The student is expected to distinguish between types of solutions such as electrolytes and nonelectrolytes and unsaturated, saturated, and supersaturated solutions AND investigate factors that influence solubilities and rates of dissolution such as temperature, agitation, and surface area.
Electrolytic solutions include dissolved ionic solutes. The charged ions in the solution are able to conduct an electric current due to the motion of charged particles. Nonelectrolytic solutions include dissolved molecular solutes and are not able to conduct an electric current due to the lack of charged particles.
Saturated solutions occur when the maximum amount of solute is dissolved in a particular amount of solvent at that temperature. If the solution contains more than the maximum amount of dissolved solute it is supersaturated, and if it contains less than the maximum amount of dissolved solute it is unsaturated.
The solubility of a substance in water, whether it is a gas or a solid, depends on the polar nature of the solute, the temperature of the solution, and (for gases) the pressure conditions. Polar solutes are more soluble in water than nonpolar solutes.
The rates of dissolution of a solute in a solvent depend on the temperature of the solution, the agitation of the solution, and surface area of the solute. Increasing any one of these three factors results in a faster rate of dissolution.
Electrolytic Solutions Conduct Electricity
Solutions may contain solutes that are either electrolytes (compounds that conduct an electric current in aqueous solution) or nonelectrolytes (compounds that do not conduct an electric current in aqueous solution). Some solvents can dissolve electrolytes and thus become conductive. Electrolytes are ionic compounds. They include salts such as NaCl that completely dissociate (break into ions) in water. When ionic compounds such as NaCl are dissolved in solution, the positive and negative ions are free to move. This dissociation of ions in solution is what allows for the conduction of an electric current. Commonly, electrolyte solutions contain acids, bases, or salts as their ionic solute.
The electricity to power a cell phone comes from a battery. Cell phone batteries use free ions to support this process, within which electrolyte ions form a closed circuit. Inside a cell phone, the battery may be labeled with a lithium-ion battery symbol. This is a type of rechargeable battery that uses an electrolytic solution.
Strong Electrolytes: These are compounds that readily form ions in solution, making them good conductors of electricity. The ions of a strong electrolyte will completely dissociate in solution.
Weak Electrolytes: These are compounds in which only a small amount of ions dissociate in solution, making them poor conductors of electricity. A weak electrolyte will partially dissociate in solution, but some ions will remain bonded together. An example of a weak electrolyte is acetic acid, which forms acetate anions and hydronium cations only to a small extent.
Non-Electrolytes: These are compounds that do not dissociate into ions in solution and do not conduct electricity. These are often molecular compounds that do not contain oppositely charged particles. Non-electrolytes dissolve in solution. They do not dissociate into ions, but instead into neutral molecules. As there are no charged particles (ions) present in a non-electrolytic solution, a current cannot be conducted. Glucose is an example of a nonelectrolyte that would simply break down into smaller units.
Amount of Dissolved Solute
A solution consists of a solvent and one or more solutes in various concentrations.
Saturated solution occurs when the maximum possible amount of solute is dissolved in a particular amount of solvent at a certain temperature.
Unsaturated solution occurs if the solution contains less than the maximum possible amount of dissolved solute.
Supersaturated solution occurs if the solution contains more than the maximum possible amount of dissolved solute. In Image 1, supersaturated carbon dioxide gas in cola releases a bubble jet when in contact with certain candy. In Image 2, a supersaturated sugar solution becomes rock candy.
Factors Affecting Solubility
The solubility of a gas or a solid in water depends on the polar nature of the solute, the temperature of the solution, and (for gases) the pressure conditions.
Polarity: Polar solutes are more soluble in water than non-polar solutes. This is true because water is polar solvent. Therefore, polar solutes, such as NaCl, dissolve more easily in water than in non-polar solutes, such as oil. This phenomenon is often referred to as “like dissolves like.” For example, non-polar solutes are more soluble in non-polar solvents than in polar solvents. Furthermore, polar and non-polar substances are not soluble with one another and do not form solutions. An example is the mixing of oil and water. The oil does not dissolve in the water, and instead remains suspended in the solvent.
Do not confuse molecules that are polar with compounds that are electrolytes. Polar molecules have partial charges, where one end of the molecule is partly positive, and the other end is partly negative. They are not, however, composed of ions that have full positive or negative charges. Unlike electrolytes, polar molecules do not break apart into ions when they dissolve, which means that they do not produce electrolytic solutions. For example, table sugar is a polar molecule that dissolves in water, but it is a nonelectrolyte.
Temperature: Temperature is a key factor in solubility. The solubility of gases decreases as temperature increases, while the solubility of solids usually increases with an increase in temperature. If you have a saturated salt solution in a bottle, such as NaCl, and you add a little bit more NaCl, then some NaCl may settle to the bottom of your bottle. However, if the water in the bottle was heated, you may observe that the settled salt would completely dissolve.
Pressure Conditions: The solubility of gases depends on varying changes of pressure. Therefore, an increase in pressure would equal higher solubility, while a decrease in pressure would mean lower solubility. This applies especially to deep sea divers. Deep under water, the high pressure of the water surrounding our bodies causes nitrogen to form in our blood and tissue. Like the bottle of soda, if we move around or come up from the water too quickly, the nitrogen will be released from our bodies too quickly and creates bubbles in our blood. This phenomenon is known as “the bends.” It can be extremely painful and, in certain cases, fatal, so divers are very careful as to how quickly they rise after deep water dives.
Factors Affecting Dissolution of a Solute
The rate of dissolution of a solute in a solvent depends on the following:
the temperature of the solution,
the agitation of the solution,
and the surface area of the solute.
Increasing any one of these three factors will result in a faster rate of dissolution. For example, when one add some sugar in a cup of coffee, and it is suddenly too sweet, a simple way to correct the solution is to stir or to add more hot coffee!