Chemical Reactions and Equations | Class 10 Science Notes
By ConceptScroll Team · Published on 17 July 2026 · 4 min read
Chemical Reactions and Equations – this guide gives you a concise, exam-ready overview of Chemical Reactions and Equations from Class 10 Science, written by ConceptScroll editors and reviewed against the latest NCERT textbook.
1.1 Chemical Equations
Chemical equations provide a concise and symbolic way to represent chemical reactions. Instead of describing reactions in long sentences, word equations and then chemical equations using formulae are used. For example, the burning of magnesium in oxygen can be written as a word equation: Magnesium + Oxygen → Magnesium oxide. Here, magnesium and oxygen are reactants, and magnesium oxide is the product. Using chemical formulae, this reaction is represented as Mg + O₂ → MgO. However, this skeletal equation is unbalanced because the number of oxygen atoms on the left (2) does not equal the number on the right (1). According to the law of conservation of mass, matter cannot be created or destroyed in a chemical reaction, so the number of atoms of each element must be the same on both sides of the equation. Balancing chemical equations involves adjusting coefficients (numbers before formulas) to ensure equal numbers of atoms on both sides without changing the formulas themselves. For example, the reaction of zinc with dilute sulphuric acid is Zn + H₂SO₄ → ZnSO₄ + H₂, which is already balanced. Another example is the reaction of iron with steam producing iron oxide and hydrogen, initially unbalanced as Fe + H₂O → Fe₃O₄ + H₂. By balancing oxygen, hydrogen, and iron atoms stepwise, the balanced equation becomes 3Fe + 4H₂O → Fe₃O₄ + 4H₂. Physical states of reactants and products can also be indicated using symbols (s) for solid, (l) for liquid, (g) for gas, and (aq) for aqueous solutions. Reaction conditions such as temperature, pressure, or catalysts can be noted above or below the reaction arrow to provide more information. Thus, chemical equations are essential tools for representing chemical reactions clearly and accurately.
📊 Diagram: Figure 1.2 Formation of hydrogen gas by the action of dilute sulphuric acid on zinc; Tables showing atom counts for balancing chemical equations.
🧪 Activity: Balancing chemical equations stepwise using atom counting and hit-and-trial method.
🔗 Connection: This section sets the foundation for understanding different types of chemical reactions, which is covered in the next section.
Table on page 3 (5×3)
| Element | Number of atoms in reactants (LHS) | Number of atoms in products (RHS) |
|---|---|---|
| Zn | 1 | 1 |
| H | 2 | 2 |
| S | 1 | 1 |
| O | 4 | 4 |
Table on page 4 (4×3)
| Element | Number of atoms in reactants (LHS) | Number of atoms in products (RHS) |
|---|---|---|
| Fe | 1 | 3 |
| H | 2 | 2 |
| O | 1 | 4 |
Table on page 4 (3×3)
| Atoms of oxygen | In reactants | In products |
|---|---|---|
| (i) Initial | 1 (in H₂O) | 4 (in Fe₃O₄) |
| (ii) To balance | 1×4 | 4 |
Table on page 4 (3×3)
| Atoms of hydrogen | In reactants | In products |
|---|---|---|
| (i) Initial | 8 (in 4 H₂O) | 2 (in H₂) |
| (ii) To balance | 8 | 2 × 4 |
Table on page 5 (3×3)
| Atoms of iron | In reactants | In products |
|---|---|---|
| (i) Initial | 1 (in Fe) | 3 (in Fe₃O₄) |
| (ii) To balance | 1×3 | 3 |
Table on page 13 (6×1)
| QUESTIONS |
|---|
| 1. Why does the colour of copper sulphate solution change when an iron nail is dipped in it? |
| 2. Give an example of a double displacement reaction other than the one given in Activity 1.10. |
| 3. Identify the substances that are oxidised and the substances that are reduced in the following reactions. |
| (i) $4\mathrm{Na}(\mathrm{s}) + \mathrm{O}_2(\mathrm{g}) \rightarrow 2\mathrm{Na}_2\mathrm{O}(\mathrm{s})$ |
| (ii) $\mathrm{CuO}(\mathrm{s}) + \mathrm{H}_2(\mathrm{g}) \rightarrow \mathrm{Cu}(\mathrm{s}) + \mathrm{H}_2\mathrm{O}(\mathrm{l})$ |
Frequently asked questions
The gas used to prevent rancidity is
Nirtogen
Carbon in presence of insufficient air produces ___________ .
Carbon monoxide
At the time of short circuit,the current in the circuit______________.
increases heavily.
For the given question, two statements are given – one labeled Assertion (A) and the other labeled Reason (R). Select the correct answer to these questions from codes (i), (ii), (iii) and (iv) as given below: Assertion: Potassium has a bigger atomic radius than Lithium Reason :Atomic radius decreases along a period
Both A and R are true but R is not the correct explanation of the assertion.
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