Solubility and Solution

INTRODUCTION

The solubility of a solute (a dissolved substance) in a solvent (the dissolving medium) is the most important chemical principle underlying three basic techniques you will study in the organic chemistry laboratory: crystallization, extraction, and TLC chromatography. In this lab, you will gain an understanding of the structural features of a substance that determine its solubility behavior and to understand the techniques that are based on this property. Understanding solubility behavior will also help you understand what is going on during a reaction, especially when there is more than one liquid phase present or when a precipitate is formed.

 

DEFINITION OF SOLUBILITY

Although we often describe solubility behavior in terms of a substance being soluble (dissolved) or insoluble (not dissolved) in a solvent, solubility can be described more precisely in terms of the extent to a which substance is soluble. Solubility may be expressed in terms of grams of solute per liter (g/L) or milligrams of solute per milliliter (mg/mL) of solvent. Consider the solubilities at room temperature for the following three substances in water:

Cholesterol 0.002 mg/mL

Caffeine 22 mg/mL

Citric acid 620 mg/mL

In a typical test for solubility, 40 mg of solute is added to 1 mL solvent. Therefore, if you were testing the solubility of these three substances, cholesterol would be insoluble, caffeine would be partially soluble, and citric acid would be soluble. Note that a small amount (0.002 mg) of cholesterol would dissolve. It is unlikely, however, that you would be able to observe this small amount dissolving, and you would report that cholesterol is insoluble. On the other hand, 22 mg (55%) of the caffeine would dissolve. It is likely that you would be able to observe this, and you would state that caffeine is partially soluble.

When the solubility of a liquid solute in a solvent is described, it is sometimes helpful to use the terms miscible and immiscible. Two liquids that are miscible will mix homogeneously (one phase) in all proportions. For example, water and ethyl alcohol (ethanol) are miscible. When they are mixed in any proportion only one layer will be observed. When two liquids are miscible, it is also true that either one of them will be completely soluble in the other one. Two immiscible liquids do not mix homogeneously in all proportions, and under some conditions they will form two layers. Water and diethyl ether are immiscible. When mixed in roughly equal amounts, they will form two layers. However, each liquid is slightly soluble in the other one. Even when two layers are present, a small amount of diethyl ether will be soluble in the water. Furthermore, if only a small amount of either one is added to the other, it may dissolve completely, and only one layer will be observed. For example, if a small amount of water (less than 1.2% at 20 °C) is added to diethyl ether, the water will dissolve completely in the diethyl ether, and only one layer will be observed. When more water is added (more than 1.2%), some of the water will not dissolve and two layers will be present.

 

Although the terms solubility and miscibility are related in meaning, it is important to understand that there is one essential difference. There can be different degrees of solubility, such as slightly, partially, very, and so on. Unlike solubility, miscibility does not have any degrees―a pair of liquids is either miscible or it is not.

 

PREDICTING SOLUBILITY BEHAVIOR

  1. Solutions in Which the Solvent and Solute are Molecular

A useful generalization in predicting solubility is the widely used rule “Like dissolves like.” This rule is most commonly applied to polar and nonpolar compounds. According to this rule, a polar solvent will dissolve polar (or ionic) compounds, and a nonpolar solvent will dissolve nonpolar compounds.

 

The reason for this behavior involves the nature of intermolecular forces of attraction. The force of attraction between polar molecules is called dipole-dipole interaction; between nonpolar molecules, forces of attraction are called van der Waals forces (also called London or dispersion forces). In both cases, these attractive forces can occur between molecules of the same compound or different compounds.

 

To apply the rule “Like dissolve like”, you must first determine whether a substance is polar or nonpolar. The polarity of a compound is dependent on both the polarities of the individual bonds and the shape of the molecule. For most organic compounds, evaluating these factors can become quite complicated because of the complexities of the molecules. However, it is possible to make some reasonable predictions just by looking at the types of atoms that a compound possesses. As you read the following guidelines, it is important to understand that although we often describe compounds as being polar or nonpolar, polarity is a matter of degree, ranging from nonpolar to highly polar.

 

Guidelines for Predicting Polarity and Solubility

 

1, All hydrocarbons are nonpolar.

 

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