Introduction  (Page 2/3)

Chemical changes in matter

When a chemical change takes place, new substances are formed in a chemical reaction. These new products may have very different properties from the substances that were there at the start of the reaction.

The breakdown of copper(II) chloride to form copper and chlorine is an example of chemical change. A simplified diagram of this reaction is shown in [link] . In this reaction, the initial substance is copper(II) chloride, but once the reaction is complete, the products are copper and chlorine.

Chemical change

The formation of new substances in a chemical reaction. One type of matter is changed into something different.

There are some important things to remember about chemical changes:

1. Arrangement of particles
   During a chemical change, the particles themselves are changed in some way. In the example of copper (II) chloride that was used earlier, the CuCl ₂ molecules were split up into their component atoms. The number of particles will change because each CuCl ₂ molecule breaks down into one copper atom (Cu) and one chlorine molecule (Cl ₂ ). However, what you should have noticed, is that the number of atoms of each element stays the same, as does the total mass of the atoms. This will be discussed in more detail in a later section.
2. Energy changes
   The energy changes that take place during a chemical reaction are much greater than those that take place during a physical change in matter. During a chemical reaction, energy is used up in order to break bonds, and then energy is released when the new product is formed. This will be discussed in more detail in "Energy changes in chemical reactions" .
3. Reversibility
   Chemical changes are far more difficult to reverse than physical changes.

We will consider two types of chemical reactions: decomposition reactions and synthesis reactions .

Decomposition reactions

A decomposition reaction occurs when a chemical compound is broken down into elements or smaller compounds. The generalised equation for a decomposition reaction is:

AB $\to$ A + B

One example of such a reaction is the decomposition of hydrogen peroxide ( [link] ) to form hydrogen and oxygen according to the following equation:

2H ₂ O ₂ $\to$ 2H ₂ O $+$ O ₂

The decomposition of mercury (II) oxide is another example.

Experiment : the decomposition of mercury (ii) oxide

Aim

To observe the decomposition of mercury (II) oxide when it is heated.

Note: Because this experiment involves mercury, which is a poisonous substance, it should be done in a fume cupboard, and all the products of the reaction must be very carefully disposed of.

Apparatus

Mercury (II) oxide (an orange-red product); two test tubes; a large beaker; stopper and delivery tube; Bunsen burner; wooden splinter.

Method

1. Put a small amount of mercury (II) oxide in a test tube and heat it gently over a Bunsen burner. Then allow it to cool. What do you notice about the colour of the mercury (II) oxide?
2. Heat the test tube again and note what happens. Do you notice anything on the walls of the test tube? Record these observations.
3. Test for the presence of oxygen by holding a glowing splinter in the mouth of the test tube.