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Chemical Reactions
ICSE Class 8 Chemistry • Chapter 6 (Detailed Master Notes)
Chapter Overview
A chemical reaction is not magic. It is simply the process where old chemical bonds break apart and new chemical bonds form, leading to the creation of entirely new substances. In this chapter, we will classify the different ways atoms rearrange themselves to react.
6.1 Characteristics of Chemical Reactions
How do you know if a chemical reaction has actually occurred? You look for specific physical observations.
- Evolution of a Gas: When zinc connects with dilute acid, bubbles of hydrogen gas vigorously escape.
- Change of Color: When copper carbonate (green powder) is strongly heated, it breaks down into copper oxide (black powder).
- Formation of a Precipitate: Mixing two clear solutions (like Silver Nitrate and Sodium Chloride) suddenly produces a thick, insoluble white solid that settles to the bottom.
- Change of State: Solid wax and oxygen gas turning into carbon dioxide gas and liquid water vapor upon burning.
6.2 Types of Chemical Reactions
Reactions follow predictable patterns. We classify them into four main categories:
1. Direct Combination (Synthesis)
Two or more simple substances (elements or compounds) combine strictly together to form a single, complex product.
$\text{A} + \text{B} \rightarrow \text{AB}$
Example: Burning of coal. Carbon combines directly with oxygen.
$\mathbf{C + O_2 \rightarrow CO_2}$
2. Decomposition Reaction
The exact opposite of combination. A single, complex compound breaks down into two or more simpler substances. This usually requires energy (heat or electricity).
$\text{AB} \rightarrow \text{A} + \text{B}$
Example: Thermal decomposition of limestone (calcium carbonate).
$\mathbf{CaCO_3 \xrightarrow{\Delta} CaO + CO_2}$ (The $\Delta$ symbol means heat is applied).
AI Image Prompt: A neat, stylized 4-part diagram showing abstract shapes acting out the core reaction types. 1. Combination (A red square and a blue circle merge into a red-blue pill). 2. Decomposition (The red-blue pill splits apart). 3. Displacement (A strong green triangle kicks the blue circle out of the red-blue pill, taking its place). 4. Double Displacement (Two different colored pills swap their halves seamlessly).
3. Single Displacement Reaction
A highly active element physically displaces (pushes out) a less active element from its compound to take its place.
$\text{A} + \text{BC} \rightarrow \text{AC} + \text{B}$
Example: Iron is more reactive than Copper. If you drop an iron nail into blue copper sulphate
solution, iron kicks the copper out.
$\mathbf{Fe + CuSO_4 \rightarrow FeSO_4 + Cu}$ (The solution turns pale green, and brown copper coats the
nail).
4. Double Displacement Reaction
Two compounds in an aqueous solution mutually exchange their positively charged ions (radicals) to form two completely new compounds.
$\text{AB} + \text{CD} \rightarrow \text{AD} + \text{CB}$
Example: Neutralization Reaction. When an acid and a base mix, they exchange ions to form salt and
water.
$\mathbf{NaOH (Base) + HCl (Acid) \rightarrow NaCl (Salt) + H_2O (Water)}$
6.3 Energy Changes (Exothermic & Endothermic)
Chemical reactions always involve energy. Breaking old bonds requires energy, and forming new bonds releases energy.
- Exothermic Reactions: Reactions that release energy (heat) into the surroundings. The container gets hot. (e.g., Burning of fuels, Respiration).
- Endothermic Reactions: Reactions that absorb energy (heat) from the surroundings. The container feels cold, or you must actively supply heat. (e.g., Photosynthesis, Decomposition of Calcium Carbonate).
Q1. Classify the following reaction: $2H_2O_2 \rightarrow 2H_2O + O_2$
Answer: This is a Decomposition Reaction. A single reactant (Hydrogen peroxide) breaks down into two simpler products (water and oxygen gas).
Q2. Why is respiration considered an exothermic reaction?
Answer: During respiration, the glucose in our cells chemically reacts with oxygen to break down. This chemical process releases a significant amount of heat energy that our body uses to survive, making it strictly an exothermic reaction.