Everyday you observe different types of changes in surroundings. Some of these changes are very simple and are of temporary nature. Some others are really complex and of permanent nature.
When ice kept in a tumbler is exposed to the atmosphere, it melts and is converted into water. When the tumbler containing this water is kept in a freezer it is converted again into ice. Thus, this is a temporary change and the substance comes to its original form. Such changes are physical changes.
However, milk once converted into curd can not be converted into milk again. Such changes are chemical changes. These changes are of permanent nature. Both physical and chemical changes are integral part of daily life. You can present these changes in the form of an equation.
A substance which undergoes a chemical change is called the reactant and the substance which is formed as a result of a chemical change is called the product.
Magnesium + Oxygen → Magnesium oxide
In the above reaction, magnesium and oxygen undergo chemical change and they are the reactants. Magnesium oxide is a new substance formed. It is the product.
Zinc + dil Sulphuric acid → Zinc sulphate + Hydrogen
Here, zinc and dilute sulphuric acid are the reactants. The product in this reaction is zinc sulphate and hydrogen.
In chemical reaction, the reactants are written on the left hand side and the products are written on the right hand side. The change of the reactant into the product is shown through an arrow. Use of + sign is made when there are more than one reactant or there are more than one product.
A chemical equation can be made more concise and useful if we use chemical formulae instead of words. You substitute formulae of magnesium, oxygen and magnesium oxide for the words:
Mg + O2 → MgO
Zn + H2SO4 → ZnSO4 + H2
According to the Law of conservation of mass, the mass and the number of atoms present in the reactants should be equal to the mass and number of atoms present in products.
Chemical equations in which the number of atoms is not equal on both sides of the arrow but still represent chemical reactions are called skeletal chemical equations. Skeletal chemical equations can be balanced by using suitable coefficients in the equation.