Grade 7 Chapter 15: Materials We Use

Introduction

From the clothes we wear to the houses we live in, and the tools we use every day, everything is made of different materials. These materials can be broadly categorized into natural materials and man-made materials. Understanding where these materials come from, how they are made, and their properties helps us appreciate their importance in our lives and use them responsibly.

15.1 Natural and Man-made Materials

Natural Materials

Natural materials are those substances that are obtained directly from nature, without significant human intervention in their formation. They can come from plants, animals, or the earth.

• Examples: Wood, cotton, silk, wool, leather, rubber (natural), soil, minerals, metals (like iron ore, gold).
• Characteristics: Often biodegradable, can be renewable (like wood from trees) or non-renewable (like minerals).

Man-made Materials (Synthetic Materials)

Man-made materials, also known as synthetic materials, are substances that are produced or synthesized by humans using natural resources as raw materials. These materials often have properties that are superior to or different from natural materials, making them suitable for specific applications.

• Examples: Plastics, synthetic fibers (nylon, rayon, terylene, dacron), glass, ceramics, cement, synthetic rubber, paper.
• Characteristics: Can be designed for specific properties (strength, water resistance, flexibility), often non-biodegradable, production involves chemical processes.

15.2 Important Man-made Materials

Rubber

Rubber is an elastic material known for its flexibility and waterproofing properties. It exists in both natural and synthetic forms.

• Natural Rubber: Obtained from the latex (a milky sap) of rubber trees. It is a natural polymer. Natural rubber is widely used for tires, gloves, and other elastic products.
• Synthetic Rubber: Man-made rubber produced from petroleum. Synthetic rubber often has enhanced properties like resistance to oil, heat, or chemicals, making it suitable for specialized applications (e.g., seals, hoses).

Paper

Paper is a thin material primarily used for writing, printing, drawing, and packaging. It is a man-made material with a long history.

• Production: Paper is mainly produced from wood pulp. The wood is processed to separate cellulose fibers, which are then mixed with water, pressed, and dried to form sheets.
• Properties: Absorbent, relatively strong, can be recycled.
• Uses: Books, newspapers, packaging, currency, stationery.
• Recycling: Paper recycling is crucial for environmental conservation as it reduces the need to cut down trees and saves energy.

Synthetic Fibers

Synthetic fibers are man-made textile fibers produced through chemical processes, often from petroleum-based raw materials. They are designed to have specific desirable properties.

• Examples:
  • Rayon: Also known as 'artificial silk', it is made from regenerated cellulose (wood pulp). It has a silky feel and is absorbent.
  • Nylon: A strong, elastic, and durable fiber. It is resistant to wrinkles, shrinking, and abrasion. Used in ropes, parachutes, clothing, and toothbrushes.
  • Terylene (Polyester): A strong, wrinkle-resistant, and quick-drying fiber. Often blended with natural fibers. Used in clothing, curtains, and sails.
  • Dacron: Another name for a type of polyester fiber.
• Properties: Strong, durable, elastic, wrinkle-resistant, quick-drying, resistant to chemicals and insects, often non-biodegradable.
• Uses: Clothing, ropes, carpets, fishing nets, industrial fabrics.

Plastics

Plastics are a vast group of synthetic materials made from polymers, which are large molecules formed by linking many smaller repeating units called monomers. Plastics are incredibly versatile and have revolutionized many industries.

• Properties: Lightweight, durable, strong, waterproof, resistant to corrosion and chemicals, electrical insulators, thermal insulators.
• Uses: Packaging, containers, pipes, electrical insulation, toys, furniture, automotive parts, construction materials.

Types of Plastics:

Plastics are generally categorized into two main types based on their behavior when heated:

• Thermoplastics:
  • Definition: These plastics can be softened by heating and hardened by cooling repeatedly. They can be melted and reshaped multiple times without significant degradation.
  • Examples: Polyvinyl Chloride (PVC), Polythene (PE), Polypropylene (PP), Polystyrene (PS).
  • Uses: Water pipes, electrical insulation, plastic bags, food containers, toys.
• Thermosetting Plastics:
  • Definition: These plastics undergo a chemical change when heated and molded, becoming permanently rigid. Once set, they cannot be softened or reshaped by further heating.
  • Examples: Bakelite, Melamine, Formica.
  • Uses: Electrical switches, handles of cooking utensils, dinnerware, floor tiles.

Environmental Impact of Plastics

While plastics offer many benefits, their widespread use and non-biodegradable nature pose significant environmental challenges.

• Plastic Pollution: Plastics do not decompose easily in nature, leading to accumulation in landfills, oceans, and natural environments. This pollution harms wildlife and ecosystems.
• Microplastics: Larger plastic pieces break down into tiny particles called microplastics, which can enter the food chain and water sources, posing potential health risks.
• Burning Plastics: Burning plastics releases toxic gases into the atmosphere, contributing to air pollution.

Responsible Use of Plastics (3R Principle)

To mitigate the negative impact of plastics, it is essential to adopt responsible practices. The "3R" principle is a guiding approach:

• Reduce: Minimize the consumption of plastic products, especially single-use plastics. Choose alternatives whenever possible.
• Reuse: Use plastic items multiple times before discarding them. For example, reusable bags, water bottles, and containers.
• Recycle: Properly dispose of recyclable plastic items in designated recycling bins. Recycling helps convert waste plastics into new products, reducing the demand for new plastic production.