Chapter-Chapter 4. The World of Metals and Non-Metals Science Curiosity class 7 in english Medium CBSE Notes

Chapter 4: The World of Metals and Non-Metals

Introduction: Everything around us is made up of different materials. These materials have different properties, which help us identify and use them in various ways. Some materials are hard, shiny, and conduct electricity, while others are soft, dull, and do not conduct electricity.

Based on their properties, materials are broadly classified into metals and non-metals. Understanding their properties helps us use them effectively in daily life.

This chapter covers the following topics:

• Metals and their properties
• Non-metals and their properties
• Uses of metals and non-metals
• Physical properties (hardness, lustre, conductivity)
• Chemical properties (basic idea)
• Comparison between metals and non-metals

What are Metals? Metals are substances that are generally hard, shiny, and good conductors of heat and electricity. They are widely used in making tools, machines, and electrical wires.

• Iron
• Copper
• Aluminium
• Gold

Example: Copper is used in electric wires because it allows current to pass easily.

Properties of Metals:

• Lustrous: Metals have a shiny surface.
• Hard: Most metals are strong and hard.
• Conductors: They allow heat and electricity to pass.
• Malleable: They can be beaten into thin sheets.
• Ductile: They can be drawn into wires.

What are Non-Metals? Non-metals are substances that are generally soft and dull. They are poor conductors of heat and electricity.

• Oxygen
• Carbon
• Sulphur
• Phosphorus

Example: Rubber is a non-metal and does not allow electricity to pass through it.

Properties of Non-Metals:

• Non-lustrous (dull)
• Soft and brittle
• Poor conductors of heat and electricity
• Cannot be beaten into sheets
• Cannot be drawn into wires

Key Point: Metals are generally strong and conductive, while non-metals are usually weak and non-conductive.

Uses of Metals: Metals are widely used in our daily life because of their strength, durability, and conductivity. Their properties make them suitable for different purposes.

• Iron: Used in construction of buildings, bridges, and tools due to its strength.
• Copper: Used in electrical wires because it is a good conductor of electricity.
• Aluminium: Used in utensils and packaging because it is light and does not rust easily.
• Gold and Silver: Used in jewellery due to their shine and resistance to corrosion.

Example: Electric wires are made of copper, while cooking utensils are often made of aluminium.

Uses of Non-Metals: Non-metals are also important and are used in many fields despite not being strong conductors.

• Oxygen: Essential for respiration.
• Carbon: Used as fuel and in making graphite (used in pencils).
• Nitrogen: Used in fertilizers to improve plant growth.
• Sulphur: Used in medicines and rubber industry.

Example: Graphite (a form of carbon) is used in pencil lead.

Exceptions in Properties: Although metals and non-metals have general properties, there are some exceptions which are important to understand.

• Mercury: A metal that is liquid at room temperature.
• Sodium and Potassium: Metals that are soft and can be cut with a knife.
• Graphite: A non-metal that conducts electricity.
• Diamond: A non-metal that is very hard.

These exceptions show that not all substances strictly follow general rules.

Chemical Behaviour (Basic Idea): Metals and non-metals also differ in their chemical reactions.

• Metals generally react with oxygen to form metal oxides.
• Non-metals react with oxygen to form non-metal oxides.
• Some metals react with water and acids to release hydrogen gas.

Example: Iron reacts with oxygen in air to form rust.

Comparison between Metals and Non-Metals:

• Metals are shiny, non-metals are dull.
• Metals conduct electricity, non-metals generally do not.
• Metals are strong, non-metals are usually weak.
• Metals are malleable and ductile, non-metals are brittle.

Key Point: While metals and non-metals have opposite properties, exceptions always exist and should be remembered.

Corrosion (Rusting): Corrosion is the process by which metals slowly get damaged due to reaction with air, water, or other substances. The most common example of corrosion is rusting of iron.

When iron is exposed to air and moisture for a long time, it reacts with oxygen and forms a reddish-brown substance called rust.

Example: Iron gates and tools left outside in rain get rusted after some time.

Why Corrosion is Harmful:

• Weakens metals
• Damages structures like bridges and buildings
• Reduces the life of metal objects

Prevention of Corrosion: To protect metals from corrosion, several methods are used.

• Painting: Prevents contact with air and moisture
• Oiling/Greasing: Forms a protective layer
• Galvanization: Coating iron with zinc to prevent rusting
• Alloying: Mixing metals to improve resistance

Example: Iron pipes are often coated with paint to prevent rusting.

Real-Life Importance: Understanding metals and non-metals helps us choose the right material for the right purpose.

• Metals are used where strength and conductivity are needed
• Non-metals are used where insulation or light weight is required
• Safety depends on correct material selection

Overall Understanding: Metals and non-metals are two important groups of materials with different properties. Their uses depend on their physical and chemical characteristics. Knowing their behavior helps us use them effectively and safely.

Key Point: Proper use and protection of metals increase their life and prevent damage caused by corrosion.

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