This grade 7 science lesson on atmospheric pressure notes gives the key ideas first, then worked examples and diagrams. After that, practise with the worksheet on atmospheric pressure and check yourself with the quiz.
Quick answer: Air is a mixture of gases, mainly 78 % nitrogen and 21 % oxygen, and it has mass: about 1.2 kg per cubic metre. Atmospheric pressure is the push of the air above us, about 101 kPa at sea level. A barometer measures it, and it falls as height increases.
Level: Grade 7 science (Year 8 · Class 7) · Curriculum: MS-ESS2-5
Atmospheric pressure notes: the key ideas
Does air have mass?
Air is invisible, but it is matter, so it has mass. Weigh a flat football, pump air into it and weigh it again: the reading rises. A gas is made of particles, and particles have mass.
m = ρ × V
m is the mass in kilograms (kg), ρ is the density in kg/m³ and V is the volume in m³. Near sea level, the density of air is about 1.2 kg/m³.
Find the mass of air in a classroom 8.0 m long, 6.0 m wide and 3.0 m high.
V = 8.0 × 6.0 × 3.0 = 144 m³. m = ρ × V = 1.2 × 144 = 172.8. m ≈ 173 kg
What is air made of?
Air is a mixture of gases. Nearly four-fifths is nitrogen, and about one-fifth is oxygen, the gas that animals need to breathe. The rest is mostly argon with a trace of carbon dioxide and a variable amount of water vapour.
The atmosphere is the layer of gases that surrounds the Earth and is held in place by gravity. Most of its mass lies within about 10 km of the surface.
| Gas | Symbol | Share of dry air |
|---|---|---|
| Nitrogen | N₂ | 78 % |
| Oxygen | O₂ | 21 % |
| Argon | Ar | 0.9 % |
| Carbon dioxide | CO₂ | 0.04 % |
What is atmospheric pressure?
Air particles move randomly and collide with every surface. Each collision pushes a little, and billions of collisions a second add up to a steady push. We call it pressure. Atmospheric pressure is caused by the weight of the column of air above you, and it pushes in all directions, not just downwards.
p = F ÷ A
p is the pressure in pascals (Pa), F the force in newtons (N) and A the area in square metres (m²). 1 Pa = 1 N/m², 1 kPa = 1000 Pa and 1 hPa = 100 Pa.
At sea level atmospheric pressure is about 101 000 Pa. Find the force of the air on a window of area 0.20 m².
F = p × A = 101 000 × 0.20 = 20 200. F = 20 200 N. The window does not break, because the air inside pushes back equally.
Why does air pressure fall as you climb?
On a mountain there is less air above you, so the weight of the air column is smaller and the air is thinner. The graph shows that pressure drops quickly at first and then more slowly. At the height of Mount Everest the pressure is only about one-third of the value at sea level, which is why climbers carry oxygen.
How does a barometer measure pressure?
A barometer measures atmospheric pressure. In a mercury barometer, a closed glass tube full of mercury is turned upside down in a dish of mercury. The mercury sinks until the pressure of the column equals the push of the air on the dish, leaving a vacuum above it. At sea level the column stands about 760 mm high. A metal-box (aneroid) barometer uses a sealed, flexible box that squashes as pressure rises.
Standard pressure equals 760 mm of mercury, or 101.3 kPa, or 1013 hPa. Pressure falling steadily often means cloud and rain are coming; rising pressure often means clearing, drier weather.
A weather station in Nairobi reads 1008 hPa. Convert this to pascals and kilopascals.
1008 × 100 = 100 800 Pa and 100 800 ÷ 1000 = 100.8 kPa. 100 800 Pa = 100.8 kPa
Do not think that the atmosphere only presses downwards, or that a gas has no mass. Air pushes equally in every direction at a point, and an inflated ball is heavier than a flat one.
When you convert pressure units, write the chain: hPa × 100 gives Pa, then ÷ 1000 gives kPa. Check your answer is sensible: sea-level pressure should come out near 101 kPa.
- Air is about 78 % nitrogen, 21 % oxygen, 0.9 % argon and 0.04 % carbon dioxide.
- Air has mass: the density of air near sea level is about 1.2 kg/m³.
- Pressure p = F ÷ A, measured in pascals (1 Pa = 1 N/m²).
- Atmospheric pressure at sea level is about 101 kPa = 1013 hPa = 760 mm of mercury.
- Atmospheric pressure decreases as height above sea level increases.
What air is made of and why it presses: frequently asked questions
Does air have mass?
Yes. Air is a mixture of gases made of particles, and particles have mass. One cubic metre of air near sea level has a mass of about 1.2 kg.
What gases make up air?
Air is about 78 % nitrogen and 21 % oxygen. The remaining 1 % is mostly argon, with a tiny amount of carbon dioxide and a variable amount of water vapour.
What is atmospheric pressure?
Atmospheric pressure is the force the air exerts on each unit of area, caused by the weight of the air above. At sea level it is about 101 000 Pa.
How does a barometer work?
A mercury barometer balances the weight of a column of mercury against the push of the air. At sea level the column is about 760 mm high, and it rises and falls as the pressure changes.
Why is air pressure lower on a mountain?
There is less air above you on a mountain, so the weight of the air column is smaller. The air is also thinner because it has fewer particles in each cubic metre.
