INTRODUCTION Chemical reaction occurs continuously in the atmosphere, in factories, in vehicles, in the environment and in our bodies. In a chemical reaction, one or more kinds of matter are changed in a new kind – or several new kinds – of matter. Life as we know it, could not exist without these processes: plants could not photosynthesize, cars could not more, pudding could thicken, muscles could not burn energy, glue could not stick and fire could not burn. With these few jesters in mind, we can define chemistry. CHEMISTRY This is said to be the (study) science of composition, structure and the properties of substances and how they react with other substances to form new substances with the addition or removal of energy in any of its forms. There are two processes by which changes take place in the subject matter of chemistry: Physical Chemical It is important to realize that many aspects of substance are not included in the subject matter of chemistry. For instance, water may be cooled, and at a certain temperature freezes and forms ice; the temperature at which this happens is (0˚) which is an important fact about water, but this is physical rather than a chemical property. This is mainly because the process of freezing water is merely changing from the liquid state to the solid and is not changing into a new substance. Freezing is therefore regarded as a physical rather than a chemical process. On the other hand, when magnesium ribbon is heated in a flame, it burns brilliantly and the white powder which is formed is a completely new substance; a chemical process has taken place in this case.
There are several important differences between physical and chemical processes:Figure i. PHYSICAL AND CHEMICAL PROCESSES PHYSICAL 1. No new substance is formed 2. Usually easily reversible 3. Energy change involved is usually small CHEMICAL 1. A new substance is always formed 2. Not easily reversible 3. Large energy change may be involved
THE CLASSIFICATION OF SUBSTANCES Matter is that which has weight and occupies space. Matter is the name given to all substances and can exist in any one of three states – solid liquid or gas. Some forms of matter eg. water may be found quite commonly in all three states, but many substances at normal temperature are found only in one or two of the states. Because the total number of substances is so large, running into millions, it is helpful to think about them grouped into different classes rather than as individuals. Substances can be grouped into three classes: i. ii. iii. Elements Element can be defined as substances that cannot be broken down by chemical treatment into any simpler substances. Elements consist of atom of the same type. Out of millions of substances that exist, this statement applies to about a hundred. Iron for example, is a familiar substance that part in many chemical processes. If it is left for some time in a moist atmosphere, it will change into a new substance, rust. Analysis of the rust shows that along with the iron, it also contains oxygen. Again, iron may be dissolved in sulphuric acid. If the resulting liquid is evaporated, green crystals are obtained. If these crystals are analyzed, they will be found to contain not only iron but sulphur and oxygen. In these two elements mixtures compounds
reactions and in all other reactions in which iron takes part, the iron is converted into a more complicated substance. Apparently there is not chemical treatment which can convert iron into a simpler substance than it already is. Substances such as iron are placed into a class called elements. By using these simplest substances as building blocks and by putting them together in two different ways, all other substances known to exist can be made. The group of elements may be divided into sub groups in several different ways – for example into solids (such as iron and sulphur), liquids (such as mercury and bromine), and gases...