2.9 Chemical Tests
Modern chemistry is built on a collection of thousands of ideas and theories proposed by numerous scientists over the past centuries. The accepted theories are backed by experimental evidence that can be replicated over and over again. Each and every concept that you are learning has undergone rigorous experimentation for it to be accepted as a fact by the scientific community. Therefore, we have to understand the importance of chemical tests.
We cant just look at a solid or solution in a test tube and identify its composition. But using s chemical test can help us quickly verify if a reaction has happened and what kind of products have been formed. We can use these chemical tests to quickly verify if the theory we are learning is actually. There are range of tests that chemists perform to confirm experimental results. The complexity of these test range from very quick simple and requiring minimal equipment to very complicated tests that require complex equipment and a longer time frame. The tests we will learn in this chapter are not very complicated, nevertheless they are very useful.
I suggest you memorize as many of these reactions and tests as possible because it will help you a lot down the line if you plan to study chemistry for A-Levels. It will also improve your understanding of the concepts since you can visualize results of experiments wihtout having to perform the reactions.
Let's now look at a couple examples of chemical tests:
Testing for Gases
Most gases are colorless and odorless. Even if they have a smell it is not a very smart idea to go about sniffing these gases hoping to identify it. The follwing test can be performed for some common gases like hydrogen, oxygen, chlorine, etc.
Oxygen is a flammable gas. If you put a glowing splint into a test tube containing oxygen, the splint will relight almost immediately.
Hydrogen is another colorless and odorless gas. Hydrogen is highly flammable; therefore, if you have a significant concentration of it in the presence of oxygen and heat it will burn explosively. If you hold a lit splint to hydrogen in a test tube, it will burn immediately with a squeaky pop sound.
Carbon Dioxide (CO2)
Carbon dioxide is a colorless gas that is weakly acidic. To perform a test for this gas you need lime water. Lime also known as calcium hydroxide is a base that can be dissolved in water to form a solution. If carbon dioxide, which a acidic, is bubbled into this lime water solution, a white precipitate will be formed. This causes the solution to turn milky white.
Chlorine gas is toxic and yellow-green in color. It has a strong smell and dissolves in water to form an acid. It has the ability to bleach dyes. For this test you can use a piece of litmus paper. The litmus paper has to be damp so that the chlorine gas can dissolve in it. If you hold a piece of blue litmus over the test tube containing the gas, then it will first turn red to show an acid has formed before being bleached to white by the dissolved gas.
Ammonia is a colorless gas with a very pungent smell. It is an alkaline gas that is soluble in water. If you hold a piece of damn red litmus paper, the gas that dissolved in the paper will turn it blue to indiccate an alkaline.
The flame test can be used to test for metal cations in a compound. Some metals show different colors when held onto a flame. This method does not work for all metals because some metals do not show any color. However, it is effective and simple method for identifying a group 1 and group 2 metals.
All you have to do is dip a nichrome or platinum wire in hydrochloric acid and then dip it into the compound being tested. Then hold it next to a flame and observe the color change of the flame.
Nichrome or platinum is used because they do not have a flame color. The hydrochloric acid is used as a way to making sure the wire is clean and also as a means to keep the compound being tested on the wire.
The flame colors of some metals are shown below:
Testing for Cations
Sometimes the flame test cannot be used because either the metal doesn't have a distinct flame color or because the unknown compound is in a solution. There could be other reasons as to why the flame test cannot be used. Therefore when testing for unknown compounds you would have to predetermine what the best test might be based of of any assumptions you have of the compound.
If you have reason to assume that the unknown compound contains metal cations or an ammonium ion the following test could be used. For this test, you need sodium hydroxide or ammonia solution. First the unknown compound has to be dissolved to form an aqueous solution so that the ions in the compound could separate. The sodium hydroxide or ammonia solution can then be added. What happens now is that the metal cation, which has dissolved and separated from the unknown compound will combine with a hydroxide anion to form a precipitate. If a precipitate forms you can be certain that the compound has a metal cation. However, for some metal cations the precipitate will be colored, observing this color will help you determine what exactly the metal is.
When testing for the ammonium ion , the unknown compound soes not have to be aqueos. If an ammonium ion is present in the unkown solid or solution then a gas a given off when aqueous sodium hydroxide is added. This gas is ammonia
The expected results of this test for different types of cations is shown below: