Solubility
Solubility is the ability of one substance to fully dissolve in another substance which is often present in a different state of matter. The word soluble comes from the fourteenth century, from the Latin word solvere meaning to dissolve.
The solubility of a substance is the amount of that substance that will dissolve in another substance under specified conditions. When solutions of solids or liquids in liquids are considered, the solubility is given in terms of the weight dissolved in a given weight or volume of the solvent at a specified temperature.
Substances are generally soluble in similar solvents. For example inorganic salts are generally soluble in water but not in organic solvents. Organic compounds are generally soluble in organic solvents but not in water. Organic solvents include such compounds as ethanol and propanone. Organic solvents are commonly encountered in many household items such as glues, paints, aerosols, and varnishes.
The most commonly encountered solutions are of solids dissolved in liquids. The solid that dissolves in a liquid is the solute and the liquid in which it dissolves is the solvent. If a solid can dissolve in a liquid, it is said to be soluble in that liquid; if not, it is said to be insoluble. As we add more solid to a liquid the solution becomes more concentrated. The greater the solubility of a substance the more concentrated it is possible to make the solution. The concentration of a solution is usually quoted in terms of mass of solute dissolved in a particular volume of solvent. This is generally expressed in grams per liter.
When we add solute to solvent a point is reached where no more will dissolve, under the specified conditions. The solution is saturated. The concentration of the solute in a saturated solution is the solubility of the solute in that solvent at that temperature. Solubility is the mass of solute that will saturate 100 g of water at the specified temperature. Saturation of a solution is also defined as the point where the solution is in equilibrium with the undissolved solute. If less solute is added to the solvent then the solution is said to be unsaturated. It is possible to dissolve more solute into a solvent than is required to give a saturated solution. This yields a supersaturated solution. Such solutions can often be prepared by utilizing the greater solubility possible at higher temperatures. The solute is dissolved into the solvent at a high temperature and then the solution is slowly cooled. Such solutions are unstable and the addition of a tiny amount of the solute will cause all of the excess solute to crystallize out of solution.
Solubility can be affected by such processes as hydrogen bonding between the solute and solvent. The greater the amount of hydrogen bonding that occurs the greater the solubility of the solute in that liquid.
A graph showing how the solubility of a solid changes with temperature is known as a solubility curve. In general the solubility of most solids will increase with an increase in temperature. At 68°F (20°C) potassium nitrate has a solubility of 30 g in 100 g of water. If the temperature is raised to 104°F (40°C) the solubility of potassium nitrate in 100 g of water will increase to 65 g. This pattern is not always the same. For example, if copper sulfate is considered under the same conditions the solubility is 21 g and 27 g. Other substances show no change.Sodium chloride has a solubility of 30 g at both temperatures.
For most purposes a substance which 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). 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 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 we 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 g of water at 68°F (20°C) the solubility of oxygen is 0.004 g, if the temperature is increased to 104°F (40°C) then the solubility decreases to 0.003 g. Under the same conditions nitrogen shows solubilities of 0.002 g and 0.0015 g.Carbon dioxide is even more soluble in water, but this 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 g and 0.05 g.
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 this escape of carbon dioxide that gives these drinks their fizz. If the can or bottle is left standing then the drink will go 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 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 divers blood than at the surface. This is due to the greater solubility of gases under pressure. If the diver comes back to the surface too quickly there will be a rapid decrease in the pressure he 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.
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