Class 8 Science Chapter 8 Nature of Matter Elements Compounds Mixtures Solutions | Curiosity NCERT Answers

 

🌟 Class 8 Science – Curiosity Chapter 8

🧪 Nature of Matter: Elements, Compounds & Mixtures

(पदार्थ की प्रकृति: तत्व, यौगिक और मिश्रण)

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🟢 1. Consider the following reaction:

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.

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✅ Answer:

👉 Correct option is (iv) A and B are elements, C is a compound, and has a fixed composition.

🧠 Explanation:

• 🧪 Elements (तत्व) → cannot be broken further
• 🔗 Compound (यौगिक) → formed by chemical combination

📌 Since A & B cannot be broken → they are elements

👉 C formed from them → compound with fixed ratio

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🟢 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.

(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.

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✅ Answer:

👉 Correct option is (i)

🧠 Explanation:

• 🌬️ Air = mixture of gases (oxygen, nitrogen, etc.)
• ❌ No chemical reaction happens

👉 So Reason perfectly explains Assertion ✔️

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🟢 3. Water vs Hydrogen & Oxygen

Water, a compound, has different properties compared to those of the elements oxygen and hydrogen. Justify this statement.

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✅ Answer:

🔥 Hydrogen (हाइड्रोजन):

• Combustible (जलने वाला) gas

🔥 Oxygen (ऑक्सीजन):

• Supports combustion (जलने में सहायता करता है)

💧 Water (पानी):

• Formed in fixed ratio (2:1)
• ❌ Does NOT burn
• 🚫 Even extinguishes fire

📌 Conclusion:

👉 Compound properties are completely different from elements

✨ This is the key feature of compounds (यौगिक)

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🟢 4. Correct Matching

(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.

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✅ Answer:

❌ (i) Incorrect

👉 Water = compound, Air = mixture

❌ (ii) Incorrect

👉 Minerals are usually compounds

✅ (iii) Correct

👉 All are pure substances (शुद्ध पदार्थ)

❌ (iv) Incorrect

👉 Air & brass = uniform mixtures

📌 Final Answer: 👉 Only (iii) is correct ✔️

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🟢 5. Classification of Substances

Iron reacts with moist air → iron oxide

Magnesium burns in oxygen → magnesium oxide

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✅ Answer:

🧱 Iron → Element

🌬️ Moist air → Mixture (air + water vapour)

🧪 Iron oxide → Compound

⚡ Magnesium → Element

🌬️ Oxygen → Element

🧪 Magnesium oxide → Compound

📌 Tip:

👉 Elements = single type

👉 Compounds = chemically combined

👉 Mixtures = physically combined

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🟢 6. Classification Table

🔹 Substance	📌 Type
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

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Answer:

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🟢 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.

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✅ Answer:

🔥 New Substance Formed:

👉 Iron sulfide (FeS) 🧪

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🔍 Difference:

🧱 Original Mixture (Iron + Sulfur):

• Properties remain same
• 🧲 Iron can be separated using a magnet
• ❌ No chemical reaction

🧪 Product (Iron Sulfide):

• Completely new substance
• ❌ Cannot be separated by magnet
• ✔️ New properties formed

📌 Conclusion:

👉 Mixture → Physical combination

👉 Compound → Chemical combination 💥

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✍️ Word Equation:

👉 Iron + Sulfur → Iron sulfide

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🟢 8. Is it possible for a substance to be classified as both an element and a compound? Explain why or why not.

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✅ Answer:

❌ No, it is NOT possible.

🧠 Explanation:

🧱 Element (तत्व):

• Made of only one type of atom
• Examples: O₂, H₂, Fe

🧪 Compound (यौगिक):

• Formed by chemical combination of elements
• Examples: H₂O, CO₂

📌 Conclusion:

👉 A substance can be either element OR compound, not both

👉 Classification is mutually exclusive (परस्पर अलग)

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🟢 9. How would our daily lives be changed if water were not a compound but a mixture of hydrogen and oxygen?

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✅ Answer:

⚠️ If water were a mixture:

💥 Hydrogen + Oxygen gases = Explosive mixture

❌ Not safe for drinking or daily use

🌍 Major Impacts:

• 🚫 No safe drinking water
• 🚫 No proper temperature regulation
• 🚫 No solvent property (घोलक गुण)

🧬 Reality:

👉 Water (H₂O) is a compound, giving it:

• 💧 Stability
• 🌡️ Temperature control
• 🧪 Universal solvent property

📌 Conclusion:

👉 Life would NOT exist without water as a compound ❌

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🟢 10. Analyse Fig. 8.24. Identify Gas A. Also, write the word equation of the chemical reaction.

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✅ Answer:

🧠 Gas A = Oxygen (ऑक्सीजन) 🌬️

👉 Usually this figure shows decomposition of water (electrolysis ⚡)

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✍️ Word Equation:

👉 Water → Hydrogen + Oxygen

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📌 Extra Concept:

• Hydrogen collects at one side 🔥
• Oxygen at another 🌬️
• Ratio = 2 : 1

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See Answer Fig. 8.24 shows iron filings reacting with dilute hydrochloric acid.

Gas A = Hydrogen gas (H₂).

Word equation:

Iron + Hydrochloric acid → Iron chloride + Hydrogen

 

 

🟢 11. Write the names of any two compounds made only from non-metals, and also mention two uses of each of them.

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✅ Answer:

🧪 1. Carbon dioxide (CO₂)

🌬️ Type: Compound of non-metals (Carbon + Oxygen)

🔹 Uses:

• 🔥 Used in fire extinguishers (आग बुझाने में)
• 🥤 Used in fizzy / aerated drinks (ठंडे पेय पदार्थ)

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🧪 2. Sulfur dioxide (SO₂)

🌫️ Type: Compound of non-metals (Sulfur + Oxygen)

🔹 Uses:

• 🧼 Used in bleaching (सफेदी करने में)
• 🏭 Used in making sulfuric acid (सल्फ्यूरिक अम्ल निर्माण)

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📌 Key Point:

👉 Non-metals combine to form covalent compounds (सहसंयोजक यौगिक)

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🟢 12. How can gold be classified as both a mineral and a metal?

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✅ Answer:

🪨 As a Mineral (खनिज):

• Gold is found naturally in Earth’s crust 🌍
• Exists in native (pure) form
  👉 इसलिए इसे mineral कहा जाता है

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🪙 As a Metal (धातु):

Gold shows typical metallic properties:

• ✨ Lustre (चमक)
• 🔄 Ductility (तार बनाने की क्षमता)
• 🔨 Malleability (पतली चादर बनाने की क्षमता)
• ⚡ Conductivity (विद्युत चालकता)

👉 इसलिए यह एक metal (धातु) भी है

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📌 Conclusion:

👉 Gold is both:

• 🪨 Mineral (occurrence based)
• 🪙 Metal (properties based)

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