Solutions in The Our Worlds

Solutions are all around us. They are the beverages we drink to the chemicals we mix in a common feed lab. The technical definition of a solution is a homogenous mixture of two or more substances. A solution can be a solid dissolved in a liquid, but it can also be a solution in cages and solids. Air is a solution of different gases including oxygen and nitrogen. Metallic items are also solutions. The rings and bracelets that many people commonly wear are homogenous mixtures of two or more kinds of metals commonly referred to as an alloy. The most common form of a solution is a substance dissolved in water. Solutions have two parts: the solute and the solvent. The solvent is the substance that is being dissolved into. The solvent is the compound that is present in the largest quantity. The solute is the second part. The solute is being dissolved into the solvent. It is present in lesser quantities than the solvent. A solution where the solvent is water is referred to as an aqueous solution. Many important chemical reactions usually take place in aqueous solutions.

Liquid solutions are clear and transparent. That is not to say that they do not have color. They may be colorless or colored depending on the characteristics of the solute and the solvent. A true solution is a homogenous mixture with uniform properties throughout. In a true solution the solute cannot be isolated form the solution by filtration. The solute particles will also not settle out of the solution over time. If the particles are not homogenous or will settle out over time, the solution is referred to as a colloidal dispersion. In a colloidal dispersion the solute particles are distributed throughout a solvent. However, the solute particle size is much larger than the solvent particle size therefore they form a precipitate or an insoluble substance formed and separated from the solution. A precipitate will be visible to the naked eye.

To the naked eye a colloidal solution and a true solution will appear identical. The solute and the colloid cannot be seen. How can an experimenter tell the difference? Light. In a colloidal solution, the colloid particles are large enough to scatter light. This causes the solution to appear hazy. Solute particles in the true solution will not be large enough to scatter light. The ability of colloidal dispersions to scatter light is called the Tyndall effect. A suspension is a heterogeneous mixture that contains particles much larger than a colloidal dispersion. Over time the particles may settle and form a second phase. A suspension is not a true solution. A suspension is also not a precipitate.

Polarity plays a role in the solubility of a solute in a solvent. The phrase “like dissolves like” is used to describe the fundamental condition of solubility. The degree of solubility is a quantitative measure of how much solute can dissolve in a given volume of solvent. Solutes that are described as polar are soluble in a polar solvent. Solutes that are described as nonpolar dissolve well in solvents that are nonpolar. This is a good guide for solubility measurements, but is can be difficult to predict the solubility of each and every compound. There are other factors to solubility. The first being the magnitude of difference between the polarity of solute and solvent. The greater the difference, the less soluble the solute is. The second is temperature. It is common knowledge that sugar dissolves better in warm tea. The same goes for many other substances. As a general rule of thumb, the greater the temperature, the easier the solute will dissolve in the solvent.

The third factor of solubility is pressure. Pressure has little to no effect on the solubility of solids and liquids in liquids. But the solubility of a gas in liquid is directly proportional to the applied pressure. Sodas are a good example of this. Carbon dioxide is dissolved in a sugar syrup/water mixture under high pressure, making the carbonated soda. When a solution contains all the solute that can be dissolved at a particular temperature it is considered a saturated solution. There is a condition where the experimenter can heat the solution up further and dissolve more solute in. If the solution is easily brought down to a cooler temperature without a precipitate settling out of the mixture, the solution is considered a supersaturated solution. These solutions are usually unstable and with time the excess solute will settle out into a precipitate, reverting to a more stable saturated solution. When an excess of solute is added to a solvent, the amounts of dissolved and undissolved solutes will establish a dynamic equilibrium. A continual exchange of solute particles between solid and liquid phases because particles are in constant motion.

Henry’s law allows us to predict the maximum amount of gas dissolved in a liquid. Henry’s law states that the number of moles (mol) of a gas dissolved in a liquid at a given temperature is proportional to the pressure of the given gas. Henrys law expressed mathematically is M=kP. Where M is the molar concentration of the gas in the liquid in units of moles/liter (mol/L). P is the pressure (in atm) of the gas over the solution at equilibrium. For a given gas, k is a constant that depends only on temperature. The constant, k, has units of mol/L · atm. In the event that more than one gas is present, P is the partial pressure. Gases are most soluble at low temperatures.