Chemical Decomposition

Introduction

The kinds of changes you've just been working with--such as solid to liquid, liquid to gas, these kinds of things--are generally referred to as physical changes. The same material is present, but the condition changes. There are other changes or processes in which the material itself is changed, not just the condition of the material. These changes that go beyond just simply changing phase or mixing and separating materials from one another are called chemical reactions. In chemical reactions the materials themselves change. In doing so they sometimes give off energy and sometimes they require energy. Let's take a look at some of them now.

Electrolysis of Water

Let's consider a specific example: water. Pure water is a material which can go through distillation without a change in it properties. That is, it is water before and it is water afterwards. If it started as impure water, as it goes through the process it is separated from the impurities and becomes pure water. You can distill pure water over and over again; you can freeze it; you can thaw it; and it remains water throughout all those phase changes. It can, however, be separated into two components by a process known as electrolysis. This process will decompose water into two components, both are gases.

We have an electrolysis apparatus set up with instructions for your operation at the demonstration table. We will get to that in a moment.
	After it has been operating for a while and a sufficient amount of each gas has been formed, you will be able to collect it in a test tube by inverting the test tube over the opening and turning the stopcock to release the gas. When almost all of the gas is released into the test tube, close the stopcock and put your thumb over the end of the test tube and you've got the gas. Try to avoid having any liquid come up into the stopcock. You will collect two gases, one at a time, in separate test tubes.
You should test these gases one at a time by two different methods. One of the gases will pop when a flaming splint (a small piece of wood) is held next to the mouth of the test tube. The gas that does this is hydrogen.
The second test is to put a glowing splint into the test tube and observe whether the glow gets brighter or goes out. Oxygen will make the splint glow brighter. Sometimes it will actually make the splint light up again.

You should do these tests in this order: Hold a burning splint by the mouth of the test tube (to test for hydrogen); then, if it doesn't pop, blow out the flame and put the still glowing splint into the test tube (to test for oxygen). If you are in the lab, take time now to observe the decomposition of water by electrolysis and test the products. It is Exercise 6. If not, you can do that later.