Grade 8 Chapter 7: Metals and Nonmetals

Introduction

Elements are the fundamental building blocks of all matter. Based on their physical and chemical properties, elements are broadly classified into metals and nonmetals. This classification helps us understand their behavior and diverse applications in our daily lives and various industries. This chapter will delve into the characteristic properties that distinguish metals from nonmetals, their chemical reactions, and their significance in the modern world.

Physical Properties of Metals and Nonmetals

Physical Properties of Metals

• Lustre (Shine): Metals generally have a shiny appearance, known as metallic lustre. They can be polished. (e.g., Gold, Silver, Copper).
• Hardness: Most metals are hard solids at room temperature (e.g., Iron, Copper, Aluminium). Exceptions: Sodium (Na) and Potassium (K) are soft and can be cut with a knife. Mercury (Hg) is a liquid at room temperature.
• Malleability: Metals are malleable, meaning they can be beaten into thin sheets without breaking (e.g., Gold, Silver, Aluminium are highly malleable). This property allows metals to be shaped into various forms.
• Ductility: Metals are ductile, meaning they can be drawn into thin wires (e.g., Gold, Silver, Copper are highly ductile). This property is used in making electric wires.
• Sonority (Sound): Metals are sonorous, meaning they produce a ringing sound when struck (e.g., bells are made of metal).
• Conduction of Heat and Electricity: Metals are generally good conductors of heat and electricity (e.g., Silver is the best conductor of electricity, followed by Copper). This is why electrical wires are made of copper and cooking utensils are made of metals.
• State at Room Temperature: Most metals are solid at room temperature. Exception: Mercury is a liquid.
• High Melting and Boiling Points: Metals generally have high melting and boiling points.

Physical Properties of Nonmetals

• Appearance: Nonmetals generally have a dull appearance (non-lustrous). Exception: Iodine is lustrous.
• Hardness: Nonmetals are generally soft (e.g., Sulfur, Carbon in the form of coal). Exception: Diamond (an allotrope of carbon) is the hardest natural substance.
• Malleability and Ductility: Nonmetals are non-malleable and non-ductile. They are brittle, meaning they break easily when hammered or stretched.
• Sonority: Nonmetals are non-sonorous; they do not produce a ringing sound when struck.
• Conduction of Heat and Electricity: Nonmetals are generally poor conductors of heat and electricity (insulators). Exception: Graphite (an allotrope of carbon) is a good conductor of electricity.
• State at Room Temperature: Nonmetals can exist as solids (e.g., Carbon, Sulfur), liquids (e.g., Bromine), or gases (e.g., Oxygen, Nitrogen) at room temperature.
• Low Melting and Boiling Points: Nonmetals generally have low melting and boiling points.

Chemical Properties of Metals and Nonmetals

Chemical Properties of Metals

• Reaction with Oxygen: 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
  • Some metal oxides are amphoteric, meaning they show both acidic and basic properties (e.g., Aluminium oxide, Zinc oxide).
  • Metals like Sodium (Na) and Potassium (K) are highly reactive and react vigorously with oxygen even at room temperature, so they are stored in kerosene.
  • Iron (Fe) reacts with oxygen and water to form rust (iron oxide).
• Reaction with Water:
  • Some metals react vigorously with cold water (e.g., Sodium, Potassium).
    2Na + 2H₂O → 2NaOH + H₂ + Heat
  • Some metals react with hot water (e.g., Magnesium).
  • Some metals react with steam (e.g., Aluminium, Iron, Zinc).
    3Fe + 4H₂O (steam) → Fe₃O₄ + 4H₂
  • Less reactive metals like Gold, Silver, Copper do not react with water.
• Reaction with Acids: 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
  • Hydrogen gas evolved burns with a 'pop' sound.
• Reaction with Bases: Some metals (e.g., Aluminium, Zinc, Lead) react with strong bases (like Sodium hydroxide) to produce hydrogen gas.
  • Example: Aluminium (Al) reacts with Sodium hydroxide (NaOH) to form Sodium aluminate (NaAlO₂) and hydrogen gas (H₂).
    2Al + 2NaOH + 2H₂O → 2NaAlO₂ + 3H₂
• Displacement Reactions: A more reactive metal can displace 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

Chemical Properties of Nonmetals

• Reaction with Oxygen: 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
  • Nonmetal oxides dissolve in water to form acids (e.g., CO₂ + H₂O → H₂CO₃ (carbonic acid)).
• Reaction with Water: Nonmetals generally do not react with water or steam.
• Reaction with Acids: Nonmetals generally do not react with dilute acids.
• Reaction with Bases: Nonmetals react with bases in a complex manner, often depending on the specific nonmetal and base. (e.g., Chlorine reacts with sodium hydroxide).
• Displacement Reactions: A more reactive nonmetal can displace a less reactive nonmetal from its salt solution. (e.g., Chlorine displaces bromine from potassium bromide solution).

Uses of Metals and Nonmetals

Uses of Metals

• Iron: Used in construction (steel), machinery, vehicles, tools.
• Copper: Used in electrical wires, plumbing, coins, utensils.
• Aluminium: Used in aircraft, utensils, foils, electrical cables, window frames.
• Gold and Silver: Used in jewelry, coins, dentistry, electrical contacts.
• Mercury: Used in thermometers, barometers.
• Zinc: Used for galvanization (coating iron to prevent rusting), batteries.

Uses of Nonmetals

• Oxygen: Essential for respiration, combustion, medical uses.
• Nitrogen: Used in fertilizers, food packaging (preservative), liquid nitrogen for cryogenics.
• Chlorine: Used as a disinfectant (water purification), in PVC, bleaching powder.
• Sulfur: Used in sulfuric acid production, vulcanization of rubber, fungicides, gunpowder.
• Carbon (Graphite): Used in pencil leads, electrodes, lubricants.
• Carbon (Diamond): Used in jewelry, cutting tools.
• Iodine: Used as an antiseptic (tincture of iodine).

Noble Metals

Noble metals are metals that are highly resistant to corrosion and oxidation in moist air. They are generally unreactive and do not easily react with acids, bases, or oxygen.

• Examples: Gold (Au), Silver (Ag), Platinum (Pt).
• Properties: Highly lustrous, resistant to corrosion, very ductile and malleable.
• Uses: Primarily used in jewelry, coinage, and high-end electronics due to their unreactive nature and attractive appearance.

Corrosion

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.

• Rusting of Iron: The most common example of corrosion. Iron reacts with oxygen and moisture (water) to form a reddish-brown flaky substance called rust (hydrated iron(III) oxide).
  4Fe + 3O₂ + 2nH₂O → 2Fe₂O₃.nH₂O (Rust)
• Green coating on Copper: Copper objects turn green when exposed to moist air for a long time due to the formation of basic copper carbonate.
• Black coating on Silver: Silver articles turn black due to the formation of silver sulfide when exposed to air containing hydrogen sulfide gas.

Prevention of Corrosion:

• Painting: Applying a coat of paint prevents the metal surface from coming into contact with air and moisture.
• Oiling/Greasing: Applying oil or grease forms a protective layer.
• Galvanization: Coating iron with a layer of zinc. Zinc is more reactive than iron, so it corrodes first, protecting the iron.
• Chromium plating: Coating with chromium, which is corrosion-resistant.
• Anodizing: Forming a thick oxide layer on aluminum to protect it.
• Alloying: Mixing metals with other elements to form alloys that are more resistant to corrosion (e.g., stainless steel).

Summary

• Elements are classified into Metals and Nonmetals based on their physical and chemical properties.
• Metals: Lustrous, hard, malleable, ductile, sonorous, good conductors of heat and electricity, mostly solids (except Mercury). Chemically, they form basic oxides, react with acids to produce H₂, and can displace less reactive metals.
• Nonmetals: Dull, soft (except Diamond), brittle, non-ductile, non-sonorous, poor conductors (except Graphite), exist as solids, liquids, or gases. Chemically, they form acidic oxides and generally do not react with water or dilute acids.
• Metals and nonmetals have diverse uses in industries and daily life.
• Noble Metals: Gold, Silver, Platinum are highly unreactive and resistant to corrosion.
• Corrosion: The gradual eating away of metals by environmental factors (air, moisture, chemicals). Rusting of iron is a common example.
• Corrosion can be prevented by painting, oiling, galvanization, chromium plating, anodizing, and alloying.

References

1. Maharashtra State Board Science and Technology Standard Eight Textbook (Specific Edition/Year) - Chapter 7: Metals and Nonmetals.
2. Maharashtra State Board 8th Standard Science Syllabus.
3. Balbharati Science and Technology Textbook.