Chemical and physical changes

Molecules

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As an atom is the smallest part of an element, and as a compound must contain at least two elements, it follows that the smallest part of a compound that can exist must contain at least two different atoms. This is called a molecule. Although the atoms of elements take part in chemical reactions, they very often do not normally exist as single atoms when they are free. The atoms of such gases as oxygen, hydrogen, nitrogen and chlorine may be considered as very friendly ones, as they almost always occur in pairs, each pair forming a molecule, the smallest particle able to exist alone. On the other hand, the atoms of such gases as argon, helium and neon seem to prefer to remain alone. As the molecules of these gases contain only one atom, they are said to be monoatomic in contrast to the former which are diatomic. Some compounds are so complex that a single molecule may contain several hundreds of atoms and be quite large in comparison with a molecule of hydrogen, the smallest of all. It is possible to take photographs showing the way in which such large molecules are made up by means of a special type of microscope called an electron microscope, but these molecules arc still too small to be seen through the most powerful of optical microscopes.

An iron rod held in the fire long enough increases in energy content until it becomes too hot to hold in the unprotected hand. Nevertheless, the rod is still iron, and when cooled to its original temperature, its properties are just as they were before.

The heating and the subsequent cooling of the rod are examples of physical changes. A physical change may result in a more or less temporary alteration of a new of the properties of a substance involved, but no change of composition results from it and most of the altered properties usually regain their former value.

Changes of this type are numerous and many of them are familiar to everyone. As an example we may take the behavior of ice when it is heated. At first when heated the ice melts, when further heated, the liquid water boils forming the gaseous water (or the steam). If the steam is cooled, the process is reversed – when cooled sufficiently, the ice results. The substance present in every instance was water. This experiment shows that there are three physical states in which the substance may exist.

If the rod concerned is placed in a container of hydrochloric acid, it will be noted that bubbles begin to form on the rod. If the rod involved is left in the acid for some time, the evolution of gas will continue. When examined, it will be found that the rod has diminished in mass or disappeared altogether. The liquid in the container if examined will have a greenish colour.

If evaporated, a mass of greenish crystals will be obtained. The crystals will have totally different properties. This is an example of a chemical change. So, a chemical change may be called a chemical reaction are called reactants. Phenomena accompanied by radical changes of substances are called chemical phenomena.

Дата добавления: 2015-01-03