Solubility as Physical Property

Exercise 1: Solubility as a Physical Property

According to Chang in his book Chemisty (2010), a solution is a homogeneous mixture of two or more substances. It is composed of a solute and a solvent. The solvent is the component with greater amount and the solute is the component with smaller amount and the one that is dissolved in the solvent.

         Solubility is a measure of how much solute will dissolve in a given amount of solvent at a specific temperature. There are 3 types of solutions based on their capacity to dissolve a solute; the saturated solutions, unsaturated solutions, and supersaturated solutions. Supersaturated solutions often form crystals, this process is called crystallization. According to Brown in his book Chemistry: The Central Science (2012), crystallization is the opposite of dissolution, it happens when the concentration of the solute in the solution increases beyond its maximum, the chances that some solute particles will collide with the surface of the solid and reattach also increases.

Zumdhal (2009) stated in his book Chemical Principles, that the cardinal rule of solubility is ‘like dissolves like’. Polar solvents dissolve polar solutes; nonpolar solvents dissolve non polar solutes (p. 849). Another factor that affect solubility is temperature.

Discussion

Part 1: Observation on the Dissolution Process [KMnO4 in water]

In the experiment, a small crystal of potassium permanganate was dropped in a 250-mL E-flask with 50 mL of water. But before this the preliminary apperance of the solid was observed first. It was observed to be dark purple, tiny crystals. Upon contact of the KMnO4 to the water, purple color started to spread out in the solvent. Then agitation on the mixture was performed, the result was the complete dissolution of the KMnO4 crystals in the solvent, a purple solution was then observed.

Potassium Permanganate is an ionic compound composed of colorless potassium ions K+, and purple permanganate ions MnO4-. When a small amount of solid potassium permanganate is added to water, the compound dissolves and dissoci ates to yield potassium ions and permanganate ions uniformly distributed throughout the mixture.

  A phase is sometimes termed to be synonymous to state of matter. It is sometimes true, the states of matter are phases but matter can exist in different phases yet the same phase of matter. A phase is a region of matter where there are uniformity in all the physical and chemical properties such as density, chemical composition and temperature.

Part 2: Description and Properties of Organic Compounds

Organic compounds are those substances that contain carbon. In the experiment, there are different organic substances used. In the second part, the observation and test for the solubility of two sets of organic compounds in petroleum ether and water at room temperature were performed. Six organic compounds were used namely; acetic acid, benzoic acid, sodium benzoate in set A, and cyclohexanol, dextrose, and cellulose (cotton fiber) in set B. These compounds were observed and recorded.

The solvent, petroleum ether was observed to be clear colorless solution. In the set A organic compounds, acetic acid appeared to be clear, colorless liquid with a strong acidic odor. Benzoic acid on the other hand, was white powder-like or crystalline in appearance. And the sodium benzoate was noted to be white crystalline powder with no odor produced.

On the set B compounds, these observations were observed: Cyclohexanol was not available during the experiment resulting to the lack of experimental observation of it but according to its theoretical appearance, it is a colorless liquid with camphor-like odor. Dextrose, was observed to be a white crystalline or granuar powder and also odorless. While the cellulose or the cotton fiber appeared to be soft fluffy white cotton balls.

Part 3: Solubility of these organic compounds in petroleum ether vs. in water at room temp

The difference between the electronegativities of the hydrogen and oxygen atoms in water is much larger than C--O bond in pet ether and the H--O bonds in this molecule are therefore polar. Water molecules have a bent, or angular shape therefore water molecules have distinct positive and negative poles. Water is therefore classified as a polar solvent. Petroleum ehe other hand is ne of the most used nonpolar organic compounds.  

In the experiment, two sets of solvents were prepared; petroleum ether and water. Six organic compounds were dissolved in each of the solvents inside the test tubes. The resulting appearance of the mixtures formed were recorded. Acetic acid, Sodium benzoate, and dextrose, all were dissolved in the water thus, formed a solution. While the benzoic acid only dissolved partially. The cellulose and cyclohexanol on the contrary, were observed to be insoluble in water. While in the pet ether solvent, acetic acid, benzoic acid, and cyclohexanol were completely dissolved. The sodium benzoate, dextrose, and cellulose remained undissolved.

The results can be explained using the concept of “like dissolves like”. Here are the polarity and properties of the sample organic compounds that will prove the validity of the results. Acetic acid (CH3COOH) is a carboxylic acid with high polarity and an almost 100% solubility in water. It is not only soluble in polar solvents but also in nonpolar solvents. Acetic acid can effectively "dimerize", form a double structure held togeter very effectively by a geometrical arrangement that accommodates double hydrogen bonding. The resulting structure has no dipole moment. The "dimer" can then serve as a solvent in nonpolar solvents.

         Sodium benzoate is made of a benzene ring with a carboxylic acid group, -COOH group attached but the hydrogen atom of this group is substituted with a sodium cation. The Na + ion makes sodium benzoate polar because the compound can easily dissociate in water, also a highly polar compound. In water sodium benzoate will yield benzoic acid and sodium hydroxide, the latter of which is an ion-ion force that is the strongest intermolecular force.

In most cases, polar compounds are soluble in polar solvents like water, but benzoic acid is a unique exception. Because of its large, hydrophobic benzene ring, benzoic acid is only slightly soluble in water when heated but this compoud when put into a nonpolar solvent like pet ether, it is soluble. In the case of cyclohexanol, it is a fact that alcohols are very polar and can are soluble in water but that’s not always the case, as the number of carbon in an alcohol increases, the solubility in water decreases. For example, ehanol (C2H6O), having only 2 carbons is a very soluble compound in water. That’s why, cyclohexanol, having 6 carbons is considered to be a nonpolar compound resulting to disolution in pet ether but notin water.  

         While in the case of cellulose, because it has high molecular weight and crystalline structure, it is insoluble in water and has a poor ability to absorb water.

Part 4: Effect of pH on solubility [HCl, water, NaHCO3, NaOH]   

On the observation of effect of pH on solubility, six organic compounds were dissolved in four solvents of diffent pH range; water, HCl, NaOH, and NaHCO3. The results are as follows: Aniline was only soluble in NaHCO3 and insoluble in water, HCl , and NaOH. Diethylamine was soluble in water, HCl and in NaHCO3 but not in NaOH. Naphthalene was observed to be not soluble in any of the solvents. Toluene was soluble in water and HCl but insoluble in NaOH and NaHCO3. Benzoic acid was soluble in NaOH only and phenol is only soluble in water.

Phenol has an acidic nature and hence are soluble in basic solvents like NaOH and slightly soluble in water because of its ability to form hydrogen bonding with water molecules. However the large part of phenol molecule is phenyl group that is non polar and hence its solubility is limited in water.

Amines generally possess basic properties. It can react with HCl, water and also NaHCO3.

Aniline, an aromatic compound, is a weak base. That’s why it would likely react with HCl and NaHCO3. Aniline is also somewhat soluble in water because of its ability to form hydrogen bonds with the water.

Benzoic acid:

Toluene is a base in nature that’s why it was dissolved in HCl and NaHCO3.

Naphthalene

Part 5: Salting out and effect to solubility

Salting out is an effect based on the electrolyte-nonelectrolyte interaction, in which the non-electrolyte could be less soluble at high salt concentrations.

Salting out is also explained as a purification method that utilizes the reduced solubility of certain molecules in a solution of very high ionic strength.

When two or more solutes are present in a solution, it can be infered that one of the solutes ‘salt-out’ the second solute according to th change in the solubility of the second solute in the presence of the first.