Decomposition Reaction MCQs Quiz | Class 10
This quiz is designed for Class X Science students (Code 086) covering Unit I: Chemical Substances – Nature and Behaviour. It specifically focuses on Decomposition Reactions, including their definition and the three major types: thermal, electrolytic, and photolytic decomposition. Attempt these 10 multiple-choice questions to test your understanding, then submit to view your score and download a detailed PDF answer sheet for revision.
Overview of Decomposition Reactions
A decomposition reaction is a type of chemical reaction where a single compound breaks down into two or more simpler substances under suitable conditions. It is the opposite of a combination reaction. The general form of a decomposition reaction is:
AB → A + B
These reactions are generally endothermic because they require energy in the form of heat, light, or electricity to break the chemical bonds holding the compound together.
Types of Decomposition Reactions
Based on the source of energy supplied, decomposition reactions are classified into three main types:
1. Thermal Decomposition (Thermolysis)
This reaction occurs when heat is supplied to the reactant. It is widely used in industries, such as in the manufacturing of cement and quick lime.
- Example 1: Calcium Carbonate (Limestone) decomposes into Calcium Oxide (Quick lime) and Carbon Dioxide gas upon heating.
- Example 2: Ferrous Sulphate crystals lose water and change color from green to white/brown, releasing Sulphur Dioxide and Sulphur Trioxide gases.
- Example 3: Lead Nitrate powder (white) decomposes on heating to produce Lead Oxide (yellow), Nitrogen Dioxide (brown fumes), and Oxygen.
2. Electrolytic Decomposition (Electrolysis)
This reaction involves the breakdown of a substance by passing an electric current through it in a molten or aqueous state.
- Example: Electrolysis of acidified water yields Hydrogen gas at the cathode and Oxygen gas at the anode. The volume ratio of Hydrogen to Oxygen collected is 2:1.
3. Photolytic Decomposition (Photolysis)
These reactions are driven by light energy (usually sunlight). They are essential in black-and-white photography.
- Example: Silver Chloride (white) turns into grey Silver metal and Chlorine gas in the presence of sunlight. Similarly, Silver Bromide decomposes into Silver and Bromine.
Comparison Table
| Reaction Type | Energy Source | Key Example | Observation |
|---|---|---|---|
| Thermal | Heat | Heating Lead Nitrate | Brown fumes of Nitrogen Dioxide |
| Electrolytic | Electricity | Electrolysis of Water | Gas bubbles at electrodes |
| Photolytic | Sunlight | Silver Chloride exposure | White turns grey |
Quick Revision Points
- Decomposition reactions require energy to break bonds; hence, they are endothermic.
- Digestion of food is a decomposition reaction where complex food molecules (like carbohydrates) break down into simple glucose.
- The decomposition of Calcium Carbonate is crucial for the cement industry.
- Hydrogen is collected at the negative electrode (cathode) and Oxygen at the positive electrode (anode) during water electrolysis.
Extra Practice Questions
- Q: Why are decomposition reactions called the opposite of combination reactions?
A: In decomposition, one reactant breaks into multiple products, whereas in combination, multiple reactants form one product. - Q: Write the color change observed when Ferrous Sulphate crystals are heated.
A: Light green to reddish-brown. - Q: Name the gas evolved as brown fumes when Lead Nitrate is heated.
A: Nitrogen Dioxide (NO2). - Q: Which metal is extracted from its molten chloride salt by electrolytic decomposition?
A: Sodium (Na). - Q: Why is Silver Chloride stored in dark-colored bottles?
A: To prevent its photolytic decomposition by light.
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