In materials science and industrial applications, silicon carbide is commonly used for which of the following purposes?

Introduction / Context:
Silicon carbide is an important industrial material known for its exceptional hardness and high thermal stability. It is widely used in engineering, manufacturing, and various abrasive applications. This question asks you to identify the correct practical use of silicon carbide from several unrelated options. Recognising such uses helps link classroom chemistry to real industrial technologies and everyday tools.

Given Data / Assumptions:
- The substance in question is silicon carbide. - Multiple uses are listed, including disinfection, casting, construction materials, and cutting hard substances. - We assume standard industrial applications of silicon carbide.

Concept / Approach:
Silicon carbide is an extremely hard compound, with hardness close to that of diamond. Because of this property, it is commonly used as an abrasive material in cutting tools, grinding wheels, sandpapers, and other devices for machining metals, stones, and ceramics. It can withstand high temperatures and mechanical stress. It is not used for disinfection of water, making statue casts, or as a primary ingredient for cement and glass. Therefore, we should select the option related to cutting and grinding very hard materials.

Step-by-Step Solution:
Step 1: Recall that silicon carbide is known for its high hardness and abrasion resistance. Step 2: Link this property to common applications such as cutting, grinding, and polishing hard surfaces. Step 3: Evaluate Option A, which mentions disinfecting pond water, a process usually involving chlorine or other disinfectants, not silicon carbide. Step 4: Evaluate Option B, making casts for statues, which typically uses materials like plaster of Paris rather than silicon carbide. Step 5: Evaluate Option C, making cement and glass, which rely mainly on limestone, sand, and other silicate materials. Step 6: Recognise that Option D, cutting and grinding very hard substances, matches the known abrasive use of silicon carbide.

Verification / Alternative check:
Reference books on materials science and engineering list silicon carbide as a key abrasive material. It is used in grinding wheels, abrasive papers, and cutting tools for metals and stone. Silicon carbide is also found in some high temperature ceramic components, furnace linings, and electronic devices, further highlighting its robust mechanical and thermal properties. None of these uses involve disinfection of water or casting statues, confirming that the use related to cutting and grinding hard substances is correct.

Why Other Options Are Wrong:
Option A (Disinfecting the water of ponds) is incorrect because water disinfection usually relies on chemical agents such as chlorine, ozone, or ultraviolet light, not on a hard abrasive solid. Option B (Making casts for statues) is wrong because artists and sculptors generally use plaster of Paris or similar moulding materials instead of silicon carbide. Option C (Making cement and glass) is incorrect because these materials are produced primarily from limestone, clay, and sand, not from silicon carbide.

Common Pitfalls:
Students sometimes guess an answer by associating a substance with any random industrial use without considering its key physical properties. Another pitfall is ignoring the hardness of silicon carbide and choosing options that have nothing to do with mechanical abrasion. Always try to connect the important property of a material, such as hardness or reactivity, with a realistic and logical application.

Final Answer:
The correct answer is: Cutting and grinding very hard substances.