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Solubility
The maximum amount of a substance that can be dissolved in a given volume of solvent is called solubility. Often, the solubility in water is expressed in gram/100 mL. A solution that has not reached its maximum solubility is called an saturated solution. This means that more solute could still be added to the solvent and dissolving would still occur.
A solution that has reached the maximum solubility is called a saturated solution. If more solute is added at this point, it will not dissolve into the solution. Instead it will remain precipitated as a solid at the bottom of the solution.
Thus, one can often tell that a solution is saturated if extra solute is present (this can exist as another phase, such as gas, liquid, or solid). In a saturated solution there is no net change in the amount of solute dissolved, but the system is by no means static.
In fact, the solute is constantly being dissolved and deposited at an equal rate. Such a phenomenon is called equilibrium. For example:
In special circumstances, a solution may be supersaturated. Supersaturated solutions are solutions that have dissolved solute beyond the normal saturation point.
Usually a condition such as increased temperature or pressure is required to create a supersaturated solution. For example, sodium acetate has a very high solubility at 270 K. When cooled, such a solution stays dissolved in what is called a meta-stable state.
However, when a seeding crystal is added to the solution, the extra solute will rapidly solidify. During the crystallization process, heat is evolved, and the solution becomes warm. Common hand warmers use this chemical process to generate heat.
Predicting the solubility of a substance
One useful classification of materials is polarity. Ionic compounds have the highest polarity forming full cations and anions within each molecule as electrons are donated from one atom to another. Covalent bonds could be polar or nonpolar in nature depending on whether or not the atoms involved in the bond share the electrons unequally or equally, respectively.
The electronegativity difference can be used to determine the polarity of a substance. Typically an ionic bond has an electronegativity difference of 1.8 or above, whereas a polar covalent bond is between 0.4 to 1.8, and a nonpolar covalent bond is 0.4 or below.
The diagram above is a guide for discerning what type of bond forms between two different atoms. By taking the difference between the electronegativity values for each of the atoms involved in the bond, the bond type and polarity can be predicted.
Note that full ionic character is rarely reached, however when metals and nonmetals form bonds, they are named using the rules for ionic bonding.
Substances with zero or low electronegativity difference such as H2, O2, N2, CH4, CCl4 are nonpolar compounds, whereas H2O, NH3, CH3OH, NO, CO, HCl, H2S, PH3 have higher electronegativity difference and therefore are polar compounds.
Typically compounds that have similar polarity are soluble in one another. This can be described by the rule: Like Dissolves Like.
This means that substances must have similar intermolecular forces to form solutions. When a soluble solute is introduced into a solvent, the particles of solute can interact with the particles of solvent.
In the case of a solid or liquid solute, the interactions between the solute particles and the solvent particles are so strong that the individual solute particles separate from each other and, surrounded by solvent molecules, enter the solution.
This process is called solvation . When the solvent is water, the word hydration, rather than solvation, is used.
In general polar solvents dissolve polar solutes whereas nonpolar solvents will dissolve nonpolar solutes. Overall, the solution process depends on the strength of the attraction between the solute particles and the solvent particles.
For example, water is a highly polar solvent that is capable of dissolving many ionic salts. When ionic compounds dissolve in a solvent they break apart into free floating ions in solution. This enables the compound to interact with the solvent.
In the case of water dissolving sodium chloride, the sodium ion is attracted to the partial negative charge of the oxygen atom in the water molecule, whereas the chloride ion is attracted to the partial positive hydrogen atoms.
The Process of Dissolving.
When an ionic salt, such as sodium chloride comes into contact with water, the water molecules dissociate the ion molecules of the sodium chloride into their ionic state,
Many ionic compounds are soluble in water, however, not all ionic compounds are soluble. Ionic compounds that are soluble in water exist in their ionic state within the solution. The sodium chloride for instance, breaks apart into the sodium ion and the chloride ion as it dissolves and interacts with the water molecules.
For ionic compounds that are not soluble in water, the ions are so strongly attracted to one another that they cannot be broken apart by the partial charges of the water molecules.
The following table can be used to help you predict which ionic compounds will be soluble in water.
SOLUBILITY RULE
Nitrates (NO3–): All nitrates are soluble.
Acetates (C2H3O2–): All acetates are soluble; silver acetate is moderately soluble.
Bromides (Br–) Chlorides (Cl–) and Iodides (I–): Most are soluble except for salts containing silver, lead, and mercury
Sulfates (SO42–): All sulfates are soluble except barium and lead. Silver, mercury(I), and calcium are slightly soluble.
Hydrogen sulfates (HSO4–) : The hydrogen sulfates (aka bisulfates) are more soluble than the sulfates
Carbonates (CO32–), phosphates (PO43–), chromates (CrO42), silicates (SiO42–): All carbonates, phosphates, chromates, and silicates are insoluble, except those of sodium, potassium, and ammonium. An exception is MgCrO4, which is soluble
Hydroxides (OH–): All hydroxides (except lithium, sodium,potassium, cesium, rubidium, and ammonium) are insoluble; Ba(OH)2, Ca(OH)2 and Sr(OH)2 are slightly soluble.
Sulfides (S2–): All sulfides (except sodium, potassium, ammonium, magnesium, calcium and barium) are insoluble. Aluminum and chromium sulfides are hydrolyzed and precipitate as hydroxides
Sodium (Na+), potassium (K+), ammonium (NH4+): All sodium, potassium, and ammonium salts are soluble. (Except some transition metal compounds.)
Silver (Ag+): All silver salts are insoluble.
Exceptions: AgNO3 and AgClO4; AgC2H3O2 and Ag2SO4 are moderately soluble
The dissociation of soluble ionic compounds gives solutions of these compounds an interesting property: they conduct electricity. Because of this property, soluble ionic compounds are referred to as electrolytes.
Many ionic compounds dissociate completely and are therefore called strong electrolytes. Sodium chloride is an example of a strong electrolyte.
Some compounds dissolve but dissociate only partially, and solutions of such solutes may conduct electricity only weakly. These solutes are called weak electrolytes.
Acetic acid (CH3COOH), the compound in vinegar, is a weak electrolyte.
Solutes that dissolve into individual neutral molecules without dissociation do not impart additional electrical conductivity to their solutions and are called nonelectrolytes.
Polar covalent compounds, such as table sugar (C12H22O11), are good examples of nonelectrolytes.
Similarly, solutions can also be made by mixing two compatible liquids together. The liquid in the lower concentration is termed the solute, and the one in higher concentration the solvent. For example, alcohol (CH3CH2OH) is a polar covalent molecule that can mix with water.
When two similar solutions are placed together and are able to mix into a solution, they are said to be miscible.
Liquids that do not share similar characteristics and cannot mix together, on the other hand, are termed immiscible. For example, the oils found in olive oil, such as oleic acid (C18H34O2) have mainly nonpolar covalent bonds which do not have intermolecular forces that are strong enough to break the hydrogen bonding between the water molecules. Thus, water and oil do not mix and are said to be immiscible.