**CHAPTER 1: CHEMICAL REACTIONS AND EQUATIONS**
**1. WHAT IS A CHEMICAL REACTION?**
• A chemical reaction occurs when the nature and identity of a substance changes completely
• Examples: milk spoiling, iron rusting, food digestion, respiration, fermentation, cooking
• Key difference from physical change: chemical reactions create NEW substances with different properties
**Signs That a Chemical Reaction Has Occurred:**
• Change in state (solid → liquid, etc.)
• Change in colour
• Evolution of gas (bubbling, fizzing)
• Change in temperature (heat absorbed or released)
• All four may not occur in every reaction — at least one indicates a chemical change
**2. WORD EQUATIONS**
• Simplest way to represent a chemical reaction using words
• Format: Reactants → Products
• Reactants: substances that undergo chemical change (LEFT side)
• Products: new substances formed (RIGHT side)
• Arrow (→) shows direction of reaction
• Multiple reactants or products joined with plus sign (+)
• Example: Magnesium + Oxygen → Magnesium oxide
**3. CHEMICAL EQUATIONS**
• More concise than word equations — uses chemical formulae instead of words
• Example: Mg + O₂ → MgO
• Skeletal chemical equation: initial equation before balancing
• Shows reactants on LHS and products on RHS
• Uses chemical symbols and molecular formulae
**4. BALANCING CHEMICAL EQUATIONS (CRITICAL FOR CBSE)**
**Law of Conservation of Mass:**
• Mass can NEITHER be created NOR destroyed in a chemical reaction
• Total mass of reactants = Total mass of products
• Number of atoms of EACH element must be SAME on both sides
**Step-by-Step Process to Balance Equations:**
Step I: Write the skeletal equation and draw boxes around each formula — DO NOT change anything inside boxes during balancing
Step II: Count atoms of each element on LHS and RHS — identify unbalanced elements
Step III: Start with the compound containing maximum atoms; balance the element with maximum atoms in that compound first
Step IV: Use coefficients (numbers BEFORE formulas) to balance — NEVER change subscripts inside formulas
• Correct: 4 H₂O (adds 4 molecules)
• Wrong: H₂O₄ or (H₂O)₄ (changes the compound)
Step V: Balance remaining elements one by one
Step VI: Check final equation — count all atoms on both sides to confirm balance
**DON'T CONFUSE:**
• Coefficients vs. Subscripts: Coefficient is written OUTSIDE and BEFORE formula (can change); subscript is INSIDE formula (cannot change)
• Example: In 4 H₂O — '4' is coefficient (can change), '2' in H₂ is subscript (cannot change)
**Example of Balancing:**
Unbalanced: Fe + H₂O → Fe₃O₄ + H₂
Step 1: Start with Fe₃O₄ (maximum atoms); balance O first
• O atoms: 1 on LHS, 4 on RHS → use 4 H₂O
• Fe + 4 H₂O → Fe₃O₄ + H₂
Step 2: Now H atoms need balancing
• H atoms: 8 on LHS (from 4 H₂O), 2 on RHS → use 4 H₂
• Fe + 4 H₂O → Fe₃O₄ + 4 H₂
Step 3: Balance Fe atoms
• Fe atoms: 1 on LHS, 3 on RHS → use 3 Fe
• 3 Fe + 4 H₂O → Fe₃O₄ + 4 H₂
Step 4: Final check — all atoms balanced ✓
Fe: 3 = 3 ✓ | H: 8 = 8 ✓ | O: 4 = 4 ✓
**5. STANDARD FORM OF BALANCED EQUATION**
Format: Reactant₁ + Reactant₂ → Product₁ + Product₂ + ...
Example Balanced Equations:
• Mg + O₂ → MgO (unbalanced)
• 2 Mg + O₂ → 2 MgO (balanced)
• Zn + H₂SO₄ → ZnSO₄ + H₂ (balanced — already has equal atoms)
• 2 Mg + O₂ → 2 MgO (magnesium burns in oxygen)
**6. CONDITIONS OF REACTION**
• Written above or below the arrow in chemical equations
• Heat (Δ or heat symbol) — reaction requires heating
• Catalyst (substance name) — substance that speeds up reaction without being consumed
• Pressure symbol — reaction occurs at high pressure
• Light symbol (hv) — light is required
• Example: 2 Mg + O₂ →(heat) 2 MgO
**7. KEY DEFINITIONS**
• **Reactant**: A substance that participates in and is changed by a chemical reaction
• **Product**: A new substance formed as a result of a chemical reaction
• **Chemical Equation**: A symbolic representation of a chemical reaction showing reactants and products using chemical formulas and coefficients
• **Balanced Chemical Equation**: An equation where the number of atoms of each element is identical on both reactant and product sides
• **Coefficient**: A number placed BEFORE a chemical formula to indicate how many molecules or atoms participate in the reaction
• **Skeletal Equation**: An initial chemical equation before balancing
**8. IMPORTANT POINTS FOR CBSE EXAMS**
• Always write balanced equations — unbalanced equations are incorrect
• Show all calculation work when balancing — examiners award partial marks
• Never alter chemical formulas while balancing — only adjust coefficients
• Include reaction conditions (heat, catalyst) where applicable
• Learn to identify which element to balance first (usually from compound with most atoms)
• Common mistake: changing subscripts instead of using coefficients
• Verify by counting atoms of EACH element separately on both sides
**9. TYPES OF INFORMATION IN CHEMICAL EQUATIONS**
A balanced equation tells us:
• Which substances react (reactants)
• Which substances are produced (products)
• The ratio in which substances react and are produced (from coefficients)
• The conditions required for the reaction (heat, catalyst, etc.)
• Type of chemical change occurring
Q1. A student performs Activity 1.1 by burning a magnesium ribbon in air and observes a dazzling white flame and white powder formation. Which observation directly indicates that a chemical reaction has occurred?
Answer: A — Change in colour is a direct sign of a chemical reaction as it indicates formation of a new substance (MgO); brightness and speed are observational details but not direct evidence of chemical change.
Q2. In Activity 1.3, zinc granules are added to dilute sulphuric acid. The student observes vigorous bubbling and the test tube becomes warm. What do these two observations together indicate?
Answer: A — Gas evolution (bubbling) and temperature increase are both characteristic signs of a chemical reaction; physical dissolution does not produce gas or heat, and external heating was not mentioned.
Q3. A lead nitrate solution is mixed with potassium iodide solution (Activity 1.3), and a bright yellow precipitate forms immediately. Based on the law of conservation of mass, which statement is true for the balanced chemical equation of this reaction?
Answer: A — The law of conservation of mass requires equal number of atoms of each element on both sides; mass is conserved overall but not necessarily distributed equally between single compounds.
Q4. A student writes the following equation for burning magnesium: Mg + O₂ → MgO. After counting atoms, they find 1 Mg on LHS but 1 Mg on RHS (balanced), yet 2 O atoms on LHS but only 1 O on RHS (unbalanced). Which step should they take next?
Answer: A — Coefficients are used to balance equations without changing chemical formulae; changing O₂ to O alters the element identity, and formulae cannot be altered during balancing.
Q5. In a laboratory, a student observes that when iron filings are added to copper sulphate solution, the blue colour gradually fades and a reddish precipitate forms. Which of the following correctly identifies what has happened?
Answer: A — Colour change from blue to colourless and appearance of reddish precipitate indicates formation of new substances (iron sulphate and copper) through a displacement reaction; physical dissolution doesn't produce a precipitate.
Q6. Assertion (A): A skeletal chemical equation for a reaction may not satisfy the law of conservation of mass. Reason (R): Skeletal equations show only the reactants and products without adjusting coefficients to balance atom numbers. Choose the correct option:
Answer: A — Skeletal equations (like Mg + O₂ → MgO) are unbalanced and violate conservation of mass until coefficients are added; R correctly explains why A is true.
Q7. Assertion (A): In the balanced equation Zn + H₂SO₄ → ZnSO₄ + H₂, the number of hydrogen atoms is conserved. Reason (R): There are 2 hydrogen atoms on both the reactant and product sides of the equation. Choose the correct option:
Answer: A — The equation is balanced with 2 H atoms on each side; R provides the evidence that supports assertion A perfectly.
Q8. Assertion (A): When magnesium burns in air, the product weighs more than the original magnesium ribbon. Reason (R): Oxygen from the air combines with magnesium to form magnesium oxide, increasing the total mass. Choose the correct option:
Answer: A — The white ash (MgO) weighs more because oxygen mass is added; R correctly explains why the product is heavier than the reactant magnesium alone.
Q9. An experiment shows: When 56 g of iron reacts with oxygen, it forms iron oxide. The total mass of the product is 80 g. What can be concluded about this reaction?
Answer: A — By conservation of mass, product mass = reactant mass; 80 g = 56 g (Fe) + 24 g (O₂), confirming the law is satisfied; options B and C wrongly suggest mass loss.
Q10. A student balances the equation Fe + H₂O → Fe₃O₄ + H₂ by first writing: 3Fe + 4H₂O → Fe₃O₄ + 4H₂. They count: Fe: 3=3 ✓, H: 8≠8 ✗, O: 4=4 ✓. What does this reveal about their balancing process?
Answer: B — The student correctly identified 8 H atoms on LHS but 4H₂ on RHS produces only 8 H atoms distributed as written, so the equation needs further adjustment; this shows the critical thinking needed in balancing.
What is a chemical reaction?
A process in which the nature and identity of substances change, forming new substances with different properties.
Name four observations that prove a chemical reaction has occurred.
Change in state, change in colour, evolution of a gas, and change in temperature.
What is the difference between reactants and products?
Reactants are substances that undergo chemical change and appear on the left side of the equation; products are new substances formed and appear on the right side.
What is a word-equation?
A chemical reaction written in sentence form using the names of substances, showing reactants on the left and products on the right separated by an arrow.
What is a skeletal chemical equation?
A chemical equation written using chemical formulae instead of words, but which may not have equal atoms on both sides.
What is a balanced chemical equation?
A chemical equation in which the number of atoms of each element is equal on both the reactant and product sides, obeying the law of conservation of mass.
State the law of conservation of mass.
Mass can neither be created nor destroyed during a chemical reaction; total mass of reactants equals total mass of products.
Why do we use coefficients when balancing equations?
Coefficients show how many molecules of each substance react or are produced, without changing the chemical formula of any compound.
In balancing Fe + H₂O → Fe₃O₄ + H₂, why is H₂O₄ incorrect?
The chemical formula of a compound cannot be altered during balancing; only coefficients before the formula can be changed.
What does the arrow (→) in a chemical equation represent?
The direction of the reaction, showing the transformation of reactants into products.
Define a chemical reaction. Name four observations that help us determine whether a chemical reaction has taken place. [2 marks]
Chemical reaction = change in nature/identity of substances forming new substances. Four observations: colour change, state change, gas evolution, temperature change (pick any four from Activity 1.1, 1.2, 1.3 results).
Differentiate between a skeletal chemical equation and a balanced chemical equation. Why is balancing necessary? [3 marks]
Skeletal equation uses formulae but atoms are unequal on both sides; balanced equation has equal atoms of each element on LHS and RHS. Balancing is necessary because it obeys the law of conservation of mass—total mass of reactants must equal total mass of products.
Balance the following chemical equation and explain the steps you followed: Fe + H₂O → Fe₃O₄ + H₂. Why is it incorrect to write H₂O₄ instead of H₂O when balancing this equation? [5 marks]
Balanced equation: 3Fe + 4H₂O → Fe₃O₄ + 4H₂. Steps: box formulas, count atoms (Fe=1 vs 3, H=2 vs 2, O=1 vs 4), balance O first (4H₂O), then H (4H₂), then Fe (3Fe), verify all atoms equal. H₂O₄ is incorrect because chemical formulas of compounds cannot be altered—they represent the fixed composition of that substance; only coefficients before formulas can be changed to balance.
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