Final answer:
Benzene, C6H6, is the substance that will not dissolve in water, as it is a nonpolar molecule and water is a polar solvent.
Step-by-step explanation:
The substance that will not dissolve in water is benzene, C6H6. Water is a polar solvent and is capable of dissolving many ionic compounds and polar molecules. Salt (NaCl), ethanol (CH3CH2OH), hydrogen chloride (HCl), and acetic acid (CH3COOH) are all examples of polar substances or substances that ionize in water, thus they are soluble in water. Benzene, on the other hand, is a nonpolar molecule and does not mix well with water due to the absence of significant positive or negative charges.