What are Gases?
A gas is a state of matter that has no fixed shape and no fixed volume. Air, oxygen, carbon dioxide, nitrogen, helium, and water vapour are familiar examples of gases.
Gas particles are far apart and move freely in all directions. Because of this, a gas spreads out and fills all the available space in its container.
Topics
Particle Arrangement in Gases
The particles in a gas are much farther apart than the particles in solids and liquids.
The attractive forces between gas particles are very weak, so the particles move rapidly and randomly in all directions.
Gas particles constantly collide with one another and with the walls of their container.
Properties of Gases
Most gases have the following properties:
- no fixed shape
- no fixed volume
- ability to flow
- particles that are far apart
- particles that move rapidly and randomly
- high compressibility
- low density
- ability to expand and fill a container
- ability to exert pressure
- ability to diffuse easily
The specific properties of a gas depend on its composition, temperature, pressure, and particle behaviour.
No Fixed Shape
A gas does not keep its own shape.
It takes the shape of the container in which it is placed.
For example, air inside a balloon takes the shape of the balloon, while air inside a room fills the shape of the room.
No Fixed Volume
A gas does not have a definite volume.
It expands until it fills all the available space in its container.
If a gas is transferred from a small container to a larger one, it spreads out to occupy the larger volume.
Ability to Flow
Gases can flow because their particles move freely from one place to another.
Moving air is called wind.
Compressibility of Gases
Gases are much easier to compress than solids and liquids.
There is a large amount of empty space between gas particles. When pressure is applied, the particles can be pushed closer together.
Compressed gases are stored in cylinders for uses such as:
- cooking
- medical oxygen
- fire extinguishers
- welding
- vehicle fuel
- diving equipment
Expansion of Gases
A gas expands to fill the space available to it.
- When a gas is heated, its particles gain energy and move faster. If the gas is free to expand, it occupies a larger volume.
- When a gas is cooled, its particles lose energy and move more slowly.
Density of Gases
Gases generally have much lower densities than solids and liquids because their particles are far apart.
Different gases have different densities.
- Helium is less dense than air, which is why helium-filled balloons rise.
- Carbon dioxide is denser than air and may collect in low areas when there is little air movement.
Pressure in Gases
Gases exert pressure on the walls of their container.
Gas pressure is produced when moving particles collide with the container walls.
The more frequent and forceful the collisions, the greater the gas pressure.
Effect of Temperature on Gas Pressure
When a gas in a fixed container is heated, its particles move faster.
They collide with the walls more frequently and with greater force, so the pressure increases.
When the gas is cooled, the particles move more slowly and the pressure decreases.
This is why gas containers should not be exposed to excessive heat.
Effect of Volume on Gas Pressure
When the volume of a gas is reduced, its particles have less space in which to move.
They collide with the container walls more frequently, causing the pressure to increase.
When the volume is increased, the particles spread out and the pressure decreases.
Gas pressure acts in all directions.
Diffusion in Gases
Diffusion is the movement of particles from a region of higher concentration to a region of lower concentration.
Gases diffuse easily because their particles move rapidly and are far apart.
Examples include:
- the smell of perfume spreading through a room
- cooking smells moving through a house
- smoke spreading through air
- oxygen moving into the bloodstream in the lungs
- carbon dioxide moving out of the bloodstream
Mixing of Gases
Gases mix easily with one another.
When two gases come into contact, their particles move and spread until they are distributed throughout the available space.
Air is a mixture of gases, mainly nitrogen and oxygen, along with smaller amounts of other gases.
Gas Pressure and Atmospheric Pressure
The air surrounding Earth has mass and exerts pressure. This pressure is called atmospheric pressure.
Atmospheric pressure acts in all directions and affects objects, liquids, living organisms, and weather systems.
Although we do not usually notice it, atmospheric pressure is constantly acting on us.
Changes in Atmospheric Pressure
Atmospheric pressure generally decreases with increasing altitude. At higher altitudes, there is less air above a given point, so the pressure is lower.
Changes in atmospheric pressure are also associated with weather conditions.
High-pressure and low-pressure regions influence winds, clouds, and rainfall.
Gases and Temperature
The behaviour of gases is strongly affected by temperature.
When a gas is heated:
- its particles move faster
- collisions become more energetic
- pressure may increase
- volume may increase if expansion is possible
When a gas is cooled:
- its particles move more slowly
- pressure may decrease
- volume may decrease
- the gas may eventually condense into a liquid
Condensation of Gases
Condensation is the change of a gas into a liquid.
When a gas is cooled, its particles lose energy and move more slowly. Attractive forces pull the particles closer together, forming a liquid.
Examples:
- water droplets forming on a cold glass
- dew forming on grass
- clouds forming in the atmosphere
- steam changing back into water
Evaporation and Gas Formation
Evaporation is the change of a liquid into a gas from the liquid’s surface.
The particles with enough energy escape from the liquid and enter the air as gas particles.
Water vapour in the atmosphere is formed partly through evaporation from oceans, lakes, rivers, soil, and plants.
Sublimation
Sublimation is the direct change of a solid into a gas without passing through the liquid state.
Examples include:
- dry ice changing directly into carbon dioxide gas
- mothballs slowly changing into gas
- ice changing directly into water vapour under certain conditions
Deposition
Deposition is the direct change of a gas into a solid without passing through the liquid state.
Frost forms when water vapour changes directly into ice on a cold surface.
Air as a Mixture of Gases
Air is not a single substance. It is a mixture of different gases.
The main gases in dry air are:
- nitrogen
- oxygen
- carbon dioxide
Air also contains variable amounts of water vapour and very small amounts of other gases.
Each gas has an important role in natural and biological processes.
Oxygen
Oxygen is essential for respiration in most living organisms.
It also supports combustion.
Oxygen is used in hospitals, industry, welding, and life-support systems.
Nitrogen
Nitrogen makes up most of Earth’s atmosphere.
It is relatively unreactive under ordinary conditions.
Nitrogen is important for plant growth and is used in fertilizers, food packaging, industrial processes, and liquid nitrogen cooling.
Carbon Dioxide
Carbon dioxide is produced during respiration, combustion, and decomposition.
Plants use carbon dioxide during photosynthesis.
It is also used in fire extinguishers, carbonated drinks, refrigeration, and some industrial processes.
Water Vapour
Water vapour is water in its gaseous state.
It enters the atmosphere through evaporation and transpiration.
Water vapour plays an important role in clouds, rainfall, humidity, weather, and Earth’s energy balance.
Noble Gases
Noble gases are a group of gases that are generally very unreactive.
They include helium, neon, argon, krypton, and xenon.
Uses include:
- helium in balloons
- neon in signs
- argon in electric bulbs and welding
- some noble gases in specialised lighting and technology
Gases in Everyday Life
Gases are used and observed in many everyday situations.
Examples include:
- oxygen used for breathing
- carbon dioxide in fizzy drinks
- cooking gas used as fuel
- air in tyres
- helium in balloons
- compressed air in tools
- gas in refrigerators and air conditioners
- aerosols and sprays
- air movement in fans and ventilation systems
Gases in Respiration
Living organisms exchange gases with their surroundings.
Humans breathe in oxygen and breathe out carbon dioxide.
In the lungs, oxygen diffuses from the air into the blood, while carbon dioxide diffuses from the blood into the air.
Plants also exchange gases through tiny openings in their leaves.
Gases in Weather
The atmosphere is made of gases that are constantly moving.
Differences in temperature and pressure cause air to move, producing winds.
Water vapour condenses to form clouds and may later fall as rain, snow, or hail.
Gases therefore play an important role in weather and climate.
Gases in Combustion
Combustion is a chemical process in which a substance reacts with oxygen and releases energy.
Many fuels require oxygen to burn.
The products of combustion depend on the fuel and the amount of oxygen available.
Combustion is used in cooking, heating, engines, and power generation.
Storage and Transport of Gases
Gases are often compressed, cooled, or liquefied for storage and transport.
They may be stored in strong cylinders, tanks, or insulated containers.
Gas containers must be handled carefully because high pressure, heat, leakage, or flammability can create hazards.
Gases can also be transported through pipes, tubes, and ducts.
Safety Around Gases
Some gases are flammable, toxic, corrosive, or able to displace oxygen.
Important safety practices include:
- keeping gas cylinders away from heat
- checking for leaks
- ensuring good ventilation
- keeping flammable gases away from sparks and flames
- using the correct regulator and equipment
- following warning labels and safety instructions
Importance of Gases
Gases are essential to life, industry, transport, medicine, weather, and technology.
They support respiration, photosynthesis, combustion, cooling, chemical reactions, communication systems, and many industrial processes.
Understanding gases helps explain the atmosphere, breathing, weather, engines, pressure systems, and changes of state
Key Points
- A gas has no fixed shape and no fixed volume.
- Gas particles are far apart and move rapidly in all directions.
- Gases expand to fill their container.
- Gases are highly compressible.
- Gas particles produce pressure by colliding with surfaces.
- Gas pressure is affected by temperature and volume.
- Gases diffuse and mix easily.
- Atmospheric pressure is caused by the air surrounding Earth.
- Gases can condense into liquids, and some solids can sublime directly into gases.
- Air is a mixture of gases.
- Gases are essential in respiration, combustion, weather, medicine, and industry.
Glossary of Key Terms
Recap of the Key Terms in Gases
- Atmospheric Pressure: The pressure exerted by the air surrounding Earth.
- Carbon Dioxide: A gas produced during respiration, combustion, and decomposition, and used by plants during photosynthesis.
- Combustion: A chemical process in which a substance reacts with oxygen and releases energy.
- Compressibility: The ability of a gas to decrease in volume when pressure is applied.
- Condensation: The change of a gas into a liquid.
- Deposition: The direct change of a gas into a solid without passing through the liquid state.
- Diffusion: The movement of particles from a region of higher concentration to a region of lower concentration.
- Expansion: The increase in the volume occupied by a gas.
- Gas: A state of matter with no fixed shape and no fixed volume.
- Gas Pressure: The force exerted by gas particles as they collide with the walls of a container.
- Helium: A light, unreactive gas commonly used in balloons.
- Noble Gases: A group of gases that are generally very unreactive, including helium, neon, argon, krypton, and xenon.
- Nitrogen: The gas that makes up most of Earth’s atmosphere.
- Oxygen: A gas essential for respiration in most living organisms and required for combustion.
- Particle: A tiny unit of matter, such as an atom or molecule.
- Pressure: The force acting on a surface per unit area.
- Sublimation: The direct change of a solid into a gas without passing through the liquid state.
- Water Vapour: Water in its gaseous state.
Questions and Answers
Recap the concepts you have learnt. Try to answer the questions. You can find the answer to any question by clicking on the icon.
What is a gas?
A gas is a state of matter with no fixed shape and no fixed volume.
How are particles arranged in a gas?
Gas particles are far apart and move freely in all directions.
Why do gases have no fixed shape?
Gases have no fixed shape because their particles move freely and spread throughout their container.
Why do gases have no fixed volume?
Gases expand to fill all the available space in their container.
Give three examples of gases.
Oxygen, nitrogen, and carbon dioxide are examples of gases.
Why can gases flow?
Gases can flow because their particles move freely from one place to another.
Why are gases easy to compress?
There is a large amount of empty space between gas particles, so they can be pushed closer together.
What happens to gas particles when a gas is heated?
The particles gain energy and move faster.
What happens to gas particles when a gas is cooled?
The particles lose energy and move more slowly.
Why do gases generally have low density?
Their particles are far apart, so there is relatively little mass in a given volume.
Why does helium rise in air?
Helium rises because it is less dense than air.
What causes gas pressure?
Gas pressure is caused by gas particles colliding with the walls of their container.
In which directions does gas pressure act?
Gas pressure acts in all directions.
What happens to gas pressure when a gas in a fixed container is heated?
The pressure increases because the particles move faster and collide more forcefully with the walls.
What happens to gas pressure when the volume of a gas is reduced?
The pressure increases because the particles collide with the walls more frequently.
What is diffusion?
Diffusion is the movement of particles from a region of higher concentration to a region of lower concentration.
Why do gases diffuse easily?
Gases diffuse easily because their particles move rapidly and are far apart.
Give one everyday example of diffusion in gases.
The smell of perfume spreading through a room is an example of diffusion.
Why do gases mix easily?
Their particles move freely and spread until they are distributed throughout the available space.
What is atmospheric pressure?
Atmospheric pressure is the pressure exerted by the air surrounding Earth.
How does atmospheric pressure change with altitude?
Atmospheric pressure generally decreases as altitude increases.
Why is atmospheric pressure lower at high altitudes?
There is less air above a given point at higher altitudes.
What is condensation?
Condensation is the change of a gas into a liquid.
Give one example of condensation.
Water droplets forming on the outside of a cold glass are an example of condensation.
What is evaporation?
Evaporation is the change of a liquid into a gas from the liquid’s surface.
What is sublimation?
Sublimation is the direct change of a solid into a gas without passing through the liquid state.
Give one example of sublimation.
Dry ice changing directly into carbon dioxide gas is an example of sublimation.
What is deposition?
Deposition is the direct change of a gas into a solid without passing through the liquid state.
Give one example of deposition.
Frost forming on a cold surface is an example of deposition.
Is air a single gas?
No. Air is a mixture of several gases.
Which gas makes up most of Earth’s atmosphere?
Nitrogen makes up most of Earth’s atmosphere.
Why is oxygen important?
Oxygen is essential for respiration in most living organisms and also supports combustion.
Why is nitrogen important?
Nitrogen is important for plant growth and is used in fertilisers and many industrial processes.
What role does carbon dioxide play in plants?
Plants use carbon dioxide during photosynthesis.
What is water vapour?
Water vapour is water in its gaseous state.
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What are noble gases?
Noble gases are a group of gases that are generally very unreactive.
Give two examples of noble gases.
Helium and neon are examples of noble gases.
How are gases involved in respiration?
Oxygen diffuses into the blood, while carbon dioxide diffuses out of the blood.
How are gases involved in weather?
Moving air produces winds, while water vapour helps form clouds and rainfall.
Why must gas cylinders be handled carefully?
Gas cylinders may contain gases under high pressure, and some gases may be flammable, toxic, or dangerous if they leak.