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Class 8 Science: Our Home: Earth, a Unique Life Sustaining Planet, 8 practice questions with answers

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These are 8 practice questions on the thirteenth and last chapter of the new NCERT Class 8 Science textbook, Curiosity, called Our Home: Earth, a Unique Life Sustaining Planet. Each has a short answer and the textbook page it comes from. They cover why Venus is the hottest planet, the habitable zone and liquid water, the Earth's orbit and the ozone layer, the magnetic field, the four spheres of the Earth, how reproduction allows change, where young animals develop, and the threats to life on Earth. For the solar system itself, see our Class 6 Beyond Earth guide. Try each question before you read the answer.

These questions are our own, drafted with AI help. Every fact was checked against the page it cites by a test script, but no practising teacher has reviewed them yet. If you find a mistake, please tell us.

What the chapter says you need

Facts from the NCERT textbook (Curiosity, Grade 8, Chapter 13, printed pages 210 to 228):

A planet like no other

Question 1. Mercury is closer to the Sun than Venus, yet Venus is the hottest planet. (a) Why? (b) The chapter's table of planets gives average temperatures of about 170 °C for Mercury, 450 °C for Venus and 15 °C for the Earth. By how many degrees is Venus hotter than Mercury, and than the Earth?

Answer: (a) Venus's thick carbon dioxide atmosphere traps heat; (b) by 280 °C and by 435 °C. (a) Distance alone does not decide it. Venus has a very thick atmosphere made almost entirely of carbon dioxide, and this gas holds the heat in instead of letting it leave the planet. This is the greenhouse effect. (b) 450 − 170 = 280 °C hotter than Mercury, and 450 − 15 = 435 °C hotter than the Earth. (Venus and the greenhouse effect, pages 213 and 214; Table 13.2, page 213.)

Question 2. Imagine a rocky planet going round another star. (a) What would happen to its water if it were much nearer the star than the Earth is to the Sun, and what if it were much farther? (b) What is the band of distances in between called? (c) Why does the chapter treat liquid water as so important?

Answer: (a) nearer, the water would evaporate; farther, it would freeze; (b) the habitable zone, or Goldilocks zone; (c) as far as we know, life needs liquid water to develop. (a) Much nearer the star, the planet would be so hot that its water would turn to vapour. Much farther away, it would be so cold that its water would freeze. (b) The range of distances from a star over which water stays liquid is the habitable zone, also called the "Goldilocks zone". (c) Some microbes can survive in frozen places, but as far as scientists know today, life needs liquid water in order to evolve and to keep going, and most plants and animals could not grow or thrive without it. (Position of the Earth and the habitable zone, pages 214 and 215.)

Question 3. (a) The Earth's orbit round the Sun is almost a circle. How does that help living things? (b) What is ozone, and how does the ozone layer protect living things?

Answer: (a) it keeps the sunlight and heat reaching the Earth nearly steady through the year; (b) a form of oxygen with three atoms in each molecule, which blocks harmful ultraviolet rays. (a) Because the orbit is nearly circular, the Earth stays at almost the same distance from the Sun all year. The amount of sunlight and heat it gets stays nearly steady, which prevents extreme summers and winters in most places. (b) Some of the oxygen in the atmosphere changes into ozone, a molecule of three oxygen atoms. This layer stops much of the Sun's ultraviolet (UV) light, which harms living cells, from reaching the ground. (Orbit and size of the Earth, pages 215 and 216; the ozone layer, page 216.)

Question 4. (a) What are cosmic rays and the solar wind? (b) Why can they be harmful? (c) What is believed to cause the Earth's magnetic field?

Answer: (a) tiny high-energy particles from space; (b) they can damage the atmosphere and the ozone layer; (c) the movement of molten iron in the Earth's core. (a) Tiny particles carrying a lot of energy keep striking the Earth from space. Those from far across the universe are cosmic rays; those from the Sun are the solar wind. (b) They can harm the atmosphere and thin the ozone layer, so that more UV light gets through. (c) The movement of molten iron in the Earth's core is believed to be the origin of the field. (Magnetic field of the Earth, page 217.)

How life is sustained

Question 5. Which of the Earth's four spheres (atmosphere, hydrosphere, geosphere, biosphere) does each belong to: (a) groundwater; (b) the oxygen we breathe; (c) rocks and minerals such as iron ore; (d) earthworms and the plants they live among; (e) the water in the oceans? About how much of the Earth's surface is covered with water?

Answer: (a) hydrosphere; (b) atmosphere; (c) geosphere; (d) biosphere; (e) hydrosphere; about 70 per cent. The hydrosphere is all of the Earth's water, whether in oceans and rivers or under the ground. The atmosphere is the layer of air. The geosphere is the solid part of the Earth: rocks, soils and minerals. The biosphere is all living beings with the places where they live. Roughly 70 per cent of the planet's surface is under water. The four spheres interact and together sustain life. (Air and water, page 218; the geosphere, pages 218 and 219; the biosphere, page 219; the four spheres together, page 225.)

Question 6. (a) Besides making more living beings of the same kind, what else does reproduction allow, according to the chapter? (b) Give the chapter's two examples of such changes helping living things survive. (c) Over many generations, what can these small changes lead to?

Answer: (a) small changes in the instructions passed from parents to young; (b) camels' humps and bacteria that resist antibiotics; (c) new features, or even new kinds of living beings. (a) Reproduction passes the instructions in the genes from parents to their young, and it also lets small changes creep into those instructions. Some of these changes make it easier for a plant or an animal to live in new surroundings. (b) Over time camels came to have humps that store fat, helping them live in deserts, and some bacteria have become resistant to antibiotics, which helps them persist. (c) Over many generations such small changes can add up to new features, or even to completely new types of living beings. (Reproduction and change, page 220.)

Question 7. For a frog, a hen and a cow, say where the young one develops from the zygote: in water, inside an egg laid by the mother, or inside the mother's body. What is the zygote?

Answer: frog, in water; hen, inside an egg laid by the mother; cow, inside the mother's body. The zygote is the cell formed when a sperm and an egg join. When a male gamete (sperm) and a female gamete (egg) combine, this is fertilisation, and the new cell is the zygote. In frogs and fish, the eggs and sperm are released into water, and the zygote also develops into an embryo there. In birds such as hens, the fertilised egg is laid, and the embryo develops in it, fed by the food the mother put in the egg, until it hatches. In most mammals, such as cows, the embryo develops inside the mother's body, which gives it food and oxygen until it is born. (Sexual reproduction in animals, pages 222 and 223.)

Question 8. (a) Which three problems together are called the "triple planetary crisis"? (b) Why does burning coal and oil warm the Earth? (c) Which international agreement helped the ozone layer to recover, and how?

Answer: (a) climate change, biodiversity loss and pollution; (b) it releases greenhouse gases that trap more heat; (c) the Montreal Protocol (1987), by cutting harmful chemicals such as CFCs. (a) The biggest environmental challenges, climate change, biodiversity loss and pollution, are together called the triple planetary crisis. (b) Burning fossil fuels releases greenhouse gases such as carbon dioxide and methane, carbon that had been locked underground for millions of years. They trap more heat in the atmosphere faster than plants and ocean plankton can take up the extra carbon, so the Earth warms. (c) Under the Montreal Protocol (1987), countries cut down on chemicals such as chlorofluorocarbons (CFCs) that destroy ozone, and the ozone layer has been healing slowly since. (Threats to life, pages 223 and 224.)

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Sources

All opened and read on 2 November 2026. Page numbers are the printed page numbers of the textbook; printed page = PDF page + 209 for this chapter.

The facts are from the textbook pages above. The questions, answers and checks are our own. This guide is general information; follow your teacher and the textbook.