Test for Ions and Gases - Video Tutorials & Practice Problems
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Below are the useful tests that can be employed to identify ions and gases within typical laboratory procedures.
Test for Ions
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Test for Anions
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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 a gas as a product. Now 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. Now 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 gonna need room guys. So let me take myself out of the image. So 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 minus, because of certain properties, it's too, dangerous to talk about in terms of these reactions. So we tend not to discuss it in terms of identifying it. Alright. So we're gonna 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. Now, what's gonna happen in each case is that the silver will combine with the halogen. K. So here, since all of them have the same exact number in terms of their charges, they're just gonna cancel one another out and then the elements combine. So we're gonna gonna get silver chloride here. We're gonna get silver bromide and silver iodide. Here's the thing. Each one of these compounds is a solid. So by adding these ingredients or reagents HNO3, HAGNO3 to each one of these ions we're gonna form these different solids or precipitates. K. So a solid is equal to a precipitate. Now if you don't know what I'm talking about, make sure you go back and take a look on my videos dealing with solution chemistry. When we talk about the solubility rules, when we talk about ions and electrolytes, when we talk about molecular equations, net ionic equations and total ionic equations. This those concepts are being brought here right now in order to talk about tests we can run to figure out which ions we have. Now here's the thing. 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. K? And that white precipitate will thicken over time. So thicken over time. So what's gonna happen here is it's gonna get cloudy within the beaker. K. So let's say or a test tube. Let's say we have a test tube here. You're gonna have your white precipitate down here. I know it's black but it's we need oh it's gonna be white precipitate. And the solution itself is gonna get cloudy over time. That's what I mean by thickening. Silver bromide, silver bromide, it's gonna be close to white precipitate like silver chloride, but it's gonna be more a creamy white precipitate. It's gonna look slightly different. So silver chloride is a pale white. Silver bromine is a creamy white. And then silver iodide, we're gonna get a yellow precipitate. All of these will cause the solution to get cloudier because of the formation of that precipitate. So that's how we're able to differentiate which one of these ions we have. So if you have an unknown solution and you're trying to investigate which one of these 3 ions you have, in all of them you add nitric acid followed by silver nitrate. And 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 or we have bicarbonate ion. So carbonate is this and bicarbonate is this. Now bicarbonate is also called hydrogen carbonate as well as, as well as bicarbonate. So what's gonna happen here is to these things we add, hydrochloric acid, HCl. Now what is going on reaction wise? So here we have carbonate and we add HCl to it. Now HCl is made up of H+ and Cl-. What's happening here is the positive h here is gonna combine with the carbonate here. And because their numbers are different, they're gonna crisscross. 2 goes there and 1 comes here to give us h2c03. This is carbonic acid which is not stable. So when it forms in solution, it doesn't stay that way. It quickly decomposes, which means it breaks down to produce water as a liquid, plus c o two gas. So So here we're gonna get the formation of carbon dioxide gas. Okay. So what we're gonna say here is we're going to get, basically c02 gas bubbles out of the solution. And gonna say, c02 is colorless, has no color to it. K. And we're gonna say here, when it does this, it could change this is gonna change the water to cloudy white. Now the same thing can be done if we used bicarbonate. Because it would also combine with the H+ from the HCl. But here since their numbers are the same, they just cancel out and combine together and again you'd make carbonic acid, which again would produce water and CO2. So both of these things, 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? What additional thing could I do? Well what I could do here additionally additionally I could add If we add magnesium sulfate, that'll help me determine if I have carbonate or bicarbonate for sure. So if I add magnesium sulfate, what it's gonna do here, it's gonna form a white precipitate if carbonate is present. And then here, if present. And then here, if bicarbonate is present, we will have no precipitate form. That's how we're able to differentiate the 2. So again, adding HCl to both will produce c o two gas. But if we wanna be sure if we have one instead of another, we do an additional step. We add magnesium sulfate. If carbonate is present, we form a white white precipitate. If no carbonate is present, we get no precipitate. Next, we have sulfate and bisulfate or what's called hydrogen sulfate. So for this one, we can add HCl and then we follow it up with barium chloride. And what happens here is if we have the barium chloride, the barium is gonna connect can connect with the sulfate to give me barium sulfate which will be a solid. The same thing would happen whether I used bisulfate or just regular sulfate. They both will produce this which would be a white precipitate. But what if I wanted to make sure I had one ion instead of another? What could I do? Well to differentiate between sulfate and bisulfate what I could do is I could add sodium carbonate. Now here, if I add actually, we don't even need to do this yet, but we could do instead of that, we can heat the solid. So we heat the barium sulfate that we have. K? And we're gonna say here, if we get s o 3 gas is produced with light heating, then that means that we had our bisulfate. But if you get s l 3 gas with only strong heating, That means we used sulfate. K. So that's how we're able to differentiate which one of these ions is truly present. Now here, like I said, so that's one thing we could do. Another thing that we could do to differentiate between the 2 is we could add sodium carbonate. Now if we add sodium carbonate, we're gonna say if a lot of CO 2 gas is evolved, meaning released, then bisulfate is present. Okay. So if you're releasing a lot of gas, bisulfate is present. If little to no c o two is present, then that means that we used sulfate. So again, these are important to know just in case you have a lab practicum or you have, an experiment that's going on where they're asking you to identify different types of anions. These are the most common types of anions and the tests that we can use to figure them out. Next, we have our sulfite ion. So to this sulfite ion we add hydrochloric acid so we add HCl. And we're gonna say here when we add that HCl so we're gonna have s03 two minus and then we have HCL which is made up of h+clminus. So the H+ and the SO 3, 2 minus are gonna react. So the 2 from here comes here and the 1 from here comes here. And what do we get? We get sulfuric acid. Now sulfuric acid is similar to carbonic acid that we saw earlier. This will undergo, decomposition as well to produce liquid as water plus SO2 gas, sulfur dioxide gas. So here we'll have the evolution or evolving of s o two gas, which indicates that we have sulfite ion present. And one way you can tell that you have s o 2 present is it'll be a strong smell, not a pleasant smell either. So you'll smell that s o two being produced and it smells really bad. So that indicates that our sulfide ion was present. Now finally we have our nitrate ion, NO3-. So to this we can add cold iron to sulfate and then we follow and then we follow-up with some sulfuric acid. Now we're gonna say that this is called the brown ring test. So basically from the name, you're gonna form a brown ring within the test tube. So this lets us know that we have the presence of nitrate ion within this solution. So again, anions are our negative ions. These are the most common ones that we fall, into contact with when we do a lab experiment. And these are just some of the tests that we can run-in order to identify them. Some of them have common tests, so you have to look at the precipitate that forms, what color is it, to help us differentiate the different types of halide anions that we have. And then for other ones we have to do an additional step in order to differentiate if it's one ion versus another. So we have covered the anion portion here. We are going to take a look at the cation portions later on.