Science Class 8 — Curiosity
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Atmospheric Pressure

🎓 Class 8 Science CBSE Theory Ch 6 — Combustion and Flame ⏱ ~30 min
🌐 Language: [gtranslate]

This MCQ module is based on: Atmospheric Pressure

[myaischool_lt_science_assessment grade_level="class_8" science_domain="chemistry" difficulty="basic"]

Probe and Ponder

Look up at the sky on a hot summer afternoon. The air looks empty — yet above your head, stretching tens of kilometres into space, is an ocean of invisible gas. And just like water in a swimming pool presses on you from all sides, so does this ocean of air. We live at its bottom, drenched in it every second of our lives.

  • Why do winds blow stronger on some days than on others?
  • Why are overhead water tanks built high up on stands?
  • Can the air around us really be strong enough to crush a metal can?
  • What actually causes storms, thunder, lightning and cyclones?
  • Would cyclones still form if the Earth stopped spinning on its axis?

By the end of this part, you will discover how the simple weight of air gives rise to a force called atmospheric pressure — and how that force explains everyday magic tricks, the design of barometers, and even why your ears pop in an aeroplane.

6.1 Atmospheric Pressure

Air is a substance. It has mass, and because it has mass, it has weight. The layer of air that surrounds the Earth is called the atmosphere. The weight of all this air, acting on every square metre of the Earth's surface, produces atmospheric pressure.

Atmospheric pressure: The force exerted by the weight of the atmosphere on a unit area of any surface beneath it. At sea level it is about 1 atmosphere (atm), which equals roughly 101325 pascal (Pa), or 105 N/m2.

If you imagine a column of air sitting on your chest, it weighs about the same as a small car — almost 10,000 kg! Why doesn't this crush us? The reason is simple: our body is filled with air and fluids that push outward with exactly the same pressure. The inside and outside are perfectly balanced, so we feel nothing.

Weight of the Air Above Us Column of air (~10,000 kg) Body pushes out → balance! Air pushes down & from all sides
Fig 6.1 — The atmosphere presses on us from all directions, but internal pressure balances it.

Pressure is defined mathematically as:

\[ \text{Pressure} = \dfrac{\text{Force}}{\text{Area}} \quad \Rightarrow \quad P = \dfrac{F}{A} \]

Its SI unit is the pascal (Pa), where 1 Pa = 1 N/m2.

🔬 Activity 6.1 — The Crushing CanL3 Apply
🤔 Predict first: If you heat water inside an empty tin can and then quickly seal it and let it cool, what do you think will happen to the can?

You need: an empty thin aluminium/tin can, a little water, a stove, tongs, a tight-fitting cap. (Do this with an adult — heating is involved.)

  1. Pour about 2 tablespoons of water into the empty can.
  2. Heat the can until steam escapes vigorously — the steam pushes most of the air out.
  3. Using tongs, quickly close the cap tightly and place the can on the table.
  4. Pour cold water over the can, or just let it cool in air.
Result: The can suddenly collapses with a loud crunch!
Why? When the steam cools, it turns back into liquid water. This leaves very little gas inside, so the pressure inside the can drops far below the outside air pressure. The atmosphere outside — which was always pressing in — now has nothing to push back against it, and crushes the can.
🔬 Activity 6.2 — The Magic Glass and CardL3 Apply
🤔 Predict first: If you fill a glass to the brim with water, cover it with a stiff card, and flip it upside down — what happens?
  1. Fill a glass completely with water (no air bubble).
  2. Place a smooth, stiff postcard on top.
  3. Hold the card in place and flip the glass upside down over a sink.
  4. Carefully remove your hand from the card.
Result: The card stays stuck to the mouth of the glass and the water does not fall!
Why? Atmospheric pressure pushing upwards on the card from below is much greater than the weight of the small amount of water pushing downwards. Air is holding the water in!
🔬 Activity 6.3 — Sipping Through a StrawL2 Understand
🤔 Predict first: When you sip juice through a straw, is it your mouth that pulls the juice up, or something else?
  1. Place a straw in a glass of juice.
  2. Sip gently. Observe the liquid rising in the straw.
  3. Now try sipping with a small hole punched in the side of the straw above the liquid. Difficult, isn't it?
Explanation: When you suck, you remove some air from inside the straw — this lowers the pressure inside. The outside atmospheric pressure, acting on the surface of the juice in the glass, pushes the liquid up into the straw. It's not your mouth pulling — it's the atmosphere pushing!
Atmospheric Pressure at Work Crushed can Glass + card (flipped) Air pushes UP Straw low P inside
Fig 6.2 — In all three demonstrations, the everyday "magic" is really atmospheric pressure pushing from outside.

6.2 Measuring Atmospheric Pressure — the Barometer

The first instrument to measure atmospheric pressure was invented in 1643 by an Italian scientist Evangelista Torricelli. An instrument that measures atmospheric pressure is called a barometer.

Torricelli's Mercury Barometer

Torricelli took a glass tube about 1 m long, closed at one end, and filled it with mercury (a heavy liquid metal). He inverted the tube into a dish of mercury. The mercury inside the tube fell — but not all the way. It stopped at a height of about 76 cm above the surface of the mercury in the dish.

Why 76 cm? Because the weight of the 76-cm mercury column exactly balances the push of the atmosphere on the mercury in the open dish. So the height of the mercury column directly measures the atmospheric pressure.

Standard atmospheric pressure at sea level = 76 cm of mercury = 760 mm Hg = 1 atm ≈ 101325 Pa.
Torricelli's Mercury Barometer vacuum 76 cm of mercury Atmospheric pressure Dish of mercury (open to air)
Fig 6.3 — Atmospheric pressure on the open dish supports a 76 cm column of mercury inside the closed tube.

The Modern Aneroid Barometer

Carrying a long glass tube of mercury is inconvenient — and mercury is toxic. The aneroid barometer (from Greek "a-neros" meaning "without liquid") solves this. Inside is a small metal box from which most of the air has been removed. When atmospheric pressure rises, the box gets squashed slightly; when pressure falls, it expands. A lever system converts this tiny movement into a needle sweeping across a dial.

Sudden drops in the barometer reading often signal a coming storm — this is how meteorologists predict weather even before the sky looks threatening.

6.3 How Pressure Varies with Altitude

If you climb a mountain, the layer of atmosphere above you becomes thinner — there is less air above your head, so less weight, and therefore less pressure. The higher you go, the lower the atmospheric pressure falls.

🌊
Sea level
~101.3 kPa (1 atm) — this is our reference pressure.
⛰️
Shimla (~2200 m)
About 78 kPa — roughly 77% of sea-level pressure.
🏔️
Mount Everest (8848 m)
Just about 33 kPa — only about one-third of sea-level pressure. Climbers need oxygen cylinders!

This is why you feel your ears "pop" in an aeroplane or while driving up a steep hill — the pressure inside your ear and outside it are briefly unequal, and your ear equalises with a small click. It is also why water boils at a lower temperature on mountains: with less atmospheric pressure pushing down, water molecules can escape into vapour more easily. Dal takes forever to cook in a plain pot at Leh!

Pressure Decreases with Altitude Pressure (kPa) Altitude (km) 0 25 50 75 100 0246810 Sea level (101 kPa) Shimla Everest (~33 kPa)
Fig 6.4 — Atmospheric pressure falls sharply as altitude increases.

🎯 Pressure Predictor — Interactive L3 Apply

Drag the slider to set an altitude. See the approximate atmospheric pressure at that height.

0.0 km ~101 kPa

You are at sea level — pressure is at its maximum.

📋 Competency-Based Questions

Aanya and her family are travelling from Chennai (sea level) to Leh (3500 m altitude) by road. On the way, Aanya notices her sealed packet of chips getting puffier and puffier as they climb. Her father's wristwatch barometer also shows a steadily falling reading. When they stop for lunch, her mother complains that the dal in the pot is taking much longer than usual to cook.

Q1. L1 Remember What is the SI unit of pressure? Write its symbol.

Answer: The SI unit of pressure is the pascal; its symbol is Pa. 1 Pa = 1 N/m2.

Q2. L2 Understand Why does the chips packet get puffier as Aanya climbs?

  • A. The temperature drops and air shrinks
  • B. Atmospheric pressure outside falls but pressure inside stays the same, so it bulges out
  • C. The chips release gas
  • D. The plastic expands on its own
Answer: B. The sealed packet has air at sea-level pressure inside. As outside pressure decreases at altitude, the inside pressure is now higher, pushing the packet outwards.

Q3. L3 Apply Why does dal take longer to cook on mountains?

Answer: Water boils when its vapour pressure equals the surrounding atmospheric pressure. At high altitude the atmospheric pressure is lower, so water boils at a temperature well below 100 °C. Cooking at this lower temperature takes much longer — a pressure cooker fixes this by raising the internal pressure artificially.

Q4. L4 Analyse If the barometer in their car shows 66 cm of mercury instead of the usual 76 cm, roughly by how much is atmospheric pressure reduced?

Answer: The reading is 10 cm of mercury lower than sea-level value. That is a reduction of (10/76) × 100 ≈ 13% of sea-level atmospheric pressure.

Q5. L5 Evaluate A classmate claims: "If atmospheric pressure is so huge — like 10,000 kg on our chest — we should be crushed. Since we aren't, the pressure must actually be very small." Evaluate this argument.

The argument is flawed. The pressure really is very large, but the fluids and gases inside our body push outwards with an equal pressure. The inside and outside are balanced, so there is no net force and therefore no crushing. If we were suddenly transported into a vacuum, the internal pressure would cause severe harm.

🔗 Assertion–Reason Questions

Assertion (A): A glass full of water covered with a stiff card does not spill when inverted.

Reason (R): Atmospheric pressure acting upward on the card is large enough to hold the weight of the water column.

  • A. Both A and R are true, and R correctly explains A.
  • B. Both A and R are true, but R does not explain A.
  • C. A is true, R is false.
  • D. A is false, R is true.
Answer: A. Atmospheric pressure on the card (~105 Pa over the card's area) easily supports the small weight of the water column.

Assertion (A): Atmospheric pressure decreases as we go higher above sea level.

Reason (R): The column of air above becomes shorter, so its weight becomes smaller.

  • A. Both A and R are true, and R correctly explains A.
  • B. Both A and R are true, but R does not explain A.
  • C. A is true, R is false.
  • D. A is false, R is true.
Answer: A. Less air above means less weight per unit area — hence lower pressure.

Assertion (A): Mercury is used in Torricelli's barometer instead of water.

Reason (R): Mercury is much denser than water, so a shorter, more convenient column can balance atmospheric pressure.

  • A. Both A and R are true, and R correctly explains A.
  • B. Both A and R are true, but R does not explain A.
  • C. A is true, R is false.
  • D. A is false, R is true.
Answer: A. A water barometer would need a tube over 10 m tall! Mercury's high density gives the compact 76 cm column.
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Science Class 8 — Curiosity
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