Solubility in the general sense refers to the property of being soluble—being able to dissolve, usually in a liquid. Chemists, however, use the word solubility to, also, mean the maximum amount of a chemical substance that dissolves in a given amount of solvent at a specific temperature. The word soluble comes from the fourteenth century Latin word solvere meaning to dissolve.
How much sugar could one dissolve in a cup of hot coffee? Certainly one teaspoonful would mix into the liquid and disappear quite easily. However, after trying to dissolve several more teaspoonfuls, there will come a point where the extra sugar one adds will simply not dissolve. No amount of stirring will make the sugar disappear and the crystals just settle down to the bottom of the cup. At this point the coffee is said to be saturated—it cannot dissolve any more sugar. The amount of sugar that the coffee now holds is the solubility of sugar in coffee at that temperature.
A sponge gets saturated when one is using it to wipe up spilled milk from a kitchen counter. At first, a dry sponge soaks up milk very quickly. However, with further use, the sponge can only push milk along the counter-its absorbing action is lost. This sponge is now holding its maximum amount of milk. Similarly, a saturated solution is one that is holding its maximum amount of a given dissolved material.
The sugar oneadds toa cupofcoffeeisknown as the solute. When this solute is added to the liquid, which is termed the solvent, the dissolving process begins. The sugar molecules separate and diffuse or spread evenly throughout the solvent particles, creating a homogeneous mixture called a solution. Unsaturated solutions are able to dissolve more solute, but eventually the solution becomes saturated.
Solubility is often expressed in grams of solute per 0.2 lb (100 g) of solvent, usually water. At 122°F (50°C), the solubility of sugar in water is approximately 130 g/sugar in 100 g water. If one were to add 0.26 lb (130 g) of sugar to 0.2 lb (100 g) of water at 122°F (50°C), the resulting solution would be saturated. Adding 0.26 lb (131 g) would mean that even with continuous stirring, 0.002 lb (1 g) of sugar would remain at the bottom of the container.
Sometimes, solubility is expressed as grams of solute per 0.2 lb (100 g) of solution. In this case the value of the solubility of sugar in 0.2 lb (100 g) of solution at 122°F (50°C) would be less than 0.26 lb (130 g), because unlike the previous example where the weight of the solvent was fixed, the weight of a solution changes as solute is added.
Other commonly used units include g/L (grams of solute per liter of solution) and m/L (moles of solute per liter of solution). Solubility units always express the maximum amount of solute that will dissolve in either a given amount of solvent, or a given amount of solution, at a specific temperature.
For most solutes, the higher the temperature of the solvent, the faster its rate of dissolving and the greater its solubility.
When making iced tea in summertime, it is best to dissolve the sugar in the hot tea before adding the ice cubes and refrigerating. Trying to dissolve sugar in a mixture of tea and ice is a much slower process and will often result in a build up of sugar at the bottom of the glass.
Figure 1 shows that the solubility of sugar and the three other compounds listed increases with rising temperature. Most solid compounds show the same behavior. One theory to explain this observation suggests that hot solvent particles, which move faster than cold ones, are on average more spread out. This creates larger spaces and increases the amount of solute that can fit into the solvent.
Bases, however, are less soluble in hot water than in cold. The solubility of carbon dioxide gas in soda pop actually decreases as temperature is increased. An open bottle of pop taken from a refrigerator soon loses its fizz if stored in a warm environment. As the pop warms up the carbon dioxide gas dissolved in it becomes less soluble.
A person may notice the same thing happening when heating a pot of water on a kitchen stove. Tap water contains dissolved air and when heated, small bubbles form, rise to the surface and leave. This reduced solubility of air is one cause of thermal pollution. Industries often use lake water as a coolant for their machinery. Before the hot water can be returned to the lake it must be allowed to cool down; otherwise it can be harmful to some fish because warm water holds less dissolved air and therefore less oxygen.
Not all substances are equally soluble at the same temperature. At 41°F (5°C), the solubility of table sugar is more than three times greater than that of table salt, as shown in Figure 1.
Even substances such as ordinary glass, which appear not to dissolve, actually do so, but their solubility values are extremely small.
The types of bonds or forces that hold sugar particles together are different from those found in glass. The interaction between the attractive forces holding these particles together and the attractive forces to the molecules of solvents accounts for the different solubilities.
For most purposes a substance that has a solubility of less than 0.01 moles per liter is generally regarded as insoluble. With ionic compounds, certain salts are generally soluble or insoluble. For example nitrates are soluble, as are most chlorides, bromides, and iodides (those with silver, mercury or lead as the cation are exceptions to this guideline). Carbonates and hydroxides are generally insoluble. The solubility of a substance whose anion is basic will be affected by the pH of the solution. As the acidity of the solution increases, the solubility of the basic anion also increases. Also, with regard to ions, the solubility of a slightly soluble salt is decreased by the presence of a second solute, if the second solute provides a common ion.
Gases, as mentioned earlier, can also dissolve in liquids. However as the temperature increases the solubility of the gas generally decreases. To illustrate the solubility of gases in water one can consider the gases found in the air. The solubility of these gases in water is quite small but the amount of oxygen that dissolves in water is sufficient to support aquatic life. Normally the composition of gases in air is 79% nitrogen and 20% oxygen. When air is dissolved in water, the composition is 61% nitrogen and 37% oxygen. This is due to the greater solubility of oxygen in water than nitrogen in water. This gives an enrichment of oxygen in the water that is of great importance for living organisms. In 100 grams of water at 68°F (20°C), the solubility of oxygen is 0.004 grams, however, if the temperature is increased to 104°F (40°C), then the solubility decreases to 0.003 grams. Under the same conditions, nitrogen shows solubilities of 0.002 grams and 0.0015 grams, respectively. Carbon dioxide is even more soluble in water, but this property is because there is a chemical reaction occurring to produce carbonic acid. With the temperature conditions previously described, carbon dioxide has a solubility of 0.17 grams and 0.05 grams, respectively.
The solubility of gases decreases with an increase in temperature but the solubility increases with an increase in pressure. Soda drinks contain carbon dioxide gas that has been dissolved under pressure. When the pressure is released, by opening the can or bottle, the carbon dioxide comes out of solution. It is escaping carbon dioxide that gives these drinks their fizz. If the can or bottle is left standing, then the drink will go
Homogeneous —Having one phase, one uniform color and texture.
Saturated —Full. Containing a maximum amount.
Solute —Usually a solid. It is the least abundant component of a solution.
Solution —A transparent, homogeneous mixture.
Solvent —The major component of a solution, for example, water in sugar water.
flat; i.e., the fizz will be lost as all of the carbon dioxide comes out of solution. This will happen more quickly if the liquid is warm.
The disease suffered by divers known commonly as the bends (decompression sickness) is an example of the same phenomenon. As the diver descends in the water the pressure increases. If the diver is breathing an air mixture, more nitrogen will dissolve in the diver’s blood below the surface than at the surface. This is due to the greater solubility of gases under pressure. If the diver returns to the surface quickly, then there will be a rapid decrease in the pressure he or she is experiencing. The nitrogen will come out of solution too quickly, producing bubbles of gas. These bubbles may stop blood flow and damage nerves. Decompression sickness can be a fatal condition. To reduce the likelihood that this problem will occur, divers may use mixtures of helium and oxygen. Helium has a much lower solubility in blood than nitrogen.
Henry’s law describes the solubility of gases in liquids. It states that the mass of gas that dissolves in a given volume of liquid at constant temperature is directly proportional to the pressure of the gas. This law only holds true if there is no chemical reaction between the liquid and the gas. The equation that describes Henry’s law is Cg = kPg where Cg is the solubility of the gas in the solution phase, usually expressed as molarity, k is the Henry’s law constant, and Pg is the partial pressure of the gas over the solution. The Henry’s law constant is different for each solute and solvent pair.