ScienceCBSE Class 8 · Nature of Matter: Elements, Compounds, and Mixtures

Mixtures, Air, Alloys and Pure Substances: Class 8 Science

Updated 17 October 2026 · Fact-checked

A mixture contains two or more substances in which each keeps its own properties and does not react chemically with the others. It is uniform if the components cannot be told apart, even with a microscope, and non-uniform if they can be seen. Air is a uniform mixture of gases, alloys are uniform mixtures of metals, and in science a pure substance has only one kind of particle.

In a non-uniform mixture the parts can be seen separately; in a uniform mixture like air or brass they are evenly spread and cannot be told apart.

Illustration drawn for QuestionBuddy.

Understand Mixtures, Air, Alloys and Pure Substances

Mixture. When two or more substances are mixed so that each substance keeps its properties, the result is a mixture; the individual substances are its components, and they do not react chemically with each other. Poha, a sprout salad, sugar in water, soups and lemonade are mixtures.

Non-uniform mixture: components generally visible with the naked eye or with a magnifying device (sprout salad).
Uniform mixture: components evenly distributed and not distinguishable even with a microscope (sugar and water).

Alloys. Stainless steel (iron, nickel, chromium and a small amount of carbon), brass (copper and zinc) and bronze (copper and tin) are uniform mixtures of metals called alloys. Mishraloha was the old Indian name for a mixture of metals with properties different from the parent metals; bronze, Kamsya, was described as 4 parts copper (Tamra) to 1 part tin (Vanga). Alloys such as stainless steel are stronger and more durable than pure iron.

Is air a mixture? Air is a uniform mixture mainly of nitrogen (about 78 per cent), oxygen, argon, carbon dioxide and water vapour. Oxygen supports life and combustion; nitrogen does not take part in combustion. Warm air touching a cool surface deposits tiny droplets of water from its vapour. Activity 8.1: calcium oxide (quick lime) added slowly to water reacts vigorously, giving calcium hydroxide and heat; after filtering, the clear solution is lime water. Left in a dish in the open, it turns milky because carbon dioxide from the air reacts with calcium hydroxide to give calcium carbonate (insoluble white particles) and water: calcium hydroxide + carbon dioxide gives calcium carbonate + water. This shows the presence of carbon dioxide in air. Activity 8.2: a clean black paper left near a window collects dust, so dust particles float in air, but they are not an integral part of air and are pollutants whose nature and number vary with time and place. Major air pollutants are particulate matter (dust, soot) and gases such as carbon monoxide, ozone, nitrogen dioxide and sulfur dioxide. The Air Quality Index (AQI) describes air quality.

Types of mixtures by physical state of components (Table 8.1). Gas in gas: air (uniform). Gas in liquid: aerated water (soda), oxygen dissolved in water (uniform). Solid in gas: carbon particles in air (non-uniform). Liquid in liquid: acetic acid in water (vinegar), uniform; oil and water, non-uniform. Solid in liquid: sand and water, non-uniform; seawater, uniform. Solid in solid: baking powder (baking soda and tartaric acid), alloys (uniform). The components of a mixture may themselves be mixtures in everyday talk, but in science all components must be pure substances. In science, a mixture is separated to obtain pure substances.

Pure substance. In everyday use 'pure' means unadulterated (adulteration is illegally adding cheaper or poor-quality material). In science a pure substance has no other substance in it: it cannot be separated into other kinds of matter by any physical process and consists of the same type of particles. So even 'pure' milk or ghee, being made of more than one substance, is a mixture in science.

Elements, compounds and the iron-sulfur activity are in the next topic.

Mixtures, Air, Alloys and Pure Substances: formulas and key facts

Mixture: Two or more substances
Each keeps its properties
No chemical reaction between components; Components can be separated by physical methods.
Types of mixture: Uniform: components not distinguishable | Non-uniform: components visible; Sugar in water is uniform; sprout salad is non-uniform.
Alloys: $S t ain l esss t ee l = F e + N i + C r + C ∣ B r a ss = C u + Z n ∣ B r o n z e = C u + S n$; Uniform mixtures of metals.
Lime water test for carbon dioxide: Calcium hydroxide + Carbon dioxide > Calcium carbonate + Water; Lime water turns milky.
Air composition in the chapter: Nitrogen about 78 percent
Oxygen
Argon
Carbon dioxide
Water vapour; Dust is a pollutant, not a part of air.
Pure substance in science: One kind of particle | Cannot be separated by physical processes; Different from the everyday meaning of 'pure'.

How to solve Mixtures, Air, Alloys and Pure Substances questions

Use this method to classify matter.

1Ask: can the components be separated by a physical method? If yes, it is a mixture.
2Ask: can the parts be seen as separate (naked eye, magnifier, microscope)? Yes: non-uniform; no: uniform.
3Identify the physical states of the components and match with Table 8.1.
4For alloys, name the metals combined.
5For air, list the gases; dust is not a part of the air.
6For 'pure', use the science meaning: only one kind of particle.

Mixtures, Air, Alloys and Pure Substances in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Classify kitchen items

Suppose a kitchen has sugar syrup, sprout salad, soda water, a jar of ghee marked 'pure', and a steel spoon.

(a) Mark each as uniform mixture, non-uniform mixture or other. (b) Say whether the ghee is pure in the science sense. (c) What type of mixture is soda water by states of components?

Try it first, then show the solutionSolution

Sugar syrup: uniform; sprout salad: non-uniform; soda water: uniform; steel spoon: uniform mixture (an alloy).
In science ghee is made of more than one substance, so it is a mixture and not a pure substance, even if unadulterated.
Soda water is a gas dissolved in a liquid.

Answer: (a) Uniform, non-uniform, uniform, uniform (alloy); (b) not pure in science; (c) gas in liquid.

Memory hook: Pure on the pack is not pure in the lab.

Bronze parts

Imagine a craftsperson mixing copper and tin in the ancient 4 : 1 proportion (copper : tin) to make 500 g of bronze.

(a) How many parts make up the bronze? (b) Find the mass of copper and tin. (c) Is bronze an element, compound or mixture?

Try it first, then show the solutionSolution

4 + 1 = 5 parts.
One part = 500 ÷ 5 = 100 g; copper = 4 × 100 = 400 g; tin = 100 g.
Mixture (an alloy), because the metals keep their properties and are only mixed.

Answer: (a) 5 parts; (b) 400 g copper and 100 g tin; (c) a mixture (alloy).

Memory hook: Parts add up, then split the total.

Tricks to remember Mixtures, Air, Alloys and Pure Substances

Short memory aids. Say each one aloud once, then use it on a question the same day.

Uniform = Uniformly smooth
Uniform: one look, no pieces. Non-uniform: you can point at pieces.
Use it when: Type of mixture.
Air = 78 Nitrogen
Think 'nearly four-fifths nitrogen'.
Use it when: Composition of air (chapter value about 78 percent).
Lime water milky
Lime + CO2 = chalk (calcium carbonate) = milky.
Use it when: Test for carbon dioxide.
Brass and bronze
Brass: copper + Zinc (Z at the end). Bronze: copper + tin.
Use it when: Alloys.

Mixtures, Air, Alloys and Pure Substances: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Uniform and non-uniform mixtures · 2 marks · Short answerBoard level

Distinguish between a uniform and a non-uniform mixture with one example of each.

Try it first, then show the solutionSolution

Uniform: components are evenly distributed and cannot be distinguished, even under a microscope; example sugar in water. (1)
Non-uniform: components can be seen with the naked eye or a magnifier; example sprout salad. (1)

Answer: Uniform: components not distinguishable (sugar in water); non-uniform: components visible (sprout salad).

Example 2: Lime water and air · 1 mark · MCQBoard level

Lime water left open to the air turns milky because
(a) oxygen reacts with calcium hydroxide
(b) carbon dioxide reacts with calcium hydroxide to form calcium carbonate
(c) nitrogen dissolves in it
(d) dust settles in it

Try it first, then show the solutionSolution

Carbon dioxide from the air gives insoluble calcium carbonate particles and water.

Answer: (b)

Example 3: Classify the mixtures · 5 marks · Long answerHarder board level

Classify the following by the physical states of the components and as uniform or non-uniform: (a) air, (b) vinegar, (c) oil and water, (d) sand and water, (e) seawater, (f) carbon particles in air.

Try it first, then show the solutionSolution

(a) Gas and gas; uniform. (1)
(b) Liquid and liquid (acetic acid in water); uniform. (1)
(c) Liquid and liquid; non-uniform. (1)
(d) Solid and liquid; non-uniform. (1)
(e) Solid and liquid (salts dissolved); uniform. (f) Solid and gas; non-uniform. (1)

Answer: (a) gas-gas uniform; (b) liquid-liquid uniform; (c) liquid-liquid non-uniform; (d) solid-liquid non-uniform; (e) solid-liquid uniform; (f) solid-gas non-uniform.

Example 4: Dust and air · 4 marks · Case-basedHarder board level

Read the passage and answer.

A clean black sheet of paper is left near an open window for a few hours; tiny particles settle on it. A student says dust is a component of air like nitrogen.
(a) What does the experiment show? (1)
(b) Is the student right? Explain. (1)
(c) Name two gases that are major pollutants. (1)
(d) Which index describes air quality? (1)

Try it first, then show the solutionSolution

(a) Dust particles are suspended in the air. (1)
(b) No: dust is not an integral part of air; it is a pollutant and its amount varies with time and place. (1)
(c) Any two: carbon monoxide, ozone, nitrogen dioxide, sulfur dioxide. (1)
(d) The Air Quality Index (AQI). (1)

Answer: (a) Dust floats in air; (b) not a component, a pollutant; (c) e.g. carbon monoxide and sulfur dioxide; (d) AQI.

Example 5: Is it pure · 4 marks · Case-basedHOTS / competency

Read the passage and answer.

A shop sells 'pure' honey, 'pure' ghee, distilled water, an iron nail and brass utensils. A scientist says only some are pure substances in science.
(a) What is the everyday meaning of 'pure'? (1)
(b) What is the scientific meaning? (1)
(c) From the list, identify one item that you expect to be a pure substance in the chapter's sense and justify. (1)
(d) Identify one that is a mixture and justify. (1)

Try it first, then show the solutionSolution

(a) Unadulterated, with nothing cheaper or inferior added. (1)
(b) A pure substance has no other substance in it and consists of the same type of particles. (1)
(c) Distilled water: it is a compound made of only one kind of particle (H2O), so it is a pure substance. (An iron nail is usually mild steel, an alloy, so it is probably a mixture.) (1)
(d) Brass: an alloy of copper and zinc, so a mixture; 'pure' ghee also has more than one substance. (1)

Answer: (a) Unadulterated; (b) one kind of particle; (c) distilled water (only one kind of particle, H2O); (d) brass (mixture of copper and zinc).

Where students lose marks in Mixtures, Air, Alloys and Pure Substances

The slips examiners see most often, and what to write instead.

Mistake: Using the everyday meaning of 'pure' in a science answer.
Packs say 'pure' for unadulterated products.
Wrong working
Pure milk is a pure substance.
Right working
Milk is made of more than one substance, so it is a mixture in science.
Do this: Pure in science means one kind of particle.
Usually costs: 1 mark.
Mistake: Counting dust as a gas of air.
Dust is always seen in air.
Wrong working
Air contains dust as one of its components.
Right working
Dust is a pollutant suspended in air, not an integral part of it.
Do this: List gases only when asked for components.
Usually costs: 1 mark.
Mistake: Saying alloys are compounds.
They are made by joining metals.
Wrong working
Brass is a compound of copper and zinc.
Right working
Brass is a uniform mixture (alloy) of copper and zinc.
Do this: Components keep properties in a mixture.
Usually costs: 1 mark.
Mistake: Using the wrong test for carbon dioxide.
Several gas tests exist.
Wrong working
Carbon dioxide makes a burning candle burn brighter.
Right working
Carbon dioxide turns lime water milky.
Do this: Lime water milky = CO2.
Usually costs: 1 mark.

Exam tips

Use 'components retain their properties' in any definition of a mixture.
Give a state-pair label (solid-liquid) and uniformity for table-type questions.
Write the lime water reaction as a word equation.
Distinguish the common and scientific meanings of 'pure' with an example.
In ratio questions, add the parts first and then divide the total.

Practice questions on Mixtures, Air, Alloys and Pure Substances

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 2 marks · Very short answerBoard level
What is an alloy? Name two alloys and the metals in each.
Hint
Think of brass and bronze.
Answer
A uniform mixture of metals (1). Brass: copper and zinc; bronze: copper and tin (1).
Q2 · 1 mark · MCQBoard level
Which of the following is a non-uniform mixture?
(a) Air
(b) Vinegar
(c) Oil and water
(d) Seawater
Hint
Can you see two layers?
Answer
(c)
Q3 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Air is a uniform mixture.
Reason (R): The components of air are evenly distributed and cannot be distinguished separately.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Definition of uniform.
Answer
(a) R explains why air is uniform.
Q4 · 4 marks · Short answerHarder board level
A 600 g sample of brass is made of copper and zinc in the ratio 3 : 1 (invented ratio for this problem). (a) Find the mass of each metal. (b) Find the percentage of copper. (c) Is brass an element, compound or mixture? (d) Name one property that stainless steel has that pure iron lacks, as mentioned in the chapter.
Hint
3 + 1 = 4 parts.
Answer
(a) One part = 600 ÷ 4 = 150 g; copper 450 g, zinc 150 g. (b) 450 ÷ 600 × 100 = 75%. (c) A mixture (alloy). (d) It is stronger and more durable (1 mark each).
Q5 · 4 marks · Case-basedHOTS / competency
Read the passage and answer.
A student prepares lime water by adding calcium oxide to water, filters it and leaves it open. A second dish is covered. After some hours the open dish is milky; the covered one is clear.
(a) Explain the difference. (b) Write the word equation for the reaction. (c) Name the substance that makes the open dish milky. (d) What does the test show about air?
Hint
Carbon dioxide is in air.
Answer
(a) Carbon dioxide from the air reaches the open dish but not the covered one. (b) Calcium hydroxide + carbon dioxide gives calcium carbonate + water. (c) Calcium carbonate, an insoluble white solid. (d) Air contains carbon dioxide (1 mark each).

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.What is a mixture?Show answer
Two or more substances mixed so that each keeps its properties and does not react chemically with the others.
2.Name an alloy of copper and tin.Show answer
Bronze.
3.Which gas forms about 78 percent of air?Show answer
Nitrogen.
4.What does lime water do with carbon dioxide?Show answer
It turns milky because calcium carbonate forms.
5.What does AQI stand for?Show answer
Air Quality Index.

Mixtures, Air, Alloys and Pure Substances: frequently asked questions

Is sugar solution a pure substance?

No. It is a uniform mixture of sugar and water.

What is Mishraloha?

An ancient Indian name for a mixture of two or more metals with properties distinct from its constituent metals.

Why is separation of a mixture done in science?

To obtain pure substances; in daily life it is often to obtain the component of interest.

Does the water vapour in air turn into liquid?

Yes. When warm air touches a cool surface, the vapour changes into tiny droplets of liquid water.