In this video, we are going to take a look at the different tests that we can use in order to identify the different types of anions within our chemical. So, anytime we have a reaction occurring, remember that a reaction only occurs if we produce a solid, liquid, or gas as a product. Based on these phases being formed, ions will also exist within the solution. So you'll make one of these forms or maybe more than one of these forms, solids, liquids, or gases. And you'll have ions floating around as well. We can identify these different ions through a series of tests. For right now, we'll take a look at the anions and later on, we'll take a look at the cations. So, we're going to need room, guys. Let me take myself out of the image.
For the first three ions, we're dealing with chloride ions, bromide ion, and iodide ion. They're all in group 7a. Remember, these are all our halogens. We're not talking about fluoride ion, F⁻, because of certain properties, it's too dangerous to talk about in terms of these reactions. We tend not to discuss it in terms of identifying it. Alright, we're going to say for our chloride ions, bromide ions, and iodide ions, they're all basically the same thing. To a solution, we add nitric acid and then we follow it up with silver nitrate. What's going to happen in each case is that the silver will combine with the halogen. Since all of them have the same exact number in terms of their charges, they're just going to cancel one another out, and then the elements combine. We're going to get silver chloride, silver bromide, and silver iodide. Each one of these compounds is a solid. By adding these reagents, HNO₃, HAgNO₃ to each one of these ions, we're going to form these different solids or precipitates. A solid is equal to a precipitate.
If you don't know what I'm talking about, make sure you go back and take a look at my videos dealing with solution chemistry. We talk about the solubility rules, ions and electrolytes, molecular equations, net ionic equations, and total ionic equations. These concepts are being brought here right now in order to talk about tests we can run to figure out which ions we have. All of them, when we add nitric acid and silver nitrate, produce all these solids. So how do we know which ion we have in particular? Well, each one of these solids has a distinct color to them. Silver chloride will be seen as a white precipitate and that white precipitate will thicken over time, meaning it's going to get cloudy within the beaker or test tube. Silver bromide, it's going to be a creamy white precipitate, slightly different from the pale white of silver chloride. Silver iodide, we're going to get a yellow precipitate. All of these will cause the solution to get cloudier because of the formation of that precipitate. That's how we're able to differentiate which one of these ions we have. If you have an unknown solution and you're trying to investigate which one of these three ions you have, in all of them you add nitric acid followed by silver nitrate. Then you look to see what is the color of the precipitate that forms. Match the color with the ion. If no precipitate forms, that means we don't have chloride ion, bromide ion, or iodine ion.
Next, we're trying to investigate if we have carbonate ion (CO₃²⁻) or bicarbonate ion (HCO₃⁻). Bicarbonate is also called hydrogen carbonate. What's going to happen here is to these things, we add hydrochloric acid, HCl. What is going on reaction-wise? Here, we have carbonate and we add HCl to it. Now HCl is made up of H⁺ and Cl⁻. The positive H⁺ here will combine with the carbonate. Because their numbers are different, they'll crisscross (2 goes to H, 1 goes to CO₃), giving us H₂CO₃. This is carbonic acid, which is not stable. It quickly decomposes, producing water as a liquid and CO₂ gas. So, we're going to get the formation of carbon dioxide gas, which bubbles out of the solution, and is colorless. It's going to change the water to a cloudy white appearance. The same reaction occurs with bicarbonate, because it can also combine with the H⁺ from the HCl, but here since their numbers are the same, they just cancel out and combine together, and you'd again make carbonic acid, which would again produce water and CO₂. So both of these, if we add HCl, will produce carbon dioxide as a gas, indicating that they're present. But how would I be able to tell if I have carbonate instead of bicarbonate? To determine this, we can add magnesium sulfate. If carbonate is present, it will form a white precipitate. If bicarbonate is present, there will be no precipitate. Adding HCl to both will produce CO₂ gas, but to be sure if we have one instead of the other, we do this additional step.
Next, we have sulfate (SO₄²⁻) and bisulfate or what's called hydrogen sulfate (HSO₄⁻). For this test, we can add HCl and then follow it up with barium chloride. The barium will combine with the sulfate to form barium sulfate, which will be a solid. The same reaction occurs whether using bisulfate or regular sulfate; both will produce barium sulfate, a white precipitate. To differentiate between sulfate and bisulfate, one approach is to heat the barium sulfate. If SO₃ gas is produced with light heating, then it means we had bisulfate. If you get SO₃ gas with only strong heating, thatł means we used sulfate. Another distinguishing approach involves adding sodium carbonate. If a lot of CO₂ gas is released, then bisulfate is present. If little to no CO₂ is present, then it means we used sulfate.
Next, we have our sulfite ion (SO₃²⁻). To this, we add hydrochloric acid (HCl). The H⁺ from HCl will react with the SO₃²⁻, crisscrossing numbers, forming sulfuric acid. This sulfuric acid undergoes decomposition to produce water and SO₂ gas. The evolution of SO₂ gas indicates that we have sulfite ion present, and it has a strong, unpleasant smell.
Finally, we have our nitrate ion (NO₃⁻). To this, we can add cold iron(II) sulfate and then follow up with some sulfuric acid. This is called the brown ring test, forming a brown ring within the test tube, indicating the presence of nitrate ion.
Anions are our negative ions. These are the most common types we encounter when performing lab experiments. We've covered the anion portion here and will take a look at the cation portions later on.
