CBSE Class 8 Science Chapter 08 Nature Of Matter Elements, Compounds, And Mixtures MCQs Set 01

Science Objective Questions and Answers: Chapter 08 Nature Of Matter Elements, Compounds, And Mixtures

Review structured MCQ sets for Class 8 Science Chapter 08 Nature Of Matter Elements, Compounds, And Mixtures. Built according to official CBSE guidelines, these downloadable questions support daily revision and core concept reinforcement.

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Question: When sugar is heated in a boiling tube, it first turns brown and then becomes blackish. What does the charcoal-like residue left behind represent?
A. a pure element
B. a mixture of carbon and water
C. carbon, which is one of the elements present in sugar
D. an entirely new compound formed during heating
Show Answer & Explanation

Answer: (C) carbon, which is one of the elements present in sugar

Explanation:
During heating, sugar decomposes and releases water vapour, leaving behind charcoal (carbon). Since sugar cannot be broken down into simpler substances and gives off carbon when heated, this shows that carbon is one of the constituent elements of sugar.

Question: In Activity 8.5, when iron filings and sulfur powder are mixed but not heated, the mixture exhibits magnetic properties. After heating this mixture, the resulting black mass does not show magnetic properties. This difference occurs because:
A. heating destroys the magnetic properties of iron permanently
B. the iron and sulfur have chemically combined to form a new compound with different properties
C. the heating process removes all iron from the mixture
D. the black mass is no longer attracted to magnets due to cooling
Show Answer & Explanation

Answer: (B) the iron and sulfur have chemically combined to form a new compound with different properties

Explanation:
Iron sulfide is a compound formed by chemical combination of iron and sulfur in a fixed ratio. Once formed, the elements lose their individual properties and cannot be separated by physical means or magnetic attraction, unlike in the original mixture where iron particles could be drawn out by a magnet.

Question: The text states that when lime water is exposed to air, it turns milky. This happens because:
A. water vapour in the air condenses on the surface of lime water
B. dust particles in the air react with lime water
C. carbon dioxide in the air reacts with calcium hydroxide to form an insoluble white precipitate
D. the lime water evaporates when exposed to open air
Show Answer & Explanation

Answer: (C) carbon dioxide in the air reacts with calcium hydroxide to form an insoluble white precipitate

Explanation:
Calcium carbonate, an insoluble compound, forms when calcium hydroxide reacts with carbon dioxide from the air. This white precipitate gives lime water a milky appearance, demonstrating the presence of carbon dioxide in the air.

Question: Which of the following best explains why a pure substance in science has a different meaning than a pure product labeled in shops?
A. Scientific purity means the product comes from a reliable manufacturer
B. In science, a pure substance consists of only one type of particle, while commercial products marked pure typically mean unadulterated but may contain multiple components
C. Shop products are always mixtures, but scientific pure substances are always compounds
D. Scientific definitions are more relaxed than commercial definitions of purity
Show Answer & Explanation

Answer: (B) In science, a pure substance consists of only one type of particle, while commercial products marked pure typically mean unadulterated but may contain multiple components

Explanation:
In everyday language, pure means free from unwanted additives, but scientifically, a pure substance must consist entirely of one type of particle with uniform properties. Even products labeled pure in shops can be mixtures of multiple substances and thus would not be pure in the scientific sense.

Question: Based on Activity 8.3, when electric current passes through water, the volume of gas collected at one terminal is approximately twice that collected at the other. This observation suggests:
A. water is composed of hydrogen and oxygen in a 2:1 ratio by volume
B. hydrogen is twice as reactive as oxygen
C. the electricity breaks water apart unevenly
D. oxygen is consumed faster during the reaction
Show Answer & Explanation

Answer: (A) water is composed of hydrogen and oxygen in a 2:1 ratio by volume

Explanation:
The electrolysis of water produces hydrogen gas at one terminal and oxygen gas at the other. Since roughly twice the volume of hydrogen is produced compared to oxygen, this indicates water molecules contain twice as many hydrogen atoms as oxygen atoms, supporting the chemical formula H₂O.

Question: Stainless steel, brass, and bronze are all described as mixtures despite appearing uniform throughout. What property distinguishes them from compounds like water or salt?
A. Their constituent metals do not chemically combine in fixed ratios
B. They are all solid at room temperature while compounds can exist in any state
C. They can be separated into their components by physical methods, whereas compound components cannot
D. They contain only metallic elements, whereas compounds must contain non-metals
Show Answer & Explanation

Answer: (C) They can be separated into their components by physical methods, whereas compound components cannot

Explanation:
• These alloys are mixtures of metals mixed uniformly but not chemically bonded in fixed ratios
• Physical methods like magnetic separation or density differences can separate alloy components
• Compounds like water cannot be separated by physical processes—only by chemical means
• The component metals retain their individual properties within the alloy structure

Question: In the chapter's discussion of minerals, gold is mentioned as a native mineral. This classification means gold is:
A. a compound found naturally in the Earth
B. a pure element that exists as a single element in nature rather than chemically combined
C. a mixture of metallic substances formed over geological time
D. a pure substance that can only be extracted through chemical reactions
Show Answer & Explanation

Answer: (B) a pure element that exists as a single element in nature rather than chemically combined

Explanation:
Native minerals are pure elements found uncombined in nature. Gold is a native mineral because it occurs naturally as the element gold itself, not as part of a chemical compound, distinguishing it from most other minerals which are compounds.

Question: When iron reacts with moist air to form rust (iron oxide), and magnesium burns in oxygen to form magnesium oxide, both reaction products are examples of:
A. elements because they contain metals
B. mixtures because they contain more than one substance
C. compounds because different elements have combined chemically in fixed ratios
D. pure substances that cannot be further broken down
Show Answer & Explanation

Answer: (C) compounds because different elements have combined chemically in fixed ratios

Explanation:
Both rust and magnesium oxide are compounds formed when elements chemically combine. They have fixed compositions and different properties from their constituent elements. They cannot be separated by physical methods, confirming their status as compounds.

Question: The chapter describes graphene aerogel as having high absorbing capacity due to being highly porous. What type of matter classification best describes graphene aerogel?
A. a pure element composed entirely of carbon atoms
B. a compound made from carbon and oxygen
C. a mixture combining carbon with other absorptive materials
D. a pure substance that is neither element nor compound
Show Answer & Explanation

Answer: (A) a pure element composed entirely of carbon atoms

Explanation:
Graphene aerogel is made exclusively from carbon atoms arranged in a special structure. Since it consists of identical particles (carbon atoms) that cannot be separated by physical processes, it qualifies as a pure element rather than a mixture or compound.

Question: In comparing Sample A (unheated iron and sulfur mixture) and Sample B (heated iron and sulfur), the magnet test shows different results. What does this difference reveal about what happened during heating?
A. The sulfur evaporated, leaving only magnetic iron behind
B. Iron and sulfur chemically combined to form iron sulfide, a new substance with different properties
C. The heat somehow reduced the amount of iron in the mixture
D. Magnetic properties were temporarily lost but would return upon cooling
Show Answer & Explanation

Answer: (B) Iron and sulfur chemically combined to form iron sulfide, a new substance with different properties

Explanation:
Sample A's iron particles are attracted to a magnet because iron retains its element properties within the mixture. In Sample B, iron has chemically bonded with sulfur to form iron sulfide, a compound with entirely different properties that no longer responds to magnetic attraction, proving a chemical change occurred.

Question: The text explains that in a mixture, components do not react chemically, whereas in a compound they do. Based on this distinction, why can salt be separated from salt water by evaporation but cannot be separated from its elements by heating?
A. Salt water is a mixture where sodium and chlorine retain separate identities, while in salt they are chemically bonded in a fixed 1:1 ratio
B. Evaporation breaks the chemical bonds in salt water but not in pure salt
C. Salt and water have different melting points, making separation possible only by heat
D. Salt dissolves when liquid, but compounds cannot dissolve
Show Answer & Explanation

Answer: (A) Salt water is a mixture where sodium and chlorine retain separate identities, while in salt they are chemically bonded in a fixed 1:1 ratio

Explanation:
In salt water, sodium chloride and water are mixed but not chemically bonded. Evaporation removes water, leaving salt crystals. However, within salt itself, sodium and chlorine are chemically combined in a fixed ratio and cannot be separated by physical heating—chemical methods would be required.

Question: When carbon dioxide is passed through lime water, a chemical reaction occurs producing calcium carbonate, which appears as a white precipitate. This demonstrates that:
A. lime water is a compound and calcium carbonate is a mixture
B. carbon dioxide is a mixture of carbon and oxygen
C. both carbon dioxide and calcium carbonate are compounds formed by chemical combinations
D. calcium carbonate exists naturally without being formed by reactions
Show Answer & Explanation

Answer: (C) both carbon dioxide and calcium carbonate are compounds formed by chemical combinations

Explanation:
Carbon dioxide (formed from carbon and oxygen elements combined chemically) reacts with lime water to produce calcium carbonate (formed from calcium, carbon, and oxygen combined in a fixed ratio). Both are compounds because their elements are chemically bonded, not just mixed.

Question: The chapter mentions that elements like silicon and boron are metalloids with intermediate properties between metals and non-metals. How does this classification affect their consideration as pure substances?
A. Metalloids are neither pure substances nor elements
B. Metalloids remain pure elements despite having intermediate properties between metals and non-metals
C. Metalloids are compounds because they have combined properties
D. Metalloids cannot be classified as substances
Show Answer & Explanation

Answer: (B) Metalloids remain pure elements despite having intermediate properties between metals and non-metals

Explanation:
Metalloids are still elements because they are pure substances made of identical atoms of a specific element. Their intermediate chemical properties do not change their fundamental classification—they consist of only one type of particle, making them pure elements like all other elements.

Question: Based on the discussion of air composition and the experiments with lime water and dust particles, air can be classified as:
A. a compound because it contains nitrogen, oxygen, and other gases
B. a uniform mixture of gases, with non-uniform suspended dust particles
C. a pure substance because it always has the same general composition
D. an element because it is essential for life
Show Answer & Explanation

Answer: (B) a uniform mixture of gases, with non-uniform suspended dust particles

Explanation:
• Air is a uniform mixture of gases like nitrogen and oxygen that are evenly distributed
• Dust particles suspended in air are separate non-uniform components
• Air components do not chemically react with each other
• While air has a relatively consistent composition, this does not make it pure—it contains multiple distinct substances mixed together

Question: When the chapter states that elements are 'building blocks of all matter,' what does this mean in terms of how matter is structured?
A. Elements are the only substances found in nature
B. All matter can ultimately be traced back to elements that cannot be broken down into simpler substances by physical or chemical means
C. Elements combine only to form compounds, never mixtures
D. Building blocks refers only to solid materials like concrete and steel
Show Answer & Explanation

Answer: (B) All matter can ultimately be traced back to elements that cannot be broken down into simpler substances by physical or chemical means

Explanation:
Elements are fundamental because they cannot be decomposed further into simpler substances through any process. All other matter—whether compounds or mixtures—is ultimately composed of elements chemically or physically arranged. This makes elements the foundation upon which all other matter is constructed.

Question: When electric current passes through water in Activity 8.3, hydrogen gas is collected at one terminal and oxygen gas at the other. What does the production of these two distinct gases reveal about water's nature?
A. Water is a mixture because two different substances are separated from it
B. Water is a compound made of hydrogen and oxygen chemically bonded in a fixed ratio
C. Water is an element that contains both hydrogen and oxygen as its components
D. Water is a pure substance that cannot undergo any chemical changes
Show Answer & Explanation

Answer: (B) Water is a compound made of hydrogen and oxygen chemically bonded in a fixed ratio

Explanation:
Activity 8.3 demonstrates that water breaks down into two different elements—hydrogen and oxygen—through an electrical process. Since water consistently yields these two elements in a predictable manner and the elements are chemically bonded together, this confirms that water is a compound composed of hydrogen and oxygen in a fixed ratio.

Question: In Activity 8.4, when sugar is heated, water droplets form and a black charcoal residue remains. Based on these observations, which statement correctly identifies what sugar contains?
A. Sugar contains only carbon and no other elements
B. Sugar is composed of carbon, hydrogen, and oxygen elements
C. Sugar is a simple element that decomposes under heat
D. Sugar is a mixture of water and carbon particles
Show Answer & Explanation

Answer: (B) Sugar is composed of carbon, hydrogen, and oxygen elements

Explanation:
The experiment shows that heating sugar produces water (hydrogen and oxygen) and charcoal (carbon). Since the decomposition of sugar yields these three distinct elements, sugar must be a compound made from carbon, hydrogen, and oxygen.

Question: When iron filings and sulfur powder are mixed without heating, they form Sample A, which remains attracted to a magnet. After heating the mixture to form Sample B, the material no longer responds to a magnet. What does this change indicate?
A. The sulfur has evaporated during heating, leaving only iron
B. Iron and sulfur have combined chemically to create a new substance with different properties
C. The heat has weakened the magnetic properties of iron temporarily
D. The mixture has separated into its components more completely
Show Answer & Explanation

Answer: (B) Iron and sulfur have combined chemically to create a new substance with different properties

Explanation:
• Sample A retains magnetic properties because iron remains as a separate element
• Sample B loses magnetism because iron and sulfur have bonded chemically
• The formation of iron sulfide (a compound) has properties completely different from its component elements
• This demonstrates that compounds formed from elements have entirely new characteristics

Question: The chapter explains that lime water turns milky when exposed to air. What substance in air causes this reaction?
A. Nitrogen gas dissolves in the solution
B. Oxygen causes calcium hydroxide to crystallize
C. Carbon dioxide reacts with calcium hydroxide to form an insoluble white substance
D. Water vapour in the air condenses and clouds the solution
Show Answer & Explanation

Answer: (C) Carbon dioxide reacts with calcium hydroxide to form an insoluble white substance

Explanation:
Lime water (calcium hydroxide solution) reacts with carbon dioxide in the air to produce calcium carbonate, which appears as white, insoluble particles, making the solution turn milky. This reaction is used as a test to confirm the presence of carbon dioxide.

Question: Common salt dissolves in water to form a uniform mixture called a solution. Salt can be recovered from this solution by evaporation. Does this mean salt is a mixture?
A. Yes, because it separates easily from water by a physical process
B. No, salt is a compound that cannot be separated into elements by evaporation
C. Yes, because salt contains sodium and chlorine mixed together
D. No, because evaporation is only used to separate mixtures, not compounds
Show Answer & Explanation

Answer: (B) No, salt is a compound that cannot be separated into elements by evaporation

Explanation:
Salt can be separated from water by evaporation, but this is a physical separation of a mixture. However, salt itself is a compound because sodium and chlorine are chemically bonded in a fixed ratio. Evaporation separates salt from water but cannot break salt into its component elements; only chemical processes can do that.

Question: According to the chapter, most rocks are mixtures of different minerals, where the individual minerals can be observed with the naked eye or magnifying glass. What distinguishes these rocks from compounds?
A. Minerals in rocks retain their individual properties; compounds have properties different from their elements
B. Rocks are harder than compounds
C. The minerals in rocks are always in equal proportions
D. Rocks can only be found in nature, whereas compounds are made artificially
Show Answer & Explanation

Answer: (A) Minerals in rocks retain their individual properties; compounds have properties different from their elements

Explanation:
Mixtures like rocks allow their components (minerals) to retain their individual properties. In contrast, compounds form when elements chemically combine and develop entirely new properties distinct from their constituent elements.

Question: The chapter mentions that oxygen supports combustion while hydrogen burns as a fuel, but water extinguishes fire. What does this observation demonstrate about compounds versus their elements?
A. Elements are always more useful than compounds in everyday applications
B. Compounds have completely different properties from the elements that form them
C. Compounds are simply mixtures of elements with similar properties
D. The properties of compounds depend only on which elements are present
Show Answer & Explanation

Answer: (B) Compounds have completely different properties from the elements that form them

Explanation:
Hydrogen (a fuel) and oxygen (combustion supporter) are very different from water (a fire extinguisher). This dramatic contrast in properties shows that when elements chemically combine to form a compound, the resulting substance has entirely new characteristics that cannot be predicted from its component elements alone.

Question: Alloys like stainless steel appear uniform throughout and their components cannot be distinguished visually. How should stainless steel be classified?
A. A non-uniform mixture because it contains multiple metals
B. A compound because the metals are chemically bonded
C. A uniform mixture where the metals remain as separate entities mixed uniformly
D. A pure element because it appears homogeneous
Show Answer & Explanation

Answer: (C) A uniform mixture where the metals remain as separate entities mixed uniformly

Explanation:
Stainless steel is described as a uniform mixture (alloy) where metals like iron, nickel, and chromium are mixed so evenly that the entire material appears uniform. However, these metals retain their individual properties and are not chemically bonded as in a compound—they remain as separate metallic components.

Question: The chapter states that atoms of most elements cannot exist independently and must combine to form molecules. What does this tell us about the structure of pure elemental substances?
A. Elements exist as single atoms floating freely in nature
B. Elements typically consist of molecules made from two or more atoms of the same element bonded together
C. Molecules can only form when different elements combine
D. Pure elements are always compounds because atoms must combine
Show Answer & Explanation

Answer: (B) Elements typically consist of molecules made from two or more atoms of the same element bonded together

Explanation:
For most elements, individual atoms bond with other atoms of the same element to form stable molecules. Oxygen forms O₂ (two oxygen atoms) and hydrogen forms H₂ (two hydrogen atoms). These molecular forms are the stable, naturally occurring state of these elements, not individual atoms.

Question: Graphene aerogel, made from carbon, is described as highly porous and extremely light. Because it is made from a single element (carbon), how should it be classified?
A. A mixture because it has many small spaces throughout
B. A compound because carbon atoms are bonded together
C. An element that has been shaped into a specific form while retaining carbon's properties
D. A pure substance that is neither element nor compound
Show Answer & Explanation

Answer: (C) An element that has been shaped into a specific form while retaining carbon's properties

Explanation:
Graphene aerogel is made entirely from carbon atoms bonded in a specific three-dimensional structure. Since it contains only one element (carbon) and carbon atoms remain bonded to each other, it is classified as an element—specifically, a form of elemental carbon with unique structural properties created by its porous arrangement.

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