Air Looks Like One Gas but Is a Mixture of Many
Learn to pick out the components of everyday mixtures, tell uniform from non-uniform mixtures, sort mixtures by the states of their components, and separate a pure substance from a mixture.
If air looks like one single gas, why is it called a mixture?
Because it is several gases sharing the same space, in no fixed proportion, each keeping its own properties. The oxygen in air still supports burning and the nitrogen still does not — exactly as they would on their own.
Looking uniform is not the same as being one substance. This page covers everything in the CBSE Class 8 Science chapter's first part: the components of mixtures, uniform and non-uniform mixtures, classifying by physical state, and pure substances.
Looking uniform is not the same as being one substance. This page covers everything in the CBSE Class 8 Science chapter's first part: the components of mixtures, uniform and non-uniform mixtures, classifying by physical state, and pure substances.
What are the components of everyday mixtures?
A mixture contains two or more substances physically mixed together, each keeping its own properties and present in any proportion.
Picking out the components:
- Air — nitrogen, oxygen, carbon dioxide, water vapour, inert gases, dust and smoke
- Sea water — water with dissolved salts, mainly common salt
- Milk — water, fat, proteins, sugar, vitamins and minerals
- Soil — sand, clay, minerals, water, air and decayed matter
- Tea — water, tea extract, sugar and milk
- Brass — copper and zinc
- Concrete — cement, sand, gravel and water
Air is a mixture for three reasons that must be stated together:
- Its proportions vary — water vapour, dust and carbon dioxide differ from place to place and season to season
- Its components keep their own properties — oxygen supports burning, carbon dioxide turns limewater milky
- It can be separated by physical means, by liquefying air and then distilling it
Most things around us are mixtures rather than pure substances, which is why separating them is such a practical skill.
The feature to remember is the variable proportion. A cup of tea can be made more or less sweet and it is still tea — whereas a compound such as water is always the same two elements in the same fixed ratio, whatever its source.
Picking out the components:
- Air — nitrogen, oxygen, carbon dioxide, water vapour, inert gases, dust and smoke
- Sea water — water with dissolved salts, mainly common salt
- Milk — water, fat, proteins, sugar, vitamins and minerals
- Soil — sand, clay, minerals, water, air and decayed matter
- Tea — water, tea extract, sugar and milk
- Brass — copper and zinc
- Concrete — cement, sand, gravel and water
Air is a mixture for three reasons that must be stated together:
- Its proportions vary — water vapour, dust and carbon dioxide differ from place to place and season to season
- Its components keep their own properties — oxygen supports burning, carbon dioxide turns limewater milky
- It can be separated by physical means, by liquefying air and then distilling it
Most things around us are mixtures rather than pure substances, which is why separating them is such a practical skill.
The feature to remember is the variable proportion. A cup of tea can be made more or less sweet and it is still tea — whereas a compound such as water is always the same two elements in the same fixed ratio, whatever its source.
What is the difference between a uniform and a non-uniform mixture?
A uniform (homogeneous) mixture has its components spread evenly throughout, so every part looks and behaves the same. A non-uniform (heterogeneous) mixture does not, and its parts can often be seen separately.
Uniform mixtures:
- Sugar dissolved in water
- Salt solution
- Air
- Brass and steel
- Vinegar
Take a spoonful from the top or the bottom of a glass of sugar solution and both taste equally sweet — the sugar is distributed evenly.
Non-uniform mixtures:
- Sand and salt
- Oil and water
- Soil
- Muddy water
- A mixture of iron filings and sulphur
- Chalk powder stirred into water
Here the parts differ from place to place. Take a spoonful from the top of oily water and you get mostly oil; from the bottom, mostly water.
An Indian kitchen holds both on the same shelf — a glass of sweetened lassi is uniform, while a bowl of chana with onions is plainly not.
The case that trips students up is air, and it is why this section follows the last one. Air is a uniform mixture — evenly spread and impossible to separate by eye — yet still a mixture, because being uniform says nothing about whether the components are chemically combined.
Uniform mixtures:
- Sugar dissolved in water
- Salt solution
- Air
- Brass and steel
- Vinegar
Take a spoonful from the top or the bottom of a glass of sugar solution and both taste equally sweet — the sugar is distributed evenly.
Non-uniform mixtures:
- Sand and salt
- Oil and water
- Soil
- Muddy water
- A mixture of iron filings and sulphur
- Chalk powder stirred into water
Here the parts differ from place to place. Take a spoonful from the top of oily water and you get mostly oil; from the bottom, mostly water.
An Indian kitchen holds both on the same shelf — a glass of sweetened lassi is uniform, while a bowl of chana with onions is plainly not.
The case that trips students up is air, and it is why this section follows the last one. Air is a uniform mixture — evenly spread and impossible to separate by eye — yet still a mixture, because being uniform says nothing about whether the components are chemically combined.
How are mixtures classified by the states of their components?
By naming the state of the substance being mixed in the state of what it is mixed into.
- Gas in gas — air, a mixture of nitrogen, oxygen and other gases
- Gas in liquid — aerated drinks, with carbon dioxide dissolved in water; also the dissolved oxygen in a pond that fish breathe
- Gas in solid — hydrogen absorbed in certain metals; air trapped in a sponge or in foam
- Liquid in liquid — vinegar in water, or alcohol in water; oil and water as a non-uniform example
- Liquid in solid — moisture held in cheese or butter; mercury absorbed in gold
- Solid in liquid — salt or sugar in water; mud in water
- Solid in solid — alloys such as brass (copper and zinc) and steel (iron and carbon)
- Solid in gas — smoke, with fine solid particles suspended in air; dust in air
- Liquid in gas — mist and fog, with tiny water droplets in air
A cold drink bottle demonstrates gas in liquid neatly: the fizz is carbon dioxide escaping the moment the pressure is released.
The order of the words matters, and reversing it names something different. Solid in liquid is salt in water, a solution, while liquid in solid is moisture inside butter — so name the smaller-quantity component first and the one doing the dissolving second.
- Gas in gas — air, a mixture of nitrogen, oxygen and other gases
- Gas in liquid — aerated drinks, with carbon dioxide dissolved in water; also the dissolved oxygen in a pond that fish breathe
- Gas in solid — hydrogen absorbed in certain metals; air trapped in a sponge or in foam
- Liquid in liquid — vinegar in water, or alcohol in water; oil and water as a non-uniform example
- Liquid in solid — moisture held in cheese or butter; mercury absorbed in gold
- Solid in liquid — salt or sugar in water; mud in water
- Solid in solid — alloys such as brass (copper and zinc) and steel (iron and carbon)
- Solid in gas — smoke, with fine solid particles suspended in air; dust in air
- Liquid in gas — mist and fog, with tiny water droplets in air
A cold drink bottle demonstrates gas in liquid neatly: the fizz is carbon dioxide escaping the moment the pressure is released.
The order of the words matters, and reversing it names something different. Solid in liquid is salt in water, a solution, while liquid in solid is moisture inside butter — so name the smaller-quantity component first and the one doing the dissolving second.
How do you tell a pure substance from a mixture?
A pure substance is made of only one kind of particle throughout, with a fixed composition and fixed properties such as a sharp melting point. A mixture contains more than one kind.
The tests:
- Composition — a pure substance has a fixed composition; a mixture's varies
- Properties — a pure substance has fixed, definite properties; a mixture shows the properties of all its components
- Melting and boiling point — a pure substance melts or boils at a sharp, definite temperature; a mixture does so over a range
- Separation — a mixture's components separate by physical methods; a pure substance's do not
Pure substances are of two kinds: elements and compounds. Everything else is a mixture.
Among given examples:
- Pure substances — distilled water, oxygen, iron, copper, common salt, sugar, carbon dioxide, gold
- Mixtures — air, sea water, milk, soil, tea, brass, muddy water, ink
The word pure means something different in a shop and in a laboratory. Packaged "pure" ghee, honey or spices are pure in the everyday sense of unadulterated, but chemically they are mixtures of many substances.
So the chemical test is composition, not cleanliness. Distilled water is chemically pure while filtered mineral water is not, even though both are perfectly clean and safe to drink — which is exactly why the melting-point test is the more reliable one in the laboratory.
The tests:
- Composition — a pure substance has a fixed composition; a mixture's varies
- Properties — a pure substance has fixed, definite properties; a mixture shows the properties of all its components
- Melting and boiling point — a pure substance melts or boils at a sharp, definite temperature; a mixture does so over a range
- Separation — a mixture's components separate by physical methods; a pure substance's do not
Pure substances are of two kinds: elements and compounds. Everything else is a mixture.
Among given examples:
- Pure substances — distilled water, oxygen, iron, copper, common salt, sugar, carbon dioxide, gold
- Mixtures — air, sea water, milk, soil, tea, brass, muddy water, ink
The word pure means something different in a shop and in a laboratory. Packaged "pure" ghee, honey or spices are pure in the everyday sense of unadulterated, but chemically they are mixtures of many substances.
So the chemical test is composition, not cleanliness. Distilled water is chemically pure while filtered mineral water is not, even though both are perfectly clean and safe to drink — which is exactly why the melting-point test is the more reliable one in the laboratory.
Exam tip
Exam tip: giving two reasons that air is a mixture
This chapter is marked on reasons, not labels, so supply them.
For "why is air a mixture", give at least two of the three: variable proportions, components keep their own properties, separable by physical means. One reason earns part of the mark.
When classifying by state, name it as X in Y in the right order — gas in liquid for an aerated drink — and give the example alongside.
For uniform against non-uniform, state the test: whether a sample taken from any part would be the same. And remember that air is uniform and still a mixture.
For pure substances, quote the sharp melting point and fixed composition rather than saying "it is clean".
And keep the everyday and chemical senses of pure apart — a question asking which examples are chemically pure expects distilled water, not packaged ghee.
For "why is air a mixture", give at least two of the three: variable proportions, components keep their own properties, separable by physical means. One reason earns part of the mark.
When classifying by state, name it as X in Y in the right order — gas in liquid for an aerated drink — and give the example alongside.
For uniform against non-uniform, state the test: whether a sample taken from any part would be the same. And remember that air is uniform and still a mixture.
For pure substances, quote the sharp melting point and fixed composition rather than saying "it is clean".
And keep the everyday and chemical senses of pure apart — a question asking which examples are chemically pure expects distilled water, not packaged ghee.
Did you know
Why does being evenly mixed not make something a single substance?
Because uniformity is about how well spread the components are, not about whether they have combined.
Air, brass and sugar solution are all perfectly even to the eye and to the taste, yet each holds substances that have merely been placed together. The oxygen in air can still be breathed, the copper in brass can still conduct, the sugar in the solution can still be recovered by evaporation.
A compound is different in kind. When sodium combines with chlorine, neither element survives as itself — the result behaves like nothing but salt. That is the real dividing line, and it has nothing to do with how uniform the material looks.
Air, brass and sugar solution are all perfectly even to the eye and to the taste, yet each holds substances that have merely been placed together. The oxygen in air can still be breathed, the copper in brass can still conduct, the sugar in the solution can still be recovered by evaporation.
A compound is different in kind. When sodium combines with chlorine, neither element survives as itself — the result behaves like nothing but salt. That is the real dividing line, and it has nothing to do with how uniform the material looks.
Key takeaways
Mixtures and pure substances: quick revision
- A mixture holds two or more substances physically, in any proportion, each keeping its own properties.
- Air is a mixture because its proportions vary, its gases keep their properties, and it separates by liquefying and distilling.
- Uniform (homogeneous) mixtures are evenly spread — sugar solution, air, brass; non-uniform ones are not — sand and salt, oil and water, soil.
- Being uniform does not make something pure; air is uniform and a mixture.
- Classify by state as X in Y: gas in gas (air), gas in liquid (aerated drink), solid in liquid (salt water), solid in solid (alloys), solid in gas (smoke), liquid in gas (fog).
- A pure substance has a fixed composition and a sharp melting point and is either an element or a compound — so distilled water is pure while sea water and packaged ghee are mixtures.
You will remember all of this far better after answering five questions on it than after reading it twice.
- Air is a mixture because its proportions vary, its gases keep their properties, and it separates by liquefying and distilling.
- Uniform (homogeneous) mixtures are evenly spread — sugar solution, air, brass; non-uniform ones are not — sand and salt, oil and water, soil.
- Being uniform does not make something pure; air is uniform and a mixture.
- Classify by state as X in Y: gas in gas (air), gas in liquid (aerated drink), solid in liquid (salt water), solid in solid (alloys), solid in gas (smoke), liquid in gas (fog).
- A pure substance has a fixed composition and a sharp melting point and is either an element or a compound — so distilled water is pure while sea water and packaged ghee are mixtures.
You will remember all of this far better after answering five questions on it than after reading it twice.