Ammonia is famous for its pungent smell and its exceptional ability to dissolve in water. Gaseous ammonia (\(\text{NH}_3\)) is highly soluble, with hundreds of grams dissolving in just one liter of water at room temperature. This solubility forms the basis of a common household product used in cleaning solutions. When encountered in stores or laboratories, this solution is often labeled by the familiar, but slightly inaccurate, term “ammonium hydroxide.” This term refers to the aqueous solution created when the gas mixes with water.
Understanding the Terminology: Aqueous Ammonia
The name “ammonium hydroxide” (\(\text{NH}_4\text{OH}\)) is a historical and commercial designation for the resulting water solution. Chemically, it is more precise to refer to this mixture as aqueous ammonia, represented by the formula \(\text{NH}_3\text{(aq)}\). This distinction is important because a stable, isolated molecule of ammonium hydroxide does not exist in significant quantity. The vast majority of ammonia molecules remain dissolved as neutral \(\text{NH}_3\) molecules surrounded by water. This dissolved ammonia is responsible for the solution’s characteristic odor and cleaning power.
The Chemistry of Dissolution
Ammonia gas dissolves easily due to the molecular structure of both the gas and water. Ammonia is a polar molecule, meaning it has slight positive and negative charges on opposite ends. Water molecules are also highly polar, driving a strong attraction between the two substances. This attraction allows the molecules to readily mix and form strong intermolecular forces, including hydrogen bonds.
Once dissolved, a small portion of the ammonia molecules reacts with the water, establishing a chemical equilibrium. This reversible reaction is represented by the equation \(\text{NH}_3 + \text{H}_2\text{O} \rightleftharpoons \text{NH}_4^+ + \text{OH}^-\). In this process, ammonia acts as a base by accepting a proton (\(\text{H}^+\)) from a water molecule. This reaction generates the ammonium ion (\(\text{NH}_4^+\)) and the hydroxide ion (\(\text{OH}^-\)).
The formation of the hydroxide ion makes the solution basic, or alkaline. However, this reaction does not go to completion because ammonia is classified as a weak base. Only a very small fraction, often less than one percent, of the dissolved ammonia molecules react to form ions. The equilibrium strongly favors the dissolved molecular ammonia (\(\text{NH}_3\text{(aq)}\)) over the ammonium and hydroxide ions. This balance defines the solution’s weak basicity and chemical properties.
Everyday Uses and Applications
The unique properties of aqueous ammonia make it a highly versatile chemical with wide-ranging applications. In the home, it is a common component in cleaning products, particularly those designed for glass and windows. Its effectiveness stems from its ability to cut through grease and oils. Its volatility ensures it evaporates cleanly without leaving streaks. Household ammonia solutions typically contain between 5% and 10% ammonia by weight.
Beyond residential cleaning, aqueous ammonia is employed on a large scale in industrial processes. The most significant global use is in the production of nitrogen-containing fertilizers, which support crop growth. Aqueous ammonia is also used in water treatment, combined with chlorine to create chloramines for disinfection. It serves as a refrigerant in large-scale industrial cooling systems.
When handling this substance, use it in a well-ventilated area due to the strong, irritating odor of the evaporating gas. A safety precaution is to never combine ammonia-based products with those containing chlorine, such as bleach. Mixing these substances creates toxic chloramine gas, which poses a serious health hazard.