Core Concepts
Aqueous solutions are one of the most important types of mixture inside and outside the lab. Check out this article to learn more about the characteristics and properties of aqueous solutions!
Topics Covered in Other Articles
- Physical & Chemical Properties of Water
- Factors in Acid Strength
- Complex Ions and Ligands in Chemistry
- What is a Buffer Solution?
- Effects of Intermolecular Forces
- Osmotic Pressure
What are Aqueous Solutions?
Chemists use the term “aqueous” to describe a solution with water as its solvent. Due to the unique chemistry of water, aqueous solutions have certain properties not shared with solutions that use other solvents common to the chemistry lab, such as hexane, acetone, or dichloromethane.
In particular, water is both polar and protic. As a polar solvent, water has a polar structure that allows the solubilization, or molecular mixing, of polar molecules such as alcohols and amines in aqueous solutions. Conversely, nonpolar molecules such as hydrocarbons often cannot become solubilized into aqueous solutions. This follows the “like dissolves like” mnemonic. As a protic solvent, water is capable of forming hydrogen bonds, one of the strongest intermolecular forces. The protic nature of water allows many salts to dissolve into aqueous solutions, due to the formation of strong ion-dipole interactions. However, while ionic compounds tend to dissolve more easily in aqueous solutions than other solvent systems, not all salts are soluble in water.
These properties of water allow chemists to use aqueous solutions in lab for reaction workups. In particular, water allows the separation of water-soluble, or hydrophilic, substances from organic solvents. Outside the lab, aqueous solutions are everywhere! The water in oceans and rivers counts as aqueous solution, carrying a broad diversity of salts and organic compounds to support the organisms that inhabit them. Aqueous solutions, with a soap or detergent solute, are used to remove dirt and germs from dishes, clothes, surfaces, and human skin. Any drink, from tapwater, milk, coffee, or soda, can be thought of as an aqueous solution carrying sugar, carbon dioxide, salts, and other nutrients into the human body, which itself has aqueous solutions such as blood, urine, and sweat.
Composition of Aqueous Solutions
Chemists define a solution as a system with two basic components, the solvent and the solute. The solvent is the liquid phase of the system; as mentioned before, aqueous solutions have water as the solvent. The solute is the substance suspended in the solvent; as mentioned before, aqueous solutions typically involve polar molecules and salts.
Ionic vs Molecular Solutes
When in suspension of water, solutes with an ionic bond, called ionic solutes, behave different than wholly covalent solutes, also called molecular solutes. In particular, ionic solutes dissociate into their respective anion and cation when dissolved in water. For example, when you submerge table salt, NaCl, into water, it separates into positively charged sodium ions and negatively charged chloride ions. This dissolution happens because the ions in NaCl have more attraction to the water solvent than each other. Specifically, the Na+ ions become attracted to the negative dipoles of water, while Cl– have attraction to the positive water dipoles.
Molecular solutes, like glucose, lack an easily breakable ionic bond, so they become solubilized in water through a different mechanism. In solid form, glucose molecules attract each other and form a crystalline matrix. When submerged in an aqueous solution, glucose molecules separate from the matrix and become surrounded by water molecules. This process of becoming surrounded by water is called solvation. In glucose, solvation involves the formation of hydrogen bonds and dipole bonds with the water molecules. Importantly, unlike ionic solutes, molecular solutes do not break down into component pieces, but rather maintain their molecular structures.
Electrolytes in Aqueous Solutions
In aqueous solution, salts and other ionic solutes can conduct electricity. This is due to the fact that dissolving ionic compounds introduces conductive charged particles into the solution. Because of this property, chemists sometimes use the term “electrolyte” to refer to dissociated ions in water. Conducting electricity is an important property for the aqueous solutions involved in batteries.
Nonelectrolytes
Solutes not capable of conducting electricity go by the name nonelectrolytes. This includes molecular solutes that do not ionically dissociate, as well as intact ionic molecules. This means that ionic solutes like silver nitrate are nonelectrolytes, but become electrolytes when dissolved into separate ions.
Strong vs Weak Electrolytes
Chemists like to classify electrolytes as “strong” or “weak” depending on how well they conduct electricity. Strong electrolytes are highly conductive, meaning that they transfer a high proportion of electric current that passes through its aqueous solution. Weak electrolytes, by contrast, transfer a lower proportion electricity.
The conductivity, and thus strength, of an electrolyte depends on the degree that it dissociates in water. An ionic compound that is highly soluble in water, such as ammonium nitrate, will dissociate almost entirely in an aqueous solution, making it a strong electrolyte. A less soluble compound, like sodium bicarbonate, doesn’t completely dissociate in water, and a high proportion of NaHCO3 remains intact, which ultimately makes it a weak electrolyte.
This concept is similar to strong-weak acid theory, and in fact, all strong acids are strong electrolytes while all weak acids are weak electrolytes.