12.8: Solutions and Solubility
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- 476619
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You might recall that we had introduced the idea of a solution as a type of homogeneous mixture in which the particles that make it up are very tiny. In this section we will discuss some of the properties of solutions.
Solute-Solvent Combinations
Although much of the time we will be focusing on water based solutions, it turns out that you do not need water in order to have a solution. In fact, you do not even necessarily need to have a liquid in order to have a solution. There are many examples of solutions that do not involve water at all, or that involve solutes that are not solids. The table below summarizes the possible combinations of solute-solvent states, along with examples of each.
| Solute State | Solvent State | Example |
|---|---|---|
| liquid | gas | water in air |
| gas | gas | oxygen in nitrogen (gas mixture) |
| solid | liquid | salt in water |
| liquid | liquid | alcohol in water |
| gas | liquid | carbon dioxide in water |
| solid | solid | zinc in copper (brass alloy) |
| liquid | solid | mercury in silver and tin (dental amalgam) |
Gas-Gas Solutions
Our air is a homogenous mixture of many different gases and therefore qualifies as a solution. Approximately \(78\%\) of the atmosphere is nitrogen, making it the solvent for this solution. The next major constituent is oxygen (about \(21\%\)), followed by the inert gas argon \(\left( 0.9\% \right)\), carbon dioxide \(\left( 0.03\% \right)\), and trace amounts of neon, methane, helium, and other gases.
Solid-Solid Solutions
Solid-solid solutions such as brass, bronze, and sterling silver are called alloys. Bronze (composed mainly of copper with added tin) was widely used in making weapons in times past, dating back to at least 2400 B.C. This metal alloy was hard and tough, but was eventually replaced by iron. Although it is possible for these solutions to exist in the gas phase, they must be formed in the liquid phase.
Liquid-Liquid Solutions
Perhaps the most familiar liquid-solid solution is dental amalgam, used to fill teeth when there is a cavity. Approximately \(50\%\) of the amalgam material is liquid mercury to which a powdered alloy of silver, tin, and copper is added. Mercury is used because it binds well with the solid metal alloy. However, the use of mercury-based dental amalgam has gone under question in recent years, because of concerns regarding the toxicity of mercury.
How Temperature Influences Solubility
The solubility of a substance is the amount of that substance that is required to form a saturated solution in a given amount of solvent at a specified temperature. Solubility is often measured as the grams of solute per \(100 \: \text{g}\) of solvent. The solubility of sodium chloride in water is \(36.0 \: \text{g}\) per \(100 \: \text{g}\) water at \(20^\text{o} \text{C}\). The temperature must be specified because solubility varies with temperature. For gases, the pressure must also be specified. Solubility is specific for a particular solvent. In this examples considered here, we will consider solubility of material in water as solvent.
The solubility of the majority of solid substances increases as the temperature increases. However, the effect is difficult to predict and varies widely from one solute to another. The temperature dependence of solubility can be visualized with the help of a solubility curve, a graph of the solubility vs. temperature (Figure \(\PageIndex{1}\)).
Notice how the temperature dependence of \(\ce{NaCl}\) is fairly flat, meaning that an increase in temperature has relatively little effect on the solubility of \(\ce{NaCl}\). The curve for \(\ce{KNO_3}\), on the other hand, is very steep, and so an increase in temperature dramatically increases the solubility of \(\ce{KNO_3}\).
Several substances—\(\ce{HCl}\), \(\ce{NH_3}\), and \(\ce{SO_2}\)—have solubility that decreases as temperature increases. They are all gases at standard pressure. When a solvent with a gas dissolved in it is heated, the kinetic energy of both the solvent and solute increase. As the kinetic energy of the gaseous solute increases, its molecules have a greater tendency to escape the attraction of the solvent molecules and return to the gas phase. Therefore, the solubility of a gas decreases as the temperature increases.
Henry's Law
Pressure has very little effect on the solubility of solids or liquids, but has a significant effect on the solubility of gases. Gas solubility increases as the partial pressure of a gas above the liquid increases. Suppose a certain volume of water is in a closed container with the space above it occupied by carbon dioxide gas at standard pressure. Some of the \(\ce{CO_2}\) molecules come into contact with the surface of the water and dissolve into the liquid. Now suppose that more \(\ce{CO_2}\) is added to the space above the container, causing a pressure increase. In this case, more \(\ce{CO_2}\) molecules are in contact with the water and so more of them dissolve. Thus, the solubility increases as the pressure increases. As with a solid, the \(\ce{CO_2}\) that is undissolved reaches an equilibrium with the dissolved \(\ce{CO_2}\), represented by the equation:
\[\ce{CO_2} \left( g \right) \rightleftharpoons \ce{CO_2} \left( aq \right)\nonumber \]
At equilibrium, the rate of gaseous \(\ce{CO_2}\) dissolution is equal to the rate of dissolved \(\ce{CO_2}\) coming out of the solution.
When carbonated beverages are packaged, they are done so under high \(\ce{CO_2}\) pressure so that a large amount of carbon dioxide dissolves in the liquid. When the bottle is open, the equilibrium is disrupted because the \(\ce{CO_2}\) pressure above the liquid decreases. Immediately, bubbles of \(\ce{CO_2}\) rapidly exit the solution and escape out of the top of the open bottle. The amount of dissolved \(\ce{CO_2}\) decreases. If the bottle is left open for an extended period of time, the beverage becomes "flat" as more and more \(\ce{CO_2}\) comes out of the liquid.
The relationship of gas solubility to pressure is described by Henry's law, named after English chemist William Henry (1774-1836). Henry's Law states that the solubility of a gas in a liquid is directly proportional to the partial pressure of the gas above the liquid. Henry's law can be written as follows:
\[\frac{S_1}{P_1} = \frac{S_2}{P_2}\nonumber \]
Section Summary
- The solubility of a substance is the amount of that substance that is required to form a saturated solution in a given amount of solvent at a specified temperature.
- A solubility curve is a graph of the solubility vs. temperature.
- The solubility of a solid in water increases with an increase in temperature.
- Gas solubility decreases as the temperature increases.
- Gas solubility increases with increasing pressure.
Glossary
- solubility
- The amount of a substance that is required to form a saturated solution in a given amount of solvent at a specified temperature.
- solubility curve
- A graph of the solubility vs. temperature.