HOTS questions - Synthetic fibres and plastics - Class 8 Science

Q2) Sania observed that a thin strip of magnesium when exposed to air catches fire and burns with a bright white flame. Can you explain why this happens?

Solution: Magnesium is a highly reactive metal that reacts with oxygen in the air to form magnesium oxide, which releases a lot of heat and light, leading to the bright white flame.

Q3) Suresh noticed that a piece of copper wire has a reddish-brown coating on its surface. Can you explain the reason behind this coating?

Solution: The reddish-brown coating on the surface of the copper wire is due to the formation of copper oxide when the copper reacts with oxygen in the atmosphere.

Q4) Shreya observed that the metal frame of her bicycle has started rusting. Can you explain how rusting of iron takes place?

Solution: Rusting of iron takes place when iron reacts with oxygen and moisture in the air to form hydrated iron(III) oxide or rust.

Q5) Radha noticed that the aluminium utensils used in her house have white patches on their surface. Can you explain the reason behind these patches?

Solution: The white patches on the surface of aluminium utensils are due to the formation of a thin layer of aluminium oxide, which forms when the aluminium reacts with alkaline substances present in the food or water.

Q6) Harish observed that a piece of lead can be easily cut with a knife. Can you explain why lead is a soft metal?

Solution: Lead is a soft metal because its atoms are held together by weak metallic bonds, which make it easy to deform and shape.

Q7) Ramesh observed that the iron gates in his house have been painted with a layer of red-brown coloured paint. Can you explain why iron is painted?

Solution: Iron is painted to protect it from rusting. The paint acts as a barrier between the iron and the atmosphere, preventing it from coming in contact with moisture and oxygen.

Q8) Priya observed that the gold ornaments she wears have a bright yellow colour. Can you explain why gold has a characteristic yellow colour?

Solution: Gold has a characteristic yellow colour due to its unique electronic structure, which absorbs certain colours and reflects yellow light.

Q9) Ravi noticed that the metal chain of his cycle has become weak and brittle over time. Can you explain the reason behind this phenomenon?

Solution: The metal chain of Ravi's cycle has become weak and brittle due to the formation of rust, which makes the metal porous and brittle.

Q10) Meena noticed that the statue of liberty, made of copper, has a green coating on its surface. Can you explain the reason behind this coating?

Solution: The green coating on the surface of the statue of liberty is due to the formation of a layer of copper carbonate, which forms when copper reacts with carbon dioxide and water in the atmosphere.

Q11) Ramesh’s father uses aluminum foil to wrap food items before putting them in the fridge. Can you explain the reason behind this?

Solution: Aluminum foil is a good conductor of heat and electricity. Wrapping food items in aluminum foil helps to keep them fresh by preventing contact with air and moisture.

Q12) Sita observed that her silver jewelry has tarnished over time. Can you explain why this happens?

Solution: Silver reacts with sulfur present in the air to form a black layer of silver sulfide on its surface. This layer makes the jewelry look tarnished and dull.

Q13) Radha’s mother uses copper vessels to store drinking water. Can you explain the reason behind this?

Solution: Copper has antibacterial properties that help to kill harmful bacteria and viruses in water, making it safe for consumption.

Q14) Ajay noticed that his bicycle chain has turned black over time. Can you explain the reason behind this?

Solution: The bicycle chain is made of iron which reacts with oxygen and moisture in the air to form a layer of iron oxide, making it look black and dull.

Q15) Meera observed that a white powder is formed on the surface of a zinc plate when it is left in the open for a few days. Can you explain the reason behind this?

Solution: Zinc reacts with oxygen and moisture in the air to form a white layer of zinc oxide on its surface. This layer makes the plate look white and powdery.

Q16) Jatin noticed that the brass door handle in his house had turned green. Can you explain the reason behind this phenomenon?

Solution: The brass may have reacted with moisture and atmospheric oxygen to form a layer of copper carbonate (verdigris), which gives it a green color.

Q17) Sana found that the aluminum foil she used to wrap food items had dark spots on it. Can you explain the reason behind this?

Solution: The aluminum foil may have reacted with acidic or salty food items, causing dark spots or discoloration due to a process called oxidation.

Q18) Shreya’s school chemistry lab has a supply of hydrochloric acid (HCl) in a glass bottle. Can you suggest a reason why the bottle is made of glass?

Solution: Hydrochloric acid is a highly corrosive substance that can react with many materials. Glass is used as a container for hydrochloric acid as it is resistant to the acid's corrosive properties.

Q19) Rahul noticed that the silver ring he wore often tarnished over time. Can you explain the reason behind this?

Solution: The silver ring may have reacted with sulfur in the air to form a layer of silver sulfide, which makes the ring look dull and tarnished.

Q20) Riya saw that the iron nail she left in water for a few days had a layer of brown rust on it. Can you explain the reason behind this?

Solution: Iron reacts with water and oxygen in the air to form iron oxide, which is a reddish-brown substance commonly known as rust.

Q21) Varun’s father was melting an iron rod in a furnace. Varun saw that the iron rod turned red hot before melting. Can you explain why the iron rod turned red hot?

Solution: When heated, iron absorbs thermal energy and its temperature increases. As the temperature increases, the iron rod emits red light, which makes it appear red hot.

Q22) Tina observed that the surface of the zinc sheet she had kept in the open turned dull over time. Can you explain the reason behind this phenomenon?

Solution: Zinc reacts with oxygen in the air to form a layer of zinc oxide on its surface, which appears dull and white.

Q23) Nishant noticed that the copper wire he used for conducting electricity was thicker than the aluminum wire of the same length. Can you suggest a reason for this?

Solution: Copper has lower electrical resistance than aluminum, which means that copper wire can conduct electricity more efficiently. Hence, a thicker copper wire is required to conduct the same amount of electricity as an aluminum wire of the same length.

Q24) Rohit’s mother gave him a gold ring as a gift. Can you explain why gold is considered a precious metal?

Solution: Gold is a precious metal as it is rare, highly valued, and has a distinctive appearance. It is also ductile, malleable, and does not corrode, making it useful for jewelry and currency.

Q25) Priya’s chemistry teacher warned her to handle magnesium ribbon with care. Can you explain the reason behind the caution?

Solution: Magnesium ribbon is highly reactive and can react with oxygen and water to produce heat, light, and hydrogen gas. This reaction can be dangerous if the ribbon is not handled properly, as it can cause burns or fires.

Q26) Reema observed that the copper vessels used in her house have become dull and dark after a few months of use. Can you suggest a reason for this?

Solution: The copper vessels may have reacted with atmospheric oxygen and formed a layer of copper oxide, which has made them look dull and dark.

Q27) Anu noticed that the iron gate in her house is covered with rust. Can you explain how the rust is formed on iron?

Solution: Rust is formed on iron when it reacts with oxygen and water in the atmosphere. The iron oxide formed is flaky and porous, making the metal weak and brittle.

Q28) Rahul bought an iron rod from the hardware store. He noticed that the rod was coated with a thin layer of zinc. Can you explain why zinc is used to coat iron?

Solution: Zinc is used to coat iron as it prevents the iron from reacting with oxygen and water in the atmosphere and forming rust. This process is called galvanization.

Q29) Sunita observed that a silver spoon left in the air turns black over time. Can you explain the reason behind this phenomenon?

Solution: The silver spoon reacts with hydrogen sulfide present in the air to form a black layer of silver sulfide on its surface. This layer makes the spoon look black and dull.

Q30) Sonu’s father works in a chemical factory. He always advises his father to wear gloves and safety goggles while working. Can you explain the reason behind his father's safety precautions?

Solution: The chemicals used in the factory may be corrosive and harmful, which can cause skin and eye damage. Wearing gloves and safety goggles can protect against these harmful effects.