This MCQ module is based on: Atmosphere and Water Essentials
TOPIC 48 OF 50
Atmosphere and Water Essentials
🎓 Class 8
Science
CBSE
Theory
Ch 13 — Chemical Effects of Electric Current
⏱ ~27 min
🌐 Language: [gtranslate]
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Myaischool Lt Science Assessment
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[myaischool_lt_science_assessment grade_level="class_8" science_domain="physics" difficulty="basic"]
The Ocean of Air Above Us
Each time you breathe in, you are taking a little sip from an invisible ocean — the atmosphere. It stretches for hundreds of kilometres above our heads, but the air we can actually breathe lies only in its lowest few kilometres. Without this thin envelope of gas, Earth would be silent, baked by day, frozen at night, and utterly lifeless.
In this part we look at the two great life-giving resources our planet alone offers in plenty — air and water.
13.7 Layers of the Atmosphere
Earth's atmosphere is not uniform. As we go higher, temperature, pressure and composition change. Scientists divide it into five main layers.
Layer by Layer
- Troposphere (0–12 km): Where we live. All weather — clouds, rain, storms — happens here. Temperature falls as we go up.
- Stratosphere (12–50 km): Contains the precious ozone layer. Aeroplanes fly here to avoid turbulence.
- Mesosphere (50–85 km): The coldest layer (drops to −90 °C). Meteors entering Earth burn up here, giving us "shooting stars".
- Thermosphere (85–500 km): Very hot but extremely thin. Home to the auroras and the International Space Station.
- Exosphere (500+ km): The fuzzy boundary with outer space. Satellites orbit here before the air finally fades away.
13.8 What's in the Air?
If you could put a sample of dry air on a chemical balance, you would find it is a mixture. Four-fifths of it is a single gas that we neither breathe in nor out — nitrogen!
| Gas | Approx. % | Role in life / Earth |
|---|---|---|
| Nitrogen (N₂) | 78% | Keeps oxygen diluted; taken in by plants through soil for proteins. |
| Oxygen (O₂) | 21% | Respiration in animals and plants; supports burning. |
| Argon (Ar) | ~0.9% | Noble gas — chemically inactive; fills some light bulbs. |
| Carbon dioxide (CO₂) | ~0.04% | Used by plants in photosynthesis; also a greenhouse gas. |
| Water vapour (H₂O) | Variable | Forms clouds, rain, snow; drives the water cycle. |
| Others (Ne, He, Kr, CH₄…) | Traces | Tiny amounts, but some are very active in atmospheric chemistry. |
Important: Although CO₂ is only about 0.04% of air, even a small rise in its amount can warm the whole planet through the greenhouse effect. This is the key reason behind climate change.
Why the Ozone Layer Matters
High up in the stratosphere, three oxygen atoms combine to form an unusual molecule — ozone (O₃). The thin film of ozone soaks up most of the Sun's dangerous ultraviolet (UV) rays before they reach the ground. Without this natural sunscreen, UV would cause skin cancers, eye damage, and destroy tiny ocean plants (plankton) at the base of the food chain.
13.9 Water — Life's Favourite Molecule
Seen from a spaceship, Earth looks mostly blue because of one remarkable molecule: H₂O. Our planet is the only one in the solar system where water is found in all three states — ice, liquid and vapour — all at the same time.
Salty or Sweet?
Earth's water is nearly everywhere, but most of it is not the kind we can drink.
- 97% of Earth's water is salty — sitting in the oceans.
- 3% is fresh water — but most of this is locked in ice caps, glaciers or deep groundwater.
- Barely 1% of the fresh water (so far less than 1% of all water on Earth!) flows in rivers and lakes where we can easily use it.
Why Water Is Special
- It stays liquid across a huge range of Earth temperatures — perfect for cells.
- It is the universal solvent — dissolves salts and nutrients so living things can use them.
- It has a high specific heat — oceans absorb heat slowly, keeping Earth's climate stable.
- It floats when frozen — ice on top insulates lakes, so fish below survive winter.
Water Cycle: Sun's heat evaporates water from oceans → vapour rises and cools → condenses into clouds → falls as rain/snow → flows back to oceans. Earth is the only planet where this cycle works continuously.
💧 Activity 13.2 — How Much Usable Water Do We Really Have?
You will need: a 1-litre jug of water, three empty glasses, a dropper, a teaspoon.
- Pour the full 1 L of water into a large bowl — this represents ALL the water on Earth.
- Carefully remove 30 mL (about 2 tablespoons) into a small glass — this is all the fresh water on Earth.
- From that 30 mL, take out just 1 mL using the dropper into another tiny container — this represents the liquid fresh water in lakes, rivers and accessible groundwater.
- Look at the tiny drop left in the dropper compared with the big bowl.
🔍 Predict: Before the activity, guess how many teaspoons (out of 1 litre) would represent the fresh water we can actually drink.
Only about 1 drop (or less) out of the entire litre represents the fresh water that is easily accessible to us. The rest is either salty ocean water or frozen in glaciers.
This activity shows why saving every litre of fresh water matters — it is a surprisingly small resource shared by eight billion humans and millions of other species.
🎯 Competency-Based Questions
Meera sees a news headline: "Ozone hole over Antarctica smaller than it was in 1990." She remembers her teacher saying the ozone layer protects us. She wonders what the ozone layer is, where it sits, and why anyone should care whether there is a "hole" in it.
Q1. L1 Remember In which layer of the atmosphere is the ozone layer found?
Answer: The ozone layer is found in the stratosphere — the second layer from Earth's surface, about 12 to 50 km above the ground.
Q2. L2 Understand Even though nitrogen makes up 78% of air, why is oxygen considered more important for animals?
Answer: Animal cells need oxygen to release energy from food during cellular respiration. Nitrogen gas in the air is inert — it cannot be used directly by most animals. Only certain soil bacteria can "fix" nitrogen into forms plants can absorb, passing it along the food chain. Thus oxygen is the immediately vital gas for animal life.
Q3. L3 Apply A news report says "the air feels heavy today". Apply your knowledge: which layer of the atmosphere is being talked about, and what is one likely cause?
Answer: The report refers to the troposphere, the layer closest to the ground where all weather happens. "Heavy air" usually means high humidity (a lot of water vapour) or high atmospheric pressure, often just before rain. Sometimes it also indicates air pollution trapping dust and gases near the surface.
Q4. L4 Analyse Analyse why, even though Earth is called the "blue planet", fresh water is considered a scarce resource.
Answer: Out of all the water on Earth, about 97% is salty ocean water that cannot be directly used for drinking or agriculture. Of the remaining 3% fresh water, the majority is trapped in ice caps and glaciers or lies deep underground. Only a tiny fraction flows in rivers and lakes where it is easily accessible. As population and industry grow, the demand on this tiny share is enormous — making fresh water scarce despite Earth's blue appearance.
Q5. L5 Evaluate Evaluate this claim: "Because water is a renewable resource — the water cycle always brings it back — we can never run out of it."
Answer: The claim is partly right and partly wrong. Water on Earth is recycled endlessly by the water cycle, so the total amount does not disappear. However, the water cycle does not guarantee clean water where and when we need it. Pollution, over-extraction of groundwater, deforestation and climate change can all make fresh water unavailable at specific places and times — even though the cycle still runs overall. So water is renewable, but not unlimited in practice.
🔗 Assertion–Reason Questions
Assertion (A): All weather phenomena like clouds, rain and storms take place in the troposphere.
Reason (R): The troposphere contains almost all of the atmosphere's water vapour and dust.
Answer: A. Water vapour and dust are essential for cloud formation — they are found mostly in the troposphere, which is why weather happens there.
Assertion (A): The ozone layer protects living things from ultraviolet (UV) radiation.
Reason (R): Ozone molecules absorb most of the UV rays reaching Earth from the Sun.
Answer: A. The absorption of UV by ozone is exactly why the ozone layer shields us — R directly explains A.
Assertion (A): Oceans help keep Earth's climate stable.
Reason (R): Water has a low specific heat capacity, so it warms up very quickly.
Answer: C. A is correct — oceans are a climate stabiliser — but R is wrong. Water has a high specific heat, so it absorbs and releases heat slowly, which is precisely why it moderates climate.
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