Class 10 Science · Chapter 1 NotesChemical Reactions and Equations
Comprehensive notes on Class 10 Science Chapter 1. Learn about balancing equations, types of chemical reactions, redox, corrosion, and rancidity as per NCERT and CBSE cur
Chemical reactions are fundamental processes where substances transform into entirely new entities with different properties. In our daily lives, we witness these changes when milk sours in summer, iron rusts in humid air, or when food is digested in our bodies. This chapter introduces students to the systematic way of representing these transformations through chemical equations. You will learn to identify the signs of a chemical change, such as gas evolution or temperature shifts, and master the essential skill of balancing equations to satisfy the Law of Conservation of Mass. By exploring various reaction types—from combination and decomposition to displacement and redox—students gain a clear understanding of how atoms rearrange and bonds break or form. The chapter also connects these laboratory concepts to real-world phenomena like the corrosion of metals and the rancidity of fats, providing a comprehensive foundation for further studies in chemistry.
What you'll learn
1Identify chemical changes using observations like color change, gas evolution, and temperature shifts.
2Represent chemical reactions symbolically using balanced chemical equations.
3Apply the Law of Conservation of Mass to balance skeletal equations using the hit-and-trial method.
4Classify reactions into combination, decomposition, displacement, and double displacement types.
5Distinguish between exothermic and endothermic reactions based on energy exchange.
6Define oxidation and reduction and identify redox reactions.
7Explain the everyday effects of oxidation, specifically corrosion and rancidity.
Chapter at a glance
01Chapter Overview
02Chemical Equations and Their Representation
03Types of Chemical Reactions
04Balancing Chemical Equations
Detailed chapter notes
01
Identifying Chemical Reactions
A chemical reaction occurs whenever the nature and identity of the initial substance change. We can determine if a reaction has taken place by looking for specific physical or chemical observations. These include a change in the state of matter, a change in color, the evolution of a gas, or a change in temperature. For instance, burning a magnesium ribbon produces a dazzling white flame and leaves behind a white powder called magnesium oxide, while reacting zinc granules with dilute acid leads to the evolution of hydrogen gas and a rise in temperature.
Change in state
Change in colour
Evolution of a gas
Change in temperature
02
Writing and Balancing Chemical Equations
Chemical equations are concise representations of reactions using chemical formulae. Reactants are written on the left-hand side (LHS) and products on the right-hand side (RHS), separated by an arrow showing the reaction's direction. According to the Law of Conservation of Mass, mass cannot be created or destroyed. Therefore, the number of atoms of each element must be equal on both sides. An unbalanced equation is called a skeletal equation. We use the hit-and-trial method to balance equations by adding coefficients before the formulae without altering the formulae themselves.
ReactantsSubstances undergoing change
ProductsNew substances formed
State symbols(s) for solid, (l) for liquid, (g) for gas, (aq) for aqueous solution
03
Combination and Exothermic Reactions
A combination reaction occurs when two or more reactants combine to form a single product. A classic example is the reaction of quick lime (calcium oxide) with water to form slaked lime (calcium hydroxide). This specific reaction releases a significant amount of heat, making it an exothermic reaction. Exothermic reactions are those in which heat is released along with the formation of products. Other examples include the burning of natural gas and the process of respiration, where glucose reacts with oxygen in our cells to provide energy.
CaO + H2O -> Ca(OH)2 + Heat
C + O2 -> CO2 (Burning of coal)
Respiration is an exothermic process
04
Decomposition Reactions
Decomposition reactions are the opposite of combination reactions; here, a single reactant breaks down into two or more simpler products. These reactions require energy in the form of heat, light, or electricity to break chemical bonds, making them endothermic. Thermal decomposition involves heat, such as breaking down limestone into quick lime and carbon dioxide. Electrolytic decomposition uses electricity, as seen in the electrolysis of water. Photolytic decomposition occurs in the presence of light, such as silver chloride turning grey as it decomposes into silver metal and chlorine gas.
ThermalCaCO3 -> CaO + CO2
Photolytic2AgCl -> 2Ag + Cl2
EndothermicReactions that absorb energy
05
Displacement and Double Displacement
In a displacement reaction, a more reactive element displaces a less reactive element from its compound. For example, when an iron nail is placed in a blue copper sulphate solution, the iron displaces the copper, forming iron sulphate and causing the blue color to fade. Double displacement reactions involve an exchange of ions between two reactants. These often result in the formation of an insoluble solid called a precipitate. For instance, mixing sodium sulphate and barium chloride solutions produces a white precipitate of barium sulphate.
PrecipitateAn insoluble substance formed during a reaction
06
Oxidation, Reduction, and Redox Reactions
Oxidation is defined as the gain of oxygen or the loss of hydrogen by a substance. Conversely, reduction is the loss of oxygen or the gain of hydrogen. In most reactions, one reactant is oxidized while the other is reduced; these are known as oxidation-reduction or redox reactions. For example, when hydrogen gas is passed over heated copper oxide, the copper oxide loses oxygen (reduction) to become copper, while the hydrogen gains oxygen (oxidation) to become water. These processes are essential in understanding chemical transformations.
OxidationGain of O2 or loss of H2
ReductionLoss of O2 or gain of H2
RedoxSimultaneous oxidation and reduction
07
Corrosion and Rancidity
Oxidation has significant effects in everyday life. Corrosion occurs when a metal is attacked by moisture, acids, or air, leading to its deterioration. Rusting of iron, the black coating on silver, and the green coating on copper are common examples. This process causes immense damage to infrastructure like bridges and ships. Rancidity refers to the oxidation of fats and oils in food. When oxidized, food develops an unpleasant smell and taste. To prevent this, manufacturers add antioxidants or flush food packages with nitrogen gas to create an oxygen-free environment.
CorrosionDeterioration of metals due to environmental attack
RancidityOxidation of fats and oils in food
PreventionUsing nitrogen gas or airtight containers
Want the complete chapter resources?Topic notes, quizzes and flashcards for Chemical Reactions and Equations.
Q1. What is a balanced chemical equation? Why should chemical equations be balanced?
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Model answer
A balanced chemical equation has the same number of atoms of each element on both sides of the equation. It must be balanced to obey the law of conservation of mass, which states that mass can neither be created nor destroyed in a chemical reaction. Therefore, the total mass of reactants equals the total mass of products.
Sample question3 marks
Q2. Define combination reaction. Give one example from daily life and write its balanced chemical equation.
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Model answer
A combination reaction is a reaction in which two or more substances combine to form a single product. Example: Burning of coal, C(s) + O2(g) → CO2(g). Another example is formation of water: 2H2(g) + O2(g) → 2H2O(l).
Sample question3 marks
Q3. Define oxidation and reduction in terms of oxygen transfer. Give one example of a redox reaction from the chapter.
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Model answer
Oxidation is the gain of oxygen by a substance, while reduction is the loss of oxygen. For example, in the reaction CuO + H2 → Cu + H2O, CuO loses oxygen (reduction) and H2 gains oxygen (oxidation). This is a redox reaction.
Sample question3 marks
Q4. Balance the following chemical equation: Fe + H2O → Fe3O4 + H2. Also, write the physical states of reactants and products.
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Model answer
The balanced equation is: 3Fe(s) + 4H2O(g) → Fe3O4(s) + 4H2(g). The reactants are solid iron and steam (gaseous water), and the products are solid ferrosoferric oxide and hydrogen gas.
Sample question3 marks
Q5. Write the balanced chemical equation for the reaction between zinc and dilute sulphuric acid. Also mention the physical states of reactants and products.
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Model answer
Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g). This is a displacement reaction where zinc displaces hydrogen from sulphuric acid. The reactants are solid zinc and aqueous sulphuric acid; products are aqueous zinc sulphate and hydrogen gas.
Want more questions with answers?Get the full practice set for this chapter.
Why should a magnesium ribbon be cleaned before burning in air?
Magnesium is a reactive metal that forms a layer of magnesium oxide on its surface when exposed to air. This layer hinders the burning process. Cleaning the ribbon with sandpaper removes this protective oxide layer, allowing the metal to react freely with oxygen and burn effectively.
What is the difference between a skeletal and a balanced chemical equation?
A skeletal equation is an informal representation where the number of atoms of each element is not equal on both sides. A balanced equation is one where the number of atoms for every element is identical for both reactants and products, ensuring the total mass remains constant.
Why is respiration considered an exothermic reaction?
During digestion, carbohydrates are broken down into glucose. This glucose reacts with oxygen in our body's cells to produce energy, carbon dioxide, and water. Because energy is released during this process to sustain life, respiration is classified as an exothermic chemical reaction.
How does nitrogen gas help in preventing rancidity in potato chips?
Rancidity occurs when the fats and oils in chips react with oxygen (oxidation), leading to a change in taste and smell. Nitrogen is an unreactive gas. By flushing chip bags with nitrogen, oxygen is removed, preventing the oxidation process and keeping the food fresh for longer.