8/31/23

Dear Rk,

In general, sugars have good solubility in water because they are poly-hydroxylated. That is, they have many hydroxy groups, usually 1 -OH group per carbon. The hydroxyls form hydrogen bonds with the hydroxyls of water, facilitating the solubilization of the sugar in water, often to very high concentrations. For example, sucrose (i.e., table sugar) is soluble to the extent of ~200 g/100 g of water at 20C. However, not all sugars have such good solubility in water. On the other hand, lactose (milk sugar) is soluble only to the extent of ~22 g/100 mL water. The structure of the sugar influences solubility. Similarly, in oligosaccharides, the component sugars and type of linkages of the sugars (e.g., alpha or beta) also affect solubility.

Like sugars, some salts are very soluble while others are not. Keep in mind that salts have cationic and anionic components, e.g., Na+ and Cl- ion NaCl. The nature of the 2 components influences solubility. E.g, NaCl is very water soluble while AgCl (silver chloride) is nearly insoluble in water. Similarly, Na2CO3 (sodium carbonate) has good water solubility whereas CaCO3 (calcium carbonate) does not.

The change in volume of a solution that occurs when a solute is dissolved in it is referred to as displacement. This phenomenon is not restricted to sugars. It happens regardless of whether the solute is sugar or salt. If you add enough of either of these to water, the solution volume will increase. The extent of displacement may differ depending on the solute, but it will still occur.

I hope this information helps you.

Bill Colonna Dept. Food Science & Human Nutrition, Iowa State University, Ames, Iowa (USA) [email protected]

Dr W J Colonna thank you for your contribution to the discussion

The level of water does not change when salt is dissolved in water because the salt particles dissociate and occupy the intermolecular spaces between the water particles. Since only the empty spaces are occupied, the level of water does not increase. When salt is mixed with water, the salt dissolves because the covalent bonds of water are stronger than the ionic bonds in the salt molecules. This happens because water molecules are not tightly packed and have space between them hence when we dissolve the salt in it the salt particles occupy the space between the molecules of the water and thus the water level doesn't rise up. The antiparticle space between the water particles is more as compared to salt. Salt also contains ions. Water helps in separating these ions by decreasing the intrinsic forces allowing the salt to disperse throughout the water. The salt doesn't literally disappear but they converted into ions from solid crystals in the aqueous medium of water. Also you may know that the ions are stable only in an aqueous medium. The NaCl crystal molecules are formed by the strong Coulombian force of attraction between Na+ and Cl- ions. This is because, when we add salt to water and stirred to dissolve, the particles of salt separate out and enter the empty spaces between the particles. This means, particles of matter have empty spaces between them. When we add a solid substance to a liquid, the particles solid get into the spaces between the liquid particles so, since the solids occupy the spaces inside liquid atoms, the volume of the liquid doesn't increase. Example: Sugar dissolves in water completely at different levels. This is because, when we add salt to water and stirred to dissolve, the particles of salt separate out and enter the empty spaces between the particles. This means, particles of matter have empty spaces between them. When salt is dissolved in fresh water, the density of the water increases because the mass of the water increases. Also, the size of the sugar molecule is greater than that of the salt molecule. Thus a single sugar molecule can attract more water molecules than the table salt leading to its faster dissolution in water.You should have noticed sugar had the highest solubility of all your tested compounds (about 200 grams per 100 milliliters of water) followed by Epsom salts (about 115 grams/100 milliliters) table salt (about 35 grams/100 milliliters) and baking soda (almost 10 grams/100 milliliters). Sugar dissolves in water because energy is given off when the slightly polar sucrose molecules form intermolecular bonds with the polar water molecules. The weak bonds that form between the solute and the solvent compensate for the energy needed to disrupt the structure of both the pure solute and the solvent. The reason for this is salt is an ionic compound, while sugar is a covalent compound. Ionic compounds generally have much higher melting points than covalent compounds. This is because to melt an ionic compound you have to weaken the ionic bonds between the ions.