Why Does Salt Increase the Density of Water More Than Sugar?

Difference in solubility of salt and sugar in water

Density of a substance is defined as its mass per unit volume. Solubility, the number of grams of solute that can saturate 100g of a solvent, of one substance in another affects the density of the resulting mixture. The substance which is present in less quantity in a solution is called a solute, and the substance present in greater quantity is called a solvent.

Solubility of Salt and Sugar in Water

Since salt contains positive and negative ions and water also has positive H+ groups and negative -OH groups, the Na+ ions of salt attract negative -OH ions of water and likewise, the Cl- ions of salt attract the positive H+ ions of water.

In case of sugar, the hydroxyl groups of carbohydrates form strong hydrogen bonding with water and let sugar dissolve in water completely.

Considerations

Density of all mixtures is greatly affected by temperature and pressure. Lower temperature and higher pressure would mean greater density.

Comparison of densities

Sugar dissociates in water completely whereas salt is not entirely soluble in water. Hence, when adding salt to water, the ions form a greater mass, while the volume of the solution increases by little. Since density is mass over volume, this means for a given volume of a salt solution, the mass of ions in the solution is greater. Thus, salt forms a higher density in water than sugar.

References

• Ask the Van -- Illinois Dept. of Physics: Salt's Effect on Water's Density
• Bodner Research Web: Why Do Some Solids Dissolve in Water?
• Chemistry Explained: Solution Chemistry