New NCERT Curiosity · Class 8 Science · Term II · Chapter 8: Nature of Matter: Elements, Compounds and Mixtures
This chapter sorts the matter around us into mixtures and pure substances. It explains uniform and non-uniform mixtures, how passing electricity through water shows that it is made of elements, and how compounds differ from mixtures.
From the Class 8 Science Short Notes, made as per the new NCERT book Curiosity · Pages 46–52 of the book.
Key points
- A mixture has two or more substances, each keeping its own properties; they do not react chemically.
- Non-uniform mixtures show their components (sprout salad, sand in water); uniform mixtures do not (sugar in water, air, alloys).
- Alloys are uniform mixtures of metals — brass is copper + zinc, bronze is copper + tin.
- Air is a uniform mixture; nitrogen is about 78% of it. Lime water turning milky shows air has carbon dioxide.
- Handpicking, sieving, sedimentation and decantation, filtration and evaporation separate mixtures.
- A pure substance in science has the same type of particles and cannot be separated by any physical process.
- Passing electricity through water breaks it into hydrogen and oxygen.
- An element cannot be broken into simpler substances; it is made of identical atoms. 118 elements are known.
- Elements are grouped as metals, non-metals and metalloids.
- A compound forms when elements combine chemically in a fixed ratio; it has new properties and cannot be separated by physical methods.
- A mineral is a natural solid with a fixed chemical composition; most rocks are mixtures of minerals.
Formulas
| Breaking down water | Water → Hydrogen + Oxygen By passing electricity. |
| Lime water and carbon dioxide | Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water |
| Heating sugar | Sugar → Carbon + Water Shows sugar is a compound. |
| Iron and sulfur | Iron + Sulfur → Iron sulfide On heating. |
| Ratio in water | Hydrogen : Oxygen = 2 : 1 |
Key terms
- Mixture: Two or more substances mixed together, where each keeps its own properties.
- Components: The individual substances that make up a mixture.
- Alloy: Metals (sometimes with a little carbon) mixed so uniformly that the mixture looks the same throughout.
- Adulteration: The illegal adding of cheaper or poor-quality substances to increase quantity or cut cost.
- Pure substance: A substance with no other substance in it, made of the same type of particles.
- Element: A pure substance that cannot be broken down into simpler substances; made of identical atoms.
- Compound: A substance formed when different elements combine chemically in a fixed ratio.
- Mineral: A natural solid found on Earth with a fixed chemical composition.
Diagrams to learn
- Passing electricity through water (Fig 8.1)
- Classification of matter (Fig 8.2)
- Table of types of mixtures by physical state
- Table of methods of separating mixtures
- Table comparing iron + sulfur mixture and iron sulfide
- Table comparing element, compound and mixture
- Milk, ghee and spices marked 'pure' are still mixtures for a scientist.
- Never smell a gas directly — waft it gently towards your nose with your hand.
- Not everything is matter — light, heat, electricity, thoughts and emotions have no mass and take up no space.
- A compound is pure; a mixture is not. Fixed ratio + new properties = compound.
A real page from the book
Get the full chapter in the book
This page is a summary. The Class 8 Science Short Notes has the complete chapter in colour, with every diagram and worked example.
Frequently asked questions
What is the difference between a compound and a mixture?
In a compound, elements are chemically combined in a fixed ratio, it has new properties and cannot be separated by physical methods. In a mixture, substances are not chemically combined, the ratio can vary and each component keeps its own properties.
Is air a mixture or a compound?
Air is a uniform mixture, mainly of nitrogen, oxygen, argon, carbon dioxide and water vapour.
What happens when electricity is passed through water?
Gas bubbles form at both terminals. One gas burns with a 'pop' (hydrogen) and the other makes a flame glow brighter (oxygen), showing water breaks down into hydrogen and oxygen.
← Term I · Chapter 7: Particulate Nature of MatterTerm II · Chapter 9: The Amazing World of Solutes, Solvents and Solutions →
