Air Has Mass and Exerts Pressure

Introduction

Air seems weightless — we walk through it without noticing. Yet the air in a typical classroom has a mass of about 200 kg, more than two adults, and the atmosphere above us presses on every square centimetre of our skin with a force of about 10 newtons. We do not feel crushed because air pushes equally from all directions, including from inside our bodies. This page explains why air has mass, where air pressure comes from, and how it changes as you go higher.

Core explanation

Air has mass. Air is matter: it is made of real molecules with mass. At sea level and 20 °C, the density of air is about 1.2 kg/m³ , or 1.2 g per litre. You can show that air has mass by weighing a sealed, rigid container, removing some of the air with a pump, and weighing it again — it is lighter. Pumping extra air into a football makes it slightly heavier.

Pressure from moving particles. In the particle model, air molecules move rapidly in all directions — at room temperature, the average speed of a nitrogen molecule is roughly 500 m/s. Every surface in contact with air is struck by an enormous number of molecules every second. Each collision gives the surface a tiny push. Added together, these pushes produce a steady force on every surface. Pressure is the force acting on each unit of area:

pressure = force ÷ area

The SI unit of pressure is the pascal (Pa) , equal to 1 newton per square metre (1 N/m²).

Atmospheric pressure. Near sea level, atmospheric pressure is about 101 kPa (101 000 Pa). Weather reports often use millibars or hectopascals: 1013 hPa is typical standard pressure. Another way to think of it is that the atmosphere pushes with a force of about 100 000 N on every square metre — roughly the weight of a 10-tonne lorry.

Why pressure acts in all directions. Because the molecules move randomly, they hit surfaces from every direction. So air pushes up on the underside of a table as hard as it pushes down on the top, and pushes outwards from inside your lungs as well as inwards from outside.

What changes the pressure of a gas? For a sealed container of gas:

- More particles in the same space → more collisions per second → higher pressure. This is why pumping air into a tyre makes it firm. - Higher temperature → particles move faster, hit harder and more often → higher pressure. - Smaller volume (same particles, same temperature) → particles hit the walls more often → higher pressure.

Pressure falls with height. Atmospheric pressure is caused by the weight of the air above you, and by the collisions of the air particles around you. Going higher, there is less air above and the air is less dense, so pressure falls. It roughly halves for every 5.5 km of height: at the summit of Mount Everest (8.8 km) it is only about one-third of sea-level pressure.

Measuring pressure. A barometer measures atmospheric pressure. In a mercury barometer, the atmosphere supports a column of mercury about 760 mm tall. An aneroid barometer uses a small sealed metal box that squashes or expands as air pressure changes. Falling pressure often signals unsettled, wet weather.

Formulae

pressure (Pa) = force (N) ÷ area (m²). Mass of air (kg) = density (kg/m³) × volume (m³).

Step-by-step reasoning

To explain a change in gas pressure using particles:

1. State what has changed: number of particles, temperature or volume. 2. Describe the effect on the particles' speed or how crowded they are. 3. Say whether collisions with the walls become more or less frequent (and harder or softer). 4. Conclude whether the pressure rises or falls.

Visual explanation

Draw a box with a few dots representing air molecules and arrows showing them moving in random directions. Mark each point where a dot strikes a wall with a small star. Now draw a second box, the same size, with twice as many dots: there are twice as many stars on the walls each second, and so twice the pressure.

Real-world analogy

Think of hailstones drumming on a car roof. One hailstone makes a tiny tap, but thousands per second produce a continuous push. Air molecules are far smaller and far more numerous, so their combined push on a surface feels perfectly smooth and steady.

Real-world example

A sealed crisp packet taken up a mountain or on an aeroplane swells up. The packet was sealed at lower altitude, where air pressure was higher. As the outside pressure falls, the air inside pushes outwards harder than the air outside pushes in, so the packet puffs out.

Why?

Why does drinking through a straw work? Sucking lowers the pressure inside your mouth and the straw. Atmospheric pressure on the surface of the drink in the glass is now greater than the pressure inside the straw, so the air outside pushes the drink up the straw. You do not really "pull" the liquid up.

Common misconception

"Air has no weight, because things fall through it." Air has mass and weight, just a small density. A cubic metre of air has a mass of about 1.2 kg. Objects fall through air because they are much denser than air, not because air is weightless.

Worked example

Question: A classroom measures 8 m × 7 m × 3 m. The density of air is 1.2 kg/m³. Calculate the mass of air in the room.

Reasoning: Volume = 8 × 7 × 3 = 168 m³. Mass = density × volume = 1.2 × 168 = 201.6 kg.

Answer: About 200 kg of air.

Quick check

1. What causes gas pressure, in terms of particles? Answer: Gas particles colliding with the surfaces of their container, each collision exerting a small force.

Exam focus

Explain pressure changes using collisions: more particles, higher temperature or smaller volume all mean more frequent collisions with the walls. Remember pressure = force ÷ area and the unit pascal. Know that atmospheric pressure is about 101 kPa at sea level and decreases with height.

Advanced insight

The pressure, volume and temperature of a gas are linked by the gas laws. For a fixed amount of gas at constant temperature, pressure × volume stays constant (Boyle's law): halve the volume and the pressure doubles. These relationships hold closely for air at everyday conditions because its molecules are far apart and attract each other only weakly.

Summary

Air has mass: its density at sea level is about 1.2 kg/m³. Gas pressure is caused by moving particles colliding with surfaces, and pressure = force ÷ area, measured in pascals. Atmospheric pressure is about 101 kPa at sea level, acts in all directions and falls with height as the air thins. More particles, higher temperature or a smaller volume increase gas pressure.

Practice questions

1. State the SI unit of pressure and define it. Answer: The pascal (Pa), equal to one newton per square metre. 2. Explain, using particles, why a bicycle tyre gets firmer as you pump in more air. Answer: More air particles are in the same volume, so they hit the inside of the tyre more often, increasing the pressure. 3. Calculate the mass of air in a box of volume 0.5 m³ if the density of air is 1.2 kg/m³. Answer: Mass = 1.2 × 0.5 = 0.6 kg. 4. Why does atmospheric pressure decrease as you climb a mountain? Answer: There is less air above you and the air is less dense, so there are fewer particle collisions and less weight of air pressing down. 5. Why does the pressure in a sealed can rise if the can is heated? Answer: The gas particles move faster, so they hit the walls more often and with more force, raising the pressure.