Acid salt
Salts that yield acidic solutions, often via partial neutralization of polyprotic acids, but also including salts like ammonium chloride from full neutralization.
NadirSH · CC BY-SA 4.0
Acid salts are a class of salts that produce an acidic solution after being dissolved in a solvent. They are typically formed during partial neutralization of diprotic or polyprotic acids, where a half-neutralization occurs due to the remaining of replaceable hydrogen atoms that have not reacted with hydroxide ions to create water molecules. Note that while acid salts often come from weak acids, they can also be derived from strong acids (e.g., sodium bisulfate from sulfuric acid).
- field
- Chemistry
- known_for
- Producing acidic solutions upon dissolution; used as leavening acids in baking powders
- examples
- Ammonium chloride, cream of tartar, monocalcium phosphate, disodium phosphate, monosodium phosphate
Lore & Background
Acid salts are formed through the partial neutralization of diprotic or polyprotic acids, leaving replaceable hydrogen atoms that have not reacted with hydroxide ions. The acid–base property of the resulting solution depends on the remaining salt products; salts containing reactive cations undergo hydrolysis, reacting with water molecules and causing deprotonation of conjugate acids. For example, the acid salt ammonium chloride is formed by the full neutralization of ammonia with hydrogen chloride: NH3(aq) + HCl(aq) → [NH4]+Cl−(aq). While this is a full neutralization, ammonium chloride is still classified as an acid salt because its cation (NH4+) can donate a proton in water, making the solution acidic.
Reader's Guide
Acid salts are significant in both chemical theory and practical applications. In food, they are often used as leavening acids, mixed with base salts like sodium bicarbonate to create baking powders that release carbon dioxide. Common leavening acids include cream of tartar and monocalcium phosphate. Leavening agents can be slow-acting (e.g., sodium aluminum phosphate) which react when heated, or fast-acting (e.g., cream of tartar) which react immediately at low temperatures. Double-acting baking powders contain both types to provide even rising throughout the baking process. Disodium phosphate is used in foods, and monosodium phosphate is used in animal feed, toothpaste, and evaporated milk. The acidity of the solution is determined by comparing the Ka and Kb values: the solution is acidic if Ka > Kb, alkaline if Ka < Kb, and neutral only when Ka = Kb. For ammonium chloride, the Ka of NH4+ is 5.6×10−10, which is greater than the Kw of water (1.0×10−14), ensuring a pH below 7 at room temperature.
Did You Know?
- Acid salts produce an acidic solution after being dissolved in a solvent.
- Ammonium chloride is an acid salt formed by full neutralization of ammonia with hydrogen chloride; its acidity comes from the ammonium ion, not from partial neutralization.
- Cream of tartar and monocalcium phosphate are common leavening acids used in baking powders.
- The acidity of an acid salt solution is determined by comparing the Ka of the cation to the Kb of the anion; if Ka > Kb, the solution is acidic.
Gallery
Frequently Asked Questions
What is an acid salt?
An acid salt is a salt that, once dissolved in a solvent like water, yields a solution with a pH below seven. It most commonly appears when a diprotic or polyprotic acid is only partially neutralized, leaving some replaceable hydrogen atoms still bound to the anion.
How are acid salts formed?
They typically result from a half-neutralization reaction in which a polyprotic acid meets just enough hydroxide to swap out some—but not all—of its acidic protons. They can also be derived from strong acids; sodium bisulfate, for example, comes from sulfuric acid rather than a weak one.
What role do acid salts play in baking?
In baking powders, acid salts such as monocalcium phosphate or cream of tartar act as the acid component that reacts with a base like sodium bicarbonate to release carbon dioxide gas. That gas is what leavens dough and gives breads and cakes their light, risen crumb.
Why are acid salts important in chemistry?
They demonstrate that a salt does not automatically produce a neutral solution; the leftover replaceable protons on the anion are what push the pH acidic. They also serve practical roles in food science, pharmaceuticals, and industrial formulations.
Are all acid salts derived from weak acids?
No, that is a widespread misconception. While well-known examples like ammonium chloride and cream of tartar do come from weak acids, strong acids can produce acid salts as well, as seen with sodium bisulfate from sulfuric acid.
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