C3 Topic 3 – Chemical Detection.
There are various reasons why you might want to analysis a mystery substance. Forensic science involves investigating substances or objects that may have been involved in a crime. For example, analysing the alcohol content of a driver’s blood or checking for the presence of toxic substances in food and drink could both provide evidence for a court case. It’s in important to know what substances are present in food. Obviously, nobody wants their food to be contaminated by toxic substances. And some people have allergic reactions to substances in certain foods e.g. some food additives, some natural substances in nuts. Every day water companies check the purity of the drinking water they provide by analysing tap water for potentially dangerous chemicals. As part of their quality control process, manufacturers need to know their materials in their products are of the required purity. For example, copper wire needs to be of high purity – if it’s not pure enough, it doesn’t conduct electricity as well as it should. But not all users of a substance need the same purity – copper for water pipes does not need to be that pure.
Qualitative analysis tells you if a particular substance is present. This might be used for detecting poisons in food. You don’t need to know how much there is because even a trace can cause problems. Quantitative analysis tells you how much of a substance is present. This sort of analysis would be used in drink driving cases when the police need to know how much alcohol is present in the driver’s blood. If the amount of alcohol is higher than 80 mg per 100ml of blood, then the driver can be prosecuted.
Testing ionic compounds means identifying two different ions. Qualitative analysis could mean doing tests to work out the identity of a mystery substance. Lots of substances are made up of different bits e.g. Ionic substances all have a positive bit and a negative bit. For these, you can identify the different bit separately. So say your mystery substance is an ionic compound – you’d carry out a series of experiments to identify the positive ion, and another series of experiments to identify the negative ion. It is possible for your mystery compound to contain more than two ions but that doesn’t matter as you do the same tests. It would be pretty useless if you do not get a unique result for each ion because there would be no point in analysing the compound. You need a definite result.
Test for positive ions.
You should test for positive ions first. Flame tests. Compounds of some metals give a characteristic colour when heated. Sodium – orange. Potassium – lilac. Calcium – brick red. Copper – blue green. Add a substance to sodium hydroxide and look for the colour precipitate. Many metal hydroxides are insoluble and precipitate out of a solution when formed. Some of these hydroxides…