8. Nature of Matter: Elements, Compounds, and Mixtures
Complete solutions to Keep the Curiosity Alive. CBSE / NCERT Class 8 Science (Curiosity).
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Keep the Curiosity Alive
Answer: (iv) A and B are elements, C is a compound, and has a fixed composition.
A and B cannot be broken down further, so they are elements. C is formed by the chemical combination of two elements, so it is a compound with a fixed composition.
Answer: (i) Both Assertion and Reason are true and Reason is the correct explanation for Assertion.
Air contains nitrogen, oxygen, carbon dioxide, water vapour and other gases. They are mixed without any chemical change, and each keeps its own properties.
- Hydrogen burns, and oxygen helps in burning. But water puts out fire.
- Hydrogen and oxygen are gases at room temperature. Water is a liquid.
So the properties of a compound are different from those of its constituent elements.
Answer: (iii) Pure substances — carbon dioxide, iron, oxygen, sugar.
All four are made of only one type of particles. Iron and oxygen are elements. Carbon dioxide and sugar are compounds.
Why the others are wrong:
- (i) Water is a compound and air is a mixture, not elements.
- (ii) Minerals have a fixed composition. They are not mixtures.
- (iv) Air and brass are uniform mixtures, not non-uniform.
| Substance | Type | Reason |
|---|---|---|
| Iron | Element | Cannot be broken down into simpler substances |
| Oxygen | Element | Cannot be broken down into simpler substances |
| Magnesium | Element | Cannot be broken down into simpler substances |
| Water (moisture) | Compound | Hydrogen and oxygen combined chemically in a fixed ratio |
| Iron oxide | Compound | Iron and oxygen combined chemically |
| Magnesium oxide | Compound | Magnesium and oxygen combined chemically |
| Moist air | Mixture | Gases and water vapour mixed without chemical change |
| Elements | Compounds | Mixtures |
|---|---|---|
| Aluminium, gold, oxygen, nitrogen, sulfur, hydrogen | Carbon dioxide, magnesium oxide, rust, iron sulfide, glucose, water, sodium chloride, baking soda | Sand, seawater, muddy water, air, fruit juice |
Pure substances: all the elements and compounds are pure substances — aluminium, gold, oxygen, nitrogen, sulfur, hydrogen, carbon dioxide, magnesium oxide, rust, iron sulfide, glucose, water, sodium chloride and baking soda.
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A new substance, iron sulfide, is formed.
- In the mixture, iron filings can be separated with a magnet. Iron sulfide is not attracted by a magnet.
- In the mixture, iron and sulfur keep their own properties. Iron sulfide has new properties.
- Iron sulfide has a fixed composition. The mixture can have iron and sulfur in any ratio.
No. An element is the simplest substance. It cannot be broken down into simpler substances. A compound is made of two or more elements combined chemically, and can be broken down into them.
A substance cannot be both breakable and unbreakable into simpler substances. So it is either an element or a compound.
- Hydrogen and oxygen would keep their own properties. The “water” would be a mixture of gases, not a liquid.
- We would not have liquid water to drink, cook, wash or grow crops.
- The mixture of hydrogen and oxygen catches fire very easily. So water would be dangerous, and it could not be used to put out fires.
Life as we know it would not be possible.
Gas A is hydrogen.
Hydrogen burns with a ‘pop’ sound when a burning matchstick is brought near it.
1. Water (hydrogen + oxygen)
- Drinking, cooking and washing.
- Used as a solvent and in agriculture.
2. Carbon dioxide (carbon + oxygen)
- Used in fire extinguishers.
- Used in soda water and soft drinks. Plants also use it for photosynthesis.
Mineral: gold is found naturally in the Earth as a solid with a fixed composition. It occurs as a pure element, so it is a native mineral.
Metal: gold shows the properties of metals. It is shiny, malleable and ductile, and it conducts heat and electricity.
Discover, Design, and Debate
The “Discover, design, and debate” projects are hands-on tasks. Do them with your classmates under the guidance of your teacher, and write your own observations.