Chemguide

Extra material needed for the 2010 exam - Chapter 11

This page lists the material from the old Edexcel syllabus which isn't covered by my new Edexcel IGCSE Chemistry book.

The fractional distillation of liquid air

I'm not the least bit sure about the status of this. On the old syllabus, you were required to know how both oxygen and nitrogen were obtained by the fractional distillation of liquid air. These were in two different syllabus statements. Bizarrely, Edexcel have said that they aren't going to test the oxygen extraction in 2010, but have left the nitrogen extraction (exactly the same process) untouched.

I suspect that they just forgot to mention it as something to be excluded - there is absolutely no logic in this otherwise. However, I will give a brief summary just in case.

• Dry air, free from carbon dioxide, is compressed to about 200 atmospheres. Compressing it causes it to heat up. The hot compressed air is cooled.

• The cooled compressed air is allowed to expand through a jet, and this expansion causes it to cool further.

• This much colder air is used to cool the next lot of compressed air before it expands. So when it does expand, it is even colder.

• This continues until the incoming compressed air is so cold that when it expands, it liquefies.

• The liquid air is now allowed to warm again. Nitrogen boils at -196°C; oxygen at -183°C. So as the liquid air warms, the nitrogen boils off as a gas first, leaving liquid oxygen behind.

The test for sulfur dioxide

Sulfur dioxide is an acidic gas, turning blue litmus paper red. You can pick out sulfur dioxide from other acidic gases because it turns potassium dichromate paper from orange to green. (Note that potassium dichromate is properly called potassium dichromate(VI), but the "(VI)" is often omitted.)

The test for sulfites

If a dilute acid such as dilute hydrochloric acid is added to a sulfite (either a solid or in solution), sulfur dioxide is formed. Sulfur dioxide is fairly soluble in water, and so the mixture is often warmed to produce enough gas to test.

The test for sulfur dioxide is described above.

If you were asked how to test for a sulfite, you would say "Add dilute hydrochloric acid, warm, and test any gas evolved with potassium dichromate paper. Sulfur dioxide turns the paper from orange to green."

Another test for copper(II) ions in solution

The simple test for copper(II) ions in solution using sodium hydroxide solution is described on page 94. There is a more sensitive test for these ions using ammonia solution, but the chemistry is seriously complicated. You would need to be a good way through an A level chemistry course before you could understand it properly.

If you add ammonia solution to a solution containing copper(II) ions, you get an initial pale blue precipitate of copper(II) hydroxide - similar to the one produced with sodium hydroxide solution.

However, when you add an excess of ammonia solution, this precipitate redissolves to give a very deep blue solution containing the ion [Cu(NH₃)₄(H₂O)₂]²⁺. This is called the tetraamminediaquacopper(II) ion if you like long words!

(Don't worry that the syllabus reverses the order of the ammonia and water molecules in this formula. The version here is the normally accepted one. The ammonia and water molecules are called ligands, and it is conventional to name and write ligands in alphabetical order - "ammine" comes before "aqua".)

If you want a couple of simple pictures of this reaction, you could have a look about half-way down this Chemguide page. It would be better to ignore the text!

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