Chemistry is one of the most important parts of SSLC Science. In the 2026 LBA (Lesson Based Assessment), Unit–1 “Chemical Reactions and Equations” plays a major role in examinations. This chapter helps students understand how substances change, how reactions occur, and how chemistry is connected to daily life. According to the LBA document, this unit includes objective questions, short answers, and long-answer problems from different difficulty levels.
Learning Points in This Chapter
The chapter mainly focuses on:
- Chemical equations
- Balancing chemical equations
- Types of chemical reactions
- Endothermic and exothermic reactions
- Oxidation and reduction reactions
- Effects of oxidation reactions in daily life
These concepts are repeatedly asked in MCQs, 1-mark, 2-mark, and 4-mark questions.
What is a Chemical Reaction?
A chemical reaction is a process in which one or more substances change into new substances with different properties. For example, rusting of iron is a chemical reaction because iron changes into iron oxide. The LBA question paper also highlights rusting of iron as an example of chemical change.
Some common signs of chemical reactions are:
- Change in colour
- Release of gas
- Formation of precipitate
- Change in temperature
- Formation of new substances
Chemical Equations
Chemical reactions are represented using chemical equations. A chemical equation shows reactants and products symbolically.
Example:
$\ce{2H2 + O2 -> 2H2O}$
This equation represents the formation of water from hydrogen and oxygen.
Why Should Chemical Equations Be Balanced ?
Chemical equations must be balanced to obey the Law of Conservation of Mass. The number of atoms on both sides should be equal.
The LBA document includes several balancing questions for practice.
Types of Chemical Reactions
1. Combination Reaction
When two or more substances combine to form a single product, it is called a combination reaction.
Example:
$\ce{CaO + H2O -> Ca(OH)2}$
Quicklime combines with water to form slaked lime.
2. Decomposition Reaction
A single compound breaks down into simpler substances.
Example:
$\ce{CaCO3 ->[\text{Heat}] CaO + CO2}$
The chapter also discusses decomposition caused by heat, light, and electricity.
3. Displacement Reaction
A more reactive element displaces a less reactive element from its compound.
Example:
$\ce{Fe + CuSO4 -> FeSO4 + Cu}$
This reaction is important in examinations and appears multiple times in the LBA questions.
4. Double Displacement Reaction
Two compounds exchange ions to form new compounds.
Example:
$\ce{BaCl2 + Na2SO4 -> BaSO4 + 2NaCl}$
Formation of white precipitate is an important observation in this reaction.
Endothermic and Exothermic Reactions
Exothermic Reaction
Reactions that release heat are called exothermic reactions.
Examples:
- Respiration
- Burning of fuels
- Compost formation
Endothermic Reaction
Reactions that absorb heat are called endothermic reactions.
These concepts are frequently asked in 2-mark and 3-mark questions.
Oxidation and Reduction Reactions
Oxidation means addition of oxygen or removal of hydrogen.
Reduction means removal of oxygen or addition of hydrogen.
When both oxidation and reduction occur together, it is called a redox reaction.
Example:
$\ce{CuO + H2 -> Cu + H2O}$
Here:
- Hydrogen is oxidized
- Copper oxide is reduced
Corrosion and Rancidity
Corrosion
Corrosion is the gradual destruction of metals due to reactions with air and moisture. Rusting of iron is a common example.
Methods to prevent corrosion:
- Painting
- Greasing
- Galvanization
Rancidity
Rancidity is the oxidation of oils and fats that causes bad smell and taste in food.
The LBA material explains that chip packets are filled with nitrogen gas to prevent oxidation and rancidity.
Important Questions for Exam Preparation
Students should practice these important areas:
- Balancing chemical equations
- Types of reactions
- Electrolysis of water
- Corrosion and rancidity
- Precipitation reactions
- Redox reactions
- Writing chemical equations
- Identifying oxidized and reduced substances
The LBA document contains Easy, Average, and Difficulty level questions to help students prepare effectively.
Preparation Tips for Students
- Learn all chemical equations properly.
- Practice balancing equations daily.
- Understand reaction types with examples.
- Revise definitions and differences.
- Practice previous LBA questions regularly.
- Focus on diagrams and observations.
