Can you imagine a cup of coffee, sugarless at the top but intensely sweet at the bottom? Likewise, imagine a bowl of soup without salt at the top but over-salted at the bottom. Would these be enjoyable? We owe the joy of uniformly sweetened coffee or perfectly seasoned soup, as well as several vital life processes, to “dissolution.”
The process of dissolution occurs when a solute (solid, liquid, or gas) is placed in a solvent (also solid, liquid, or gas) and dissolves to form a solution which is a homogeneous mixture. A homogeneous mixture is uniform in composition and properties throughout. Pure substances, whether they are elements or compounds, are rarely found in nature. Most materials we encounter are mixtures of two or more substances. A mixture of sand and salt is not homogeneous, while a mixture of salt and water forms a homogeneous mixture (or solution) when enough water is added to dissolve all of the salt present. Likewise, gases dissolve in liquids to form solutions. Fish can survive in water by using their gills to extract the dissolved oxygen. Much of the world around us is made up of aqueous solutions. The oceans and our blood are only two of many examples.1
Aqueous solutions are the solutions in which water is the solvent. Water is called the "universal solvent" because it dissolves more substances than any other liquid. The kidneys and water's solvent properties play an essential role in keeping us alive and healthy. The kidneys are responsible for filtering out substances that enter our bodies from the foods and drinks we consume. However, the kidneys have to get rid of these substances when they accumulate. This is where water helps out: being such a great solvent, water washing through the kidneys dissolves these substances and sends them on their way out of our bodies.2
Polarity is the property whose job is to ensure water to be such an excellent solvent. Water molecules have a polar arrangement of oxygen and hydrogen atoms—one side (hydrogen) has a positive electrical charge and the other side (oxygen) has a negative charge. This allows the water molecule to become attracted to many polar molecules as well as less-polar molecules. Water can become strongly attracted to a polar compound, such as salt (NaCl); this allows it to disrupt the attractive forces that hold the sodium and chloride ions in the salt compound together, and, thus, dissolves it.2
Similarly, glucose, which is a carbohydrate and the most important simple sugar, dissolves in water because polar water molecules attach to the glucose molecules. The six O-H (hydroxyl-) groups in glucose are the polar centers of a glucose molecule. The oxygen in each -O-H has a slight negative charge, and the hydrogen end of the -O-H has a slight positive charge. They are attracted to the water molecules by dipole-dipole forces. When the attractive forces of the water molecules for the glucose exceeds the attractive forces between the glucose and its neighboring glucose molecules, the water can pull the sugar molecule out of the crystal. It is said that water has "dissolved" the sugar molecule. This process continues until either the sugar is completely dissolved or the unattached water molecules are exhausted; in other words, when “the solution is saturated.”3
"Like dissolves like" is an expression used by chemists to state that polar solvents dissolve polar solutes; non-polar solvents dissolve non-polar solutes.For example: water is polar, oil is non-polar. Water will not dissolve oil. Water is polar, and table salt (NaCl) is ionic, which is extremely polar. According to the rule of thumb, like dissolves like, and polar dissolves polar; thus, water dissolves table salt.4
Polar water molecules are attracted to ions (atoms or groups of atoms with a charge) where "cations" are ions with a positive charge and "anions" have a negative charge. Most ionic compounds have high solubility in water, which means that large concentrations of those compounds can dissolve before the capacity for water molecules to isolate the ions is exceeded. The point at which Na+ and Cl-, for example, would begin to precipitate a salt in seawater is termed "saturation." For NaCl (the mineral "halite"), this only occurs when seawater evaporates and is reduced to about 10% of its original volume. Besides Na+ and Cl- ions, which account for over 85% of the total dissolved solids (salts), seawater contains other important ions such as Mg2+, Ca2+, K+, Sr2+, B+3, SO42-, HCO3-, Br-, and F-, where the positive and negative charges are balanced. As a sign of mercy in its creation, seawater is electrically neutral; otherwise, the flow of current from the sea would be shocking!5
All other dissolved substances in seawater are at very low concentrations (parts per million or billion, ppm or ppb: 10-6 to 10-9). This includes important nutrients such as phosphate and nitrate that are cycled by organisms (ions called "bio-limiting") and essential for life. Metals are also found in trace concentrations. There are about 9 million tons of gold dissolved in seawater, which is about equal to all the gold mined on earth throughout history.5
The evaporation of about 81-96% of the mass of seawater produces a predictable sequence of mineral salts (as minerals become saturated at a certain point) such as CaCO3 (calcite), CaSO4 (gypsum), NaCl (halite), and the K+ and Mg2+ salts (w/ SO42- and Cl-). There is enough salt in the ocean to cover the earth’s land with a layer 170 meters thick. Such natural deposits from ancient oceans are called "evaporites."
Even though most ionic compounds are highly soluble in water, there are some that are insoluble (or very slightly soluble). Soluble substances can form a solution of at least 0.1M (0.1 moles per liter) at 25 C while insoluble substances cannot.For an ionic substance to dissolve in water, there are two competing factors that determine the enthalpy of the solution ΔHsol, which is the enthalpy (energy) change when a solute is dissolved in a solvent (which is water, in this case):
- The lattice energy LE, or the energy of the formation of the crystal between infinitely separated ions. LE is proportional to the charges of its ions. This value is always positive, as energy is required to separate the ions.
- The hydration energy of gaseous ions, which is the enthalpy change when gaseous ions dissolve in sufficient water to give an infinitely dilute solution. These values are always negative, as energy is released upon the hydration of ions.
As an example, the LE of calcium carbonate (CaCO3, or calcite), which is insoluble in water, is so large, a great amount of free energy would be required to break the strongly attracted ions apart, and this energy has to come from the enthalpy of hydration. However, the enthalpy of hydration is not large enough to overcome the large lattice energy, hence it does not dissolve in water and exists as a solid (a precipitate form).6
It would be very hard to predict whether a precipitate is formed in an aqueous reaction if there were not a number of patterns in the data obtained from measuring the solubility of various salts.These patterns form the basis for the solubility rules7 which can guide predictions of whether a salt will dissolve in water. This sense of order and harmony in creation is an exceptional gift to human beings, for it makes our lives much easier.