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Elements, Compounds, and Mixtures

NCERT Class 8 · Science Based on NCERT Class 8 Science textbook · Free CBSE study kit

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Chapter Notes

Overview
This chapter explores the fundamental classification of matter into mixtures, compounds, and elements. It explains that matter around us is rarely pure; most things are mixtures of different substances. The big idea is that pure substances are either elements (the simplest building blocks) or compounds (elements chemically combined in fixed ratios), while mixtures are physical combinations where each component retains its own properties. Understanding this helps us appreciate the materials we use daily, from air and water to alloys and medicines.
Key Concepts (14)
Matter
Anything that has mass and occupies space.
e.g. The staircase, air, water, food, clothes, books, trees, and a ball are all examples of matter.
Mixture
When two or more substances are mixed together, where each substance retains its own properties and does not react chemically with the other.
e.g. Poha, sprout salad, sugar dissolved in water, air.
Components of a mixture
The individual substances that make up a mixture.
e.g. In a sprout salad, the components are green gram, chickpeas, onion, and tomato.
Non-uniform mixture (heterogeneous mixture)
A mixture where the different components are generally visible with the naked eye or with a magnifying device, and are not evenly distributed.
e.g. Sprout salad, sand and water, oil and water.
Uniform mixture (homogeneous mixture)
A mixture where the components are evenly distributed and cannot be distinguished, even with a microscope.
e.g. Sugar and water solution, air, stainless steel.
Alloy
A uniform mixture of two or more metals, or a metal and a non-metal, that appears the same throughout.
e.g. Stainless steel (iron, nickel, chromium, carbon), brass (copper and zinc), bronze (copper and tin).
Pure substance (in science)
A kind of matter that consists of the same type of particles and cannot be separated into other kinds of matter by any physical process.
e.g. Sugar, baking soda, oxygen, gold.
Element
A pure substance that cannot be further broken down into simpler substances by chemical reactions. It is made up of identical atoms.
e.g. Hydrogen, oxygen, gold, silver, sulfur, carbon, iron.
Atom
The smallest particle of an element that retains its identity. Atoms of different elements are different from each other.
e.g. An atom of hydrogen is different from an atom of oxygen.
Molecule
A stable particle of an element formed when two or more atoms of that element combine.
e.g. Two atoms of hydrogen combine to form a molecule of hydrogen (H₂). Two atoms of oxygen combine to form a molecule of oxygen (O₂).
Compound
A pure substance formed when two or more different elements combine chemically in a fixed ratio, resulting in properties different from the constituent elements.
e.g. Water (hydrogen and oxygen in 2:1 ratio), sodium chloride (sodium and chlorine in 1:1 ratio), iron sulfide.
Mineral
A natural, solid substance found on Earth with a fixed chemical composition. Most minerals are compounds, but some are pure elements (native minerals).
e.g. Quartz, calcite, mica, talc, gold (native mineral), sulfur (native mineral).
Metalloid
An element that has intermediate properties between those of metals and non-metals.
e.g. Silicon, boron.
Adulteration
The illegal process of adding cheaper or poor-quality substances to a product to increase quantity or reduce cost, making it hazardous to health.
e.g. Adding water to milk or mixing cheaper oils in ghee.
How It Works
Demonstrating the presence of carbon dioxide in air (Activity 8.1)
  1. Fill a glass tumbler half with water.
  2. Slowly add a small amount of calcium oxide (quick lime) to the water and stir to make lime water (calcium hydroxide solution).
  3. Filter the solution and leave it colourless in a petri dish for a few hours, stirring at regular intervals.
  4. Observe that the lime water turns milky.
  5. Explanation: Carbon dioxide from the air reacts with calcium hydroxide to form insoluble calcium carbonate (white particles) and water.
💡 The milky colour is due to the formation of calcium carbonate, not because of any change in the lime water itself.
Demonstrating the presence of dust particles in air (Activity 8.2)
  1. Take a black sheet of paper free from visible dust.
  2. Place it undisturbed near an open window or in the garden for a few hours.
  3. Observe tiny particles settled on its surface, which can be examined with a magnifying glass.
  4. Conclusion: Dust particles are suspended in air and are considered pollutants.
💡 Dust particles are not an integral part of air; their nature and number vary with time and place.
Electrolysis of water — breaking water into hydrogen and oxygen (Activity 8.3)
  1. Fill a beaker 2/3rd with water and add a few drops of dilute sulfuric acid.
  2. Fill two small test tubes completely with water from the beaker.
  3. Place a 9 V battery inside the beaker.
  4. Carefully invert each water-filled test tube over one terminal of the battery without spilling water.
  5. Wait for 10–15 minutes. Observe gas bubbles forming at both terminals.
  6. Note that the volume of gas collected in one test tube is double that in the other.
  7. Test the gases: bring a burning candle near the mouth of each test tube.
  8. A pop sound indicates hydrogen (smaller volume). A brighter flame indicates oxygen (larger volume).
💡 Water is a compound, not a mixture. It cannot be separated into hydrogen and oxygen by physical means; only a chemical change (electrolysis) can break it down.
Heating sugar to show it is a compound (Activity 8.4)
  1. Put a teaspoon of sugar in a boiling tube.
  2. Heat it gently. Observe the sugar turning brown and then black (charring).
  3. Notice small droplets of water forming near the open end of the tube.
  4. The black residue left behind is carbon (charcoal).
  5. Conclusion: Sugar decomposes on heating to give carbon and water. Since water contains hydrogen and oxygen, sugar is a compound of carbon, hydrogen, and oxygen.
💡 The water droplets come from the sugar itself, not from the air, because the tube is being heated.
Formation of iron sulfide from iron and sulfur (Activity 8.5)
  1. Take 5.6 g of iron filings and 3.2 g of sulfur powder on a watch glass. Observe them separately.
  2. Mix them thoroughly to form Sample A (a non-uniform mixture).
  3. Take half of Sample A in a china dish and heat it gently with continuous stirring until a black mass forms. Let it cool and grind it to get Sample B.
  4. Compare Sample A and Sample B:
  5. Appearance: Sample A shows separate yellow (sulfur) and black (iron) particles; Sample B is a uniform black mass.
  6. Magnet test: A magnet attracts iron filings from Sample A but has no effect on Sample B.
  7. Gas test with dilute hydrochloric acid: Sample A produces hydrogen gas (colourless, odourless, burns with pop sound). Sample B produces hydrogen sulfide gas (colourless, rotten egg smell).
💡 Sample A is a mixture (iron and sulfur retain their properties). Sample B is a compound (iron sulfide) with entirely new properties — it is not magnetic and reacts differently with acid.
Exam Traps — Don't Make These Mistakes
⚠ Confusing 'pure' in everyday life with 'pure' in science
✗ Milk is a pure substance because the packet says 'pure milk'.
→ In science, 'pure' means only one type of particle. Milk contains water, fat, proteins, and lactose — it is a mixture, not a pure substance.
✓ In science, milk is a mixture because it contains several components (water, fat, proteins, etc.) that can be separated by physical processes.
⚠ Thinking air is a compound because it is uniform
✗ Air is a compound because it is a uniform mixture of gases.
→ Uniformity does not mean chemical combination. In air, each gas (nitrogen, oxygen, etc.) retains its own properties and they are not chemically bonded in a fixed ratio.
✓ Air is a uniform mixture (homogeneous mixture) of gases like nitrogen, oxygen, argon, carbon dioxide, and water vapour. It is not a compound because the components are not chemically combined.
⚠ Believing water can be separated by physical means
✗ We can separate hydrogen and oxygen from water by boiling or filtering.
→ Boiling water only changes its state to vapour; filtering removes solid impurities. Hydrogen and oxygen are chemically bonded in water and can only be separated by a chemical change like electrolysis.
✓ Water is a compound. Its constituent elements, hydrogen and oxygen, cannot be separated by any physical method. They can only be separated by a chemical reaction, such as passing an electric current through water (electrolysis).
⚠ Mistaking alloys for compounds
✗ Stainless steel is a compound because it is made of iron, nickel, and chromium.
→ In an alloy, the metals are mixed physically and retain their individual properties. They are not chemically combined in a fixed ratio.
✓ Stainless steel is a uniform mixture (alloy) of iron, nickel, chromium, and carbon. It is not a compound because the components are not chemically bonded in a fixed ratio.
⚠ Assuming all gases in air are chemically combined
✗ Oxygen and nitrogen in air are chemically combined to form a compound.
→ Oxygen and nitrogen in air are separate gases that retain their own properties. They do not react with each other under normal conditions.
✓ In air, oxygen and nitrogen are present as separate gases. They are not chemically combined; they form a mixture.
Compare & Contrast
Nature of combination
Mixture
Compound
In a mixture, components are physically mixed and retain their own properties. In a compound, elements are chemically combined in a fixed ratio and the compound has entirely new properties different from its elements.
Ability to be broken down
Element
Compound
An element cannot be broken down into simpler substances by chemical reactions. A compound can be broken down into its constituent elements by chemical reactions (e.g., electrolysis of water).
Visibility of components
Uniform mixture (homogeneous)
Non-uniform mixture (heterogeneous)
In a uniform mixture, components are evenly distributed and cannot be seen separately even with a microscope. In a non-uniform mixture, components are unevenly distributed and can be seen with the naked eye or a magnifying device.
Exam Focus
Marks pattern: 2 MCQs (1 mark each), 1 Very Short Answer (1 mark), 1 Short Answer (3 marks), 1 Long Answer (5 marks)
Most asked: Definition and examples of elements, compounds, and mixtures · Differences between mixtures and compounds · Types of mixtures (uniform and non-uniform) with examples · Activity-based questions on electrolysis of water and heating of sugar · Activity 8.5: Iron and sulfur mixture vs. iron sulfide compound
Don't skip: Differentiate between mixture and compound with examples and a table.; Describe an activity to show that water is a compound (electrolysis).; Describe an activity to show that sugar is a compound (heating sugar).; Describe an activity to show the difference between a mixture and a compound (iron and sulfur).
One-Line Revision (read aloud before the exam)
  1. Matter is anything that has mass and occupies space.
  2. A mixture is formed when two or more substances are mixed without a chemical reaction, and each substance retains its properties.
  3. In a uniform mixture (homogeneous), components are evenly distributed and cannot be seen separately.
  4. In a non-uniform mixture (heterogeneous), components are unevenly distributed and can be seen with the naked eye.
  5. Air is a uniform mixture of gases like nitrogen, oxygen, argon, carbon dioxide, and water vapour.
  6. An alloy is a uniform mixture of two or more metals, e.g., stainless steel, brass, bronze.
  7. A pure substance in science consists of only one type of particle and cannot be separated by physical processes.
  8. An element is a pure substance that cannot be broken down into simpler substances by chemical reactions.
  9. Atoms of most elements combine to form molecules, e.g., two hydrogen atoms form a hydrogen molecule (H₂).
  10. A compound is a pure substance formed when two or more elements combine chemically in a fixed ratio.
  11. Water is a compound of hydrogen and oxygen in a 2:1 ratio, and it can be broken down by electrolysis.
  12. Sugar is a compound of carbon, hydrogen, and oxygen; on heating it gives carbon and water.
  13. Iron sulfide is a compound formed by heating iron and sulfur; it is not magnetic and has a rotten egg smell with acid.
  14. A mineral is a natural solid with a fixed chemical composition; most are compounds, but some are native elements like gold.
  15. Metalloids like silicon and boron have properties between metals and non-metals.

MCQs — 10 Questions with Answers

Q1. Which of the following is a pure substance according to science?

  • A. Milk
  • B. Sugar ✓
  • C. Soil
  • D. Air

Answer: B — Sugar consists of only one type of particle (sucrose) and cannot be separated by physical process, making it a pure substance; others are mixtures.

Q2. What is the main characteristic of a uniform mixture?

  • A. Components are easily visible to the naked eye
  • B. Components are evenly distributed and cannot be distinguished separately ✓
  • C. Components undergo chemical reaction
  • D. Components have different colours

Answer: B — In uniform mixtures like sugar in water, components are completely mixed and appear as one homogeneous substance throughout.

Q3. Why does lime water turn milky when exposed to air for some hours?

  • A. Water in lime water evaporates
  • B. Carbon dioxide in air reacts with calcium hydroxide to form calcium carbonate precipitate ✓
  • C. Oxygen in air oxidizes the lime water
  • D. Nitrogen in air dissolves in lime water

Answer: B — The white milky appearance is due to insoluble calcium carbonate formed by reaction: calcium hydroxide + carbon dioxide → calcium carbonate + water.

Q4. Which of the following is an example of a non-uniform mixture?

  • A. Seawater
  • B. Vinegar
  • C. Sprout salad ✓
  • D. Salt dissolved in water

Answer: C — In sprout salad, individual components like green gram, chickpeas, onion, and tomato are easily visible and distinguishable by naked eye.

Q5. What type of mixture is stainless steel and what does it contain?

  • A. Non-uniform mixture of iron and carbon only
  • B. Uniform mixture (alloy) of iron, nickel, chromium, and carbon ✓
  • C. Pure substance of iron
  • D. Non-uniform mixture of copper and tin

Answer: B — Stainless steel is a uniform alloy where iron, nickel, chromium, and carbon are mixed so evenly that the entire mixture appears the same throughout.

Q6. Your mother makes tea by dissolving sugar in hot water. After cooling, you cannot see sugar particles separate from water. What type of mixture is this?

  • A. Non-uniform mixture because sugar was added to water
  • B. Uniform mixture because sugar particles are evenly distributed and invisible ✓
  • C. Pure substance because sugar dissolved completely
  • D. Alloy because two substances mixed together

Answer: B — Sweetened tea is a uniform mixture where sugar dissolves completely in water, components are evenly distributed, and cannot be seen separately.

Q7. In Activity 8.3, when electricity is passed through water, why do different volumes of gases collect in the two test tubes?

  • A. One gas is hydrogen and other is oxygen in 1:2 volume ratio ✓
  • B. The test tubes have different sizes
  • C. One gas dissolves faster in water than the other
  • D. The battery terminals have different charges

Answer: A — Water breaks down into hydrogen (H₂) and oxygen (O₂) in 2:1 volume ratio, so more hydrogen gas (double volume) collects compared to oxygen.

Q8. Brass is used to make many household utensils in India. Brass is which type of substance?

  • A. A pure substance made of copper only
  • B. A non-uniform mixture of copper and zinc
  • C. A uniform mixture (alloy) of copper and zinc with properties different from its constituent metals ✓
  • D. A compound of copper bonded with zinc

Answer: C — Brass is an alloy (uniform mixture) where copper and zinc are mixed uniformly, displaying properties distinct from pure copper or pure zinc.

Q9. Activity 8.2 with black paper placed near a window shows dust settling on it. What does this tell us about air composition?

  • A. Air contains equal amounts of dust and gases
  • B. Dust is an integral part of air composition
  • C. Dust particles are pollutants suspended in air and vary with time and place ✓
  • D. Dust makes air a non-uniform mixture only

Answer: C — The activity demonstrates that dust particles are suspended pollutants in air, not part of air's natural composition, and their amount varies by location and time.

Q10. According to the ancient Indian text mentioned in the chapter, an alloy called Bronze (Kamsya) was made with specific proportions. Why were such alloys created instead of using pure metals?

  • A. To reduce the cost of metals
  • B. To create new substances with properties different and often superior to constituent metals for medicinal and practical use ✓
  • C. Because pure metals were not available
  • D. To make metals softer for easy shaping

Answer: B — Alloys were created because mixing metals produces new properties (like Bronze improving digestion and immunity) that individual metals did not possess alone.

Flashcards

Define a mixture in science

A mixture is formed when two or more pure substances are mixed such that each substance retains its individual properties and components do not react chemically.

What is the difference between uniform and non-uniform mixtures?

Uniform mixtures have components evenly distributed and invisible separately (like sugar in water), while non-uniform mixtures have visible components distinguishable by naked eye (like sprout salad).

Why does lime water turn milky when exposed to air?

Lime water (calcium hydroxide) reacts with carbon dioxide present in air to form insoluble calcium carbonate, which appears as white milky particles in water.

What is a pure substance according to science?

A pure substance is matter that consists of only one type of particle and cannot be separated into other kinds of matter by any physical process.

Give one example each of uniform and non-uniform mixtures from your home

Uniform: sugar dissolved in tea or seawater; Non-uniform: poha with vegetables or baking powder (mixture of baking soda and tartaric acid).

What is an alloy and name two examples

An alloy is a uniform mixture of two or more metals with properties different from constituent metals; examples are brass (copper + zinc) and bronze (copper + tin).

What happens when electricity is passed through water?

Water breaks down into hydrogen gas and oxygen gas; hydrogen burns with a pop sound and oxygen makes a burning candle glow brighter.

Is air a mixture or a pure substance and why?

Air is a uniform mixture because it contains mainly nitrogen, oxygen, argon, carbon dioxide, and water vapour mixed together without chemical reaction.

What does the Activity 8.2 with black paper demonstrate?

The activity demonstrates that dust particles are suspended in air as pollutants and are not an integral part of air composition.

How can you separate components of a mixture and why is it done?

Mixtures are separated by physical methods (like filtration or evaporation) to obtain pure substances and to get components of scientific interest.

Important Board Questions

What is meant by a pure substance in science? [1 mark]

Define: consists of one type of particles only, cannot be separated by physical process, all particles are identical.

Distinguish between uniform and non-uniform mixtures with one example each from your kitchen. [2 marks]

Uniform: components evenly mixed, invisible separately (like salt in water); Non-uniform: components visible, distinguishable (like rice and dal). Give kitchen examples for both.

Explain with the help of Activity 8.1 why lime water turns milky when exposed to air. Write the chemical equation involved. [3 marks]

Lime water = calcium hydroxide solution; turns milky due to carbon dioxide in air; equation: Ca(OH)₂ + CO₂ → CaCO₃↓ (white precipitate) + H₂O. Explain that insoluble calcium carbonate causes milky appearance.

Describe Activity 8.3 on electrolysis of water. What gases are produced at each terminal and how would you identify them? Draw and label a diagram of the experimental setup. [5 marks]

Setup: battery in water with dilute sulfuric acid, test tubes on terminals. Gases collected: hydrogen (pop sound with burning candle) and oxygen (glowing candle). Diagram should show: beaker with water, battery, two test tubes inverted on terminals, gas bubbles. Write that hydrogen:oxygen volume ratio = 2:1. Explain how gases are identified using burning candle test.

Assertion-Reason Questions for Elements, Compounds, and Mixtures

Q1. Assertion (A):

Lime water turns milky when exposed to air, indicating the presence of carbon dioxide.

Reason (R):

Carbon dioxide reacts with calcium hydroxide to form calcium carbonate, which is insoluble and appears as white precipitate.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both statements are true, and the reason correctly explains why the assertion is true. The chemical reaction between CO₂ and calcium hydroxide directly causes the milky appearance, making reason the correct explanation of the assertion.

Q2. Assertion (A):

Stainless steel is a uniform mixture, while a sprout salad is a non-uniform mixture.

Reason (R):

Uniform mixtures always contain metals, while non-uniform mixtures always contain food items.

Answer: C — A is true but R is false — The assertion is true—stainless steel (metals evenly mixed) is uniform; sprout salad (visible separate components) is non-uniform. However, the reason is false because uniformity doesn't depend on whether components are metals or food. Many non-metallic uniform mixtures exist (sugar solution, vinegar), and the reason oversimplifies the concept.

Q3. Assertion (A):

Air contains dust and soot particles that are not integral parts of the air's composition.

Reason (R):

These particles can be collected on a black sheet of paper, proving they are pollutants that settle from the air due to gravity.

Answer: B — Both A and R are true, but R is NOT the correct explanation of A — Both statements are true. Dust and soot are indeed pollutants that settle on surfaces (proven by Activity 8.2). However, the reason is not the only or complete explanation for why they are not integral—they are pollutants because they vary by location and time, making them external additions rather than fundamental atmospheric components. The settling is a consequence, not the defining reason.

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Case-Based Questions (CBQ) for Elements, Compounds, and Mixtures

Case Study:

A chemistry teacher conducted Activity 8.1 in class to detect carbon dioxide in the air. First, students added calcium oxide (quick lime) slowly to water, which reacted vigorously, releasing heat and forming calcium hydroxide. The clear solution (lime water) was then filtered and left in a petri dish for 3 hours near an open window. Students stirred the solution at regular intervals. After 3 hours, the originally colourless solution turned distinctly milky white, confirming the presence of carbon dioxide in atmospheric air. The students then compared this result with a control sample of lime water kept in an airtight container for the same duration, which remained colourless.

Q1. Why did the lime water turn milky when exposed to air near the open window?

  • A. Oxygen in the air reacted with calcium hydroxide
  • B. Carbon dioxide from the air reacted with calcium hydroxide, forming insoluble calcium carbonate ✓
  • C. Water vapour condensed in the lime water solution
  • D. Dust particles from the air mixed with the solution

Answer: B

Q2. What was the purpose of the control sample kept in an airtight container?

  • A. To verify that lime water naturally turns milky over time
  • B. To prove that calcium hydroxide is unstable when isolated
  • C. To demonstrate that the milky appearance requires contact with atmospheric air (CO₂), not just time passing ✓
  • D. To show that airtight containers preserve all chemical reactions

Answer: C

Q3. Which property of calcium carbonate is directly responsible for the milky appearance?

  • A. It is a colourless gas
  • B. It is insoluble in water and forms tiny white particles suspended in the solution ✓
  • C. It is a liquid that reflects light
  • D. It is a compound that absorbs light

Answer: B

Q4. Why was it important to stir the lime water solution at regular intervals during the experiment?

  • A. To dissolve the calcium hydroxide completely
  • B. To prevent the water from evaporating
  • C. To ensure continuous contact between atmospheric air and the lime water, allowing maximum carbon dioxide absorption ✓
  • D. To accelerate the chemical reaction rate artificially

Answer: C

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