Why do solutions form

In that case, the salt would be the solvent and the water the solute. But this type of mixture would be useless so why bother to make it???

When we do place solutes and solvents together, there is what we call the solution process. You can think of it as being similar to what you would experience if you tried to squeeze into an already packed elevator. Everyone has to adjust to "find their space" again. Now just like in the elevator, molecules will adjust differently dependent on the type of molecule making an entrance.

And also like in an elevator there will come a point when no more people can be added. For a solution, this point is called the saturation point and the solution itself is called a saturated solution. At the point of saturation, no more solute will dissolve in the solvent. Rather the process of dissolving and precipitation are both occurring simultaneously and at the same rate. Generally speaking only certain molecules will dissolve in water to begin with.

The old phrase "like dissolves like" or "birds of a feather flock together" is very true with respect to what degree solutes are soluble or miscible in different solvents.

At very low concentrations, almost all molecules are somewhat soluble in all solvents. But by trend, ionic and polar solutes are more soluble in polar solvents and non-polar molecules are soluble in non-polar mostly organic solvents. The units of concentration we just discussed are used to describe the degree to which a solute is soluble in a solvent.

When you place a non-polar molecule in a polar solvent like oil in water the molecules try to minimize surface contact between them. This is actually the basis for the cells in our bodies. The lipids oily fatty acids form our cell membranes so that their non-polar tails face inward away from the polar cytoplasm and the polar heads face towards the polar cytoplasm.

Although much of the explanation for why certain substances mix and form solutions and why others do not is beyond the scope of this class, we can get a glimpse at why solutions form by taking a look at the process by which ethanol, C 2 H 5 OH, dissolves in water.

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. The substances solids, liquids, or gasses in a solution make up two phases, the solvent and the solute. The solvent is the substance which typically determines the physical state of the solution solid, liquid or gas. The solute is the substance which is dissolved by the solvent.

For example, in a solution of salt and water , water is the solvent and salt is the solute. Solutions are formed because the molecules of the solute are attracted to the molecules of the solvent. When the attractive forces of the solvent are greater than the molecular forces holding the solute together, the solute dissolves. There are no rules which will determine whether substances will dissolve however, the cardinal rule of solubility is "like dissolves like.

The substances which make up a solution can be either solids, liquids, gasses, or a combination of any of these. Brass is a solution of solid copper and zinc. Gasoline is a complex solution of liquids. Air is a solution of gasses. Soda pop is a solution of solid sugar, liquid water and carbon dioxide gas. The properties of solutions are best understood by studying solutions with liquid solvents. When water is the solvent, the solutions are called aqueous solutions.

The entropic contribution, though favorable, is usually too small to overcome the unfavorable enthalpy term. Hence we expect that a solution will not form readily. In contrast to liquid solutions, the intermolecular interactions in gases are weak they are considered to be nonexistent in ideal gases. Consequently, all gases dissolve readily in one another in all proportions to form solutions. In contrast, naphthalene is a nonpolar compound, with only London dispersion forces holding the molecules together in the solid state.

Hence we do not expect naphthalene to be very soluble in water, if at all. Benzoic acid has a polar carboxylic acid group and a nonpolar aromatic ring.

The strength of the interaction of benzoic acid with water should also be intermediate between those of LiCl and naphthalene. Solutions are homogeneous mixtures of two or more substances whose components are uniformly distributed on a microscopic scale. The component present in the greatest amount is the solvent, and the components present in lesser amounts are the solute s. Substances that are miscible, such as gases, form a single phase in all proportions when mixed. Substances that form separate phases are immiscible.

Solvation is the process in which solute particles are surrounded by solvent molecules. When the solvent is water, the process is called hydration. In addition, the change in entropy, the degree of disorder of the system, must be considered when predicting whether a solution will form. An increase in entropy a decrease in order favors dissolution. Learning Objectives To understand how enthalpy and entropy changes affect solution formation.

To use the magnitude of the changes in both enthalpy and entropy to predict whether a given solute—solvent combination will spontaneously form a solution. Forming a Solution The formation of a solution from a solute and a solvent is a physical process, not a chemical one.

The Role of Enthalpy in Solution Formation Energy is required to overcome the intermolecular interactions in a solute, which can be supplied only by the new interactions that occur in the solution, when each solute particle is surrounded by particles of the solvent in a process called solvation or hydration when the solvent is water.