MCQ (Solution) - Nature of Matter: Elements, Compounds, and Mixtures | Science Curiosity Class 8 - New NCERT PDF Download

Question 1:

Which description correctly defines a mixture in scientific terms?
Option A: A combination of substances where each component retains its own properties and no chemical reaction occurs
Option B: A single substance with a fixed composition that cannot be separated physically
Option C: Any item labeled “pure” in shops
Option D: A substance formed only when elements react in fixed ratios

View Answer  

Answer: Option A
Solution:

• A mixture forms when two or more pure substances are combined without chemical change, so each component keeps its identity and can often be separated by physical methods.
• Examples include air, seawater, sprout salad, sugar solution, and alloys like brass and stainless steel.
• Why others are incorrect: B describes pure substances (elements/compounds); C is marketing language and may still be a mixture (e.g., “pure” milk); D describes compounds, not mixtures.

Question 2:

Air is best classified as:
Option A: A pure compound because nitrogen and oxygen are bonded
Option B: A uniform (homogeneous) mixture of gases with variable pollutants
Option C: A non-uniform (heterogeneous) mixture because dust is visible
Option D: A single element because it is mostly nitrogen

View Answer  

Answer: Option B
Solution:

• The main gaseous components (N₂, O₂, Ar, CO₂, water vapour) are uniformly mixed, making air a homogeneous mixture; suspended dust/soot are pollutants, not integral parts.
• Composition can vary with humidity and pollution, consistent with a mixture rather than a compound.
• Why others are incorrect: A requires chemical bonding and fixed composition; C confuses transient particulates with the underlying uniform gas mixture; D confuses predominance with purity.

Question 3:

Which set contains only pure substances as defined in science?
Option A: Milk, seawater, air, soil
Option B: Fruit juice, muddy water, deodorant spray, soil
Option C: Brass, bronze, stainless steel, baking powder
Option D: Iron, oxygen, carbon dioxide, sodium chloride

View Answer  

Answer: Option D
Solution:

• Iron and oxygen are elements; carbon dioxide and sodium chloride are compounds—each has fixed composition throughout and cannot be separated by physical methods.
• Why others are incorrect: A, B, and C list mixtures (solutions, alloys, suspensions, propellant+solute systems).

Question 4:

During electrolysis of water, the gases collected at the electrodes are in the volume ratio:
Option A: 1:1 hydrogen:oxygen
Option B: 2:1 hydrogen:oxygen
Option C: 1:2 hydrogen:oxygen
Option D: Only water vapour is formed

View Answer  

Answer: Option B
Solution:

• Water (H₂O) decomposes to hydrogen and oxygen; twice as much hydrogen forms compared to oxygen by volume, reflecting the 2:1 atomic ratio.
• Hydrogen gives a ‘pop’ with a flame; oxygen supports brightening of a glowing splint/ flame.
• Why others are incorrect: A/C contradict stoichiometry; D is false—electrolysis yields H₂ and O₂, not vapour condensation.

Question 5:

Which statement best distinguishes an element from a compound?
Option A: Elements contain identical atoms; compounds contain fixed ratios of different elements chemically combined
Option B: Elements are separated by filtration; compounds by evaporation 
Option C: Elements and compounds are both mixtures
Option D: Compounds always resemble their constituent elements

View Answer  

Answer: Option A
Solution:

• An element is made of one kind of atom; compounds form when different elements combine in fixed ratios with new properties and need chemical methods for decomposition.
• Why others are incorrect: B confuses physical separations; C misclassifies; D is false (e.g., NaCl vs metallic sodium and chlorine gas).

Question 6:

Stainless steel is classified as a mixture (alloy) because:
Option A: It has a fixed chemical formula of FeCrNiC
Option B: It is a homogeneous solid solution of metals (Fe, Cr, Ni) with small carbon, not a single compound
Option C: It is 100% iron with better polishing
Option D: It is a non-metallic ceramic

View Answer  

Answer: Option B
Solution:

• Alloys are homogeneous mixtures whose composition can vary to tune properties; components are uniformly distributed without forming a single fixed-stoichiometry compound.
• Why others are incorrect: A implies fixed stoichiometry; C and D misidentify the material.

Question 7:

Which pairing correctly matches the mixture type, an example, and its uniformity?
Option A: Solid–solid mixture → brass → uniform
Option B: Liquid–liquid mixture → oil and water → uniform
Option C: Gas–liquid mixture → muddy water → uniform
Option D: Solid–gas mixture → smoke-free air → non-uniform

View Answer  

Answer: Option A
Solution:

• Brass (Cu+Zn) is a homogeneous solid solution (uniform alloy).
• Why others are incorrect: B oil and water are heterogeneous; C muddy water is solid–liquid heterogeneous; D smoke-free air is gas–gas homogeneous.

Question 8:

Which statement about “pure” on food labels vs scientific purity is correct?
Option A: “Pure” always means a single substance scientifically
Option B: In science, pure means one kind of particle only; “pure” milk or ghee can still be mixtures of many substances
Option C: Science does not classify foods
Option D: All edible products are elements

View Answer  

Answer: Option B
Solution:

• Scientific purity requires one substance with identical particles; everyday “pure” often means unadulterated but may contain multiple components (e.g., fats, proteins, water).
• Why others are incorrect: A is not generally true for consumer labels; C/D are incorrect generalizations.

Question 9:

Why is water a compound whereas an unreacted blend of hydrogen and oxygen would be a mixture?
Option A: Water can be filtered into hydrogen and oxygen easily
Option B: Water has chemically bonded elements in a fixed 2:1 ratio and new properties; an unreacted blend lacks bonding and retains component properties
Option C: Hydrogen and oxygen are solids at room temperature
Option D: Mixtures always share the same properties as water

View Answer  

Answer: Option B
Solution:

• Compounds form via chemical combination with fixed ratios and unique properties; physical mixing does not create bonds, so the blend remains gases with original properties.
• Why others are incorrect: A is false; C is wrong at room conditions; D contradicts definitions.

Question 10:

Which observation proves Sample B (from heating iron + sulfur) is a compound, not a mixture?
Option A: It is uniformly black
Option B: It is still attracted to a magnet
Option C: It is not attracted to a magnet and releases a rotten-egg smelling gas with dilute HCl
Option D: Iron and sulfur can be separated by sieving

View Answer  

Answer: Option C
Solution:

• Formation of iron sulfide (FeS) yields new properties: non-magnetic and evolves H₂S (rotten egg odour) with dilute HCl—evidence of a new chemical substance.
• Why others are incorrect: A alone is insufficient; B matches the mixture (iron magnetic), not the compound; D applies to mixtures, not compounds.

Question 11:

Which list correctly classifies items as elements, compounds, or mixtures?
Option A: Elements—water, nitrogen, iron, air
Option B: Pure substances—CO₂, iron, oxygen, sugar
Option C: Uniform mixtures—minerals, seawater, bronze, air
Option D: Non-uniform mixtures—air, sand, brass, muddy water

View Answer  

Answer: Option B
Solution:

• CO₂ (compound), iron (element), oxygen (element), sugar (compound) are pure substances in the scientific sense.
• Why others are incorrect: A wrongly lists water/air as elements; C “minerals” are mostly compounds/mixtures, not necessarily uniform; D air and brass are homogeneous, not non-uniform.

Question 12:

Which statement accurately describes minerals?
Option A: Minerals are only pure elements found in rocks
Option B: Minerals are natural solids with fixed composition; most are compounds, some are native elements
Option C: Minerals are man-made alloys
Option D: Minerals are any dissolved substances in water

View Answer  

Answer: Option B
Solution:

• Minerals like quartz, calcite, mica are compounds; native minerals like gold/sulfur are pure elements; all are naturally occurring solids with definite composition.
• Why others are incorrect: A ignores compound minerals; C/D misdefine minerals.

Question 13:

Which row shows a correct “type → example → separation possibility (physical)”? 
Option A: Mixture → seawater → can separate salt by evaporation
Option B: Compound → sodium chloride → separate Na and Cl by filtration
Option C: Element → oxygen → separate into smaller substances by decantation
Option D: Compound → water → separate into H₂ and O₂ by sieving

View Answer  

Answer: Option A
Solution:

• Mixtures can be separated by physical methods (e.g., evaporating water from seawater leaves salt).
• Why others are incorrect: Compounds need chemical methods (not filtration/decantation/sieving) to break into elements; elements cannot be broken down chemically.

Question 14:

Which scenario aligns with using matter to address environmental challenges as highlighted?
Option A: Developing a carbon-based aerogel that absorbs oil efficiently due to high porosity
Option B: Mixing random elements for instant medicines
Option C: Replacing water with H₂/O₂ gas mix for safety
Option D: Calling air a compound to regulate emissions

View Answer  

Answer: Option A
Solution:

• Graphene aerogel is an engineered material with extreme lightness and porosity, suitable for cleaning oil spills—an example of applying compounds/mixtures to environmental problems.
• Why others are incorrect: B is unscientific; C is hazardous; D misclassifies air and offers no practical control mechanism.

Question 15:

Which conclusion about the iron–sulfur system is correct?
Option A: The unheated mixture (Sample A) is non-magnetic
Option B: The heated product (Sample B) shows the same properties as iron and sulfur
Option C: Sample A is a mixture whose components retain properties; Sample B is a compound with new properties and fixed composition
Option D: Both A and B can be separated by magnets

View Answer  

Answer: Option C
Solution:

• Sample A (iron + sulfur) is heterogeneous; iron is separable by magnet and reacts with dilute HCl to give H₂; sulfur remains unreacted in HCl; Sample B (FeS) is non-magnetic and gives H₂S with dilute HCl—clear new properties and fixed composition.
• Why others are incorrect: A false—iron in A is magnetic; B false—properties change; D only A is separable by magnet, B is not.