Types of Chemical Change or Chemical Reaction - Decomposition Reactions

A decomposition reaction is a chemical process in which a single compound breaks down into two or more simpler substances. This reaction is the opposite of a combination reaction and often requires an external energy source such as heat, light, or electricity.

$$AB\to A+B$$

For example, in the electrolysis of water, water (H₂O) decomposes into hydrogen (H₂) and oxygen (O₂) when an electric current is passed through it. This can be represented by the following chemical equation:

\[2H_2O\xrightarrow{\text{electricity}}2H_2+O_2\]

1. Aim: To observe and understand decomposition reactions, where a single reactant breaks down into two or more simpler substances.

2. Requirements

• Apparatus: evaporating dish, test tubes, bent tube, rubber cork, Bunsen burner.
• Chemicals: sugar, calcium carbonate (CaCO₃), sulphuric acid, freshly prepared lime water.

4. Procedure

Experiment 1: Decomposition of Sugar

• Take some sugar in an evaporating dish.
• Heat it using a Bunsen burner and observe the changes.
• A black residue (carbon) forms, and water (H₂O) vapour is released.

\[\mathrm{C}_{12}\mathrm{H}_{22}\mathrm{O}_{11}\xrightarrow{\mathrm{Heat}}12C+11H_2O\]

Experiment 2: Decomposition of Calcium Carbonate

• Take some calcium carbonate (CaCO₃) in a test tube.
• Fit a bent tube into the test tube using a rubber cork.
• Insert the other end of the bent tube into another test tube containing freshly prepared lime water.
• Heat the calcium carbonate strongly. The lime water turns milky, indicating the presence of carbon dioxide (CO₂) gas.

\[\mathrm{CaCO}_3(s)\xrightarrow{\Delta}\mathrm{CaO}(s)+\mathrm{CO}_2\uparrow\]

Decomposition of calcium carbonate

Experiment 3: Electrolysis of Water

1. Pass electric current through acidulated water.
2. Observe the formation of hydrogen (H₂) and oxygen (O₂) gases.

\[2H_2O(l)\xrightarrow{\text{Electrical Energy}}2H_2\uparrow+O_2\uparrow\]

5. Conclusion: A decomposition reaction occurs when a single reactant breaks down into two or more simpler substances. These reactions can be thermal (heat-driven), electrolytic (electricity-driven), or biological (natural degradation by microorganisms). Decomposition reactions are crucial in nature, industry, and energy production, such as in the formation of biogas from organic waste.