What do we mean by saying that some molecular compounds dissolve in water and also dissociate? Give an example of such a compound.

Dissolve means that a molecular compound mixes uniformly with a liquid (solvent, often water in this case), forming a homogenous solution. When dissolved, the solute molecules are evenly distributed between water molecules, and the attractions between solute and solvent (such as hydrogen bonding) help maintain this dispersion.

Dissociate means that a molecular compound breaks down into its individual constituents (usually ions), when dissolved in water. In other words, the compound's molecules separate into individual ions, which are then surrounded by water molecules. This usually occurs because the ions have a higher affinity for the water molecules than for each other.

A common example of a molecular compound that both dissolves and dissociates in water is hydrogen chloride (HCl). When HCl is added to water, it first dissolves in the water and interacts with water molecules through hydrogen bonding. Then, it dissociates into hydrogen ions (H+) and chloride ions (Cl-). When HCl dissociates in water, the reaction can be represented by the following equation: \[HCl_{(aq)} + H_2O_{(l)} \rightarrow H_3O^{+}_{(aq)} + Cl^{-}_{(aq)}\] In the equation above, "aq" denotes that the compound is dissolved in water (aqueous) and "l" denotes that the substance is in its liquid state. The HCl molecule dissociates into H+ ions which then combine with water molecules to form hydronium ions (H3O+), and Cl- ions remain surrounded by water molecules.