We need a solution of halide ions. The step is adding a dilute nitric acid to acidify the solution. The nitric acid starts reacting with and removes the other ions present that might form precipitates with silver nitrate. The second step is to add silver nitrate solution and the following products will be identified by the halide. The formed precipitates of chloride, bromide and iodide precipitates are shown in the photograph below.
The precipitates of chloride are identified easily but the rest of the two are quite similar to each other. They can be differentiated only in a side-by-side comparison. If the precipitates are exposed to light, all of them will change their colors i. The fluoride ions will not show any precipitates and the absence of a precipitate ion is unhelpful unless it is well known that a halogen is present otherwise, it will show that the chloride, bromide, or iodide is absent.
We can test for individual ions using ion-selective electrodes connected to a meter. These use an electrical method to determine the concentration of a specific ion. However, these devices are not common in the "A" Level laboratory.
Cations positive ions can be identified by flame tests. It's also worth checking the tests for gases. A number of simple chemical tests used to identify different inorganic substances follow:. Warming with dilute sodium hydroxide solution releases ammonia gas. This is an alkaline gas so turns damp red litmus paper blue. It also has a characteristic smell of urine. The addition of a dilute acid releases carbon dioxide gas, so effervescence is seen. Bubbling may be sufficient evidence to identify a carbonate, or the gas identity may be confirmed using the test with limewater.
A carbonate can be distinguished from a hydrogencarbonate by the pH of the aqueous solution. The exact pH will depend on the concentration, but a hydrogencarbonate is typically pH 7. Whereas, a soluble carbonate is typically pH The silver halides except silver fluoride are insoluble in water, so we use a precipitation method to identify the halides.
To test a solution for a halide it is first acidified with dilute nitric acid, then silver nitrate solution is added. Failure to add acid can confuse the test, as alkaline solutions produce a light brown precipitate of silver oxide when the silver nitrate solution is added. Some specifications add the acid after the silver nitrate solution. The silver halide remains insoluble in the added acid.
Look at the way the solubility products vary from silver chloride to silver iodide. You can't quote a solubility product value for silver fluoride because it is too soluble. Solubility products only work with compounds which are very, very sparingly soluble. You can see that the compounds are all pretty insoluble, but become even less soluble as you go from the chloride to the bromide to the iodide. This is a reversible reaction, but the complex is very stable, and the position of equilibrium lies well to the right.
A solution in contact with one of the silver halide precipitates will contain a very small concentration of dissolved silver ions. The effect of adding the ammonia is to lower this concentration still further. What happens if you multiply this new silver ion concentration by the halide ion concentration? If the answer is less than the solubility product, the precipitate will dissolve.
That happens with the silver chloride, and with the silver bromide if concentrated ammonia is used. The more concentrated ammonia tips the equilibrium even further to the right, lowering the silver ion concentration even more. Add five drops of lead nitrate TOXIC solution to the test tube containing potassium chloride solution. A white precipitate of lead II chloride forms. Heat the mixture carefully over a gentle flame until it boils. Avoid using a yellow tipped flame as it will make the tube sooty.
The precipitate dissolves. Place the boiling tube in a beaker of cold water to cool. Fine crystals of lead chloride appear. Repeat steps 2—4 with potassium bromide solution. A white precipitate of lead II bromide forms, which dissolves on heating and recrystallises on cooling.
Repeat steps 2—4 with potassium iodide solution. A yellow precipitate of lead II iodide forms which dissolves on heating to give a colourless solution.
On cooling, fine shimmering yellow crystals of lead II iodide form. Teaching notes Any spillages of silver or lead nitrate on the skin should be washed off with plenty of water.
Additional information This is a resource from the Practical Chemistry project , developed by the Nuffield Foundation and the Royal Society of Chemistry. Level years years. Use Practical experiments Demonstrations. Category Organic chemistry Analysis Reactions and synthesis Separation. The trend in solubility of the silver halides in ammonia.
Students should be able to explain why: silver nitrate solution is used to identify halide ions. The silver nitrate solution is acidified. Ammonia solution is added. Chemical bonding 2.
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