Chapter 7: Metals and Nonmetals
Solved textbook questions with step-by-step explanations.
Grade 8 Q&A: Chapter 7: Metals and Nonmetals
Welcome to the Questions and Answers section for Grade 8 Science, Chapter 7: "Metals and Nonmetals." This chapter focuses on the classification of elements into metals and nonmetals based on their distinct physical and chemical properties, their various uses, and important related concepts like noble metals and corrosion.
Important Questions and Answers
Q1: List five physical properties of metals.
Answer:
- Lustre: Metals are shiny.
- Hardness: Most metals are hard solids (except Na, K, Hg).
- Malleability: Can be beaten into thin sheets.
- Ductility: Can be drawn into thin wires.
- Conductors: Good conductors of heat and electricity.
- Sonorous: Produce a ringing sound when struck.
Q2: List five physical properties of nonmetals.
Answer:
- Dull appearance: Generally non-lustrous (except Iodine).
- Softness: Generally soft (except Diamond).
- Brittle: Break easily when hammered.
- Non-ductile: Cannot be drawn into wires.
- Poor conductors: Generally poor conductors of heat and electricity (except Graphite).
- Non-sonorous: Do not produce a ringing sound.
Q3: Which metal is liquid at room temperature?
Answer: Mercury (Hg) is a metal that is liquid at room temperature.
Q4: Which nonmetal is lustrous?
Answer: Iodine is a nonmetal that has a lustrous (shiny) appearance.
Q5: Which nonmetal is the hardest natural substance?
Answer: Diamond, an allotrope of carbon, is the hardest natural substance.
Q6: Name a nonmetal that is a good conductor of electricity.
Answer: Graphite, an allotrope of carbon, is a good conductor of electricity.
Q7: How do metals react with oxygen? Give an example.
Answer: Metals react with oxygen to form metal oxides, which are generally basic in nature.
Example: Magnesium (Mg) reacts with oxygen (O₂) to form Magnesium oxide (MgO). 2Mg + O₂ → 2MgO
Q8: How do metals react with dilute acids? Give an example.
Answer: Metals generally react with dilute acids to produce hydrogen gas and a metal salt.
Example: Zinc (Zn) reacts with dilute hydrochloric acid (HCl) to form Zinc chloride (ZnCl₂) and hydrogen gas (H₂). Zn + 2HCl → ZnCl₂ + H₂
Q9: What is a 'Displacement Reaction' involving metals? Give an example.
Answer: A displacement reaction is a chemical reaction where a more reactive metal displaces a less reactive metal from its salt solution.
Example: Iron (Fe) displaces copper (Cu) from copper sulfate solution because iron is more reactive than copper. Fe + CuSO₄ → FeSO₄ + Cu
Q10: How do nonmetals react with oxygen? What is the nature of the oxides formed?
Answer: Nonmetals react with oxygen to form nonmetal oxides, which are generally acidic in nature.
Example: Carbon (C) reacts with oxygen (O₂) to form Carbon dioxide (CO₂). C + O₂ → CO₂
Q11: Do nonmetals generally react with water or dilute acids?
Answer: No, nonmetals generally do not react with water or dilute acids.
Q12: List three uses of metals.
Answer:
- Iron: Used in construction (steel), machinery, tools.
- Copper: Used in electrical wires, plumbing, utensils.
- Aluminium: Used in aircraft, utensils, foils.
- Gold and Silver: Used in jewelry, coins.
Q13: List three uses of nonmetals.
Answer:
- Oxygen: Essential for respiration, combustion.
- Nitrogen: Used in fertilizers, food packaging.
- Chlorine: Used as a disinfectant, in PVC.
- Carbon (Graphite): Used in pencil leads, electrodes.
Q14: What are 'Noble Metals'? Give examples.
Answer: Noble metals are metals that are highly resistant to corrosion and oxidation (reaction with oxygen) in moist air. They are generally unreactive. Examples: Gold (Au), Silver (Ag), Platinum (Pt).
Q15: What is 'Corrosion'? Give an example.
Answer: Corrosion is the process by which metals are gradually eaten away by the action of air, moisture, or chemicals present in the environment. It is a slow chemical change. Example: Rusting of iron.
Q16: Explain the process of 'Rusting of Iron'.
Answer: Rusting is the corrosion of iron. It occurs when iron reacts with oxygen and moisture (water) in the air to form a reddish-brown flaky substance called rust, which is hydrated iron(III) oxide (Fe₂O₃.nH₂O).
Q17: List three methods to prevent corrosion.
Answer:
- Painting: Applying a coat of paint.
- Oiling/Greasing: Applying oil or grease.
- Galvanization: Coating iron with a layer of zinc.
- Chromium plating: Coating with chromium.
- Alloying: Mixing metals to form corrosion-resistant alloys.
Q18: Why are sodium and potassium stored in kerosene?
Answer: Sodium and potassium are highly reactive metals. They react vigorously with oxygen and moisture present in the air, even at room temperature, potentially catching fire. To prevent this, they are stored immersed in kerosene, which prevents their contact with air and water.
Q19: What happens when copper objects are exposed to moist air for a long time?
Answer: When copper objects are exposed to moist air for a long time, they develop a dull green coating. This is due to the formation of basic copper carbonate, which is a result of copper reacting with carbon dioxide and moisture in the air.
Q20: Why are electrical wires made of copper?
Answer: Electrical wires are made of copper because copper is an excellent conductor of electricity, meaning it allows electric current to flow through it with very little resistance. It is also ductile, allowing it to be drawn into thin wires.
Q21: What is 'galvanization'? How does it prevent rusting?
Answer: Galvanization is a method of preventing rusting by coating iron objects with a thin layer of zinc. Zinc is more reactive than iron, so it preferentially reacts with oxygen and moisture in the environment, corroding itself and protecting the underlying iron from rusting (sacrificial protection).
Q22: Give an example of an amphoteric oxide.
Answer: Aluminium oxide (Al₂O₃) and Zinc oxide (ZnO) are examples of amphoteric oxides, meaning they show both acidic and basic properties.
Q23: How can you test for the presence of hydrogen gas evolved when a metal reacts with an acid?
Answer: Hydrogen gas evolved when a metal reacts with an acid can be tested by bringing a burning splinter or matchstick near the mouth of the test tube. If hydrogen gas is present, it will burn with a characteristic 'pop' sound.
Q24: Why are noble metals preferred for making jewelry?
Answer: Noble metals like gold, silver, and platinum are preferred for making jewelry because they are highly resistant to corrosion and tarnishing. They retain their metallic lustre and do not react with air, moisture, or common chemicals, ensuring their beauty and durability over time.
Q25: What is the difference in state at room temperature for metals and nonmetals?
Answer: Most metals are solid at room temperature, with the exception of mercury (liquid). Nonmetals, however, can exist in all three states at room temperature: solids (e.g., carbon, sulfur), liquids (e.g., bromine), and gases (e.g., oxygen, nitrogen).
Exercise Solutions (From Screenshot Page 53)
Q1: Fill in the blanks.
- Metals are generally hard solids at room temperature.
- The property of metals to be beaten into thin sheets is called malleability.
- The property of metals to be drawn into thin wires is called ductility.
- Metals are good conductors of heat and electricity.
- Nonmetals are generally dull in appearance.
- Diamond is an allotrope of carbon and is the hardest natural substance.
- Graphite is a nonmetal that is a good conductor of electricity.
- Metals react with oxygen to form basic oxides.
- Nonmetals react with oxygen to form acidic oxides.
- Gold, Silver, and Platinum are called noble metals.
- The process of gradual eating away of metals by air, moisture, or chemicals is called corrosion.
- Rust is hydrated iron(III) oxide.
Q2: Match the pairs.
(Note: As an AI, I cannot create interactive matching. I will provide the correct pairs.)
- Gold - Noble metal
- Mercury - Liquid metal
- Diamond - Hardest nonmetal
- Graphite - Good conductor (nonmetal)
- Sodium - Highly reactive metal
- Oxygen - Nonmetal gas
- Iron - Used in construction
- Copper - Used in electrical wires
- Aluminium - Used in aircraft
- Chlorine - Used as disinfectant
Q3: Give scientific reasons.
- Sodium and potassium are stored in kerosene.
Reason: Sodium and potassium are highly reactive metals. They react vigorously and exothermically with oxygen and moisture present in the air, even at room temperature, which can lead to fires. To prevent this dangerous reaction, they are stored immersed in kerosene, which prevents their contact with air and water. - Electrical wires are made of copper.
Reason: Copper is an excellent conductor of electricity, meaning it allows electric current to flow through it with very low resistance, minimizing energy loss. Additionally, copper is highly ductile, allowing it to be easily drawn into thin wires, and it is relatively affordable and abundant. - Bells are made of metals.
Reason: Bells are made of metals because metals possess the property of sonority. This means that when metals are struck, they produce a characteristic clear, ringing sound. Nonmetals, being non-sonorous, would not produce such a desirable sound. - Aluminium objects do not corrode easily.
Reason: Aluminium objects do not corrode easily because when aluminium is exposed to air, it quickly reacts with oxygen to form a thin, tough, and protective layer of aluminium oxide (Al₂O₃) on its surface. This oxide layer is highly stable and prevents further reaction of the underlying aluminium metal with air and moisture, thus protecting it from corrosion. This process can be enhanced by anodizing. - Noble metals are used for making jewelry.
Reason: Noble metals like gold, silver, and platinum are used for making jewelry because they are highly unreactive and resistant to corrosion and tarnishing. They do not react with atmospheric gases, moisture, or common chemicals, which ensures that they retain their metallic lustre and beauty for a very long time without losing their shine or getting discolored.
Q4: Answer the following questions.
- Explain the physical properties of metals.
Answer:- Lustre: Shiny appearance.
- Hardness: Generally hard (except Na, K, Hg).
- Malleability: Can be beaten into sheets.
- Ductility: Can be drawn into wires.
- Sonority: Produce ringing sound.
- Conductors: Good conductors of heat and electricity.
- State: Mostly solid at room temperature (except Hg).
- Melting/Boiling Points: Generally high.
- Explain the physical properties of nonmetals.
Answer:- Appearance: Dull (except Iodine).
- Hardness: Generally soft (except Diamond).
- Brittle: Break easily.
- Non-ductile: Cannot be drawn into wires.
- Non-sonorous: Do not produce ringing sound.
- Poor Conductors: Poor conductors of heat and electricity (except Graphite).
- State: Can be solid, liquid, or gas at room temperature.
- Melting/Boiling Points: Generally low.
- Explain the chemical properties of metals.
Answer:- Reaction with Oxygen: Form basic oxides (e.g., Mg + O₂ → MgO). Some are amphoteric (Al₂O₃, ZnO).
- Reaction with Water: Varying reactivity; some react with cold water (Na, K), some with hot water (Mg), some with steam (Fe, Al, Zn), and some do not react (Au, Ag, Cu).
- Reaction with Acids: Generally react with dilute acids to produce hydrogen gas and metal salt (e.g., Zn + HCl → ZnCl₂ + H₂).
- Reaction with Bases: Some metals (Al, Zn, Pb) react with strong bases to produce hydrogen gas.
- Displacement Reactions: More reactive metals displace less reactive metals from their salt solutions.
- Explain the chemical properties of nonmetals.
Answer:- Reaction with Oxygen: Form acidic oxides (e.g., C + O₂ → CO₂). These oxides dissolve in water to form acids.
- Reaction with Water: Generally do not react with water or steam.
- Reaction with Acids: Generally do not react with dilute acids.
- Reaction with Bases: React in complex ways, often depending on specific nonmetal and base.
- Displacement Reactions: More reactive nonmetals can displace less reactive nonmetals from their salt solutions.
- What is corrosion? How can it be prevented?
Answer: Corrosion: It is the process by which metals are gradually eaten away due to the action of air, moisture, or chemicals present in the environment. It is a slow chemical change. A common example is the rusting of iron. Prevention: Corrosion can be prevented by:- Painting: Applying a coat of paint to create a barrier.
- Oiling/Greasing: Applying a layer of oil or grease.
- Galvanization: Coating iron with a layer of zinc.
- Chromium plating: Electroplating with chromium.
- Anodizing: Forming a thick oxide layer (especially on aluminum).
- Alloying: Mixing metals with other elements to form corrosion-resistant alloys (e.g., stainless steel).
Q5: Differentiate between.
- Metals and Nonmetals (Physical Properties)
Property Metals Nonmetals Lustre Shiny (metallic lustre) Dull (non-lustrous), except Iodine Hardness Generally hard (except Na, K, Hg) Generally soft (except Diamond) Malleability Malleable (can be beaten into sheets) Non-malleable (brittle) Ductility Ductile (can be drawn into wires) Non-ductile (brittle) Conduction Good conductors of heat & electricity Poor conductors (insulators), except Graphite State (RT) Mostly solid (except Hg) Solid, liquid, or gas - Metals and Nonmetals (Chemical Properties)
Property Metals Nonmetals Reaction with Oxygen Form basic oxides (some amphoteric) Form acidic oxides Reaction with Water Varying reactivity (some react, some don't) Generally do not react Reaction with Dilute Acids Generally react to produce H₂ gas Generally do not react Nature of Oxides Basic (or amphoteric) Acidic
References
- Maharashtra State Board Science and Technology Standard Eight Textbook (Specific Edition/Year) - Chapter 7: Metals and Nonmetals.
- Maharashtra State Board 8th Standard Science Syllabus.
- Balbharati Science and Technology Textbook.