Classifying an Unknown Solid from Its Properties
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Classifying an unknown crystalline solid means checking, in sequence, whether it conducts as a solid (metallic), conducts only when melted or dissolved (ionic), resists melting below a very high temperature (covalent network), or melts at a comparatively low temperature without conducting in any state (molecular).
A hard, colorless solid that does not conduct electricity as a solid but conducts once dissolved in water, and that shatters rather than bends, is classified as ionic; a similarly hard solid that does not conduct in any state and resists melting to well above 1000 °C is classified as covalent network instead.
Hardness alone cannot distinguish these categories — ionic and covalent network solids are both hard, so conductivity behavior and melting point must be checked before an identification is final.