Contents / विषय-सूची (8)
  1. 1. Chemical Changes रासायनिक परिवर्तन
  2. 2. Chemical Equations रासायनिक समीकरण
  3. 3. Types of Chemical Reactions रासायनिक अभिक्रियाओं के प्रकार
  4. 4. Everyday Oxidation दैनिक जीवन में उपचयन
  5. Checkpoint Quiz ज्ञान की परीक्षा
  6. What You Can Now Do अब आप ये कर सकते हैं
  7. NCERT Solutions अभ्यास प्रश्नोत्तर Print Solutions
  8. CBSE Board PYQs विगत वर्षों के प्रश्न
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🌐 Language / भाषा:
Science — Class X

Chapter 1: Chemical Reactions and Equations

Chemical reactions - where substances lose their original identity and change into new substances! Let's explore chemical equations, balancing techniques, types of reactions, and redox processes in action. 🧪 Chemical reactions (chemical reactionएँ) - जहाँ substance अपनी मूल पहचान खोकर नए substanceों में बदलते हैं! चलो, chemical equations, balancing techniques, types of reactions, और redox processes को detail में समझते हैं। 🧪

Remember from before:

1. Chemical Changes रासायनिक परिवर्तन

Goal: After this section, you will be able to identify the criteria that determine whether a chemical reaction has taken place based on laboratory observations [NCERT p.2]. इस section के बाद, आप laboratory observations के आधार पर उन मानदंडों की पहचान कर पाएंगे जो यह तय करते हैं कि कोई chemical reaction हुई है या नहीं [NCERT p.2]।

In our daily life, we witness chemical changes constantly: when milk is left at room temperature during summers and turns sour, when an iron pan or nail is left exposed to a humid atmosphere and rusts, when grapes get fermented, when food is cooked, when food gets digested in our body, and when we respire [NCERT p.1]. In all these situations, the nature and identity of the initial substance have changed. We say a chemical reaction has taken place [NCERT p.1]. रोजमर्रा की जिंदगी में हम लगातार chemical changes (chemical change) देखते हैं: जैसे गर्मी में दूध को कमरे के temperature पर रखने पर वह खट्टा हो जाता है, जब लोहे के तवे या कील को नम हवा (moisture) में छोड़ दिया जाता है और उसमें जंग लग जाता है, जब organूरों का fermentation (किण्वन) होता है, जब खाना पकाया जाता है, जब हमारे शरीर में खाना digest होता है, और जब हम respire (साँस लेते हैं) [NCERT p.1]। इन सभी स्थितियों में, शुरुआती substance का स्वभाव और पहचान बदल जाती है। हम कहते हैं कि यहाँ एक chemical reaction हुई है [NCERT p.1]।

Lego Analogy (लेगो ब्लॉक) Think of a chemical reaction like rearranging Lego blocks. The individual blocks (atoms) are not destroyed, nor do new blocks magically appear. Instead, you break apart the model you built (reactants) and assemble the blocks into a completely different structure (products). Chemical reaction को Lego blocks को फिर से व्यवस्थित करने (rearrange) की तरह समझो। अलग-अलग ब्लॉक्स (atoms) न तो नष्ट होते हैं और न ही कोई नए ब्लॉक्स जादुई रूप से आ जाते हैं। इसके बजाय, आप अपने द्वारा बनाए गए मॉडल (reactants) को अलग करते हैं और उन ब्लॉक्स को मिलाकर एक बिल्कुल नया स्ट्रक्चर (products) बना देते हैं।

To determine experimentally whether a chemical reaction has occurred, scientists perform specific activities in the laboratory: प्रयोग के जरिए यह पता लगाने के लिए कि क्या कोई chemical reaction हुई है, scientists laboratory में कुछ activities करते हैं:

Burning of Magnesium Ribbon
Figure 1.1 — Burning of a magnesium ribbon in air and collection of white magnesium oxide powder in a watch-glass. Adapted from NCERT Figure 1.1, p. 1.

From these activities, we establish that a chemical reaction has taken place if we observe any of the following changes [NCERT p.2]: इन activities से हम यह conclusion निकालते हैं कि यदि हमें निम्नलिखित में से कोई भी बदलाव दिखाई दे, तो वहाँ एक chemical reaction हुई है [NCERT p.2]:

  1. Change in state Change in state (state में change)
  2. Change in colour Change in colour (रंग में change)
  3. Evolution of a gas Evolution of a gas (गैस का निकास)
  4. Change in temperature Change in temperature (temperature में change)
Worked Example: Identify the chemical change in the following: (a) Boiling of water, (b) Rusting of an iron gate.
Solution:
(a) Boiling of water is a physical change because it only changes the state from liquid to gas without creating new substances.
(b) Rusting of an iron gate is a chemical change because iron reacts with moisture and oxygen in the air to form a new reddish-brown compound (hydrated iron oxide, rust) with entirely different chemical properties [NCERT p.13]. निम्नलिखित में से chemical change की पहचान करें: (a) पानी का उबलना, (b) लोहे के फाटक में जंग लगना।
Solution:
(a) पानी का उबलना एक physical change है क्योंकि यह केवल liquid से gas में state बदलता है, कोई नया substance नहीं बनता।
(b) लोहे के फाटक में जंग लगना एक chemical change है क्योंकि लोहा हवा की moisture और oxygen के साथ react करके एक नया लाल-भूरा compound (hydrated iron oxide, जंग) बनाता है जिसके chemical properties पूरी तरह से अलग होते हैं [NCERT p.13]।
Checkpoint (ज्ञान की जाँच)

Why is a magnesium ribbon rubbed with sandpaper before burning in air? [NCERT p.6] जलने से पहले magnesium ribbon को sandpaper से क्यों रगड़ा जाता है? [NCERT p.6]

Section Summary (रासायनिक परिवर्तन का सारांश):
  • Chemical changes yield new substances with entirely different chemical properties [NCERT p.1]. Chemical changes से नए substances बनते हैं जिनकी chemical properties बिल्कुल अलग होती हैं [NCERT p.1]।
  • Signs of a chemical reaction include color changes, state changes, gas evolution, or temperature changes [NCERT p.2]. Chemical reaction के लक्षणों में रंग में बदलाव, state में बदलाव, गैस का निकलना या temperature में बदलाव शामिल हैं [NCERT p.2]।
  • Activity 1.1 shows magnesium ribbon burns with a dazzling white flame to form magnesium oxide powder [NCERT p.1]. Activity 1.1 दर्शाता है कि magnesium ribbon एक चमकदार सफेद लौ के साथ जलकर magnesium oxide का सफेद पाउडर बनाती है [NCERT p.1]।

2. Chemical Equations रासायनिक समीकरण

Goal: Learn how to represent chemical reactions symbolically and balance skeletal equations using the hit-and-trial coefficient method [NCERT pp.2-5]. सीखें कि कैसे chemical reactions को प्रतीकात्मक (symbolically) रूप से दर्शाया जाता है और hit-and-trial method का उपयोग करके skeletal equations को balance किया जाता है [NCERT pp.2-5]।

Instead of describing a reaction in a long sentence, we represent it using a word-equation [NCERT p.2]:
Magnesium + Oxygen → Magnesium oxide The substances that undergo chemical change are the reactants on the left-hand side (LHS), and the new substances formed are the products on the right-hand side (RHS) [NCERT p.2]. किसी reaction को लंबे वाक्यों में समझाने के बजाय, हम इसे एक word-equation (शब्द-समीकरण) के रूप में दर्शा सकते हैं [NCERT p.2]:
Magnesium + Oxygen → Magnesium oxide वे substance जिनमें chemical change होता है, उन्हें left-hand side (LHS) पर reactants कहा जाता है, और जो नए substance बनते हैं, उन्हें right-hand side (RHS) पर products कहा जाता है [NCERT p.2]।

We make chemical equations more concise by using chemical formulas instead of words [NCERT p.3]:
Mg + O2 → MgO     [Skeletal Equation] [NCERT p.3] This is a skeletal chemical equation because the number of oxygen atoms on the LHS (2) does not equal the number of oxygen atoms on the RHS (1) [NCERT p.3]. हम शब्दों के स्थान पर chemical formulas का उपयोग करके chemical equations को अधिक संक्षिप्त बना सकते हैं [NCERT p.3]:
Mg + O2 → MgO     [Skeletal Equation] [NCERT p.3] यह एक skeletal chemical equation (ढांचागत समीकरण) है क्योंकि LHS पर oxygen atoms की संख्या (2) RHS पर oxygen atoms की संख्या (1) के बराबर नहीं है [NCERT p.3]।

Scales Analogy (संतुलन तराजू) Think of a balanced chemical equation like a balance scale. The mass and number of atoms on the left pan (reactants) must exactly equal the mass and number of atoms on the right pan (products) to satisfy the Law of Conservation of Mass [NCERT p.3]. एक balanced chemical equation को एक तराजू की तरह समझो। Law of Conservation of Mass को संतुष्ट करने के लिए बाएं पलड़े (reactants) पर mass और atoms की संख्या दाएं पलड़े (products) पर मौजूद mass और atoms की संख्या के बिल्कुल बराबर होनी चाहिए [NCERT p.3]।

Balancing Step-by-Step

To satisfy the Law of Conservation of Mass, the total number of atoms of each element must remain equal on both sides [NCERT p.3]. Law of Conservation of Mass को संतुष्ट करने के लिए, प्रत्येक तत्व के atoms की कुल संख्या दोनों ओर समान होनी चाहिए [NCERT p.3]।

Worked Example: Balance the skeletal equation for iron reacting with steam:
Fe + H2O → Fe3O4 + H2
Solution:
  1. Step I: Draw boxes around each formula. Do not change anything inside the boxes [NCERT p.4]. Step I: प्रत्येक formula के चारों ओर बक्से (boxes) बनाएं। बक्से के अंदर कुछ भी न बदलें [NCERT p.4]।
  2. Step II: List the atoms of each element: Fe (LHS: 1, RHS: 3), H (LHS: 2, RHS: 2), O (LHS: 1, RHS: 4) [NCERT p.4]. Step II: प्रत्येक तत्व के atoms की सूची बनाएं: Fe (LHS: 1, RHS: 3), H (LHS: 2, RHS: 2), O (LHS: 1, RHS: 4) [NCERT p.4]।
  3. Step III: Balance the element with the maximum atoms (Oxygen). Multiply H2O on the LHS by 4 to get 4 oxygen atoms [NCERT p.4]:
    Fe + 4H2O → Fe3O4 + H2     Step III: सबसे अधिक atoms वाले तत्व (Oxygen) को balance करें। LHS पर H2O को 4 से गुणा करें ताकि 4 oxygen atoms हो सकें [NCERT p.4]:
    Fe + 4H2O → Fe3O4 + H2
  4. Step IV: Now balance Hydrogen. We have 8 H atoms on the LHS, so multiply H2 on the RHS by 4 [NCERT p.4]:
    Fe + 4H2O → Fe3O4 + 4H2     Step IV: अब Hydrogen को balance करें। हमारे पास LHS पर 8 H atoms हैं, इसलिए RHS पर H2 को 4 से गुणा करें [NCERT p.4]:
    Fe + 4H2O → Fe3O4 + 4H2
  5. Step V: Finally, balance Iron. Multiply Fe on the LHS by 3 [NCERT p.5]:
    3Fe + 4H2O → Fe3O4 + 4H2     Step V: अंत में, Iron को balance करें। LHS पर Fe को 3 से गुणा करें [NCERT p.5]:
    3Fe + 4H2O → Fe3O4 + 4H2
  6. Step VI: Count atoms to verify. LHS has 3 Fe, 8 H, 4 O. RHS has 3 Fe, 8 H, 4 O. The equation is balanced! [NCERT p.5]. Step VI: सत्यापन के लिए atoms की गणना करें। LHS पर 3 Fe, 8 H, 4 O हैं। RHS पर 3 Fe, 8 H, 4 O हैं। समीकरण संतुलित है! [NCERT p.5]।
  7. Step VII (State Symbols): Write the physical states: solid (s), liquid (l), gas (g), and aqueous (aq) [NCERT p.5]:
    3Fe(s) + 4H2O(g) → Fe3O4(s) + 4H2(g) [NCERT p.5] Here, (g) is used with H2O to indicate that water is used in the form of steam [NCERT p.5].     Step VII (State Symbols): उनकी physical stateएँ (physical states) लिखें: solid (s), liquid (l), gas (g), और aqueous (aq) [NCERT p.5]:
    3Fe(s) + 4H2O(g) → Fe3O4(s) + 4H2(g) [NCERT p.5] यहाँ, H2O के साथ (g) का उपयोग यह दर्शाने के लिए किया गया है कि पानी का उपयोग भाप (steam) के रूप में किया गया है [NCERT p.5]।
Interactive Chemical Balancer Lab इंटरैक्टिव केमिकल बैलेंसर लैब Hands-On Lab
Select a chemical reaction below. Click the ▲ and ▼ buttons to adjust the coefficients. Watch the atom count grid update. Achieve balance to verify conservation of mass! नीचे दी गई chemical reaction को चुनें। गुणांकों (coefficients) को बदलने के लिए ▲ और ▼ बटनों पर क्लिक करें। परमाणु संख्या ग्रिड को अपडेट होते हुए देखें। Mass के conservation को सत्यापित करने के लिए संतुलन प्राप्त करें!

Reactant Side (LHS)

Product Side (RHS)

Unbalanced. Adjust coefficients to balance the scale! असंतुलित। पैमाने को संतुलित करने के लिए गुणांकों को समायोजित करें!
Checkpoint (ज्ञान की जाँच)

In the balanced chemical equation 3Fe(s) + 4H2O(g) → Fe3O4(s) + 4H2(g), why is water written with state symbol (g)? [NCERT p.5] संतुलित chemical समीकरण 3Fe(s) + 4H2O(g) → Fe3O4(s) + 4H2(g) में पानी को physical state (g) के साथ क्यों लिखा गया है? [NCERT p.5]

Section Summary (रासायनिक समीकरण का सारांश):
  • A chemical equation is a symbolic representation of a chemical reaction using chemical formulas [NCERT p.3]. Chemical equation chemical सूत्रों का उपयोग करके chemical reaction का एक symbolic representation है [NCERT p.3]।
  • A skeletal equation is unbalanced, while a balanced equation has equal atoms of each element on both sides [NCERT p.3]. एक skeletal equation असंतुलित होती है, जबकि balanced equation में दोनों तरफ प्रत्येक तत्व के atoms की संख्या समान होती है [NCERT p.3]।
  • Equations must be balanced to satisfy the Law of Conservation of Mass. State symbols (s, l, g, aq) add physical detail [NCERT p.5]. Law of Conservation of Mass को संतुष्ट करने के लिए समीकरणों को संतुलित करना आवश्यक है। physical state के संकेत (s, l, g, aq) physical विवरण जोड़ते हैं [NCERT p.5]।

3. Types of Chemical Reactions रासायनिक अभिक्रियाओं के प्रकार

Goal: Classify reactions into combination, decomposition, displacement, double displacement, and redox reactions, and write equations for each type [NCERT pp.6-13].

During chemical reactions, atoms of one element do not change into those of another element, nor do they disappear [NCERT p.6]. Instead, chemical reactions involve the breaking and making of bonds between atoms to produce new substances [NCERT p.6]. Chemical reactions के दौरान, एक तत्व के atoms दूसरे तत्व के atoms में नहीं बदलते हैं, और न ही वे गायब होते हैं [NCERT p.6]। इसके बजाय, chemical reactions में नए substanceों को बनाने के लिए atoms के बीच bonds के टूटने और बनने की प्रक्रिया होती है [NCERT p.6]।

Overview of Types of Chemical Reactions (रासायनिक अभिक्रियाओं के प्रकार)
Combination
(संयोजन)
•
Decomposition
(अपघटन)
•
Displacement
(विस्थापन)
•
Double Displacement
(द्वि-विस्थापन)
•
Redox (Oxidation/Reduction)
(अपचयोपचय)
Dance Analogy (नृत्य समानता) Think of reaction types like dance partners:
  • Combination: Two single dancers join to dance as a pair.
  • Decomposition: A dancing couple splits to dance alone.
  • Displacement: A dancer comes in and displaces one partner from an active couple.
  • Double Displacement: Two dancing couples switch partners [NCERT pp.6-12].

3.1 Combination Reaction संयोजन अभिक्रिया

A reaction in which a single product is formed from two or more reactants is called a combination reaction [NCERT p.6].
Example: Quick lime (calcium oxide) reacts vigorously with water to produce slaked lime (calcium hydroxide) [NCERT p.6]:
CaO(s) + H2O(l) → Ca(OH)2(aq) + Heat [NCERT p.6] This reaction is highly exothermic because a large amount of heat is evolved, making the reaction mixture warm [NCERT p.6].

Whitewashing Application: Slaked lime is applied to walls. It reacts slowly with CO2 in air over 2–3 days to form a thin, shiny layer of calcium carbonate (marble, CaCO3) on the walls [NCERT p.7]:
Ca(OH)2(aq) + CO2(g) → CaCO3(s) + H2O(l) [NCERT p.7]

3.2 Decomposition Reaction अपघटन अभिक्रिया

A reaction in which a single reactant breaks down to give simpler products is called a decomposition reaction [NCERT p.8]. It is the opposite of a combination reaction.

Virtual Chemistry Lab & Atomic Explorer Lab Simulator
Microscopic Atomic Lattice View
Magnesium metal atoms (Mg) lose their outer electrons to Oxygen molecules (O2) from air, instantly forming a highly stable, white ionic crystal lattice of magnesium oxide (Mg2+O2−).
Burning Magnesium Ribbon (Activity 1.1)
Observe the magnesium ribbon oxidation. Magnesium burns in oxygen with a brilliant white light.
Ionic Lattice Crash View
When colorless Pb2+ ions and I− ions mix in water, they collide and instantly bind, forming a yellow, highly insoluble ionic crystal lattice of lead iodide (PbI2) which precipitates out of the solution.
Lead Iodide Precipitation (Activity 1.2)
Observe lead nitrate solution reacting with potassium iodide solution to form an insoluble yellow salt.
Electron Displacement View
Zinc atoms (Zn) are more reactive than hydrogen. They release two electrons to hydrogen ions (H+) from the acid, transforming into zinc ions (Zn2+) while hydrogen ions pair up to form hydrogen gas bubbles (H2 ↑).
Zinc Granules in Acid (Activity 1.3)
Observe the evolution of hydrogen gas bubbles and feel the exothermic temperature rise as zinc reacts with sulphuric acid.
6V + - H₂ O₂
Ionic Migration View
Electric current splits water molecules (H2O). H+ ions migrate to the negative cathode to gain electrons and form H2 gas. OH− ions migrate to the positive anode to lose electrons and form O2 gas. Cathode gas volume is double the anode gas volume.
Electrolysis of Water (Activity 1.7)
Apply electric current through acidulated water and observe the chemical decomposition. Note the 2:1 volume ratio.
Atomic Electron Transfer View
Iron atoms (Fe) are more reactive than copper. They transfer electrons to blue copper ions (Cu2+) in solution. Iron becomes soluble green Fe2+ ions, while copper ions become solid metallic copper (Cu), depositing as a brown coat on the nail.
Metal Displacement (Activity 1.9)
Dip a grey iron nail into blue copper sulphate solution. Watch the solution fade to green and a brown copper coat deposit on the nail.

3.3 Displacement Reaction विस्थापन अभिक्रिया

A reaction in which a more reactive element displaces or removes a less reactive element from its salt solution is known as a displacement reaction [NCERT p.11].
Example: Dipping an iron nail in copper sulphate solution. Iron, being more reactive than copper, displaces copper ions [NCERT p.11]:
Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s) [NCERT p.11] Similarly, zinc and lead are more reactive than copper, so they displace copper from its compounds [NCERT p.11]:
Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s) [NCERT p.11]

3.4 Double Displacement Reaction द्वि-विस्थापन अभिक्रिया

Reactions in which there is an exchange of ions between the reactants to form new compounds are called double displacement reactions [NCERT p.12].
Example: When sodium sulphate solution is mixed with barium chloride solution, a white, water-insoluble solid is formed instantly [NCERT p.11]. This insoluble substance is called a precipitate (अवक्षेप), and any reaction producing a precipitate is called a precipitation reaction (अवक्षेपण अभिक्रिया) [NCERT p.11]:
Na2SO4(aq) + BaCl2(aq) → BaSO4(s) ↓ + 2NaCl(aq) [NCERT p.11] Here, the white precipitate of BaSO4 is formed by the reaction of barium ions (Ba2+) and sulphate ions (SO42-) [NCERT p.12].

3.5 Oxidation and Reduction उपचयन एवं अपचयन / रेडॉक्स

If one reactant gets oxidised while the other gets reduced during a reaction, it is called an oxidation-reduction reaction or redox reaction (रेडॉक्स अभिक्रिया) [NCERT p.12].
Example: When black copper oxide (CuO) is heated in the presence of hydrogen gas, brown metallic copper is formed [NCERT p.12].

CuO + H₂ → Cu + H₂O Heat Reduction (Loss of O) Oxidation (Gain of O)
Figure 1.2 — Redox Reaction showing concurrent oxidation of hydrogen and reduction of copper oxide. Adapted from NCERT Figure 1.30, p. 12.

In this reaction, CuO loses oxygen and is reduced to Cu. Hydrogen (H2) gains oxygen and is oxidised to H2O [NCERT p.12].

Worked Example: Identify the reaction type for CaO(s) + H2O(l) → Ca(OH)2(aq).
Solution: Since two reactants (calcium oxide and water) combine to form a single product (calcium hydroxide), this is a Combination reaction [NCERT p.6].
3D Molecule Structure
Redox Atom Transfer Explorer
Cu O H₂
Click to heat copper oxide in hydrogen stream.
Checkpoint (ज्ञान की जाँच)

Which of the following is correct regarding the respiration process in our body? [NCERT p.7]

Section Summary (अभिक्रिया प्रकार का सारांश):
  • Combination reactions form one product. Decomposition splits one reactant using heat, light, or electricity [NCERT pp.6-9].
  • Displacement reactions swap a less reactive metal with a more reactive one. Double displacement exchanges ions, often forming a precipitate [NCERT pp.11-12].
  • Redox reactions involve simultaneous oxidation (oxygen gain/hydrogen loss) and reduction (oxygen loss/hydrogen gain) [NCERT p.12].

4. Everyday Oxidation दैनिक जीवन में उपचयन

Goal: Describe the harmful effects of oxidation in daily life, specifically corrosion and rancidity, and outline preventive measures [NCERT p.13].

4.1 Corrosion संक्षारण

When a metal is attacked by substances around it, such as moisture, air, and acids, it is said to corrode, and this process is called corrosion [NCERT p.13].

Corrosion causes massive damage to car bodies, bridges, iron railings, ships, and all objects made of metals, especially iron [NCERT p.13]. Corrosion के कारण कार की बॉडी, पुल, लोहे की रेलिंग, जहाजों और धातुओं (विशेषकर लोहे) से बनी सभी वस्तुओं को भारी नुकसान पहुँचता है [NCERT p.13]।

4.2 Rancidity विकृतगंधिता

When fats and oils are oxidised, they become rancid, and their smell, taste, and appearance change [NCERT p.13].

To prevent rancidity in food products, manufacturers use several techniques [NCERT p.13]:

  1. Adding antioxidants (उपचयन-रोधी) to foods containing fats and oils [NCERT p.13].
  2. Storing food in air-tight containers to slow down oxidation [NCERT p.13].
  3. Flushing packaging bags (like potato chips bags) with an inert gas like nitrogen to prevent oxidation of the fats [NCERT p.13].
Fruit Browning Analogy (फल का भूरा होना) Think of corrosion and rancidity like an apple slice turning brown in air. Oxygen in the atmosphere slowly attacks the fruit's surface, chemical bonds break, and a new oxidized compound forms, spoiling the taste and look.
Worked Example: Why do copper vessels develop a green coat when exposed to air?
Solution: Copper reacts with moist carbon dioxide and oxygen in the air, undergoing slow oxidation (corrosion) to form basic copper carbonate [CuCO3 • Cu(OH)2], which has a green color [NCERT p.13].
Checkpoint (ज्ञान की जाँच)

Why are potato chips packets flushed with nitrogen gas? [NCERT p.13]

Section Summary (दैनिक जीवन ऑक्सीकरण का सारांश):
  • Corrosion is the slow eating away of metals by moist air, oxygen, or acids (e.g. rusting, tarnishing) [NCERT p.13].
  • Rancidity is the oxidation of fats and oils in food, spoiling its taste, smell, and appearance [NCERT p.13].
  • We prevent rancidity by adding antioxidants, vacuum packing, or flushing with inert nitrogen gas [NCERT p.13].

Checkpoint Quiz ज्ञान की परीक्षा

Question 1 of 6
Question: Which of the following is NOT a physical change? [NCERT p.1]

What You Can Now Do अब आप ये कर सकते हैं

Verify your understanding of this chapter by checking off these skills. You should be able to:

NCERT Solutions अभ्यास प्रश्नोत्तर

Below are the official exercise solutions for Class 10 Science Chapter 1: Chemical Reactions and Equations based on the CBSE curriculum. Click the button above to print or download these solutions as a clean PDF document.

In-Text p.6 Q1. Why should a magnesium ribbon be cleaned before burning in air?
Magnesium ribbon reacts with moisture and oxygen in the air to form a protective layer of basic magnesium carbonate [MgCO3 • Mg(OH)2] on its surface. This layer prevents magnesium from reacting with oxygen during burning. Cleaning the ribbon with sandpaper removes this protective layer, allowing the metal to burn smoothly with a dazzling white flame.
In-Text p.6 Q2. Write the balanced equation for the following chemical reactions:
(i) Hydrogen + Chlorine → Hydrogen chloride
(ii) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride
(iii) Sodium + Water → Sodium hydroxide + Hydrogen
(i) H2(g) + Cl2(g) → 2HCl(g)
(ii) 3BaCl2(aq) + Al2(SO4)3(aq) → 3BaSO4(s) + 2AlCl3(aq)
(iii) 2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g)
In-Text p.10 Q3. A solution of a substance 'X' is used for whitewashing.
(i) Name the substance 'X' and write its formula.
(ii) Write the reaction of the substance 'X' with water.
(i) The substance 'X' is Calcium Oxide (also known as quick lime). Its chemical formula is CaO.
(ii) Calcium oxide reacts vigorously with water to produce slaked lime (calcium hydroxide), releasing a large amount of heat:
CaO(s) + H2O(l) → Ca(OH)2(aq) + Heat
In-Text p.10 Q4. Why is the amount of gas collected in one of the test tubes in Activity 1.7 double the amount collected in the other? Name this gas.
Water (H2O) consists of hydrogen and oxygen combined in a 2:1 ratio by volume (2H2O → 2H2 + O2). During the electrolysis of water, the chemical decomposition splits water molecules such that two volumes of hydrogen gas are released at the cathode for every one volume of oxygen gas released at the anode. The gas collected in double quantity is Hydrogen (H2).
Exercise Q1 Q1. Which of the statements about the reaction below are incorrect?
2PbO(s) + C(s) → 2Pb(s) + CO2(g)
(a) Lead is getting reduced.
(b) Carbon dioxide is getting oxidised.
(c) Carbon is getting oxidised.
(d) Lead oxide is getting reduced.
Options: (i) a and b, (ii) a and c, (iii) a, b and c, (iv) all
Answer: (i) a and b.
Reason: Carbon oxide (CO2) is the product, not a reactant, so it is not oxidized. Lead (Pb) is a product, not a reactant, so it is not reduced. The actual reactants undergoing change are Carbon (getting oxidised to CO2) and Lead oxide (getting reduced to Pb).
Exercise Q2 Q2. Fe2O3 + 2Al → Al2O3 + 2Fe. The above reaction is an example of a:
(a) combination reaction, (b) double displacement reaction, (c) decomposition reaction, (d) displacement reaction.
Answer: (d) displacement reaction.
Reason: Aluminium (Al) is more reactive than iron (Fe). It displaces iron from its oxide (Fe2O3) to form aluminium oxide (Al2O3).
Exercise Q3 Q3. What happens when dilute hydrochloric acid is added to iron filings? Tick the correct answer.
(a) Hydrogen gas and iron chloride are produced.
(b) Chlorine gas and iron hydroxide are produced.
(c) No reaction takes place.
(d) Iron salt and water are produced.
Answer: (a) Hydrogen gas and iron chloride are produced.
Equation: Fe(s) + 2HCl(aq) → FeCl2(aq) + H2(g) ↑
Exercise Q4 Q4. What is a balanced chemical equation? Why should chemical equations be balanced?
A balanced chemical equation is a chemical equation in which the number of atoms of each element is equal on both the reactant side (LHS) and the product side (RHS).
Equations must be balanced to satisfy the Law of Conservation of Mass (discovered by Lavoisier), which states that matter cannot be created or destroyed in a chemical reaction. Therefore, the total mass of the reactants must equal the total mass of the products.
Exercise Q5 Q5. Translate the following statements into chemical equations and then balance them:
(a) Hydrogen gas combines with nitrogen to form ammonia.
(b) Hydrogen sulphide gas burns in air to give water and sulphur dioxide.
(c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.
(d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.
(a) 3H2(g) + N2(g) → 2NH3(g)
(b) 2H2S(g) + 3O2(g) → 2H2O(l) + 2SO2(g)
(c) 3BaCl2(aq) + Al2(SO4)3(aq) → 2AlCl3(aq) + 3BaSO4(s)↓
(d) 2K(s) + 2H2O(l) → 2KOH(aq) + H2(g)
Exercise Q9 Q9. What does one mean by exothermic and endothermic reactions? Give examples.
  • Exothermic Reactions (ऊष्माक्षेपी अभिक्रिया): Reactions in which heat is released along with the formation of products.
    Example: Burning of natural gas:
    CH4(g) + 2O2(g) → CO2(g) + 2H2O(g) + Heat
  • Endothermic Reactions (ऊष्माशोषी अभिक्रिया): Reactions in which energy is absorbed from the surroundings (in the form of heat, light, or electricity) to break bonds.
    Example: Thermal decomposition of calcium carbonate:
    CaCO3(s) + Heat → CaO(s) + CO2(g)
Exercise Q10 Q10. Why is respiration considered an exothermic reaction? Explain.
During digestion, food containing carbohydrates is broken down into glucose (C6H12O6). During respiration, this glucose combines with oxygen inhaled in our body cells to release carbon dioxide, water, and large amounts of energy (in the form of ATP) to power cellular functions:
C6H12O6(aq) + 6O2(aq) → 6CO2(aq) + 6H2O(l) + Energy
Since energy/heat is released in this process, respiration is classified as an exothermic reaction.
Exercise Q11 Q11. Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.
In a combination reaction, two or more simple reactants combine to form a single complex product:
2H2(g) + O2(g) → 2H2O(l)
In contrast, in a decomposition reaction, a single complex reactant splits apart under the influence of energy to form two or more simpler products:
2H2O(l)   Electricity  → 2H2(g) + O2(g)
Thus, the two reaction types are exact chemical opposites.
Exercise Q13 Q13. What is the difference between displacement and double displacement reactions? Write equations for these reactions.
  • Displacement Reaction: A single, more reactive element displaces a less reactive element from its compound:
    Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s)
    Here, iron (Fe) alone displaces copper (Cu).
  • Double Displacement Reaction: Two chemical compounds react by exchanging their respective ions to form two new compounds:
    Na2SO4(aq) + BaCl2(aq) → BaSO4(s) + 2NaCl(aq)
    Here, sodium and barium exchange their partner anions (sulphate and chloride).
Exercise Q15 Q15. What do you mean by a precipitation reaction? Explain by giving examples.
A precipitation reaction is any chemical reaction occurring in aqueous solution where two soluble salts react to produce an insoluble solid compound called a precipitate.
Example: Mixing sodium sulphate and barium chloride:
Na2SO4(aq) + BaCl2(aq) → BaSO4(s)↓ + 2NaCl(aq)
The barium ions (Ba2+) and sulphate ions (SO42-) react to form white, water-insoluble barium sulphate (BaSO4), which settles down at the bottom of the test tube.

CBSE Board PYQs विगत वर्षों के प्रश्न

1 Mark CBSE 2024 Q1. Name the type of reaction that takes place when quick lime is added to water.
It is a Combination reaction (which is also highly exothermic).
2 Marks CBSE 2023 Q2. A student heated green ferrous sulphate crystals in a dry boiling tube.
(a) What changes in color will be observed?
(b) Identify the chemical formula of the residue left behind.
(a) The green color of the ferrous sulphate crystals first fades to white (due to loss of water of crystallization) and then turns reddish-brown.
(b) The reddish-brown residue left behind is Ferric oxide (Fe2O3).
3 Marks CBSE 2022 Q3. Identify the substances that are oxidized and reduced in the following reactions:
(i) H2S + Cl2 → 2HCl + S
(ii) 3MnO2 + 4Al → 3Mn + 2Al2O3
(i) Oxidized: H2S (hydrogen sulphide) is oxidized to Sulphur (S) because it loses hydrogen.
Reduced: Cl2 (chlorine) is reduced to HCl because it gains hydrogen.
(ii) Oxidized: Al (aluminium) is oxidized to Al2O3 because it gains oxygen.
Reduced: MnO2 (manganese dioxide) is reduced to Mn because it loses oxygen.
5 Marks CBSE 2020 Q4. (a) Design an activity to demonstrate the electrolysis of water.
(b) Why is dilute sulphuric acid added to water during this process?
(c) Write the chemical equations representing the reactions at anode and cathode.
(a) Electrolysis Activity Setup:
1. Take a plastic mug, drill two holes at its base, and fit rubber stoppers. Insert carbon electrodes.
2. Fill the mug with water so the electrodes are fully immersed. Add a few drops of dilute sulphuric acid.
3. Invert two test tubes filled with water over the electrodes. Connect electrodes to a 6V battery.
4. Switch on the current. You will observe bubbles collecting in both test tubes. The volume of gas at the cathode is double the volume at the anode.

(b) Reason for adding Acid: Pure water is a poor conductor of electricity (non-electrolyte). Adding dilute sulphuric acid makes the water highly conductive by releasing free hydronium (H3O+) and sulphate (SO42−) ions, facilitating electron flow.

(c) Electrode Reactions:
  • At Cathode (Negative Electrode): 4H+(aq) + 4e- → 2H2(g) ↑ (Hydrogen gas)
  • At Anode (Positive Electrode): 4OH-(aq) → 2H2O(l) + O2(g) ↑ + 4e- (Oxygen gas)