Factors Affecting Solubility

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The concept of solubility is of major importance in chemistry. Solubility is when a solid, liquid or gaseous substance (solute) dissolves in a liquid (solvent) to form a homogeneous solution. . The extent of the solubility of a substance in a solvent is measured as the saturation concentration where adding more solute does not increase the concentration of the solution.

Factors that affect the degree of solubility are many and include temperature, pressure, nature of the solute and solvent (polarity), and rate of dissolution, which will now be discussed with examples.

Common salt (solid NaCl) dissolves in water (solvent) to the extent of 36.0 grams in 100 grams (100 mls) of water at 20 degrees Celsius. Furthermore, when temperature is raised to 100 degrees, the solubility increases to 39.2 grams of NaCl. This is a comparatively small increase. Some solutes show a decrease of solubility over this temperature range e.g., Na sulfate (62 grams decreases to 41). The poorly soluble Ca sulfate and carbonate (calcite) show similar retrograde solubility and cause fouling, or blocking of hot water pipes.

The ionic solutes or salts tend to ionize in solution but most can be retained intact as salt or crystals when the solvent (water) is evaporated off. Organic solutes (covalent) e.g. sugar, do not ionize in solution but are retained intact on evaporation of water.

In Nature, chemical systems are more complex in the sense that they are not binary systems of the type A + B such as we can easily experiment with in a laboratory. For example, sea water which is our largest solvent, contains a host of ions of which sodium and chloride are predominant. Total solutes amount to (on average) 34.47 grams per liter (salinity ca 3.4%). When sea water evaporates first a little calcite and dolomite crystallizes out followed by gypsum, or calcium sulfate dihydrate, having the low solubility of ca 0.24 gm/100 ml water, eventually followed by sodium chloride, the main solute, when the saturation point has been reached (ca. 36 gm /100 ml).

Nature does this on a colossal scale. The Mediterranean Sea is underlain by salt deposits of 1 to 2 km thick as a result of prolonged evaporation in a closed basin, but now covered by sediments. The UK and EU countries mine similar ancient evaporite deposits for use as valuable inputs to their chemical industries.

Crystal growing, whether in industry or Nature is an operation of considerable importance which is dependent on solubility and what affects it. Most of our mineral deposits have been formed by changes in factors that affect the solubility of valuable components (solutes) such as gold, silver, copper, uranium ores etc., in aqueous or hydrothermal solutions. Mining geologists are well aware of how reducing water pressure and temperature results in the formation of valuable ore deposits, due to solubility products of ore minerals being exceeded.

Crystal growers know too how to take advantage of these factors that affect solubility. The solvent may be aqueous or hydrothermal, for growth of quartz and emerald, at 350 and 600 C respectively, or a molten salt at say 800 degrees C, such as lithium molybdate, being one of the best. A temperature gradient is employed to dissolve the solute at a high temperature and deposit or grow a crystal at lower temperature, maybe 20 degrees less, resulting in an overall decrease in free energy.

Hydrothermal systems are more complex because at P, T conditions above the critical point of water (e.g., 500 bars water pressure and 550 degrees C) regions of retrograde solubility for minerals exist. It is possible to grow quartz, emerald and other crystals with the dissolving nutrient in the cool region and have the crystals grow in the hotter region.

Natural systems are frequently in a non-equilibrium state with solutes (crystals, gases) either dissolving or exsolving (crystallizing out). This is not an instantaneous process but depends on the surface area of the solute. A large crystal (small surface area) will take a long time to dissolve compared to the same weight of powder (large surface area). In crystal growing it is necessary to consider the factor of grain size to control solubilities of components in the system.

Water also dissolves gases of which carbon dioxide and oxygen are important. In general a rise in temperature will decrease the solubility of the gas. A rise in gas pressure will increase its solubility in water (Henry’s Law). Sparkling wines are bottled at 3 to 5 atmospheres pressure so that when the bottle is opened bubbles of carbon dioxide form on reduction of pressure to ambient conditions. A cold beer on warming will release bubbles of carbon dioxide gas and become "flat".

Fish require a certain level of dissolved oxygen to survive. If the water temperature is increased then the oxygen level may decrease to the extent that fish die, such as in a natural heat wave or drought, or "thermal pollution" from power stations releasing warmed cooling water into rivers.

Alcohol (ethanol) and water are completely miscible liquids. We are all familiar with beer (4 to 6% alcohol) and spirits (brandy, whisky etc. with 37 to 45% alcohol by volume), however water and gasoline are not miscible which is attributed to water being a strongly polar molecule and gasoline hydrocarbons being non-polar; ethanol is of intermediate polarity. In general, "like attracts like", if the molecular structure of the solute and solvent are similar then solubility will occur. Ethanol is miscible with gasoline and the fuel "Gasohol" may contain 10% ethanol and 90% gasoline.

At high temperatures of geological systems is found the phenomenon of solid state solubility where two solids (crystals) may be completely miscible at high temperature but on slow cooling they exsolve into separate component crystals of continuously changing composition, the reaction taking place in the solid state. This is of common occurrence, such as with the feldspar and pyroxene mineral families.

In conclusion, understanding solubility and what affects it is very important since it is the basis of so many chemical procedures and hence industrial processes. What happens in Nature too is determined by the solubility of the components present and the changes of temperature and pressure encountered over maybe millions of years. The study of solubility is fascinating and knows no boundaries.

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