Consider the following statements : Glass can be etched or scratched by I. diamond. II. hydrofluoric acid. III. aquaregia. IV. cone, sulphuric acid. Which of these statements are correct ?

Diamond, being the hardest natural material, will scratch typical glass surfaces (Mohs hardness: diamond 10 vs glass ≈6–7), so statement I is correct. Hydrofluoric acid chemically etches glass by reacting with silica (SiO2) to form volatile SiF4 and soluble hexafluorosilicic species, making statement II correct. Aqua regia is a powerful oxidizing mixture effective at dissolving noble metals but is not used to etch silica-based glass; concentrated sulfuric acid is a strong dehydrating acid but is far less effective for attacking glass than HF, so statements III and IV are not correct. Therefore, only I and II are correct.