CBSE Class 10 Chemistry: Chemistry in Your Kitchen – Reactions You Encounter While Cooking (2025–2026)

Studying CBSE Class 10 Chemistry becomes significantly easier when you realize that your home kitchen functions as an active chemical laboratory. For students gearing up for the 2025–2026 board exam 10, mastering Chapter 1 (Chemical Reactions and Equations) requires connecting theoretical formulas to real-world phenomena. When you ignite the stove, bake bread, cook acidic vegetables, or store cooking oils, you are directly observing exothermic reactions, thermal decompositions, oxidation, displacement, and neutralization in action.

Key Concepts: Cooking Processes as Chemical Reactions

Chemical reactions in cooking involve the breaking and forming of chemical bonds, accompanied by energy changes, gas evolution, color transitions, and state alterations. Understanding these fundamental kitchen phenomena helps you answer application-based questions accurately in CBSE Chemistry.

1. Combustion and Exothermic Reactions

An exothermic reaction is a chemical process in which heat energy is released into the surroundings along with the formation of products. In your kitchen, the blue flame on your gas burner is the combustion of Liquefied Petroleum Gas (LPG), primarily composed of butane (C₄H₁₀) and propane (C₃H₈), or piped natural gas containing methane (CH₄):

CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g) + Heat + Light

Similarly, the energy required for cooking food is mirrored inside the human body during the biochemical digestion of carbohydrates into glucose, followed by cellular respiration:

C₆H₁₂O₆(aq) + 6O₂(aq) → 6CO₂(aq) + 6H₂O(l) + Energy

Because energy is liberated to maintain body temperature and power metabolic processes, respiration is classified as an exothermic reaction in the NCERT curriculum.

2. Thermal Decomposition and Gas Evolution in Baking

Baking cakes, dhoklas, and bread relies on the decomposition of baking soda (sodium hydrogen carbonate, NaHCO₃). When heated during cooking, it undergoes thermal decomposition:

2NaHCO₃(s) —Δ→ Na₂CO₃(s) + H₂O(g) + CO₂(g)

The released carbon dioxide (CO₂) gas gets trapped in the batter, creating tiny bubbles that make baked foods soft and spongy. To prevent the bitter taste of sodium carbonate (Na₂CO₃), bakers use baking powder, which contains baking soda mixed with a mild edible acid like tartaric acid:

NaHCO₃ + H⁺ (from edible acid) → Na⁺ + H₂O + CO₂↑ + Sodium salt of acid

3. Food Spoilage: Oxidation and Rancidity

When cooking oils and fatty foods are left exposed to air for extended periods, they undergo oxidation, leading to a condition called rancidity. Oxidation alters the molecular structure of lipids, producing volatile aldehydes and carboxylic acids with an unpleasant smell and taste.

Common preventive measures in food preservation include:

• Adding Antioxidants: Substances like BHA (butylated hydroxyanisole) and BHT (butylated hydroxytoluene) retard oxidation.
• Flushing with Inert Gas: Manufacturers flush potato chip packets with unreactive nitrogen gas (N₂) to create an oxygen-free environment.
• Airtight Packaging and Refrigeration: Lowering temperature and blocking atmospheric oxygen drastically slows down reaction rates.

4. Reactivity and Displacement Reactions in Cookware

Why should you avoid cooking sour curds, tomato gravies, or pickles in brass, copper, or unlined aluminum cookware? Organic acids (such as citric acid, lactic acid, and acetic acid) react with active metals, releasing hydrogen gas and producing toxic metallic salts that can cause food poisoning.

In CBSE Class 10 Chemistry, this concept extends to single displacement reactions, where a more reactive metal displaces a less reactive metal from its aqueous solution based on the metal reactivity series:

Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)

Here, iron displaces copper because iron is positioned higher in the reactivity series, causing the solution color to turn from blue to light green while reddish-brown copper deposits on the iron.

Important CBSE Questions with Answers

The following questions are selected from official CBSE board exam question banks to test your conceptual clarity and answer-writing precision.

1. Question: Define exothermic reactions. Give one example.

   Answer: Chemical reactions that release heat energy into the surroundings along with the formation of products are called exothermic reactions.

   Example: Combustion of methane gas:

   CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g) + Heat

2. Question: Why is respiration considered an exothermic reaction?

   Answer: During digestion, ingested food containing carbohydrates is broken down into simpler sugars, mainly glucose. In the cells of our body, glucose combines with inhaled oxygen during cellular respiration to release energy along with carbon dioxide and water:

   C₆H₁₂O₆(aq) + 6O₂(aq) → 6CO₂(aq) + 6H₂O(l) + Energy

   Since energy is released in this process to fuel vital bodily functions, respiration is classified as an exothermic reaction.

3. Question: What happens when zinc granules are treated with dilute H₂SO₄?

   Answer: Zinc reacts with dilute sulphuric acid to form zinc sulphate and hydrogen gas. Rapid effervescence (bubbling) is observed, and the reaction mixture becomes warm because the reaction is exothermic:

   Zn(s) + H₂SO₄(dil) → ZnSO₄(aq) + H₂(g)↑

   The evolved gas burns with a characteristic 'pop' sound when a burning splinter is brought near the mouth of the test tube.

4. Question: Define rancidity. How can it be prevented?

   Answer: Rancidity is the slow aerial oxidation of fats and oils present in food items, resulting in foul odors, bad taste, and changes in nutritional quality.

   Methods to prevent rancidity:

   • Adding synthetic or natural antioxidants to oily foods.
   • Packaging fat-containing foods in airtight containers to limit oxygen exposure.
   • Flushing packaged food packets with an inert gas like nitrogen (N₂).
   • Storing food items in a refrigerator to reduce the rate of oxidation.

5. Question: What is a displacement reaction? Give a chemical equation to illustrate.

   Answer: A chemical reaction in which a more reactive element displaces a less reactive element from its aqueous salt solution is called a displacement reaction.

   Equation:

   Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)

   In this reaction, iron is chemically more reactive than copper; therefore, it displaces copper from copper(II) sulphate solution, changing the color from blue to pale green.

6. Question: Balance the equation: Fe + H₂O → Fe₃O₄ + H₂

   Answer: Step-by-step balancing:

   • Equalize Iron (Fe) atoms: 3Fe on LHS.
   • Equalize Oxygen (O) atoms: 4H₂O on LHS to provide 4 oxygen atoms for Fe₃O₄.
   • Equalize Hydrogen (H) atoms: 4H₂ on RHS to match 8 hydrogen atoms from 4H₂O.

   Balanced Chemical Equation:

   3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g)

7. Question: Why do silver chloride and silver bromide turn grey on exposure to sunlight?

   Answer: Silver chloride (AgCl) and silver bromide (AgBr) undergo photochemical decomposition (decomposition triggered by light energy). When exposed to sunlight, they break down into elemental silver metal and halogen gases:

   2AgCl(s) —Sunlight→ 2Ag(s) + Cl₂(g)

   2AgBr(s) —Sunlight→ 2Ag(s) + Br₂(g)

   The elemental silver (Ag) formed is grey in color, causing the original white AgCl and pale yellow AgBr to turn grey. This reaction forms the basis of black-and-white photography.

8. Question: Identify the type of chemical reaction in each case and write balanced equations: (a) Decomposition of calcium carbonate on heating (b) Reaction of sodium with water

   Answer:

   (a) Decomposition of calcium carbonate on heating:

   • Reaction Type: Thermal Decomposition Reaction
   • Balanced Equation: CaCO₃(s) —Δ→ CaO(s) + CO₂(g)

   (b) Reaction of sodium with water:

   • Reaction Type: Displacement / Exothermic Redox Reaction
   • Balanced Equation: 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g) + Heat

How to Prepare for This Topic in CBSE Class 10 Chemistry

To secure full marks in Chapter 1 for the 2025–2026 board examination, implement these high-yield revision strategies:

1. Master Reaction Types with Physical State Symbols: Always write states — (s), (l), (g), (aq) — and reaction conditions (heat Δ, sunlight, catalyst) above the reaction arrow. Examiners award marks for complete chemical representations.
2. Practice Balancing by the Algebraic or Hit-and-Trial Method: Count each atom on both sides of the arrow before and after balancing to verify conservation of mass.
3. Memorize Characteristic Visual and Sensory Observations: Be ready for question prompts describing color changes (e.g., green FeSO₄ crystals turning reddish-brown Fe₂O₃ on heating), precipitation (white BaSO₄ precipitate), or gas identification tests (limewater test for CO₂ and pop sound test for H₂).
4. Connect Everyday Case Studies: Practice case-based and assertion-reasoning questions derived from daily life, such as corrosion of iron gates, rancidity of butter, and neutralization of ant bites or indigestion.

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