HOTS Questions - Class 8 Science - Materials: Metals and non-metals

HOTS Questions - Class 8 Science - Chapter 4 - Metals and non-metals

Aakash noticed that the surface of an aluminium foil became blackened and dull after being used to wrap food items for a few days. Can you explain the reason behind this phenomenon?

Solution: The aluminium foil reacts with acids and salts present in the food items to form a thin layer of aluminium oxide, which makes the surface of the foil look black and dull.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.

HOTS Comprehension - Synthetic fibres and plastics - Passage 1

Q1) Why do metals generally have high melting and boiling points?
Solution: Metals generally have high melting and boiling points because of the strong metallic bonds between the metal atoms. These bonds require a lot of energy to break, leading to high melting and boiling points.

Solution: Non-metals generally do not have free electrons in their outermost shells, which are required for electrical conduction. Therefore, they are generally poor conductors of electricity.

Q3) Why is hydrogen not considered a metal, despite being placed in group 1 of the periodic table?
Solution: Although hydrogen is placed in group 1 of the periodic table, it is not considered a metal because it does not exhibit the typical metallic properties such as ductility, malleability, and high electrical conductivity.

Q4) Why do metals react with acids to form salts and hydrogen gas?
Solution:Metals react with acids to form salts and hydrogen gas because the metal atoms lose electrons and are oxidized, while the hydrogen ions from the acid gain electrons and are reduced. The resulting products are salts and hydrogen gas.

Q5) Why do non-metals generally form covalent bonds?
Solution:Non-metals generally form covalent bonds because they tend to gain electrons and form negative ions, which can attract other non-metal atoms to share electrons in covalent bonds. Covalent bonds involve the sharing of electrons, allowing non-metals to achieve a stable electron configuration.

Q6) Why is aluminium used in the construction of airplanes and spacecraft?
Solution: Aluminium is used in the construction of airplanes and spacecraft because it is lightweight, strong, and resistant to corrosion. This makes it an ideal material for building structures that need to be both strong and light.

Q7) Why do metals react with oxygen to form metal oxides?
Solution:Metals react with oxygen to form metal oxides because they tend to lose electrons and form positive ions, which can combine with oxygen to form metal oxides. The resulting metal oxides are generally stable and do not react further with oxygen.

Q8) Why is sodium chloride a compound and not a mixture?
Solution:Sodium chloride is a compound because it is made up of two different elements (sodium and chlorine) that are chemically bonded together in a fixed proportion. A mixture, on the other hand, is made up of two or more different substances that are not chemically bonded together.

Q9) Why do non-metals generally have low melting and boiling points?
Solution:Non-metals generally have low melting and boiling points because they do not have strong metallic bonds that require a lot of energy to break. Instead, they have weak intermolecular forces that are easily overcome at low temperatures.

Q10) Why is mercury used in thermometers?
Solution:Mercury is used in thermometers because it has a low freezing point and a high boiling point, making it an ideal liquid for measuring temperature. It also has a uniform expansion rate, making it an accurate indicator of temperature changes.

Q11) Why is copper used for making electrical wires?
Solution: Copper is a good conductor of electricity. It has low electrical resistance and high tensile strength, making it an ideal material for making electrical wires.

Q12) Why is carbon used as a reducing agent in the extraction of metals from their oxides?
Solution: Carbon has a strong affinity for oxygen and can remove it from metal oxides, forming carbon dioxide. This process is called reduction. Therefore, carbon is used as a reducing agent in the extraction of metals from their oxides.

Q13) Why do non-metals generally form negative ions?
Solution:Non-metals have high electronegativity, which means they have a strong tendency to attract electrons towards themselves. When they react with metals, they gain electrons and form negative ions.

Q14) Why is gold used for making jewellery?
Solution:Gold is a noble metal and does not corrode easily, making it an ideal material for making jewellery. It is also very malleable and ductile, which means it can be easily shaped into various designs.

Q15) Why does magnesium react with water but not with alcohol?
Solution:Magnesium is a very reactive metal and reacts with water to form magnesium hydroxide and hydrogen gas. However, it does not react with alcohol because alcohol does not contain any hydrogen ions that can react with magnesium.

Q16) Why do metals generally have high densities?
Solution: Metals generally have high densities because their atoms are closely packed together, resulting in a high mass per unit volume.

Q16) Why are non-metals generally brittle in nature?
Solution:Non-metals are generally brittle in nature because they tend to form covalent bonds, which are strong in one direction but weak in other directions. This leads to a lack of structural stability, making them brittle and easily breakable.

Q17) Why are metals good conductors of heat?
Solution:Metals are good conductors of heat because of the free electrons present in their outermost shells, which can transfer heat energy by colliding with each other and with the metal ions.

Q18) Why do non-metals not react with acids?
Solution:

Q19) Why do metals have a tendency to form positive ions?
Solution:Metals have a tendency to form positive ions because they tend to lose electrons from their outermost shells to achieve a stable electron configuration. This results in the formation of positive ions with a lower number of electrons than protons.

Q20) Why are non-metals generally poor conductors of electricity?
Solution: Non-metals are generally poor conductors of electricity because they tend to have tightly bound electrons that are not free to move, limiting their ability to conduct electricity.>

Q21) Why do metals tend to have high boiling points?
Solution:Metals tend to have high boiling points because the metallic bonds between the metal atoms are strong and require a lot of energy to break.

Q22) Why do non-metals generally have low densities?
Solution: Non-metals generally have low densities because their atoms have few electrons in their outermost shells, resulting in a low mass per unit volume.

Q23) Why do metals generally have a shiny appearance?
Solution: Metals generally have a shiny appearance because of the free electrons present in their outermost shells, which can absorb and reflect light energy.

Q24) Why do non-metals generally have a dull appearance?
Solution:Non-metals generally have a dull appearance because they tend to form covalent bonds, which do not allow the free movement of electrons required for absorption and reflection of light energy.

PDF Sample papers - Metals and non-metals - Class 8 Science

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Revision notes - CBSE Class 8 Science Chapter 4 - Metals and non-metals

Introduction: Metals and non-metals are two main categories of elements found in nature. They have different physical and chemical properties and are used in various applications. This chapter covers the physical and chemical properties of metals and non-metals, their uses, and their significance in our daily life.

Physical Properties of Metals: Metals are generally hard, strong, and malleable. They are good conductors of heat and electricity. They have a high melting and boiling point, and are shiny in appearance. Metals are ductile, which means they can be drawn into thin wires. Examples of metals include iron, copper, silver, and gold.

Physical Properties of Non-metals: Non-metals are generally dull and brittle in nature. They are poor conductors of heat and electricity. Non-metals have a low melting and boiling point, and are not malleable or ductile. Examples of non-metals include carbon, sulfur, and oxygen.

Chemical Properties of Metals: Metals react with acids to produce hydrogen gas. They also react with oxygen to form metal oxides. Metal oxides react with water to form metal hydroxides, which are bases. Some metals like iron and copper can displace other metals from their salt solutions.

Chemical Properties of Non-metals: Non-metals do not react with acids. They react with oxygen to form non-metal oxides. Non-metal oxides react with water to form acidic solutions. Some non-metals like chlorine and fluorine are highly reactive and can displace other non-metals from their compounds.

Uses of Metals: Metals are used in various applications, such as construction, transportation, and electrical appliances. Iron is used to make bridges, buildings, and automobiles. Copper is used in electrical wires and plumbing. Gold and silver are used in jewelry and currency.

Uses of Non-metals: Non-metals are also used in various applications, such as fertilizers, medicines, and plastics. Carbon is used in the production of steel and as a fuel. Sulfur is used in the production of fertilizers and rubber. Oxygen is used in respiration and combustion.

Other Facts: Some elements have properties of both metals and non-metals and are called metalloids. Silicon and germanium are examples of metalloids. The reactivity series is a list of metals arranged in order of their decreasing chemical reactivity. The most reactive metal is at the top of the series, and the least reactive metal is at the bottom.

In conclusion, understanding the properties and uses of metals and non-metals is important as they play a significant role in our daily lives. By learning about their chemical and physical properties, we can appreciate their unique qualities and their impact on our world.