Class 8 Science Chapter- 8 Nature of Matter : Elements, Compounds, and Mixtures
Keep the curiosity alive Page no: 131
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 of the following statements is correct?
(i) A, B, and C are all compounds and only C has a fixed composition.
(ii) C is a compound, and A and B have a fixed composition.
(iii) A and B are compounds, and C has a fixed composition.
(iv) A and B are elements, C is a compound, and has a fixed composition.
Correct Answer: (iv) A and B are elements, C is a compound, and has a fixed composition.
Explanation: Since A and B cannot be broken down into simpler substances, they must be elements. When they combine to form C, which has a fixed composition, C is classified as a compound.
Question: 2. Assertion: Air is a mixture. Reason: A mixture is formed when two or more substances are mixed, without undergoing any chemical change.
(i) Both Assertion and Reason are true and Reason is the correct explanation for Assertion.
(ii) Both Assertion and Reason are true, but Reason is not the correct explanation for Assertion.
(iii) Assertion is true, but Reason is false.
(iv) Assertion is false, but Reason is true.
Correct Answer: (i) Both Assertion and Reason are true and Reason is the correct explanation for Assertion.
Explanation: Air is indeed a mixture of gases (like nitrogen, oxygen, and carbon dioxide), and it is formed without any chemical change. Therefore, the assertion is true, and the reason correctly explains why air is classified as 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: Justification of the Statement: Water as a Compound
Water is classified as a compound because it is formed from the chemical combination of two elements: hydrogen and oxygen. While both hydrogen and oxygen are gases at room temperature, water exhibits properties that are distinct and different from those of its constituent elements. Here’s a detailed justification:
1. Chemical Composition:-
Water (H₂O): Each molecule of water consists of two hydrogen atoms and one oxygen atom, combined in a fixed ratio of 2:1. This specific arrangement leads to unique properties.
2. Physical State:-
- Hydrogen: A colorless, odorless gas that is highly flammable.
- Oxygen: A colorless gas that supports combustion.
- Water: A liquid at room temperature, essential for life, and does not support combustion. This change in state from gas to liquid is a significant transformation.
3. Properties:-
- Boiling and Freezing Points:
- Hydrogen has a boiling point of -252.9 °C, and oxygen has a boiling point of -183 °C.
- Water, however, has a boiling point of 100 °C and a freezing point of 0 °C under standard atmospheric pressure.
- Solvent Properties: Water is often referred to as the “universal solvent” because it can dissolve many substances, which neither hydrogen nor oxygen can do in their gaseous forms.
4. Chemical Behavior:-
- Reactivity: Hydrogen is a fuel, and oxygen supports combustion. In contrast, water is stable and does not react with most substances under normal conditions. It can also participate in chemical reactions, such as hydrolysis, where it breaks down compounds.
5. Biological Importance:-
- Water is crucial for all known forms of life. It participates in biochemical reactions, regulates temperature, and serves as a medium for transporting nutrients and waste in biological systems. Neither hydrogen nor oxygen alone can fulfill these roles.
Conclusion
In summary, the properties of water are fundamentally different from those of hydrogen and oxygen due to the chemical bonding and molecular structure that occurs when these elements combine. This transformation illustrates the concept that compounds can exhibit entirely new characteristics that are not present in their individual elements.
Question: 4. In which of the following cases are all the examples correctly matched? Give reasons in support of your answers.
(i) Elements — water, nitrogen, iron, air.
(ii) Uniform mixtures— minerals, seawater, bronze, air.
(iii) Pure substances— carbon dioxide, iron, oxygen, sugar.
(iv) Non-uniform mixtures — air, sand, brass, muddy water.
Answer: Let’s evaluate each option to determine if all examples are correctly matched:
(i) Elements — Water, Nitrogen, Iron, Air
Incorrect:
– Water is a compound (H₂O), not an element.
– Nitrogen and Iron are indeed elements, but Air is a mixture of gases, primarily nitrogen and oxygen.
(ii) Uniform mixtures — Minerals, Seawater, Bronze, Air
Incorrect:
– Minerals can be either pure substances or mixtures, depending on their composition.
– Seawater is a uniform mixture of water and dissolved salts.
– Bronze is an alloy, which is a type of uniform mixture of metals.
– Air is also a uniform mixture of gases.
(iii) Pure substances — Carbon Dioxide, Iron, Oxygen, Sugar
Correct:
– All listed are pure substances.
– Carbon Dioxide (CO₂), Iron (Fe), Oxygen (O₂), and Sugar (C₁₂H₂₂O₁₁) consist of the same type of particles throughout.
(iv) Non-uniform mixtures — Air, Sand, Brass, Muddy Water
Incorrect:
– Air is a uniform mixture of gases.
– Sand is a non-uniform mixture of various particles.
– Brass is a uniform mixture (alloy) of copper and zinc.
– Muddy Water is a non-uniform mixture of water and suspended particles.
Summary of Answers
(i) Incorrect
(ii) Incorrect
(iii) Correct
(iv) Incorrect
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 above reactions as elements, compounds or mixtures, with justification.
Answer: Reactions:
1. Iron reacts with moist air to form iron oxide.
2. Magnesium burns in oxygen to form magnesium oxide.
Classification:
– Iron (Fe): Element : It cannot be broken down into simpler substances and is a fundamental building block of matter.
– Moist Air: Mixture: Contains various gases (nitrogen, oxygen, carbon dioxide) and water vapor, hence it is a mixture.
– Iron Oxide (Fe₂O₃): Compound: Formed by the chemical reaction of iron with oxygen, it has distinct properties different from its constituent elements.
– Magnesium (Mg): Element: Like iron, it is a pure substance that cannot be broken down further.
– Oxygen (O₂): Element: A diatomic molecule, it is a pure substance essential for combustion.
– Magnesium Oxide (MgO): Compound: Formed when magnesium reacts with oxygen, it has properties that differ from both magnesium and oxygen.
Summary of Classification for Question 5:
1. Iron: Element
2. Moist Air: Mixture
3. Iron Oxide: Compound
4. Magnesium: Element
5. Oxygen: Element
6. Magnesium Oxide: Compound
Page no: 132
Question: 6. Classify the following as elements, compounds, or mixtures in Table 8.3. Carbon dioxide, sand, seawater, magnesium oxide, muddy water, aluminium, gold, oxygen, rust, iron sulfide, glucose, air, water, fruit juice, nitrogen, sodium chloride, sulfur, hydrogen, baking soda.
Answer:
| Substance | Classification |
| Carbon dioxide | Compound |
| Sand | Mixture |
| Seawater | Mixture |
| Magnesium oxide | Compound |
| Muddy water | Mixture |
| Aluminium | Element |
| Gold | Element |
| Oxygen | Element |
| Rust | Compound |
| Iron sulfide | Compound |
| Glucose | Compound |
| Air | Mixture |
| Water | Compound |
| Fruit juice | Mixture |
| Nitrogen | Element |
| Sodium chloride | Compound |
| Sulfur | Element |
| Hydrogen | Element |
| Baking soda | Compound |
Explanation of Classifications
- Elements are pure substances that cannot be broken down into simpler substances. Examples include aluminium, gold, oxygen, nitrogen, sulfur, and hydrogen.
- Compounds are substances formed when two or more elements combine chemically in fixed ratios. Examples include carbon dioxide, magnesium oxide, rust (iron oxide), iron sulfide, glucose, water, sodium chloride, and baking soda.
- Mixtures consist of two or more substances mixed together, where each retains its individual properties. Examples include sand, seawater, muddy water, air, and fruit juice.
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 a mixture of iron filings and sulfur powder is heated, a new substance called iron sulfide is formed. This compound is different from the original mixture because:
1. Properties: Iron sulfide has different physical and chemical properties compared to the individual components (iron and sulfur). For example, iron filings are magnetic, while sulfur is not.
2. Separation: In the original mixture, iron and sulfur can be separated using a magnet, but once they form iron sulfide, they cannot be separated by physical means.
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: A substance cannot be classified as both an element and a compound simultaneously.
1. Element: An element is a pure substance that cannot be broken down into simpler substances by chemical means. It consists of only one type of atom.
2. Compound: A compound is formed when two or more different elements chemically combine in a fixed ratio. It has distinct properties that differ from the individual elements.
Thus, by definition, a substance can only belong to one category at a time.
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 a mixture of hydrogen and oxygen instead of a compound, our daily lives would be significantly affected:
1. Properties: The properties of a mixture of hydrogen and oxygen would be different from those of water. For instance, hydrogen is flammable, and oxygen supports combustion, making the mixture highly volatile and dangerous.
2. Usage: Water is essential for life, and its unique properties (like being a solvent, having a high specific heat, etc.) are crucial for biological processes. A mixture would not provide the same benefits.
3. Chemical Reactions: The chemical reactions involving water would also change, affecting everything from cooking to biological functions.
Question: 10. Analyse Fig. 8.24. Identify Gas A. Also, write the word equation of the chemical reaction.
Answer: In the context of the experiment shown in Fig. 8.24, Gas A is likely hydrogen gas.
Word Equation for the Reaction: Iron+Dilute Hydrochloric Acid→Iron Chloride+Hydrogen Gas
Question: 11. Write the names of any two compounds made only from non-metals, and also mention two uses of each of them.
Answer: Two examples of compounds made only from non-metals are:
1. Carbon Dioxide (CO₂)
Uses:
– Used in carbonated beverages.
– Employed fire extinguishers.
2. Glucose (C₆H₁₂O₆)
Uses:
– Serves as a primary energy source for living organisms.
– Used in food and beverages as a sweetener.
Question: 12. How can gold be classified as both a mineral and a metal?
Answer: Gold can be classified as both a mineral and a metal because:
– Mineral: Gold is found in nature as a native mineral, meaning it occurs in its pure, metallic form without being combined with other elements.
– Metal: As a metal, gold exhibits typical metallic properties such as conductivity, malleability, and ductility.
This dual classification highlights gold’s unique position in both geological and material science contexts.