Aqueous ammonia, often found in homes as a common cleaning agent, is fundamentally a mixture, specifically classified as a homogeneous solution. This substance, also known as ammonia water or aqua ammonia, is ammonia gas (\(\text{NH}_3\)) dissolved in liquid water (\(\text{H}_2\text{O}\)). The liquid is clear and colorless, and its pungent odor results from the dissolved gas easily escaping into the air.
How Chemists Classify Matter
Chemists categorize all forms of matter into two main groups: pure substances and mixtures. Pure substances include elements and compounds, which are formed when two or more elements are chemically bonded together. A compound, such as pure water (\(\text{H}_2\text{O}\)), always has a fixed, definite ratio of its constituent elements.
Mixtures, by contrast, are created when two or more substances are physically combined without forming a new chemical bond. The components retain their individual chemical identities and can be separated by physical means. A key feature of mixtures is that the ratio of the components can vary widely.
Solutions are a specific type of homogeneous mixture, meaning the components are uniformly distributed at a molecular level. In a solution, the solute is completely dissolved into the solvent. This uniform dispersal makes the substance appear as a single phase.
Why Aqueous Ammonia is a Solution
Aqueous ammonia fits the definition of a mixture because ammonia gas and water are physically combined, not chemically bonded into a new compound. Ammonia gas acts as the highly soluble solute, and water is the solvent. Its classification as a solution is due to the uniform distribution of \(\text{NH}_3\) molecules throughout the \(\text{H}_2\text{O}\) at a molecular level.
The most definitive proof of its mixture status is its variable composition, as the amount of ammonia dissolved can change significantly. Household ammonia often contains 5% to 10% ammonia by weight, while concentrated laboratory stock can contain up to 35%. This wide range of possible concentrations is impossible for a true chemical compound.
There is a common misconception that aqueous ammonia is a compound called ammonium hydroxide (\(\text{NH}_4\text{OH}\)). Only a very small portion of the dissolved \(\text{NH}_3\) reacts with water to form ammonium and hydroxide ions. The vast majority remains unreacted \(\text{NH}_3\) molecules physically surrounded by \(\text{H}_2\text{O}\) molecules. Therefore, chemists classify the overall substance as an aqueous solution of ammonia.
Behavior and Concentration of Ammonia Solutions
The physical combination of ammonia and water directly impacts the behavior of aqueous ammonia. Because it is a mixture, its strength depends entirely on its concentration, which varies based on its intended use. Diluted versions used for cleaning glass are weaker than concentrated grades used in industrial processes or laboratories.
The physical nature of the dissolved ammonia gas explains its noticeable properties, particularly its odor. Since the ammonia is only physically dissolved, it is highly volatile. The gas molecules readily escape the solution and enter the surrounding air, causing the characteristic pungent smell.
This volatility makes it useful as a cleaning agent because the ammonia gas evaporates completely, leaving surfaces streak-free. Aqueous ammonia is also used in industrial applications, including as a precursor for certain chemical syntheses and as a base to regulate acidity.