The solubility of a compound in water depends on the polarity of the compound and the ability of the solvent to solvate the compound. NaF is an ionic compound, which means that it consists of positively and negatively charged ions. Water is a polar solvent, which means that it has a positive end and a negative end. The positive end of the water molecule will be attracted to the negative ions in NaF, and the negative end of the water molecule will be attracted to the positive ions in NaF. This attraction between the water molecules and the ions in NaF will cause NaF to dissolve in water.

MgO is also an ionic compound, but it is less soluble in water than NaF. This is because the ions in MgO are more strongly attracted to each other than the ions in NaF. The stronger attraction between the ions in MgO means that it is more difficult for the water molecules to solvate the compound, and therefore MgO is less soluble in water.