Final answer:
The unknown substance with a melting point of 735 ˚C is most likely CoCl₂, as it is an ionic compound that is hard, has a high melting point, soluble in water, and does not conduct heat well as a solid.
Step-by-step explanation:
The unknown substance can be identified based on its properties such as melting point, solubility in water, heat conductivity, and hardness. The provided options include:
- (A) K - Potassium
- (B) diamond (a form of carbon, C)
- (C) CHCl₃ - Chloroform
- (D) CoCl₂ - Cobalt(II) chloride
Based on the information provided, option (D) CoCl₂ is the most likely identity of the unknown substance because it matches the melting point and other characteristics. CoCl₂ is an ionic compound, which is generally hard, has a high melting point (CoCl₂ has a melting point of 735 °C), and is soluble in water. Although ionic compounds typically conduct electricity when molten, they do not conduct heat well as solids, which is consistent with the described properties of the unknown substance.