What is chemical reaction?
The reactions which are not reversible are called chemical reactions . They are chemically changed. Before we understand the significance of a chemical change, let us demonstrate it with the help of a few activities.
ACTIVITY-1
We are all familiar with the phenomenon of rusting of iron. When articles made up of iron are left exposed in air for a long time, a brown patch or layer appears on their surface. This is known as rust. In order to demonstrate rusting, place a few nails of iron in a glass tube. Pour some water in it and place a rubber cork on its mouth. Leave the tube undisturbed for a few days, Brown deposit will appear on the surface of the nails. On analysis, it is found to be hydrated ferric oxide (fe2o3.xH2o) and is formed as a result of chemical reaction which iron undergoes with water and oxygen (present in air in the empty space).
ACTIVITY-2
In two separate glass beakers, dissolve small amount of lead nitrate and potassium iodide in water. Both these solutions are colourless. Now, add potassium iodide solution slowly to the lead nitrate solution. A yellow precipitate is formed at the bottom of the beaker as shown in the figure. On analysis, it is found to be lead iodide. It does not dissolve in water and therefore, separates as a precipitate.
Characteristic Features of Chemical Changes:
If we analyse the different activities which have studied, we conclude that in all these, the original substances lose their identity as well as nature and form new chemical substances. These are known as chemical changes. Based upon the observations, the features associated with these changes are summed up as follows:
1. There is generally a change in colour of the reacting species taking part in the change.
Example: In the rusting of iron nails, rust is brown in colour whereas the nails are black.
2. The physical state of the substance normally changes.
Example: In the burning of the magnesium ribbon, the ash formed is a white powder whereas magnesium is a metal and the reacting air or oxygen is a gas.
3. In some cases, a gas may evolve.
Example: Hydrogen gas is evolved when granules of zinc react either with dilute hydrochloric acid or with dilute sulphuric acid.
4. In some cases, a precipitate may be formed.
Example: When an aqueous solution of lead nitrate is mixed with an aqueous solution of potassium iodide, a yellow precipitate of lead iodide is immediately formed.
5. The chemical change is of permanent nature and cannot be easily reversed.
Example: Zinc chloride does not react with hydrogen to form zinc and hydrochloric acid.
6. In most of the chemical changes, the temperature changes.
Example: The temperature increases in case the change is exothermic in nature while there is a decrease in temperature during endothermic change.
So, The factors for determining chemical reaction:
- Change in state
- Change in colour
- Change in temperature
- Evolution of gas