Grade 7 Q&A: Chapter 15: Materials We Use

Concept Questions

Q1: What are natural materials? Give two examples.

Answer: Natural materials are substances obtained directly from nature without significant human processing. Examples include wood, cotton, and silk.

Q2: What are man-made materials? Give two examples.

Answer: Man-made materials (or synthetic materials) are substances produced or synthesized by humans using natural resources as raw materials. Examples include plastics, glass, and synthetic fibers like nylon.

Q3: From what is natural rubber obtained? What is its primary use?

Answer: Natural rubber is obtained from the latex (milky sap) of rubber trees. Its primary uses include making tires, gloves, and other elastic products.

Q4: How is paper primarily made?

Answer: Paper is primarily made from wood pulp. Cellulose fibers are extracted from wood, mixed with water, pressed into sheets, and then dried.

Q5: Name three synthetic fibers.

Answer: Three synthetic fibers are Rayon, Nylon, and Terylene (or Dacron).

Q6: What are the main characteristics of synthetic fibers?

Answer: Synthetic fibers are generally strong, durable, elastic, wrinkle-resistant, quick-drying, and resistant to chemicals and insects.

Q7: What are plastics? How are they generally categorized?

Answer: Plastics are a vast group of synthetic materials made from polymers. They are generally categorized into two main types based on their behavior when heated: thermoplastics and thermosetting plastics.

Q8: What is a thermoplastic? Give an example.

Answer: Thermoplastics are plastics that can be softened by heating and hardened by cooling repeatedly. They can be melted and reshaped multiple times. Examples include Polythene (PE) and Polyvinyl Chloride (PVC).

Q9: What is a thermosetting plastic? Give an example.

Answer: Thermosetting plastics undergo a chemical change when heated and molded, becoming permanently rigid. Once set, they cannot be softened or reshaped by further heating. Examples include Bakelite and Melamine.

Q10: List three disadvantages of plastics.

Answer: Three disadvantages of plastics are:

1. They are non-biodegradable and cause significant environmental pollution.
2. When burned, they release toxic gases into the atmosphere.
3. They can break down into microplastics, which can enter the food chain.

Q11: Explain the '3R' principle for responsible plastic use.

Answer: The '3R' principle stands for:

• Reduce: Minimize the consumption of plastic products, especially single-use items.
• Reuse: Use plastic items multiple times before discarding them (e.g., reusable bags, bottles).
• Recycle: Properly sort and dispose of recyclable plastic items so they can be processed into new products.

This principle aims to minimize plastic waste and its environmental impact.

Q12: Why is recycling paper important?

Answer: Recycling paper is important because it reduces the need to cut down new trees, thereby conserving forests. It also saves energy, reduces water pollution, and decreases the amount of waste sent to landfills.

Q13: What is the difference between natural rubber and synthetic rubber?

Answer: Natural rubber is a natural polymer obtained from the latex of rubber trees. Synthetic rubber is man-made, produced from petroleum, and often engineered to have enhanced properties like resistance to oil, heat, or chemicals, making it suitable for specialized applications.

Q14: Give two examples of how plastics are used in daily life.

Answer: Plastics are used for packaging (e.g., plastic bags, bottles) and in toys (e.g., plastic dolls, building blocks).

Q15: What are polymers and monomers in the context of plastics?

Answer: Plastics are made from polymers, which are large molecules. These polymers are formed by linking many smaller repeating units called monomers.

Exercise Solutions (from Page 103)

1. Fill in the blanks:

a. Natural rubber is obtained from the latex of trees.

b. Man-made materials are made by chemical processing of natural materials.

c. Bakelite is an example of a thermosetting plastic.

d. Polythene (or PVC) is an example of a thermoplastic.

e. Nylon and Terylene (or Rayon) are synthetic fibers.

2. Match the pairs:

a. Rayon - 3. Artificial silk

b. Nylon - 4. Synthetic fiber

c. Bakelite - 2. Thermosetting plastic

d. Rubber - 1. Natural rubber

3. Answer the following:

a. What is the difference between natural and man-made materials?

Answer:

• Natural Materials: These are substances found and obtained directly from nature, without significant human alteration. Examples include wood, cotton, silk, wool, and minerals. They are often biodegradable.
• Man-made Materials: These are substances produced or synthesized by humans through chemical processes, using natural resources as raw materials. Examples include plastics, synthetic fibers, glass, and cement. They are often designed for specific properties and may not be biodegradable.

b. What are the advantages of synthetic fibers?

Answer: The advantages of synthetic fibers include:

• They are generally very strong and durable.
• They are elastic and resistant to wrinkles.
• They dry quickly and are easy to maintain.
• They are often resistant to shrinking, stretching, and abrasion.
• They are less susceptible to damage from insects and mildew compared to natural fibers.

c. What are the disadvantages of plastics?

Answer: The disadvantages of plastics are:

• Non-biodegradable: Most plastics do not decompose naturally, leading to long-term environmental pollution in landfills and oceans.
• Environmental Pollution: Plastic waste harms wildlife, contaminates ecosystems, and can break down into harmful microplastics.
• Toxic Fumes: Burning plastics releases harmful and toxic gases into the atmosphere, contributing to air pollution and health hazards.
• Resource Depletion: Production of many plastics relies on fossil fuels, which are non-renewable resources.

d. What is meant by '3R' principle? Explain.

Answer: The '3R' principle is an environmental guideline to minimize waste, especially plastic waste, and promote sustainability. It stands for:

• Reduce: This means minimizing the consumption of materials, particularly single-use items. For example, carrying your own reusable water bottle or shopping bag.
• Reuse: This involves using items multiple times for their original purpose or a new purpose instead of discarding them after a single use. For example, reusing plastic containers for storage.
• Recycle: This is the process of collecting and processing waste materials to convert them into new products, preventing the waste of potentially useful materials and reducing the consumption of fresh raw materials. For example, sending plastic bottles to a recycling plant.

e. What are the differences between thermoplastics and thermosetting plastics?

Answer:

• Thermoplastics:
  • Can be softened repeatedly by heating and hardened by cooling.
  • Can be melted and reshaped multiple times.
  • Examples: Polythene (PE), Polyvinyl Chloride (PVC).
  • Used for items like plastic bags, toys, and pipes.
• Thermosetting Plastics:
  • Undergo a chemical change upon heating and molding, becoming permanently rigid.
  • Cannot be softened or reshaped once they have been set.
  • Examples: Bakelite, Melamine.
  • Used for items like electrical switches, handles of cooking utensils, and dinnerware.

f. Why should we use paper carefully?

Answer: We should use paper carefully because:

• Paper is primarily made from wood pulp, meaning trees are cut down to produce it. Careful use helps conserve forests.
• The paper manufacturing process consumes a significant amount of water and energy, and can also lead to pollution. Reducing paper consumption helps save these resources and reduce pollution.
• Even though paper can be recycled, reducing its use in the first place is the most effective way to lessen its environmental impact.

g. What is vulcanization?

Answer: Vulcanization is a chemical process for converting natural rubber or related polymers into more durable materials by adding sulfur or other equivalent curatives or accelerators. This process involves heating rubber with sulfur, which forms cross-links between the rubber molecules, making the rubber harder, stronger, more elastic, and more resistant to temperature changes and wear. It significantly improves the properties of natural rubber, making it suitable for various applications like tires.

4. Give scientific reasons:

a. Articles made from thermosetting plastics do not get spoiled when hot water is poured on them.

Answer: Articles made from thermosetting plastics (like Bakelite or Melamine) do not get spoiled when hot water is poured on them because thermosetting plastics, once molded and set by heating, undergo an irreversible chemical change. They form a rigid, cross-linked structure that cannot be softened or melted by further heating. This property makes them heat-resistant and suitable for applications exposed to high temperatures.

b. Plastic containers are used to store chemicals.

Answer: Plastic containers are used to store chemicals because plastics are generally unreactive and resistant to corrosion by many chemicals, including acids and bases. Unlike metals, plastics do not rust or react with the stored substances, ensuring the chemicals remain pure and the containers do not degrade.

c. Handles of pressure cookers are made of Bakelite.

Answer: Handles of pressure cookers are made of Bakelite because Bakelite is a thermosetting plastic and an excellent insulator of heat. It does not soften or melt when heated, ensuring that the handles remain cool to the touch even when the pressure cooker is hot. This prevents burns and allows for safe handling.

d. We should use cotton clothes in summer.

Answer: We should use cotton clothes in summer because cotton is a natural fiber that is highly absorbent and breathable. It absorbs sweat from the body, allowing it to evaporate easily, which helps in cooling the body. Its porous nature also allows air to circulate, making it comfortable to wear in hot and humid conditions.

e. We should avoid using plastic bags.

Answer: We should avoid using plastic bags because:

• They are non-biodegradable and persist in the environment for hundreds of years, causing significant land and water pollution.
• They can harm wildlife, as animals may ingest them or get entangled in them.
• Their production consumes fossil fuels, and their disposal (especially burning) releases toxic chemicals into the atmosphere.
• They contribute to the accumulation of waste in landfills.

Using reusable bags helps mitigate these environmental problems.