Discussing Ions/Three Ion Tests

This is another revision article that I might want to come back to here and then, but I also want to put up on this blog - it's interesting, after all.

Ions are charged particles which come about as a result of elements either losing or gaining electrons in order to become more stable. This is as they can then have a full outer shell of electrons, much like atoms of noble gases, which are well known for not being reactive. 

Most elements form ions, with the only exceptions being the noble gases as well as elements in Group 14 of the Periodic Table, because they have four valence electrons, so instead they form covalent bonds. Some ions contain a variety of elements, and are complex - these include SO₄^2- (a sulphate ion) and NH₄⁺ (an ammonium ion). Sometimes these ions may appear unusual, such as H₃O⁺ (a hydronium ion), which occurs due to water undergoing protonation; and some ions are more complex than others, usually consisting of a metal ion bonded with a series of non-metal ligands (such as Al(H₂O)₆³⁺ - six water molecules bonded to a central aluminium ion, and some electrons don't bond, hence the charge). More can be found on those kinds of complex ions here.

Some elements can be rather indicisive when it comes down to forming ions, especially the transition metals, which can often form ions of various charges. Take iron, for example - there is the Fe²⁺ ion and the Fe³⁺ ion, along with many other examples. So iron oxide could be FeO or Fe₂O₃, and the ions could only be differentiated by noticing their oxidation number, which corresponds to the ionic charge. But even these aren't consistently applied, and there are a few exceptions (one relating to hydrogen, which could form a cation (H⁺) or an anion (H^-, hydride)). But that's to be expected in chemistry.

There are two types of ion - those which are positively charged (cations) and those which are negatively charged (anions). Cations contain fewer electrons than protons, and the opposite is true with anions. And as generally works in charged particles, oppositely charged ions will attract each other to form ionic bonds. But that's all rather obvious, and I'd rather blog about ion tests instead.

One great thing about studying chemistry is that you get to occasionally react some chemicals with each other and see what happens - and some of those reactions can be used to find out which ions are present in a compound, usually as one of the products will contain that ion. 

Carbonate (CO₃^2-)

Take for example the reaction of a solution with acid, which occurs in a test tube. Provided you bubble the products of this solution through a boiling tube containing limewater (calcium hydroxide solution), which can be done by connecting a gas syringe as in the picture attached, you might be able to spot a white precipitate forming, due to the production of calcium carbonate. If this happens, there ought to be carbonate ions present in the solution. I did this test a few days ago, albeit slightly differently, because instead of following that methid, you could simply react the solution with acid and spot if there were any bubbles - if so, carbonate ions should also be present. 

Halides (Cl^-, Br^-, I^-)

Unlike the carbonate test, these tests involve a colour change.

The process is simple - take your solution and react it with aqueous ammonia to remove any ions which could distort the test results. Afterwards, react the solution with silver nitrate, and any product should be a different colour.

• If no colour change occurs, fluoride ions are present (since silver fluoride is soluble);
• If the resulting product (a silver halide) is white, chloride ions are present;
• If the silver halide is cream, bromide ions are present;
• If the silver halide is yellow, iodide ions are present.

Maybe you want to be completely certain, however, with the last three - the resulting colours could be subjective, after all. If so, react the solution with ammonia solution, and you should get this:

• The silver chloride dissolves completely;
• The silver bromide will only dissolve in highly concentrated solutions of ammonia;
• The silver iodide shouldn't dissolve at all. 

 A four step process to confirm the halide present.

Flame tests

I didn't do any flame tests, but they're interesting so I'll mention them here. They only work with cations.

Certain elements will produce different colours if a solution of a compound is exposed to a Bunsen burner flame. These tests do however depend on whether the device used to apply the solution to the flame is clean or not, as you could get unreliable results, as well as there being the risk of a more reactive element masking a less reactive element. It's thus better to clean the device using an acid before each flame test.

The colours that occur as a result of each flame test is due to the electrons in the tested element becoming over-excited when it encounters the flame, which will cause them to move between energy levels. If they move closer to the nucleus, energy is absorbed, and if they move further away, energy is released - and this causes about one photon of light to be released. The greater the frequency of the photons, the greater its energy (which is also the same if the electrons are further from the nucleus, and so have greater movement), and thus the ions will come off as more blue than with photons with a smaller frequency. A better, more detailed explanation can be seen here.

One place where these colours come up is in flame emission spectroscopy, which is a more expensive, albeit more effective, method of identifying elements in compounds. As each colour corresponds to a different wavelength, you can compare the different wavelengths of elements with the compound to identify which ones are present. However, in many humble labs, flame tests will continue to be more used (until the costs go down, that is).

The photo of flame tests is courtesy of Wikipedia - https://en.wikipedia.org/wiki/File:Coloured_flames_of_methanol_solutions_of_metal_salts_and_compounds.jpg