Exercise Solutions (From Screenshot Page 121)

Q1: Fill in the blanks.

1. Materials manufactured by humans are called man-made materials.
2. Plastics are synthetic materials derived from petroleum and natural gas.
3. Plastics that can be softened repeatedly by heating are called thermoplastics.
4. Plastics that cannot be softened after molding are called thermosetting plastics.
5. Polythene is an example of a thermoplastic.
6. Bakelite is an example of a thermosetting plastic.
7. Most plastics are non-biodegradable.
8. Rayon is also known as artificial silk.
9. Nylon is the first fully synthetic fiber.
10. Acrylic is also known as artificial wool.
11. Glass is an amorphous solid.
12. Soda-lime glass is also known as soft glass.
13. Borosilicate glass is also known as hard glass or Pyrex.
14. Ceramics are inorganic, non-metallic materials processed by heating at high temperatures.
15. Concrete is a mixture of cement, aggregate (sand and gravel), and water.

Q2: Match the pairs.

(Note: As an AI, I cannot create interactive matching. I will provide the correct pairs.)

• Polythene - Plastic bags
• PVC - Pipes
• Polystyrene - Disposable cups
• Bakelite - Electrical switches
• Melamine - Dinnerware
• Rayon - Artificial silk
• Nylon - Ropes, parachutes
• Polyester - Fabrics (terylene)
• Acrylic - Artificial wool (sweaters)
• Soda-lime glass - Window panes
• Borosilicate glass - Laboratory glassware
• Ceramics - Bricks, tiles
• Concrete - Construction material

Q3: Give scientific reasons.

1. Handles of cooking utensils are made of Bakelite.
   Reason: Handles of cooking utensils are made of Bakelite because it is a thermosetting plastic. Bakelite is a poor conductor of heat and is resistant to high temperatures. Unlike thermoplastics, it does not soften or melt upon heating, making it safe and durable for handling hot cooking utensils without transferring heat to the user's hand.
2. Borosilicate glass is used for laboratory glassware.
   Reason: Borosilicate glass (e.g., Pyrex) is used for laboratory glassware because it has a very low coefficient of thermal expansion. This means it is highly resistant to thermal shock (sudden changes in temperature) and does not crack easily when subjected to rapid heating or cooling. It is also chemically inert and resistant to most chemicals, which is crucial for laboratory experiments.
3. Most plastics are non-biodegradable.
   Reason: Most plastics are non-biodegradable because their long polymer chains are very stable and cannot be broken down by natural processes (like bacteria or fungi) in the environment. This makes them persist in landfills and the environment for hundreds of years, leading to significant waste accumulation and pollution problems.
4. Synthetic fibers are preferred over natural fibers for certain uses.
   Reason: Synthetic fibers are preferred over natural fibers for certain uses because they possess several advantageous properties. They are generally stronger, more durable, more elastic, wrinkle-resistant, quick-drying, and less susceptible to damage from moths, mildew, or chemicals compared to natural fibers. For example, nylon's strength makes it ideal for ropes and parachutes, while polyester's wrinkle resistance is good for clothing.

Q4: Answer the following questions.

1. Explain the types of plastics with examples.
   Answer: Plastics are broadly classified into two types:
   • Thermoplastics: These plastics can be softened repeatedly by heating and hardened by cooling. This property allows them to be reshaped and recycled. Examples include:
     • Polythene (Polyethylene): Used for plastic bags, bottles, and toys.
     • PVC (Polyvinyl Chloride): Used for pipes, electrical insulation, and raincoats.
     • Polystyrene: Used for disposable cups, packaging foam, and insulation.
   • Thermosetting Plastics: Once molded and hardened by heating, these plastics cannot be softened or reshaped again, even on reheating. They form a rigid, cross-linked structure. They cannot be recycled. Examples include:
     • Bakelite: Used for electrical switches, handles of cooking utensils, and telephone casings due to its heat resistance.
     • Melamine: Used for making dinnerware (crockery), floor tiles, and fire-resistant fabrics due to its resistance to heat and fire.
2. Explain the types of synthetic fibers with examples.
   Answer: Synthetic fibers are man-made fibers produced from chemical substances. They are known for their strength, durability, and other desirable properties.
   • Rayon (Artificial Silk): Made from chemically treated wood pulp. It has a silky feel, absorbs moisture, and can be dyed easily. Used for clothing, carpets, and bandages.
   • Nylon: The first fully synthetic fiber. It is very strong, elastic, lightweight, wrinkle-resistant, and easy to wash. Used for ropes, fishing nets, parachutes, stockings, and toothbrushes.
   • Polyester: A strong, wrinkle-resistant, easy-to-wash, and quick-drying fiber. Examples include Terylene and Terrycot. Used for fabrics, bottles (PET), and films.
   • Acrylic (Artificial Wool): Lightweight, soft, warm, and has a wool-like feel. It is resistant to moths, chemicals, and shrinking. Used for sweaters, blankets, shawls, and upholstery.
3. Explain the types of glass and their uses.
   Answer: Glass is an amorphous solid primarily made from sand (silica), soda ash, and limestone. Its properties can be varied by changing its composition.
   • Soda-lime glass (Soft glass): This is the most common and inexpensive type of glass. It has a relatively low melting point. It is used for making everyday items like window panes, bottles, jars, light bulbs, and ordinary glassware.
   • Borosilicate glass (Hard glass/Pyrex): This type of glass contains boron oxide, which makes it highly resistant to heat and thermal shock (sudden changes in temperature). It is also more chemically resistant than soda-lime glass. It is widely used for laboratory glassware (beakers, test tubes), bakeware, pharmaceutical bottles, and high-temperature applications.
4. Explain concrete and its uses.
   Answer: Concrete is a widely used composite construction material. It is made by mixing four main components:
   • Cement: Acts as the binding agent.
   • Aggregate: Consists of fine aggregate (sand) and coarse aggregate (gravel or crushed stone), which provide bulk and strength.
   • Water: Reacts with cement (hydration) to form a paste that binds the aggregates together.
   Properties: Concrete has high compressive strength (resists crushing forces), is durable, fire-resistant, and can be molded into various shapes before it hardens. Uses: Concrete is extensively used in construction due to its strength and versatility. It is used for:
   • Building foundations, walls, columns, and slabs.
   • Constructing bridges, dams, and tunnels.
   • Paving roads and sidewalks.
   • Making precast concrete products like pipes and blocks.
   Reinforced concrete, which incorporates steel bars, is even stronger and used for major structures.

Q5: Differentiate between.

1. Thermoplastics and Thermosetting Plastics
   

   Feature	Thermoplastics	Thermosetting Plastics
   Heating Effect	Soften on heating, harden on cooling	Harden on heating, do not soften on reheating
   Reshaping	Can be reshaped repeatedly	Cannot be reshaped once molded
   Recycling	Can be recycled	Cannot be recycled
   Bonding	Linear or branched polymer chains (weak intermolecular forces)	Cross-linked, rigid 3D network
   Examples	Polythene, PVC, Polystyrene	Bakelite, Melamine

2. Soda-lime Glass and Borosilicate Glass
   

   Feature	Soda-lime Glass (Soft Glass)	Borosilicate Glass (Hard Glass/Pyrex)
   Composition	Mainly silica, soda ash, limestone	Contains boron oxide in addition to basic components
   Heat Resistance	Low resistance to heat and thermal shock	High resistance to heat and thermal shock
   Melting Point	Relatively low melting point	Higher melting point
   Uses	Window panes, bottles, ordinary glassware	Laboratory glassware, bakeware, pharmaceutical bottles