The question asks for the classification of 'pure substances' within the broader classification of matter. We need to recall the fundamental categories into which matter is divided in chemistry.
Correct Option: B) Elements and Compounds
The question asks to identify the option where all listed substances are correctly classified as compounds. To answer this, we need to recall the definition of a compound and differentiate it from elements and mixtures.
A) Water (H₂O), Common salt (NaCl), Calcium carbonate (CaCO₃), Methane (CH₄), Carbon dioxide (CO₂) are all examples of compounds because they are formed by the chemical combination of two or more different elements in a fixed proportion.
The question asks to differentiate between a mixture and a compound based on their composition. We need to evaluate each option against the fundamental definitions of mixtures and compounds in chemistry.
Correct Option: C) A mixture can be made in any ratio of its components; a compound always has a fixed, definite ratio of its elements by mass
The question asks to identify a key characteristic that distinguishes a compound from its constituent elements. We need to understand the fundamental nature of compounds and how they differ from the elements that form them.
A) A compound has completely different properties from its constituent elements (e.g., water is a liquid while H₂ and O₂ are gases). This statement accurately describes a fundamental characteristic of compounds. The chemical combination leads to new substances with distinct physical and chemical properties.
The question asks about the properties of a mixture compared to a pure substance. To answer this, we need to understand the fundamental definition and characteristics of a mixture.
D) The properties of a mixture are the combined properties of its constituent substances, each retaining its original chemical properties. This statement accurately describes a mixture. The components are physically mixed, and they do not undergo a chemical reaction, hence they retain their individual chemical properties, and the mixture exhibits properties that are a combination of these individual properties.
This question tests the understanding of mixtures versus compounds and how their properties differ, specifically regarding magnetic attraction. Iron is magnetic, while sulfur is not. When they are mixed, they retain their individual properties. When heated, they react to form a new compound with entirely different properties.
B) Before heating: magnet attracts the iron filings (mixture — each component retains its property). After heating (compound FeS forms): magnet does NOT attract the black solid because FeS is a compound with new properties. This option correctly describes the behavior of the mixture before heating and the compound after heating, highlighting the change in properties due to chemical reaction.