Class 9 – Science Extra Questions – Ch. 2 – Is Matter Around Us Pure

Ans: D

Explanation: A substance with a uniform composition and properties throughout is called a homogeneous mixture. A heterogeneous mixture has a non-uniform composition. Elements are pure substances made of only one type of atom. Compounds are pure substances made of two or more different elements chemically bonded together.
Correct Option: D

Ans: D

Explanation: Sieving is a physical separation method that uses a sieve with a specific mesh size to separate particles of different sizes. In this case, a sieve with a mesh size smaller than the stones but larger than the rice grains would allow the rice to pass through while retaining the stones. Filtration is used to separate insoluble solids from liquids. Decantation is used to separate a liquid from a solid that has settled at the bottom of a container. Evaporation is used to separate a soluble solid from a liquid by heating the mixture to vaporize the liquid.
Correct Option: D

Ans: C

Explanation: Alum (potassium aluminum sulfate) dissolves completely in water to form a clear solution. Common salt (sodium chloride) and sucrose (sugar) are also highly soluble in water and form clear solutions. Albumin, a protein found in egg whites and blood, is not readily soluble in water. When shaken with water, albumin will form a cloudy or opaque suspension rather than a clear solution.
Correct Option: C

Ans: A

Explanation: When two substances combine to form a new substance with different properties and energy is released (exothermic reaction), it indicates a chemical change. This chemical change results in the formation of a compound. Elements are pure substances that cannot be broken down into simpler substances by chemical means. A mixture is a physical combination of substances that retain their individual properties and can be separated by physical means. Since a new substance C is formed with distinct properties, it is a compound.
Correct Option: A

Ans: A

Explanation: Diffusion is the net movement of anything generally from a region of higher concentration to a region of lower concentration. In this case, the ink molecules are highly concentrated in the drop and less concentrated in the surrounding water. Over time, the ink molecules will spread out from the area of high concentration to the area of low concentration until they are evenly distributed throughout the glass of water. Osmosis specifically refers to the movement of water across a semipermeable membrane. Evaporation is the process of a substance in a liquid state changing to a gaseous state due to an increase in temperature and/or pressure, and condensation is the reverse process. Neither of these accurately describes the spreading of ink in water.
Correct Option: A

Ans: B

Explanation: A chemical change involves the formation of a new substance with different properties. Sublimation of dry ice is a physical change where solid carbon dioxide directly turns into gaseous carbon dioxide without forming a new substance. Dissolving sugar in water is also a physical change; the sugar molecules are dispersed in water, but they remain sugar molecules. Burning of wood is a chemical change where wood reacts with oxygen to produce ash, smoke (containing carbon dioxide and water vapor), and heat. Tarnishing of silver is a chemical change where silver reacts with sulfur compounds in the air to form silver sulfide, a dark layer with different properties. Therefore, burning of wood and tarnishing of silver involve chemical changes.
Correct Option: B

Ans: B

Explanation: A pure solid material will have a sharp and distinct melting point. Impurities tend to lower the melting point and broaden the melting range, meaning the solid will melt over a range of temperatures rather than at a single point. While solubility can be a characteristic property, it’s less definitive for determining purity compared to melting point. Boiling point is a property of liquids, not solids, and solubility in water or alcohol can vary for both pure and impure substances without a clear indication of purity.
Correct Option: B

Ans: A

Explanation: The burning of a candle involves both physical and chemical transformations. The melting of the wax is a physical change, as it transitions from solid to liquid. However, the combustion of the wax (the actual burning) is a chemical change, where the wax reacts with oxygen to produce new substances like carbon dioxide and water vapor. Rusting of iron is purely a chemical change. Cooking of food involves complex chemical changes and some physical changes like boiling and evaporation. Boiling of water is a physical change as water changes from liquid to gas (steam) and back to liquid without changing its chemical composition.
Correct Option: A

Ans: C

Explanation: The chemical equation \(2X + Y \rightarrow X_2Y\) shows that element X and element Y react to form a new substance \(X_2Y\). The fixed ratio of 2 atoms of X to 1 atom of Y (implied by the chemical formula \(X_2Y\)) indicates that a chemical bond is formed between X and Y, creating a new compound. Compounds have properties that are distinct from the properties of their constituent elements. Option A is incorrect because properties of compounds are not a simple average. Option B is incorrect because the formation of \(X_2Y\) implies a chemical reaction and a fixed composition, not a variable mixture. Option D is incorrect because the properties of the compound \(X_2Y\) will be different from element X. Therefore, \(X_2Y\) is a compound with unique properties.
Correct Option: C

Ans: D

Explanation: The question draws an analogy between removing microscopic plastic particles from water and removing small metal fragments from engine oil. This suggests a method that separates particles based on density or size through centrifugal force. Distillation and evaporation involve phase changes and are not suitable for separating solid particles. Chromatography separates based on different affinities for a stationary and mobile phase, which is generally used for dissolved substances, not suspended particles. Centrifugation uses centrifugal force to separate components of different densities and sizes, effectively settling heavier particles (like metal fragments or larger plastic particles) at the bottom. While microscopic plastic particles can be very small, centrifugation is a standard technique for separating particulate matter from liquids, and smaller particles can be separated with high-speed centrifuges or by choosing appropriate centrifugation times and speeds.
Correct Option: D