Chemical Reaction:
A chemical reaction is a process in which one or more substances are converted into different substances with different chemical and physical properties. In a chemical reaction, the atoms of the reactants rearrange to form new substances called products.

Expression of Chemical Reactions:
Chemical reactions can be expressed using chemical equations. A chemical equation is a symbolic representation of a chemical reaction, showing the reactants on the left side and the products on the right side. The equation must be balanced, meaning that the number of atoms of each element must be the same on both sides of the equation.

Examples:
1. Combustion Reaction:
\[2C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O\]
In this reaction, glucose and oxygen react to form carbon dioxide and water.
2. Neutralization Reaction:
\[HCl + NaOH \rightarrow NaCl + H_2O\]
In this reaction, hydrochloric acid and sodium hydroxide react to form sodium chloride and water.
3. Decomposition Reaction:
\[2HgO \rightarrow 2Hg + O_2\]
In this reaction, mercury(II) oxide decomposes to form mercury and oxygen gas.
4. Single Replacement Reaction:
\[Zn + 2HCl \rightarrow ZnCl_2 + H_2\]
In this reaction, zinc reacts with hydrochloric acid to form zinc chloride and hydrogen gas.

Conclusion:
Chemical reactions are essential for life and play a crucial role in various processes in nature and industry. By understanding and expressing chemical reactions using chemical equations, scientists can predict the outcome of reactions and develop new materials and compounds.