Barium, Calcium, Magnesium and Flame Tests

Transcript

0:00Introduction

Hello everyone! Let us start with a small puzzle from the chemistry lab. Calcium, barium and magnesium come from the same family of the periodic table. Put a calcium salt in the flame, and the flame turns brick red. Barium turns it apple green. And magnesium gives no colour at all. Same family, so why does magnesium stay silent? Keep that puzzle in mind, because we will solve it today. We will also learn how to catch these three cations in the test tube, one by one. These tests come up often in JEE practical chemistry. So, let's begin.

0:36Group V: barium first, with potassium chromate

Let us begin with group five; the capital V in the heading is the Roman numeral five. In group five, barium and calcium come down as carbonates, barium carbonate and calcium carbonate. In both formulas, the small three below the O is a subscript, for three oxygen atoms. Okay? Next, we dissolve these carbonates in hot dilute acetic acid, and then we test the solution in a fixed order, with barium first. Right? For barium, add potassium chromate. Barium two plus gives a yellow precipitate of barium chromate; the raised two plus on screen is the charge of the barium ion. So yellow here means barium. Clear? Now let me ask you this. Hydrochloric acid also dissolves carbonates, so why do we use the weak acetic acid here, and not hydrochloric acid? Because barium chromate is soluble in strong acids. In hydrochloric acid, H C l on screen, the yellow precipitate would simply not form, and you would miss barium. In the weak acetic acid, it does precipitate. Got it?

1:42Calcium: white oxalate, but only after barium

Now let us test for calcium. Add ammonium oxalate, and calcium two plus gives a white precipitate of calcium oxalate; in its formula, C two O four is the oxalate group. And calcium oxalate is insoluble in acetic acid, so it stays as a precipitate. With me? So here is a quick question. You add ammonium oxalate straight to the group five solution, and a white precipitate appears. That means calcium, surely? That is exactly the trap, because barium also gives a white oxalate. So a white oxalate alone does not prove calcium. Right? That is why barium is tested first. We remove barium completely as the yellow chromate. Calcium chromate, in round brackets on screen, is soluble, so calcium stays behind in the solution. Clear? Then we filter, and test the filtrate with ammonium oxalate. Now a white precipitate really does mean calcium. That is the whole logic of the order: barium first, then calcium.

2:44Magnesium: group VI, a white crystalline phosphate

Got it? So where did magnesium go? It never came down in group five. The ammonium salts in the group five reagent keep the concentration of carbonate ion too low for magnesium carbonate. The square brackets mean concentration, and the raised two minus is the charge on the carbonate ion. Okay? So magnesium waits for group six, the Roman six on screen. The reagent is disodium hydrogenphosphate, with ammonium chloride and ammonium hydroxide, N H four C l and N H four O H on screen. With me? And then, on scratching the tube, a white crystalline precipitate appears. This is magnesium ammonium phosphate; the ammonium group, N H four, sits in round brackets in its formula. Remember both words, white and crystalline. Clear? Here is the equation for this confirmatory test. Magnesium two plus, the hydrogenphosphate ion with its two minus charge, and ammonia give magnesium ammonium phosphate; the arrow means gives. Right?

3:47The flame test

Now let us look at the flame test. The figure on the right shows the flame colours of the cations in our course. The salt is first moistened with concentrated hydrochloric acid. Then, as the note under the figure says, a clean platinum wire is dipped in the acid, then in the salt, and held in the non-luminous flame. Okay? Let me ask you this. Why add concentrated hydrochloric acid at all? Why not hold the dry salt straight in the flame? Because the acid turns the salt into the metal chloride, and chlorides are volatile enough to colour the flame. The acid also cleans the wire. Right? Next, the wire itself. We use platinum because it is unreactive, it has a high melting point, and it gives no flame colour of its own. So any colour you see comes from the salt. Clear? Now, the origin of the colour. The heat of the flame raises electrons of the metal atoms to higher levels. When they fall back, they give out light of definite wavelengths, and that light is the colour we see. With me? So, calcium two plus gives a brick red flame, and barium two plus an apple green flame, just as in the figure. The figure also shows two cations from our earlier lessons: copper, green with a blue centre, and lead, pale bluish white. Okay? Now, back to our opening puzzle. Calcium and barium colour the flame beautifully. So why does magnesium give no colour at all? The answer is in its electrons. The small magnesium atom holds its electrons too tightly, so the flame cannot excite them to levels whose light lies in the visible region. Beryllium two plus behaves the same way. So there is no colour, and that solves our puzzle. Got it?

5:34Example 1: apple-green flame and yellow chromate precipitate

Alright, example one, an easy one. Here, a salt gives an apple green flame, and a yellow precipitate with potassium chromate in acetic acid. Which cation is present? Pause the video and try it first. Done? Okay, let's check. The first clue is the yellow precipitate with potassium chromate in acetic acid. That is barium chromate. Right? The second clue is the apple green flame, and that points to barium again. Okay? So the cation present is barium two plus. Two clues, and both point to the same answer. Very good if you got that!

6:12Example 2: identify a salt from group VI and BaCl2 tests

Now a harder one, which mixes cations and anions. Here, a colourless solution gives no precipitate in groups one to five. But with disodium hydrogenphosphate and ammonia, it gives a white crystalline precipitate. It also gives a white precipitate with barium chloride, which does not dissolve in hydrochloric acid. Identify the salt. Pause the video and try it first. Done? Okay, let's check. Nothing came down in groups one to five, so the cation must wait for group six. Okay? And in group six, the white crystalline precipitate with disodium hydrogenphosphate and ammonia is magnesium ammonium phosphate. So the cation is magnesium two plus. Right? Now look at the barium chloride clue. Some students write: white precipitate with barium chloride, so the salt has barium. Is that right? Not at all, because that barium came from the reagent itself. This test tells us the anion. Barium ions meet sulphate ions and give white barium sulphate, which does not dissolve in hydrochloric acid. We saw this test in the anions chapter. So the anion is sulphate, S O four two minus. With me? Put the two together. Magnesium two plus and sulphate: the salt is magnesium sulphate. See how each test answers one small question? That is how salt analysis works.

7:41Quick revision: Ba, Ca and Mg tests and their flame colours

So, quick revision. Barium gives a yellow chromate in acetic acid, and an apple green flame. Calcium gives a white oxalate, but only after barium is removed, and a brick red flame. Magnesium gives white crystalline magnesium ammonium phosphate, and no flame colour, because its electrons are held too tightly. Now go and try the four practice problems on this page. All the best!