Question

Question: Although large crystals of sugar (rock candy) and large crystals of salt (rock salt) have different geometric shapes, they look much the same to the untrained observer. What physical tests other than taste might be performed to distinguish between these two crystalline substances?

Short answer

Checking the electrical conductivity in an aqueous solution is a physical test other than taste to distinguish between these two crystalline substances.

Step-by-step solution

Testing the Rock salt.

Rock salt (NaCl) is commonly known as Common salt or Table salt.

When rock salt is dissolved in pure water, it forms two ions (polar solution).

$\text{NaCl}\to {\text{Na}}^{+}+{\text{Cl}}^{-}$

These ions present in NaCl have the freedom to move randomly in the solution.

Therefore, it conducts electricity in an aqueous solution.

Testing the Rocky candy.

The ordinary table sugar, sucrose (${\text{C}}_{\text{12}}{\text{H}}_{\text{22}}{\text{O}}_{\text{11}}$), when dissolved in water, gets dissolved in water but does not form ions. As the solution remains non-polar, therefore it does not conduct electricity.

Hence, the sugar molecules are nonpolar and neutral and do not move to the opposite ends of the electrodes.