Silver nitrate is a white crystalline substance that is one of the few silver compounds to dissolve in water. It is light-sensitive and is used to make photographic film, but also has medical uses as an antiseptic and as a cauterising agent to remove unwanted skin growth. Hydrochloric acid is a highly reactive liquid with many industrial uses such as metal refining. The reaction of silver nitrate with hydrochloric acid demonstrates some fundamental chemical processes.
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A 0.025 normal solution of silver nitrate consists of 4.2 grams of the compound in one litre of distilled water. When this is poured into 5 percent by weight dilute hydrochloric acid, the first effect is a milky iridescence. Known chemically as opalescence because it appears to diffract light like an opal, this is the first sign of silver chloride precipitation.
When dissolved in water, silver nitrate dissociates into positively charged silver ions and negatively charged nitrate ions. The hydrochloric acid similarly dissociates into positively charged hydrogen ions and negatively charged chloride ions. Each substance is no longer the original chemical compound but a solution of freely moving ions in water.
When the two solutions are mixed, the silver and chloride ions exert the strongest electrical attractions and bond to form silver chloride. As more silver and chloride ions bond together, the water is unable to keep them in solution, causing silver chloride to precipitate as a solid to the bottom of the laboratory vessel. This exchange of ions between two compounds is called double displacement. Hydrogen and nitrate ions remain in solution as a dilute nitric acid. Any solution containing chloride ions, such as sodium chloride in common salt, creates a similar reaction with silver nitrate.
Changes in energy occur during chemical reactions although the total energy in the system remains constant. Silver nitrate and hydrochloric acid create an exothermic reaction, or one that generates heat. Energy initially stored as chemical bonds is released as heat and makes the vessel containing the two compounds feel hot.
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- National Health Service: Silver Nitrate
- National Pollutant Inventory: Hydrochloric acid; overview
- Journal of the American Chemical Society: Studies of the precipitation of silver chloride
- Salt Lake Metals: How to make your own silver nitrate standard solutions
- CSHC Student Resources: Precipitation reactions
- Auburn University: Chemical Reactions