SCIENCE CLASS- 8
CHAPTER-8 (Nature of Matter : Elements, Compounds, and Mixtures)
Question 1. Consider the following reaction where two substances, A and B, combine to form a product C.
A + B → C
Assume that A and B cannot be broken down into simpler substances by chemical reactions. Based on this information, which statement is correct?
Answer: (iv) A and B are elements, C is a compound, and has a fixed composition.
Explanation:
Substances that cannot be broken down into simpler substances by chemical methods are called elements.
Since A and B cannot be decomposed further, they are elements.
When two elements chemically combine, they form a compound.
Therefore, C is a compound having a fixed composition.
Question 2. Assertion–Reason
Assertion: Air is a mixture.
Reason: A mixture is formed when two or more substances are mixed without undergoing any chemical change.
Answer: (i) Both Assertion and Reason are true and Reason is the correct explanation for Assertion.
Explanation:
Air contains nitrogen, oxygen, carbon dioxide, water vapour and other gases mixed physically.
No chemical reaction takes place among these gases.
Therefore, air is a mixture.
Question 3. Water, a compound, has different properties compared to those of the elements oxygen and hydrogen from which it is formed. Justify this statement.
Answer:
Water is formed when hydrogen and oxygen combine chemically.
The properties of water are completely different from those of hydrogen and oxygen.
| Hydrogen | Oxygen | Water |
|---|---|---|
| A combustible gas | Supports burning | Used to extinguish fire |
| Gas | Gas | Liquid at room temperature |
This shows that compounds have properties different from the elements that form them.
Question 4. In which of the following cases are all the examples correctly matched?
(i) Elements — water, nitrogen, iron, air.
Answer: Incorrect
Water is a compound and air is a mixture.
(ii) Uniform mixtures — minerals, seawater, bronze, air.
Answer: Incorrect
Minerals are generally pure substances, not mixtures.
(iii) Pure substances — carbon dioxide, iron, oxygen, sugar.
Answer: Correct
All these substances have a fixed composition and are pure substances.
(iv) Non-uniform mixtures — air, sand, brass, muddy water.
Answer: Incorrect
Air and brass are uniform mixtures.
Final Answer: Only (iii) is correctly matched.
Question 5. Iron reacts with moist air to form iron oxide, and magnesium burns in oxygen to form magnesium oxide. Classify all the substances involved in the reactions as elements, compounds or mixtures.
Answer:
| Substance | Classification |
|---|---|
| Iron | Element |
| Moist air | Mixture |
| Iron oxide | Compound |
| Magnesium | Element |
| Oxygen | Element |
| Magnesium oxide | Compound |
Question 6. Classify the following as elements, compounds or mixtures.
| Elements | Compounds | Mixtures |
|---|---|---|
| Aluminium | Carbon dioxide | Sand |
| Gold | Magnesium oxide | Seawater |
| Oxygen | Rust | Muddy water |
| Nitrogen | Iron sulfide | Air |
| Sulfur | Glucose | Fruit juice |
| Water | ||
| Sodium chloride | ||
| Baking soda |
Pure Substances
Pure substances include both elements and compounds.
Pure substances:
- Aluminium
- Gold
- Oxygen
- Nitrogen
- Sulfur
- Carbon dioxide
- Magnesium oxide
- Rust
- Iron sulfide
- Glucose
- Water
- Sodium chloride
- Baking soda
Question 7. What new substance is formed when a mixture of iron filings and sulfur powder is heated, and how is it different from the original mixture? Also, write the word equation for the reaction.
Answer:
When iron filings and sulfur powder are heated strongly, a new compound called iron sulfide is formed.
Differences from the original mixture:
- Iron sulfide is a compound, while iron and sulfur together form a mixture.
- The properties of iron sulfide are different from those of iron and sulfur.
- A magnet can separate iron from the mixture, but not from iron sulfide.
- Iron sulfide has a fixed composition.
Word Equation:
Iron + Sulfur → Iron Sulfide
Question 8. Is it possible for a substance to be classified as both an element and a compound? Explain why or why not.
Answer:
No, a substance cannot be both an element and a compound.
An element contains only one type of atom.
A compound contains two or more elements chemically combined in a fixed ratio.
Since their definitions are different, a substance cannot belong to both categories simultaneously.
Question 9. How would our daily lives be changed if water were not a compound but a mixture of hydrogen and oxygen?
Answer:
If water were simply a mixture of hydrogen and oxygen:
- It would not have the properties of water.
- Hydrogen is highly combustible.
- Oxygen supports combustion.
- Rivers, lakes and oceans would not exist in their present form.
- Life on Earth would not be possible.
- Drinking, cooking and agriculture would become impossible.
Therefore, water being a compound is essential for life.
Question 10. Analyse Fig. 8.24. Identify Gas A. Also, write the word equation of the chemical reaction.
Answer:
The figure shows iron filings reacting with dilute hydrochloric acid.
The gas produced is Hydrogen Gas.
Word Equation:
Iron + Hydrochloric Acid → Iron Chloride + Hydrogen
Explanation:
When iron reacts with dilute hydrochloric acid, hydrogen gas is released and iron chloride is formed.
Question 11. Write the names of any two compounds made only from non-metals, and also mention two uses of each of them.
Answer:
(i) Water (H₂O)
Uses:
- Used for drinking.
- Used for cooking and cleaning.
(ii) Carbon Dioxide (CO₂)
Uses:
- Used in fire extinguishers.
- Used in soft drinks.
Question 12. How can gold be classified as both a mineral and a metal?
Answer:
Gold is classified as a metal because it shows metallic properties such as:
- Lustre
- Malleability
- Ductility
- Good conductivity
Gold is also classified as a mineral because it occurs naturally in the Earth's crust and is obtained through mining.
Therefore, gold can be regarded both as a naturally occurring mineral and as a metallic element.
Important Concepts from the Chapter
- Elements contain only one kind of atom.
- Compounds are formed by chemical combination of elements in a fixed ratio.
- Mixtures are formed by physical mixing of substances.
- Properties of compounds differ from those of their constituent elements.
- Pure substances include both elements and compounds.
- Mixtures can be homogeneous or heterogeneous.
- Chemical reactions produce new substances with new properties.