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An acidic solution can be simply defined as an aqueous solution in which the solute is an acid. Acid solutions are characterized by the following properties that allow them to be distinguished from basic solutions and from neutral solutions:
- They generally have a bitter, sometimes spicy taste.
- They are irritating solutions.
- They neutralize the bases.
- They are capable of dissolving some metals.
Although many acid solutions have all of these characteristics, others have only a few, which makes it not an adequate way to define an acid solution. To develop an unambiguous chemical concept of an acid solution, we must first understand what an acid is from a chemical point of view.
The concept of acid
There are three different concepts of acids and bases that have been developed throughout history. The first was Arrhenius’s concept of acids and bases, who defined an acid as a substance capable of releasing protons (H + ) in solution; while, he affirmed that a base was any substance capable of releasing hydroxide ions (OH – ).
Brønsted and Lowry then developed their own theory, modifying the Arrhenius concept of a base, but keeping that of an acid. Finally, the currently accepted concept of acidity is the ability of an electron deficient substance to accept an electron pair from a base, thus forming a dative covalent bond .
Although the Lewis concept of acids and bases is the broadest and most precise, its greatest applicability is when analyzing acidity and basicity outside of aqueous media. Since an acid solution is, by definition, an aqueous solution of an acid, the Arrhenius concept or that of Brønsted and Lowry will suffice to establish a more adequate concept of acid solution.
The ionic balance of water
The acidity of an aqueous solution is determined by two factors: the acid, which acts as the solute, and the water, which is the solvent. Between the two, water represents the most important part of the concept of acidity and basicity. This is because our scale of measurement for acidity, the pH scale, comes from the ionic balance of water, also called autoprotolysis of water:
Water is what we consider a neutral substance. From the stoichiometry of the previous reaction, we know that water produces the same amount of hydronium and hydroxide ions, which allows us to define a neutral solution as one that, like water, has equivalent concentrations of these two ions.
Definition of acid solution based on the relative concentrations of hydronium and hydroxide ions
When dissolving an acid that contributes a greater amount of hydronium ions to the solution than those that come from the dissociation of water, the ionic balance of the water is affected and it is displaced towards the reactants, reducing the concentration of hydroxide ions in the process. For this reason, a solution is obtained in which there are more hydroniums than hydroxides. This allows the following definition to be established:
An acid solution is defined, from a chemical point of view, as any aqueous solution that contains a higher concentration of hydronium ions than hydroxide ions.
Definition of acid solution based on the concentration of hydronium ions
The equilibrium constant from the above equation can be used to determine the equilibrium concentrations of hydronium and hydroxide ions for any solution. In the case of pure water or any neutral solution, since we know that both concentrations are equal, we can write that:
When the concentration of hydronium ions in a solution increases, the concentration of hydroxide ions necessarily decreases in order to keep their product equal to 10 -14 . Combining this fact with the previous definition we can establish another even more precise definition in terms only of the concentration of hydronium ions:
An acidic solution is defined as any aqueous solution that contains a concentration of hydronium ions that is greater than 10 -7 molar.
This definition implicitly contains the previous definition, since, if the concentration of hydronium ions is greater than 10 -7 , then that of hydroxide ions must necessarily be less than 10 -7 and, therefore, less than that of the hydronium ions.
Definition of acid solution based on the pH scale
Finally, we can determine when we are in the presence of an acidic, basic, or neutral solution by using the pH scale as a measure of acidity. The pH of a solution is defined, for an ideal solution, as the negative of the base 10 logarithm of the molar concentration of hydronium ions in a solution, that is:
For a neutral solution, whose hydronium ion concentration is 10 -7 , the pH has a value of 7. Also, because of the negative sign, when the hydronium ion concentration increases, the pH actually decreases. For this reason, for acid solutions in which, we know, the hydronium ion concentration is greater than 10 -7 , the pH will necessarily be less than 7. Consequently, we can give a fourth definition of an acid solution:
An acid solution is defined as any aqueous solution whose pH is less than 7.
This last solution is, in turn, the most succinct, the simplest, and yet the one that implicitly includes all the other definitions that we have presented here. If the pH is less than 7, then the hydronium concentration will be greater than 10 -7 , which implies that there will be a higher concentration of hydronium ions than hydroxide ions.
References
- assolea. (2020, May 2). 7.6: The pH Scale . Retrieved from https://assolea.org/es/7-6-la-escala-de-ph/
- Brown, T. (2021). Chemistry: The Central Science (11th ed.). London, England: Pearson Education.
- Chang, R., Manzo, Á. R., Lopez, PS, & Herranz, ZR (2020). Chemistry (10th ed.). New York City, NY: MCGRAW-HILL.
- Flowers, P., Theopold, K., Langley, R., & Robinson, W. (2019a). 14.1 Brønsted-Lowry Acids and Bases – Chemistry 2e . Retrieved from https://openstax.org/books/chemistry-2e/pages/14-1-bronsted-lowry-acids-and-bases
- pH and pOH . (2020, October 30). Spanish LibreTexts. Retrieved from https://espanol.libretexts.org/@go/page/1911
