NCERT Solutions Class 8 Science Chapter 13 Our Home Earth, a Unique Life Sustaining Plane

 

Question 1. What is one major reason Mars cannot currently support life like Earth?
(i) It has too many volcanoes.
(ii) It is too close to the Sun.
(iii) It lacks a thick atmosphere and liquid water.
(iv) Its magnetic field is too strong.
Answer: (iii) It lacks a thick atmosphere and liquid water.
Mars's atmosphere is roughly 100 times thinner than Earth's, which is not strong enough to hold life as we know it or to trap heat well. Though scientists think Mars may have had liquid water long ago, it does not have any on its surface now. Liquid water is needed for life to form and thrive.
In simple words: Mars is missing two key things that life needs - a thick air layer and liquid water on the ground.

Exam Tip: Always link Mars's lack of atmosphere to weak gravity and the loss of heat-trapping ability - examiners look for these specific connections.

 

Question 2. Which of these is an example of geodiversity?
(i) Variety of bird chirping in a forest.
(ii) Different landforms like mountains, valleys, and deserts.
(iii) Changing weather during monsoons.
(iv) Number of different types of fish in a pond.
Answer: (ii) Different landforms like mountains, valleys, and deserts.
Geodiversity means the range of non-living natural features, such as rocks, soil, and different land shapes. The other choices show biodiversity (the range of living things) or weather changes, not geodiversity.
In simple words: Geodiversity means different types of rock, soil, and land shapes - not living things.

Exam Tip: Remember - "geo" = Earth/non-living, "bio" = life. This quick memory trick helps you pick the right answer every time.

 

Question 3. If the Earth were smaller with the same density, what might happen to its atmosphere?
(i) It would become thicker and hotter.
(ii) It would escape into space due to weaker gravity.
(iii) It would become frozen.
(iv) It would cause stronger winds.
Answer: (ii) It would escape into space due to weaker gravity.
A smaller planet would have less gravitational pull. This weaker pull would not be strong enough to keep the gas molecules in the atmosphere, and they would slowly drift away into space - the same thing that took place on Mercury and largely on Mars.
In simple words: A smaller Earth would have weaker gravity, so it could not hold onto its air, and the air would float away into space.

Exam Tip: Link gravity to planetary mass and size - emphasize that a smaller planet simply cannot hang onto its gases as well.

 

Question 4. In sexual reproduction, why are offspring different from their parents?
(i) They grow in different climates.
(ii) They eat different food.
(iii) They acquire new instructions after birth.
(iv) They get mixed instructions (genes) from both parents.
Answer: (iv) They get mixed instructions (genes) from both parents.
When a new baby is formed through sexual reproduction, it comes from two special cells, one from each parent, called gametes. Each gamete carries half of the parent's genetic code. When these two halves join together, they make a brand new set of instructions in the baby that is different from both its mother and father - and different from its brothers and sisters too.
In simple words: Babies get genes from both parents mixed together, which makes them look and act different from their parents.

Exam Tip: Always mention the word "gametes" and stress that genes from two sources create a unique mix in every child.

 

Question 5. You notice tiny green plants growing in cracks on your school wall after the monsoon. Where do you think the seeds came from? What conditions helped these plants grow there?
Answer: This is a wonderful observation! Seeds probably got there in one of two ways: wind may have carried them and stuck them in the crack, or a bird that ate fruit may have dropped them there. As the book tells us, a bird eats a fruit and then drops the seed in the wall crack. The monsoon brought perfect growing conditions. Dust and soil collected in the cracks acted as a growing medium. The monsoon rains gave the water needed, and the plants also got sunlight and air. It shows how strong and flexible life can be!
In simple words: Seeds got there from birds or wind. Rain, soil, sunlight, and air all helped them grow in a small crack.

Exam Tip: Credit students for recognizing seed dispersal agents (wind and birds) and naming all four growing essentials - water, light, air, and soil or growing medium.

 

Question 6. A city has recently cut down a large patch of forest to build new roads and buildings. Discuss the possible effects this could have on the local climate and biodiversity. How might this affect water availability or quality in the area?
Answer: Cutting down trees, known as deforestation, brings many serious problems.

For local climate and biodiversity: Forests work like a natural cooler - when you remove them, local heat goes up. Trees play a big job in the water cycle by releasing water into the air, so losing them means less rain falls nearby. For living things, this is terrible. The forest was home to many animals, birds, bugs, and plants. Destroying it makes these creatures lose food and shelter, forcing them to leave or disappear. This greatly lowers the number of living things in the area.

For water supply and quality: Forest dirt is very good at soaking up rain, which seeps slowly into the ground to refill underground water. Without tree roots to hold the soil, rain runs off bare ground fast, which causes dirt to wash away and raises flood danger. This running water picks up waste from the city and carries it into rivers and lakes, hurting water quality. When less water soaks into the ground, the water table below gets lower, which means less water is available in the future.

In simple words: Cutting down forests makes the area hotter and drier, kills animals, and makes water dirty and harder to find.

Exam Tip: Structure your answer into three clear parts - climate effects, biodiversity loss, and water impacts - to show complete understanding of deforestation's many harms.

 

Question 7. A friend says, "The Earth has always had climate changes in the past, so today's global warming is nothing new." How would you respond using what you've learnt in this and other chapters of your science book?
Answer: You make a fair point, but the warming we see now is very different and worrying for two big reasons - how fast it is happening and what is causing it. The warming today is going much, much quicker than any natural warming the Earth went through before. More importantly, human actions are driving this change. We burn huge amounts of coal and oil, which releases old carbon dioxide that was locked underground for millions of years. Our planet's natural systems like forests and oceans cannot take in this extra carbon quickly enough. This traps more heat in the air and causes the fast warming we are seeing right now.
In simple words: Yes, Earth's temperature has changed before, but this time it is happening much faster because humans are burning fossil fuels.

Exam Tip: Distinguish between natural climate swings and human-caused warming by stressing the speed of change and the burning of fossil fuels as the key difference.

 

Question 8. Imagine Earth's magnetic field suddenly disappeared. What kinds of problems could arise for life on Earth? Explain.
Answer: If Earth's magnetic field went away, we would face major danger! The magnetic field works as an unseen guard that shields us from bad radiation. Without it, two serious troubles would show up. First, a steady flow of high-power bits from the Sun, named the solar wind, would slam into our air directly. This would slowly peel away the air over time, making it thinner and weaker. Second, the shield keeps us safe from risky rays that come from way out in space. Without the magnetic field, these bits and other bad UV rays from the sun would get through to the ground. This extra radiation could harm the cells of all living things, leading to big health troubles and making it very hard for life as we know it to make it through.
In simple words: Earth's magnetic field blocks bad rays from the sun and space. Without it, radiation would hurt all life and strip away our air.

Exam Tip: Name the solar wind and cosmic rays specifically, and explain the two-fold threat - both to the atmosphere and to living cells - to earn full marks.

 

Question 9. You are tasked with designing a new settlement for humans on Mars. Name three things you would need to recreate from Earth to support human life there. Which of these do you think is the hardest to replicate, and why?
Answer: To build a home for people on Mars, I would need to recreate three key things from Earth:

1. A breathable air mix: We would need a sealed living space filled with gases like oxygen and nitrogen that people can safely breathe.

2. Liquid water: We would need to find and dig out frozen water on Mars and build tools to melt it, clean it, and use it again for drinking, farming, and washing.

3. A steady food supply: We would need to set up a way to grow plants, maybe using water farming, inside the living space to give us food and also clean the air.

I think the hardest to recreate and keep going on a big scale is the breathable air. We can make it in a small, shut-off space, but making and keeping a thick, safe air layer for a whole world is a task so big that our current tools cannot do it. It would mean changing the whole planet's make-up and weather, which is way too hard for us now. Water and food farming are tough jobs too, but they are challenges on a much smaller, easier scale.

In simple words: The hardest thing to make on Mars would be a breathable air layer for the whole planet - water and food farming would be easier.

Exam Tip: Go beyond just naming the three items - explain why atmosphere creation is harder than the others by discussing planetary-scale challenges versus localized solutions.

 

Question 10. In a village, the temperature has been increasing and rainfall has become unpredictable over the past few years. What could be causing this change? Suggest two ways the village could adapt to these new conditions.
Answer: The rising heat and uneven rain are clear signs of a shift in the local weather, which comes from the larger issue of warming across the world. Burning of coal and oil all over the planet is the main driver. In the area itself, loss of trees in nearby lands could also be making things worse by changing rain patterns and making it hotter.

To handle these shifts, the village could take two main actions:

1. Rain Collection: To deal with uneven rain, they could gather water from roofs and other spots into big tanks or send it down to wells underground to fill up the groundwater. This water kept in storage could be used when there is little or no rain.

2. Planting Trees: The people could start a campaign to plant lots of trees in and around the village. Trees give shade, which makes the area cooler. They also make the soil better at holding water and can help bring back steady rain over time.

In simple words: The village should gather rain in tanks and plant lots of trees to stay cool and keep water for dry times.

Exam Tip: Show how adaptation methods directly address the stated problems - rainwater harvesting solves unpredictable rainfall, afforestation solves temperature and rainfall together.

 

Question 11. If there were no atmosphere on the Earth, would it affect life, temperature and water on the planet? Explain.
Answer: An Earth without an air layer would be a very different and dead world.

Effect on Life: Life as we know it would not be able to exist. The air holds oxygen, which nearly all living things use to breathe. It also holds the ozone layer, which keeps the Sun's bad ultraviolet (UV) light away from us. Without it, no life could be here.

Effect on Temperature: The world would go through extreme heat and cold swings. In the day, it would be burning hot, and at night, it would be very cold. The air acts like a warm cover, holding in some of the Sun's heat through the trap heat effect and keeping the world warm and steady. Without it, all the heat would go back out into space when the sun sets, making the temperature drop fast and hard.

Effect on Water: All the liquid water would go away. The extreme cold at night would turn it to ice, and the lack of air push would let the water bubble up into space when the sun shines. An air layer is key for holding the right push and heat so water can stay as a liquid.

In simple words: Without an atmosphere, there would be no air to breathe, wild temperature swings day and night, and all water would freeze or float away.

Exam Tip: Address all three factors - life, temperature, and water - separately and link each to a specific role the atmosphere plays (respiration, heat retention, pressure maintenance).

 

Question 12. Discuss five examples of vegetative propagation.
Answer: Vegetative propagation is a wonderful way for plants to make new plants without seeds, using parts like stem, root, or leaf. Here are five common cases:

1. Potato: You can grow a brand new potato plant from a bit of potato that has an 'eye'. The eye is truly a bud that shoots into a fresh stem and leaves.

2. Ginger: Ginger grows from a unique below-ground stem named a rhizome. If you put a piece of the rhizome into dirt, it will shoot roots and new growth to turn into a fresh plant.

3. Money Plant: This is simple to try at home. If you cut a stem bit and set it in water or damp dirt, it will grow roots and become a totally fresh plant.

4. Sugarcane: Crop workers plant short bits of the sugarcane stalk, called setts, into the dirt. Buds on these bits grow into fresh sugarcane plants.

5. Rose: You can make a fresh rose bush by taking a piece of stem from a live plant and setting it in the dirt. It will make its own roots and grow.

In simple words: Plants can make babies from their stems, roots, and other parts - not just from seeds.

Exam Tip: For each example, name the plant part that is used for propagation and what it grows into - this specificity is what examiners mark as complete understanding.