Hello! In this video, you'll learn about the different kinds of solutions and the factors that influence the solubility of one compound in another. A solution is a homogeneous mixture of two or more substances. It may be composed of a solid and a liquid, such as seawater, but it may also be composed of a gas and a liquid, two different liquids, or even other combinations.
In aqueous solutions, water is the solvent and a solid, liquid, or gas is the solute. For example... sugar water and salt water are both aqueous solutions. Similarly, ethyl alcohol, the alcohol in alcoholic beverages, readily mixes with water to form a solution, and carbon dioxide dissolves in water to form the aqueous solution that we know as carbonated water or club soda.
You probably know from experience that a particular solvent, such as water, does not dissolve all possible solutes. For example, you can't clean your greasy hands with just water because the water does not dissolve the grease. However, another solvent, such as paint thinner, can easily dissolve the grease.
The grease is insoluble in water but soluble in the paint thinner. The solubility of a substance is the amount of the substance it will dissolve in a given amount of solvent. The solubility of grease in water is nearly zero. The solubility of one substance in another depends both on nature's tendency towards mixing. and on the types of intermolecular forces that exist between the solute and the solvent.
So far, we have seen that many physical systems tend towards lower potential energy. For example, two particles with opposite charges, such as a proton and electron, or a cation and an anion, move toward each other because their potential energy decreases as their separation decreases, according to Coulomb's law. The formation of a solution, however, does not necessarily lower the potential energy of its constituent particles. The clearest example of this phenomenon is the formation of a homogeneous mixture—a solution—of two ideal gases.
Suppose that we enclose neon and argon in a container with a removable barrier between them. As soon as we remove the barrier, the neon and argon mix together to form a solution. Why?
At low pressures and moderate temperatures, both neon and argon behave as ideal gases. They do not interact with each other in any significant way. That is, there are no significant intermolecular forces between their constituent particles. When the barrier is removed, the two gases mix, but their potential energy remains unchanged.
In other words, we cannot think of the mixing of two ideal gases as lowering their potential energy. Rather, the tendency to mix is related to a concept called entropy. Entropy is a measure of energy dispersal.
or energy randomization in a system. Recall that a gas at any temperature above zero Kelvin has kinetic energy due to the motions of its atoms. When neon and argon are confined to their individual compartments, their kinetic energies are also confined to those compartments. When the barrier between the compartments is removed, each gas, along with its kinetic energy, becomes spread out or dispersed over a larger volume.
Thus, the mixture of the two gases has greater energy dispersal. or greater entropy than the separated components. The pervasive tendency for energy to spread out or disperse whenever it is not restrained from doing so is the reason that two ideal gases mix. Another common example of the tendency towards energy dispersal is the transfer of thermal energy from hot to cold.
If we heat one end of an iron rod, the thermal energy deposited at the end of the rod will spontaneously spread along the entire length of the rod. In similarity to the mixing of two ideal gases, where the kinetic energy of the particles becomes dispersed over a larger volume as the particles become dispersed, the thermal energy in the rod, initially concentrated in relatively fewer particles, becomes dispersed as it is distributed over a larger number of particles. The tendency for energy to disperse is why thermal energy flows from the hot end of the rod to the cold one, and not the other way around. Imagine a metal rod that became spontaneously hotter on one end and ice cold on the other. That doesn't happen because energy does not spontaneously concentrate itself.
We have just seen that in the absence of intermolecular forces, two substances spontaneously mix to form a homogeneous solution. However, we also know that solids and liquids exhibit a number of different kinds of intermolecular forces including dispersion forces, dipole-dipole forces, hydrogen bonding, and ion-dipole forces, all described in previous videos. These forces and the possible localized structures that result from them may promote the formation of a solution or prevent it, depending on the nature of the forces in the particular combination of solute and solvent. In a solution, intermolecular forces exist between the solvent and solute particles, the solvent particles themselves, and the solute particles themselves.
If the solvent-solute interactions are stronger than the solvent-solvent and solute-solute interactions, a solution forms. If the solvent-solute interactions are about equal to the solvent-solvent and solute-solute interactions, then a solution also forms, driven by entropy. However, solvent-solute interactions are weaker than solvent-solvent and solute-solute interactions, then a solution may or may not form. If the disparity is small, the tendency to mix results in the formation of a solution, even though the process is energetically uphill. If the disparity is large, however, a solution does not form.
In general, we can use the rule of thumb that like dissolves like when predicting the formation of solutions. Polar solvents, such as water, tend to dissolve many polar or ionic solutes. and nonpolar solvents, such as hexane, tend to dissolve many nonpolar solutes.
Similar kinds of solvents dissolve similar kinds of solutes. Alright, here's a practice question for you. Is methanol, CH3OH, likely to be more soluble in water or in a nonpolar solvent?
A for water, B for the nonpolar solvent. The correct answer is A, water. Since methanol is polar and contains hydrogen bonding, it is more likely to be soluble in water which is polar and contains hydrogen bonding.