Class 10 Science Chapter 1 Notes: Chemical Reactions and Equations

Class 10 Science Chapter 1 Chemical Reactions and Equations Notes

🔹 1. Chemical Reaction

A chemical reaction is a process in which one chemical substance changes into another chemical substance.

✦ Key Points

• New substance is formed

• Properties of product are different from reactants

• Only rearrangement of atoms takes place

• It is a chemical change

✦ Examples

• Rusting of iron

• Setting of milk into curd

• Digestion and respiration

• Burning of magnesium

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🔹 2. Reactants and Products

• Reactants: Substances that take part in a chemical reaction

• Products: New substances formed after the reaction

Example:

2Mg(s) + O₂(g) → 2MgO(s)

• Reactants: Magnesium (Mg), Oxygen (O₂)

• Product: Magnesium oxide (MgO)

📝 Magnesium ribbon is cleaned before burning to remove the protective layer of basic magnesium carbonate.

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🔹 3. Characteristics of Chemical Reactions

(i) Evolution of Gas

Zn + H₂SO₄ → ZnSO₄ + H₂↑

(ii) Change in Colour

• KMnO₄ (purple) → colourless

• K₂Cr₂O₇ (orange) → green

(iii) Change in State

Example: Burning of candle wax
(solid → liquid → gas)

(iv) Change in Temperature

CaO + H₂O → Ca(OH)₂ + heat

(Exothermic reaction)

(v) Formation of Precipitate

BaCl₂ + H₂SO₄ → BaSO₄↓ + 2HCl

(White precipitate formed)

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🔹 4. Chemical Equation

A chemical equation represents a chemical reaction using symbols and chemical formulae.

Example:

Word equation

Hydrogen + Oxygen → Water

Chemical equation

H₂ + O₂ → H₂O

➡️ Chemical equations are short, clear and informative.

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🔹 5. Balanced and Unbalanced Chemical Equations

(a) Balanced Chemical Equation

• Same number of atoms on both sides

• Obeys Law of Conservation of Mass

Example:

Zn + H₂SO₄ → ZnSO₄ + H₂

(b) Unbalanced Chemical Equation

• Number of atoms is not equal

Example:

Fe + H₂O → Fe₃O₄ + H₂

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🔹 6. Balancing a Chemical Equation

Example:

Fe + H₂O → Fe₃O₄ + H₂

Balanced Equation:

3Fe + 4H₂O → Fe₃O₄ + 4H₂

✦ Steps:

1. Write number of atoms on both sides

2. Balance metal first

3. Balance oxygen and hydrogen last

4. Use smallest whole numbers

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🔹 7. Writing More Informative Chemical Equations

Physical State Symbols

• Solid → (s)

• Liquid → (l)

• Gas → (g)

• Aqueous → (aq)

Example:

NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)

Conditions like heat, light, electricity are written above the arrow.

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🔹 8. Types of Chemical Reactions

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(i) Combination Reaction

Two or more substances combine to form a single product.

General form:

A + B → AB

Examples:

2Mg + O₂ → 2MgO
C + O₂ → CO₂

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(ii) Decomposition Reaction

A single compound breaks into two or more substances.

General form:

AB → A + B

Examples:

CaCO₃ → CaO + CO₂
2Fe(OH)₃ → Fe₂O₃ + 3H₂O

Types:

• Thermal decomposition (heat)

• Electrolytic decomposition (electricity)

• Photochemical decomposition (sunlight)

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(iii) Displacement Reaction

A more reactive element displaces a less reactive element.

Zn + CuSO₄ → ZnSO₄ + Cu

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(iv) Double Displacement Reaction

Exchange of ions between two compounds.

BaCl₂ + Na₂SO₄ → BaSO₄↓ + 2NaCl

✔ Precipitation reactions
✔ Neutralisation reactions

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(v) Oxidation and Reduction (Redox Reaction)

• Oxidation: Gain of oxygen / loss of hydrogen

• Reduction: Loss of oxygen / gain of hydrogen

Example:

CuO + H₂ → Cu + H₂O

• CuO → Reduced

• H₂ → Oxidised

⚠️ Oxidation and reduction always occur together.

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(vi) Exothermic and Endothermic Reactions

Exothermic Reaction

Heat is released.

C + O₂ → CO₂ + heat

✔ Respiration is exothermic

Endothermic Reaction

Heat is absorbed.

CaCO₃ + heat → CaO + CO₂

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🔹 9. Effects of Oxidation in Everyday Life

(a) Corrosion

Slow destruction of metals by air, moisture or chemicals.

Rusting of Iron is a redox reaction.

Prevention of Rusting:

• Painting

• Greasing and oiling

• Galvanisation

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(b) Rancidity

Oxidation of fats and oils causing bad smell and taste.

Prevention:

• Adding anti-oxidants

• Vacuum packing

• Nitrogen flushing

• Refrigeration

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✨ Quick Revision Points

• Chemical reactions form new substances

• Chemical equations must be balanced

• Major reactions: Combination, Decomposition, Displacement, Double displacement

• Oxidation & reduction always occur together

• Corrosion and rancidity are harmful oxidation processes

✍️ Top 10 Short Question–Answers

Q1. What is a chemical reaction?
Ans: A chemical reaction is a process in which one chemical substance changes into another new substance.

Q2. Why is a chemical reaction called a chemical change?
Ans: Because a new substance with different properties is formed.

Q3. Name any two examples of chemical reactions.
Ans: Rusting of iron and burning of magnesium.

Q4. What are reactants?
Ans: Reactants are the substances that take part in a chemical reaction.

Q5. What are products?
Ans: Products are the new substances formed after a chemical reaction.

Q6. What does evolution of gas indicate in a chemical reaction?
Ans: It indicates that a chemical reaction has taken place.

Q7. What is a chemical equation?
Ans: A chemical equation represents a chemical reaction using symbols and chemical formulae.

Q8. What is a balanced chemical equation?
Ans: A balanced chemical equation has an equal number of atoms on both sides.

Q9. What is oxidation?
Ans: Oxidation is the gain of oxygen or loss of hydrogen.

Q10. What is corrosion?
Ans: Corrosion is the slow destruction of metals by air, moisture or chemicals.

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📝 Long Answer Questions

Q1. Explain the characteristics of chemical reactions.

Ans:
The main characteristics of chemical reactions are:

1. Evolution of Gas: Gas is released during some reactions (e.g. Zn + H₂SO₄ → ZnSO₄ + H₂).

2. Change in Colour: Colour of substances changes during reaction (e.g. KMnO₄ turns colourless).

3. Change in State: State of matter may change (solid → liquid → gas).

4. Change in Temperature: Heat may be released or absorbed (exothermic or endothermic).

5. Formation of Precipitate: An insoluble solid is formed (e.g. BaSO₄ precipitate).

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Q2. What is a chemical equation? Explain balanced and unbalanced chemical equations.

Ans:

Chemical Equation:
A chemical equation is a symbolic representation of a chemical reaction using chemical formulae.

Balanced Chemical Equation:

• Same number of atoms on both sides

• Obeys law of conservation of mass

• Example: Zn + H₂SO₄ → ZnSO₄ + H₂

Unbalanced Chemical Equation:

• Unequal number of atoms on both sides

• Example: Fe + H₂O → Fe₃O₄ + H₂

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Q3. Explain different types of chemical reactions.

Ans:

1. Combination Reaction: Two or more substances combine to form one product.

2. Decomposition Reaction: One compound breaks into simpler substances.

3. Displacement Reaction: More reactive element displaces a less reactive one.

4. Double Displacement Reaction: Exchange of ions between compounds.

5. Redox Reaction: Oxidation and reduction occur together.

6. Exothermic & Endothermic Reactions: Heat is released or absorbed.

📝 Previous Years’ Questions (PYQs)

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🔹 1 Mark PYQs

1. What is meant by a chemical reaction?

2. Write the chemical formula of quicklime.

3. Name the gas evolved when zinc reacts with dilute sulphuric acid.

4. What does a precipitate indicate in a chemical reaction?

5. Write the symbol used for aqueous state in a chemical equation.

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🔹 2 Marks PYQs

6. Define reactants and products with an example.

7. What is a balanced chemical equation? Why should chemical equations be balanced?

8. Write one difference between oxidation and reduction.

9. What is corrosion? Name any one method to prevent rusting.

10. Why is respiration considered an exothermic reaction?

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🔹 3 Marks PYQs

11. Write any three characteristics of chemical reactions.

12. What is a displacement reaction? Explain with an example.

13. Explain rancidity. Write any two methods to prevent it.

14. Differentiate between combination reaction and decomposition reaction.

15. What is meant by double displacement reaction? Write one example.

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🔹 5 Marks PYQs (Long Answer Type)

16. Explain the different types of chemical reactions with suitable examples.

17. Describe the process of balancing a chemical equation with an example.

18. What is oxidation and reduction? Explain with the help of a chemical equation.

19. Explain corrosion and rancidity. How can they be prevented?

20. Write the characteristics of chemical reactions with examples.

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🔹 Very Important Board Repeated Questions ⭐

✔ Balance the following chemical equation:
Fe + H₂O → Fe₃O₄ + H₂

✔ Identify the type of reaction:
CaCO₃ → CaO + CO₂

✔ Why should magnesium ribbon be cleaned before burning?

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❓ FAQs (Concept Clarity – 5)

Q1. Why is magnesium ribbon cleaned before burning?
Ans: To remove the protective layer of basic magnesium carbonate.

Q2. Why must chemical equations be balanced?
Ans: To obey the law of conservation of mass.

Q3. Can oxidation and reduction occur separately?
Ans: No, oxidation and reduction always occur together.

Q4. Why is respiration an exothermic reaction?
Ans: Because it releases energy in the form of heat.

Q5. What is rancidity?
Ans: Rancidity is the oxidation of fats and oils that causes bad smell and taste.