Many ionic and covalent compounds dissolve well in water

Answer is: the attraction between the solute and solvent is about as strong as the attraction between the solvent particles.

For example, dissociation of potassium iodide in water:  

KI(aq) → K⁺(aq) + I⁻(aq).

Solvation is the interaction of solvent (water) with molecules or ions (in this example K⁺ and I⁻) in a solute.  

Anion I⁻ has negative charge, so positive end (hydrogen atoms) of a water molecul attracts them.

Solvent (usually a liquid, but can also be a solid or a gas) is a substance that dissolves a solute.  

the crab is multicellular

the pond organism is unicellular