Answer:
The unknown substance is CoF2
Explanation:
Diamond is insoluble in water. It does not conduct electricity. Every atom in a diamond is bonded to its neighbours by four strong covalent bonds, leaving no free electrons and no ions .
Most diamonds are extremely efficient thermal conductors, but electrical insulators. Diamond conducts heat well as a result of the strong covalent bonds between carbon atoms in a diamond crystal.
The melting point of diamond is about 4027 °C.
Since the melting point of diamond is way higher, it's not soluble in water and diamond does conduct heat. The unknown substance is not diamond.
SF6 is slightly soluble in water.
SF6 is not hard but it's a gas.
The melting point of SF6 is about -64 °C
Since SF6 is a gas and not a solid and has a melting point way lower than the unknown substance; the unknown substance is not SF6
Aluminum is an excellent heat and electricity conducto
The melting point of Al is 660 °C
Since Al has a lower melting point and is a good heat conductor; the unknown substance is not aluminium.
CoF2 is sparingly soluble in water.
CoF2 has a melting point between 1100 °C and 1200 °C
CoF2 is a solid but a bad heat conductor
⇒ The unknown substance is CoF2
