Sugar (C12H22O11) is a polar covalent molecule. When it dissolves in water, the weak bonds between the carbon, hydrogen and oxygen atoms are broken. In the process, the bonds in the water molecules are also broken, and intermolecular bonds between the water and sugar molecules can then form. This allows up to 1800 grams of sucrose to dissolve in a mere liter of water..
Salt (NaCl) is an ionic molecule which is formed between a positive ion and a negative ion. The bond between atoms is much stronger than those in a covalent molecule. Consequently, it takes more energy to break the bond between sodium and chlorine. Salt molecules are also smaller than sugar molecules which leaves fewer opportunities for intermolecular bonds to form between salt and water.Learn more about Chemistry