and animals cannot adapt in time.
• The risks are serious: melting ice caps, rising sea levels that could flood coastal cities, extreme weather, and loss of plants and animals.

The shield that keeps high-energy particles away from the planet would be gone, and the damage would work its way inward, step by step.

1. Cosmic rays and the solar wind would no longer be pushed away and would reach the atmosphere directly.
2. They would damage the atmosphere and reduce the ozone layer.
3. More harmful ultraviolet rays would reach the surface and damage living cells — harming skin and eyes, crops and ocean plankton.
4. Over a long time the atmosphere itself could be stripped away, and with it the warmth that keeps water liquid.

Compasses would also stop pointing north.

The three I would recreate:

The hardest to replicate: protection from radiation.

Air, water and heat can be made, stored and recycled inside a sealed dome. But Earth’s protection comes from a planet-wide magnetic field and ozone layer, which cannot be carried or built. The only option is to live under metres of soil or rock, which is heavy, dark and very costly.

Cause: these are signs of climate change. Burning fossil fuels releases greenhouse gases that cause global warming and change rainfall patterns; cutting down trees around the village adds local heating and makes rain less dependable.

Two ways to adapt:

1. Save rainwater when it falls — repair ponds, build check dams, and use rooftop rainwater harvesting to recharge groundwater.
2. Grow crops that need less water and can bear heat, such as millets (bajra, jowar) and pulses, and use drip irrigation instead of flooding the fields.

Yes — and in each of the three ways the question names. The atmosphere is not just something we breathe; it is what keeps the other two possible.

• Life: no oxygen for respiration, no carbon dioxide for photosynthesis, and no ozone layer to block harmful ultraviolet rays — life as we know it could not exist.
• Temperature: no greenhouse effect, so Earth would lose its heat to space and become too cold; the sunlit side would scorch while the dark side froze.
• Water: with no air pressure, surface water would boil away or freeze, and there would be no water cycle — no clouds, rain or snow to refill rivers, lakes and groundwater.

Vegetative propagation is asexual reproduction in plants: a leaf, stem or root of the parent grows into a whole new plant. Five clear examples, each using a different part:

The kit must stand in for each of Earth’s four systems — atmosphere, hydrosphere, geosphere and biosphere — and also supply warmth and shielding, with everything recycled so that nothing runs out.

Sample answer — the Earth Survival Kit:

Water alone would not be enough — but lunar soil plus water plus Earth-like air, warmth and light might well work, and that is exactly what an experiment should test.

Sample experiment:

1. Take four identical transparent containers with the same amount of soil and the same number of fast-growing seeds such as moong: A — lunar (or simulated lunar) soil; B — ordinary garden soil (the control); C — lunar soil with a little compost; D — lunar soil with no water.
2. Give all of them the same water (except D), air, temperature and light.
3. Record germination, height after two weeks, and the number and colour of leaves. Repeat each set three times.

They are advertisements — aimed at insects, birds and other animals that will carry pollen from one flower to another.

Bright colours and scent attract insects, birds and other animals. While an animal feeds on nectar, pollen from the anther sticks to its body and is carried to the next flower — this is pollination. The male and female gametes can then combine (fertilisation), and seeds and fruit are formed. Colour is seen from a distance in daylight; scent works even at night.

Because in fish and frogs almost nothing protects the eggs — so survival is a matter of numbers.

Their eggs are fertilised and develop in open water with no protection, so most are eaten or destroyed. Laying thousands makes sure that at least a few survive. Animals such as birds lay only a few eggs because each egg is packed with food and then guarded and cared for.

Each individual sparrow chick has a far better chance of surviving than each individual snake hatchling — but the snake makes up for it with numbers.