Chemical Reactions and Equations – Concept Booster | Class 10 Science CBSE

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Home Concept Boosters CBSE CBSE Class 10 Science Chemical Reactions & Equations

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How to Use This Page

Read each concept carefully, then check the formula, common mistake, and exam tip before moving to the next. This page completely covers Chemical Reactions and Equations for CBSE Class 10 Science, laying the groundwork for all of Chemistry.

Key Concepts

Class 10 · Science · Chemistry

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Chemical Reactions

The transformation of matter

Class 10 · Ch 1

Chemical Reaction vs. Equation Definition

A chemical reaction is a process where substances (reactants) transform into new substances (products) with different properties. A chemical equation is the shorthand symbolic representation of this process.

$$\text{Reactants} \rightarrow \text{Products}$$

Balanced Chemical Equation Rule

An equation where the number of atoms of each element is equal on both sides. This is strictly required to satisfy the Law of Conservation of Mass.

$$\mathrm{2Mg + O_2 \rightarrow 2MgO}$$

Combination Reaction Formula

A reaction in which two or more reactants combine to form a single new product. Example: Burning of coal or formation of water.

$$\mathrm{A + B \rightarrow AB}$$

$$\mathrm{CaO_{(s)} + H_2O_{(l)} \rightarrow Ca(OH)_{2(aq)}}$$

Decomposition Reaction Formula

A reaction where a single reactant breaks down to give simpler products. It requires energy in the form of heat (thermal), light (photolytic), or electricity (electrolytic).

$$\mathrm{AB \rightarrow A + B}$$

$$\mathrm{2FeSO_{4(s)} \xrightarrow{Heat} Fe_2O_{3(s)} + SO_{2(g)} + SO_{3(g)}}$$

Displacement Reaction Formula

A reaction in which a more reactive element displaces a less reactive element from its compound. Governed by the reactivity series.

$$\mathrm{A + BC \rightarrow AC + B}$$

$$\mathrm{Fe_{(s)} + CuSO_{4(aq)} \rightarrow FeSO_{4(aq)} + Cu_{(s)}}$$

Double Displacement Reaction Formula

A reaction in which there is an exchange of ions between the reactants. These often result in the formation of an insoluble solid called a precipitate.

$$\mathrm{AB + CD \rightarrow AD + CB}$$

$$\mathrm{Na_2SO_{4(aq)} + BaCl_{2(aq)} \rightarrow BaSO_{4(s)} + 2NaCl_{(aq)}}$$

Oxidation Definition

The process involving the addition of oxygen to a substance or the removal of hydrogen from a substance.

$$\mathrm{2Cu + O_2 \xrightarrow{Heat} 2CuO} \quad \text{(Cu is oxidized)}$$

Reduction & Redox Reactions Definition

Reduction is the addition of hydrogen or the removal of oxygen. A reaction where oxidation and reduction occur simultaneously is called a Redox reaction.

$$\mathrm{CuO + H_2 \xrightarrow{Heat} Cu + H_2O}$$

Corrosion Concept

The slow degradation of metals by the action of air, moisture, or a chemical on their surface. Rusting of iron is the most common example.

$$\text{Rust: } \mathrm{Fe_2O_3 \cdot \mathit{x}H_2O}$$

Rancidity Concept

The slow oxidation of oils and fats present in food, resulting in a bad smell and foul taste. Prevented by adding antioxidants, flushing bags with nitrogen, or keeping food in airtight containers.

$$\text{Fats/Oils } + \text{ Oxygen } \rightarrow \text{ Rancid Food}$$

Concept Deep Dive

Why Do We Balance Equations?

Atoms don’t magically appear or disappear

Core Concept

If you write $\mathrm{H_2 + O_2 \rightarrow H_2O}$, you have 2 oxygen atoms on the left, but only 1 on the right. Where did the other oxygen atom go? Did it vanish?

The Law of Conservation of Mass states that mass can neither be created nor destroyed in a chemical reaction. The total mass of elements present in the products must strictly equal the total mass of elements in the reactants. By placing coefficients in front of the molecules ($\mathrm{2H_2 + O_2 \rightarrow 2H_2O}$), we account for every single atom, proving nothing was lost or created out of thin air.

The Redox Paradox

Agents do the opposite of what happens to them

Crucial Understanding

Identifying the Oxidizing and Reducing agents is the trickiest part of Chapter 1. Think of a “travel agent.” A travel agent books trips for other people, not themselves.

An Oxidizing Agent is the substance that oxidizes the other guy. To do this, it must steal oxygen from somewhere, meaning the oxidizing agent itself gets Reduced.
A Reducing Agent reduces the other guy. To do this, it must grab oxygen for itself, meaning the reducing agent itself gets Oxidized.

Rule of thumb: Find what is oxidized. That exact substance is your reducing agent!

Compare & Contrast

✗ Exothermic Reactions

Reactions that release energy (usually heat) along with the formation of products.
The temperature of the surroundings increases.
Example: Burning of natural gas ($\mathrm{CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O + Heat}$).
Example: Respiration (glucose breaking down to give energy).

✓ Endothermic Reactions

Reactions that absorb energy from the surroundings to proceed.
The temperature of the surroundings decreases (feels cold).
Example: All decomposition reactions (thermal, electrolytic, photolytic).
Example: Photosynthesis (plants absorb sunlight to make glucose).

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Remember

A Precipitation Reaction is just a special type of Double Displacement reaction where one of the new compounds formed is insoluble in water and settles down as a solid (the precipitate).

Common Mistakes to Avoid

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Mistake 1

Changing Subscripts to Balance Equations: To balance $\mathrm{H_2 + O_2 \rightarrow H_2O}$, students often change $\mathrm{H_2O}$ to $\mathrm{H_2O_2}$. This is completely wrong! $\mathrm{H_2O}$ is water. $\mathrm{H_2O_2}$ is Hydrogen Peroxide—a completely different chemical. You can ONLY change the coefficients (the big numbers in front).

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Mistake 2

Forgetting Physical States: In board exams, a balanced equation without physical states is often considered incomplete. Always add $(s)$ for solid, $(l)$ for liquid, $(g)$ for gas, and $(aq)$ for aqueous (dissolved in water). A precipitate is always $(s)$.

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Mistake 3

Choosing the Product as the Agent: When asked “Name the substance oxidized,” students often pick the product. Oxidation/Reduction happens TO the reactants. The answers for “substance oxidized”, “substance reduced”, “oxidizing agent”, and “reducing agent” MUST all be chosen from the left side (Reactants) of the arrow!

Exam Tips

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Tip 1

Learn the Precipitation Colors: Board examiners love color changes!
– $\mathrm{BaSO_4}$ (Barium Sulphate) = White precipitate.
– $\mathrm{PbI_2}$ (Lead Iodide) = Yellow precipitate.
– $\mathrm{CuSO_4}$ solution = Blue, $\mathrm{FeSO_4}$ solution = Pale Green.

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Tip 2

The Photography Question: If a question asks about a compound used in black and white photography, it is always Silver Chloride ($\mathrm{AgCl}$) or Silver Bromide ($\mathrm{AgBr}$). They undergo photolytic decomposition when exposed to sunlight: $\mathrm{2AgCl_{(s)} \xrightarrow{Sunlight} 2Ag_{(s)} + Cl_{2(g)}}$.

Expected Exam Questions

Board Pattern Questions

Class 10 · Science · CBSE Exam

Class 10 · Chemistry

Why is respiration considered an exothermic reaction? Explain. [2 marks]

Answer Because it releases energy. 📝

Explanation

During digestion, food is broken down into simpler substances like glucose. During respiration, this glucose combines with oxygen in the cells of our body and breaks down to form carbon dioxide and water, simultaneously releasing a large amount of energy. Since energy is given out, respiration is an exothermic reaction.
Equation: $\mathrm{C_6H_{12}O_{6(aq)} + 6O_{2(aq)} \rightarrow 6CO_{2(aq)} + 6H_2O_{(l)} + Energy}$

Translate the following statement into a chemical equation and then balance it:
Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate. [3 marks]

Answer $\mathrm{3BaCl_2 + Al_2(SO_4)_3 \rightarrow 2AlCl_3 + 3BaSO_4 \downarrow}$ 📝

Explanation

Step 1: Write the skeletal equation.
$\mathrm{BaCl_2 + Al_2(SO_4)_3 \rightarrow AlCl_3 + BaSO_4}$

Step 2: Balance the atoms.
– There are 3 $\mathrm{SO_4}$ groups on the left, so put a $3$ before $\mathrm{BaSO_4}$ on the right.
– Now there are 3 Ba on the right, so put a $3$ before $\mathrm{BaCl_2}$ on the left.
– Now there are 6 Cl on the left ($3 \times 2$), so put a $2$ before $\mathrm{AlCl_3}$ on the right.
– Check Al: There are 2 Al on the left and 2 Al on the right. It’s balanced!

Final Equation: $\mathrm{3BaCl_{2(aq)} + Al_2(SO_4)_{3(aq)} \rightarrow 2AlCl_{3(aq)} + 3BaSO_{4(s)}}$

Identify the substances that are oxidized and the substances that are reduced in the following reaction:
$\mathrm{ZnO + C \rightarrow Zn + CO}$ [2 marks]

Answer Oxidized: $\mathrm{C}$, Reduced: $\mathrm{ZnO}$ 📝

Explanation

Look strictly at the reactants (left side).
– Carbon ($\mathrm{C}$) gains oxygen to become Carbon Monoxide ($\mathrm{CO}$). Therefore, Carbon is oxidized.
– Zinc Oxide ($\mathrm{ZnO}$) loses oxygen to become Zinc ($\mathrm{Zn}$). Therefore, Zinc Oxide is reduced.
Note: The reducing agent is Carbon, and the oxidizing agent is Zinc Oxide.

Concept Map

Chemical Reactions connects to →

Foundational Chemistry

Acids, Bases & Salts (Neutralization reactions)

Metals & Non-Metals (Reactivity series & displacement)

Carbon and its Compounds (Combustion reactions)

Chemical Reactions and Equations – Concept Booster | Class 10 Science CBSE

Key Concepts

Chemical Reactions