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General Chemistry

Learn the toughest concepts covered in Chemistry with step-by-step video tutorials and practice problems by world-class tutors

4. BONUS: Lab Techniques and Procedures

Test for Ions and Gases

Below are the useful tests that can be employed to identify ions and gases within typical laboratory procedures. 

Test for Ions

Test for Anions

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in this video, we're gonna take a look at the different tests that we can use in order to identify the different types of an ions within our chemical solution. So any time we have a reaction occurring, remember that a reaction on Lee occurs if we produced 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 gasses, 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 and ions and later on we'll take a look at the cat ions. 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 seven A. Remember, these are all our Hala Jin's. 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. All right, so we're gonna say for chloride ions, bromide ions and iodide ions, they're all basically the same thing to a solution. We add nitric acid, and then we followed up with silver nitrate. Now, what's gonna happen in each case is that the silver will combine with the halogen. Okay, 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 combined. So we're 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 re agents a Channel three h a g n 03 to each one of these ions, we're gonna form these different solids or precipitates. Okay, 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 soluble ity rules. When we talk about ions on 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 test, we can run to figure out which ions we have. Now, here's the thing. All of them when we add nitric acid in silver nitrate produce all these solid. So how do we know which I on 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. Okay. And that white precipitate will thicken over time. So thinking over time. So what's gonna happen here is it's gonna get cloudy within the beaker. Okay, so let's say or 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 we need it's gonna be a 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 toe white precipitate like silver chloride, but it's gonna be more of a creamy white precipitated if it's gonna look slightly different. So silver chloride is a pale white silver Brahman is a creamy white. And then silver are you die. We're gonna get a yellow precipitate. All of these will cause the solution 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 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, 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 I'd I'd iron 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 buy carbon is also called hydrogen carbonate as well Azaz well as bicarbonate. So what's gonna happen here is to these things we add, um, 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 plus and seal minus 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. Two goes there and one comes here to give us hte 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 co two gas. So here, we're gonna get the formation off carbon dioxide gas. Okay, so what we're gonna say here is we're going to get basically sealed to gas bowls out of a solution, and I'm gonna say co two is colorless has no color to it. Okay? And we're gonna say here when it does this, it could change. This is going to change the water. Two cloudy, white. Now the same thing can be done if we used bicarbonate because it would also combined with the H plus from the HCL. But here, since their numbers are the same, they just cancel out and combined together and again you'd make carbonic acid, which again would produce water and co two. So both of these things, if we had 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 just not really. I could add if we add magnesium sulfate that'll hopefully determine if I have carbonate or bicarbonate for sure. So if I add magnesium sulfate, what it's gonna do here is gonna form a white precipitate if carbonate is present and then here. If bicarbonate is present, we will have no precipitate form. So that's how we're able to differentiate the two. So again, adding HCL to both will produce sealed to 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 president, Well, former white precipitate. If no carbonate is present, we get no precipitate. Next we have sulfate and by sulfate or what's called hydrogen sulfate. So for this one, we can add hcl and then we followed up with barium chloride. And what happens here is if we have the barium chloride, the Bering is gonna connect connect with the sulfate to give me barium sulfate, which will be a solid. The same thing would happen whether I used by sulfate or just regular sulfate, they both were produced. This which would be a white precipitate. But what if I wanted to make sure I had one eye on instead of another? What could I do? Well, to differentiate between sulfate and by sulfate. 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. Okay? And we're gonna say here if we get eso three gas is produced with light heating, then that means that we had are by sulfate. But if you get Essel three gas with Onley strong heating, that means we used sulfate. Okay, 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 two as we could add sodium carbonate Now, if we add sodium carbonate, we're gonna say if a lot off co two gas is evolved, meaning released than by sulfate is present. Okay, So if you're releasing a lot of gas by sulfite is present. If little to no co two is present, then that means that we used to sulfate So again, these air important to know, just in case you have a lab practicum or you have a new experiment that's going on when they're asking to identify different types of on an ions. These are the most common types of an ions and the tests that we can use to figure them out. Next, we have our sulfide ion. So to this sulfide ion, we add hydrochloric acid. So we had HCL and we're gonna say here when we add that hcl so we're not gonna have s 03 to minus. And then we have HCL, which is made up of H plus and C L minus. So the age plus in the S +03 to minus, they're gonna react. So the two from here comes here and the one from here comes here and what do we get? We get sulfurous acid. Now, sulfurous acid is similar to carbonic acid that we saw earlier. This will undergo decomposition as well to produce liquid water plus eso to gas sulfur dioxide gas. So here we'll have the evolution or evolving off eso to gas, which indicates that we have sulfide ion present. And one way you can tell that you have eso to present is it will be a strong smell, not a pleasant smell either. So you'll smell that s 02 being produced and it smells really bad, so that indicates that our sulfide ion was present. Now, finally, we have our nitrate ion and all three minus. 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 going to say that this is called the brown ring test, So basically from the name you're gonna form a brown rain within the test tube. So this lets us know that we have the presence of nitrate ion within this solution. So again and ions are negative ions, these are the most Commons 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 orderto identify them. Some of them have common tests, so you have to look at the precipitate that forms. What color is it? Help us differentiate different types of Hallett on ions 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've covered the an eye on portion here. We're gonna take a look at the cat ion portions later on.