What makes a solute dissolve

Ethanol is actually miscible in water, which means that the two liquids can be mixed in any proportion without any limit to their solubility. Much of what we now know about the tendency of particles to become more dispersed can be used to understand this kind of change as well. Picture a layer of ethanol being carefully added to the top of some water Figure below. Because the particles of a liquid are moving constantly, some of the ethanol particles at the boundary between the two liquids will immediately move into the water, and some of the water molecules will move into the ethanol.

In this process, water-water and ethanol-ethanol attractions are broken and ethanol-water attractions are formed. The attractions that form between the ethanol and water molecules are also hydrogen bonds Figure below. Because the attractions between the particles are so similar, the freedom of movement of the ethanol molecules in the water solution is about the same as their freedom of movement in the pure ethanol.

The same can be said for the water. Because of this freedom of movement, both liquids will spread out to fill the total volume of the combined liquids. In this way, they will shift to the most probable, most dispersed state available, the state of being completely mixed.

There are many more possible arrangements for this system when the ethanol and water molecules are dispersed throughout a solution than when they are restricted to separate layers. Figure below. We can now explain why automobile radiator coolants dissolve in water. These substances mix easily with water for the same reason that ethanol mixes easily with water. The attractions broken on mixing are hydrogen bonds, and the attractions formed are also hydrogen bonds.

There is no reason why the particles of each liquid cannot move somewhat freely from one liquid to another, and so they shift toward the most probable most dispersed , mixed state. We have a different situation when we try to mix hexane, C 6 H 14 , and water. If we add hexane to water, the hexane will float on the top of the water with no apparent mixing.

The reasons why hexane and water do not mix are complex, but the following gives you a glimpse at why hexane is insoluble in water. There actually is a very slight mixing of hexane and water molecules. The natural tendency toward dispersal does lead some hexane molecules to move into the water and some water molecules to move into the hexane.

When a hexane molecule moves into the water, London forces between hexane molecules and hydrogen bonds between water molecules are broken. New attractions between hexane and water molecules do form, but because the new attractions are very different from the attractions that are broken, they introduce significant changes in the structure of the water.

It is believed that the water molecules adjust to compensate for the loss of some hydrogen bonds and the formation of the weaker hexane-water attractions by forming new hydrogen bonds and acquiring a new arrangement.

Overall, the attractions in the system after hexane and other hydrocarbon molecules move into the water are approximately equivalent in strength to the attractions in the separate substances. For this reason, little energy is absorbed or evolved when a small amount of a hydrocarbon is dissolved in water.

To explain why only very small amounts of hydrocarbons such as hexane dissolve in water, therefore, we must look at the change in the entropy of the system. It is not obvious, but when hexane molecules move into the water layer, the particles in the new arrangement created are actually less dispersed lower entropy than the separate liquids.

The natural tendency toward greater dispersal favors the separate hexane and water and keeps them from mixing. This helps explain why gasoline and water do not mix. Gasoline is a mixture of hydrocarbons, including hexane. Gasoline and water do not mix because the nonpolar hydrocarbon molecules would disrupt the water in such a way as to produce a structure that was actually lower entropy ; therefore, the mixture is less likely to exist than the separate liquids. Certain substances are soluble in all proportions with a given solvent, such as ethanol in water.

Under various conditions, the equilibrium solubility can be exceeded to give a so-called supersaturated solution, which is metastable. Solubilities range widely, from infinitely soluble such as ethanol in water, to poorly soluble, such as silver chloride in water. The term insoluble is often applied to poorly soluble compounds, though strictly speaking there are very few cases where there is absolutely no material dissolved.

The process of dissolving, called dissolution, is relatively straightforward for covalent substances such as ethanol. When ethanol dissolves in water, the ethanol molecules remain intact but form new hydrogen bonds with the water. When, however, an ionic compound such as sodium chloride NaCl dissolves in water, the sodium chloride lattice dissociates into separate ions which are solvated wrapped with a coating of water molecules. Nonetheless, NaCl is said to dissolve in water, because evaporation of the solvent returns crystalline NaCl.

Reference Terms. It is measured in terms of the maximum amount of solute dissolved in a solvent at equilibrium. The resulting solution is called a saturated solution. This property is known as miscibility. Let's explore why this happens. The phenomenon that polar solvents dissolve ionic and polar solutes, nonpolar solvents dissolve nonpolar solutes, and polar solvents don't dissolve nonpolar solutes and vice-versa is often summed up by the phrase "Like dissolves like.

As mentioned earlier, polar solvents are good at dissolving polar solutes. To explain this, we'll describe the process that occurs when table salt sodium chloride dissolves in water. As we learned in The Mole , water is a polar molecule with partial positive charge on each hydrogen atom and partial negative charge on the oxygen atom.

This polarity is shown in the following figure:. Figure In some cases, the attraction of water molecules for the polar solute isn't strong enough to pull the solute molecules apart. As a result, some polar solutes don't dissolve in water. Ionic solids like sodium chloride, by definition, contain cations and anions. As a result, when an ionic solid such as sodium chloride is placed into water, we see the following take place:. When sodium chloride is placed into water, the partial positive charges on the hydrogen atoms in water are attracted to the negatively charged chloride ions.

Likewise, the partial negative charges on the oxygen atoms in water are attracted to the positively charged sodium ions. Because the attractions of the water molecules for the sodium and chloride ions are greater than the forces holding the crystal together, the salt dissolves. When a solute dissolves in water, the process is referred to as hydration. Similarly, we find that polar solutes such as methanol, ethanol, and isopropanol are highly soluble in water because both are highly polar.

The "like dissolves like" rule indicates that polar solvents will do a poor job of dissolving nonpolar solutes. We can understand this by looking at the following figure:.