CBSE Class 8 Science Chapter 08 Nature Of Matter Elements, Compounds, And Mixtures MCQs Set 03

Answer: (B) Water is a compound because it breaks down into two different elements with contrasting properties

Explanation:
Activity 8.3 shows that water decomposes into hydrogen and oxygen—two substances with opposite combustion properties. Hydrogen burns with a pop sound while oxygen makes a flame burn brighter. Since water breaks down into simpler substances with a fixed ratio (2:1), it meets the definition of a compound.

Answer: (B) The heating process created a new substance with different properties, indicating compound formation in Sample B

Explanation:
Sample A remains a mixture where iron particles retain their magnetic properties. When heated, iron and sulfur chemically combine to form iron sulfide (Sample B), a compound with entirely different properties including loss of magnetism. This transformation demonstrates that heating can cause elements to combine chemically, forming compounds with new characteristics.

Answer: (B) It provides visible evidence of carbon dioxide presence, confirming air is a mixture of specific gases rather than a pure substance

Explanation:
Lime water turns milky when carbon dioxide reacts with it to form calcium carbonate (a white precipitate). This visible color change provides direct evidence that carbon dioxide exists in air. Since air contains multiple distinct gases that can be detected separately, this supports classifying air as a uniform mixture rather than a single pure substance or element.

Answer: (C) Mixtures because they contain multiple substances that retain their individual properties

Explanation:
Although milk and fruit juice may be labeled as 'pure' in stores (meaning unadulterated), a scientist would classify them as mixtures because they contain multiple components—water, proteins, sugars, minerals—mixed together. These components retain their properties and are not chemically bonded in fixed ratios like compounds are.

Answer: (B) The substances retain their individual properties in the mixture, confirming Sample A is a mixture

Explanation:
When iron reacts with the acid but sulfur does not, this shows each substance maintains its own characteristic properties within the mixture. This selective reactivity is a key indicator that Sample A is indeed a mixture—the components have not combined chemically and can be identified by their distinct behaviors.

Answer: (B) To demonstrate that air's different gaseous components retain their individual chemical properties even when mixed together

Explanation:
By highlighting that nitrogen remains non-combustible and oxygen remains combustion-supporting within air, the chapter illustrates a key characteristic of mixtures: components retain their original properties. If air were a compound, the properties of its constituents would change fundamentally, as occurs when hydrogen and oxygen combine to form water.

Answer: (B) Sample A is a mixture where iron alone reacts with acid, while Sample B is a compound with new properties formed by chemical combination

Explanation:
Sample A produces the characteristic smell-free hydrogen gas because only iron (from the mixture) reacts with the acid. Sample B produces hydrogen sulfide because the iron and sulfur are now chemically bonded in a compound—the acid breaks this compound differently, releasing a new gas with its own distinctive odor. This shows a compound has fundamentally different behavior than its component elements in a mixture.

Answer: (C) An element because it is composed only of carbon atoms arranged in a special structure

Explanation:
Graphene aerogel consists entirely of carbon atoms, making it an element despite its unique properties. The chapter illustrates that the same element can form different substances depending on how its atoms are arranged. Structure and arrangement affect properties, but the substance remains classified as an element if it contains only one type of atom.

Answer: (B) The appearance of water (hydrogen and oxygen) and charcoal (carbon) shows sugar contains these elements chemically combined

Explanation:
Heating caused sugar to decompose, releasing water vapor (containing hydrogen and oxygen from sugar) and leaving charcoal (carbon). This decomposition demonstrates that these three elements were chemically bonded within the sugar molecules. The production of different substances from heating reveals sugar's internal composition—a defining characteristic of identifying compounds.

Answer: (B) Sodium (a reactive soft metal) and chlorine (a poisonous hazardous gas) combine to form edible common salt with completely different properties

Explanation:
Sodium is dangerously reactive and chlorine is a poisonous gas, yet their compound—sodium chloride—is a harmless, essential food. This dramatic transformation of properties demonstrates the fundamental principle that compounds possess characteristics entirely distinct from their elements, unlike mixtures where properties remain unchanged.

Answer: (B) The components of stainless steel retain their individual properties and are not chemically bonded in fixed ratios like compound components must be

Explanation:
Alloys like stainless steel are uniform mixtures where metals are blended together physically rather than chemically combined. The constituent metals retain their properties and are not joined in the fixed, definite ratios that characterize compounds. If stainless steel were separated, individual metals could theoretically be recovered, unlike compounds where constituent elements cannot be separated by physical means.

Answer: (B) To explain the preparation of lime water, which is then used as a reagent to test for carbon dioxide in air

Explanation:
The chapter uses Activity 8.1's lime preparation as a prerequisite for demonstrating carbon dioxide detection. First, calcium oxide reacts with water to produce calcium hydroxide solution (lime water). This lime water is then used in the subsequent steps to detect carbon dioxide, establishing a logical sequence where one activity's product becomes another activity's starting material.

Answer: (B) Elements can have varying physical properties that do not always match their chemical classification as metals or non-metals

Explanation:
Bromine's liquid state at room temperature challenges the assumption that physical state always correlates with chemical classification. While most non-metals are gases, bromine demonstrates that elements defy simple categorizations. This exemplifies that elements exhibit diverse properties, and mercury (a liquid metal) similarly shows that being a metal does not determine physical state.

Answer: (B) Native minerals are composed of single pure elements, while other minerals are compounds made of multiple elements chemically bonded

Explanation:
Native minerals represent elements in their pure, elemental form found in nature—gold occurring as metallic gold, sulfur as pure sulfur. Other minerals like quartz or calcite are compounds of multiple elements. The distinction emphasizes that some deposits contain pure elements while most natural minerals result from elements chemically combining over time.

Answer: (B) Calcium carbonate forms tiny insoluble particles suspended in the solution, scattering light and creating a milky appearance

Explanation:
Calcium carbonate is insoluble in water, so when formed from the reaction, it exists as microscopic particles suspended throughout the liquid. These particles scatter light passing through the solution, creating the milky or turbid appearance. This physical property—the formation of a fine suspension—provides the visual evidence for detecting carbon dioxide in air.

Answer: (B) Alloys like brass and bronze have improved properties such as greater strength and durability compared to pure metals, and they produce the desired golden color

Explanation:
The chapter notes that stainless steel is 'stronger and more durable than pure iron,' illustrating a general principle: alloys often have superior properties to their constituent pure metals. Brass and bronze retain this advantage while also offering the aesthetic golden color valued in Dhokra craftsmanship, making them ideal for creating both functionally strong and visually beautiful art pieces.

Answer: (B) Most elements exist as molecules made of two or more atoms bonded together in their stable form

Explanation:
Most elemental substances like oxygen (O₂) and hydrogen (H₂) naturally exist as molecules—two or more atoms of the same element bonded together. A single oxygen atom cannot exist stably in air; instead, oxygen molecules form. This explains why Activity 8.3 collects oxygen gas as O₂ molecules rather than individual atoms, reflecting how elements actually exist in nature and in the substances we encounter.

Answer: (B) Minerals serve as raw materials from which useful substances are extracted and combined to create products with desired properties

Explanation:
Cement production exemplifies how minerals (which are natural minerals or elements from minerals) are extracted, processed, and combined in specific proportions to create a useful material with properties suited for construction. This illustrates the practical importance of understanding elements, compounds, and mixtures—knowledge that engineers and manufacturers apply to develop materials for everyday use.

Answer: (B) The gases were collected in test tubes over water, so any water vapor would have dissolved or condensed back into liquid water rather than remaining as a gas

Explanation:
Test tubes filled with water and inverted over a water-filled beaker create conditions where water vapor would immediately condense or dissolve. Since the collected gases remained as distinct gases with identifiable properties (hydrogen's pop sound, oxygen's flame brightening), they could not be water vapor. This reasoning helps identify the gases as the products of water's decomposition—hydrogen and oxygen—rather than merely water in a different form.

Answer: (B) Elements can have melting points close to room temperature, showing variability in their physical properties

Explanation:
Most solids remain solid at room temperature, making gallium and caesium unusual. Their low melting points demonstrate that elements exhibit diverse physical properties beyond simple categories. Temperature affects physical state, but an element's classification does not change—gallium and caesium remain elements regardless of their state, illustrating that the boundary between solid and liquid phases can fall near everyday temperatures for certain substances.

Answer: (B) Carbon dioxide from air must accumulate in the lime water over time and stirring helps distribute it evenly throughout the solution for the reaction to become visibly apparent

Explanation:
Carbon dioxide exists in air at relatively low concentrations (about 0.04%). For a visible change, adequate time allows enough CO₂ to diffuse and react with the lime water, and stirring ensures uniform distribution of both reactants and products throughout the solution. Without these conditions, the milky precipitate may form slowly or unevenly, making the observation unclear.

Answer: (C) Water is a compound with a definite, fixed composition reflecting chemical combination

Explanation:
A fixed ratio of atoms (2:1) is a defining characteristic of compounds. Elements combine in whole-number ratios to form compounds with specific formulas. Water's invariable 2:1 ratio demonstrates that hydrogen and oxygen are chemically bonded in a definite proportion, establishing water as a compound rather than a mixture or element. This fixed composition means every water molecule contains exactly two hydrogen atoms and one oxygen atom.

Answer: (B) To illustrate that air's components have distinct, independent properties that are retained even when mixed together

Explanation:
By contrasting oxygen's essential role in respiration and combustion with nitrogen's inert behavior, the chapter demonstrates a key characteristic of mixtures: components retain their individual properties. If air were a compound, the properties of oxygen and nitrogen would merge into something entirely new. Instead, each gas maintains its original chemical behavior within the mixture, exemplifying what makes air a mixture rather than a compound.

Answer: (B) Iron in Sample B has chemically combined with sulfur to form a new compound with different properties, including loss of the magnetic attraction characteristic of elemental iron

Explanation:
Iron's magnetism is a property of elemental iron atoms. When iron chemically combines with sulfur to form iron sulfide (Sample B), a new compound forms with entirely different properties—including loss of magnetic character. This demonstrates that compounds possess fundamentally different characteristics from their constituent elements, not merely a combination of their original properties.

Answer: (B) Native minerals demonstrate that pure elements exist naturally and can form complete materials by themselves, showing elements as the fundamental units from which all matter is constructed

Explanation:
Native minerals like pure gold deposits show that elements exist in nature as complete, self-contained substances with useful properties. Their existence illustrates that matter ultimately consists of elements—the simplest, irreducible forms of substance. Elements can stand alone as materials (like gold in jewelry), supporting the definition of elements as the building blocks from which all other matter, including compounds and mixtures, is constructed.