(B) Change in shape

The textbook lists change in state, change in colour, evolution of a gas, and change in temperature as indicators of a chemical reaction — change in shape is not listed.

Source: Chapter 1, Chemical Reactions and Equations (Activity 1.1–1.3 summary)

The NCERT text explicitly names four indicators: change in state, change in colour, evolution of gas, and change in temperature. "Change in shape" is a physical change only and is never mentioned as evidence of a chemical reaction. Examiners expect you to recall this specific list from the chapter.

The product is magnesium oxide (MgO).

$$2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}$$

Source: Chapter 1, Section 1.1 / Chapter 3, Section 3.2.1

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• The question asks two things: name the product and write the equation — both must be included even in a 1-mark answer (they together form one complete response).
• Always write the balanced equation; the skeletal form `Mg + O₂ → MgO` is unbalanced and will lose marks.
• The product name "magnesium oxide" and its formula "MgO" are both acceptable; writing both is safest.
• No need for state symbols here unless specifically asked.

No, $\text{Mg} + \text{O}_2 \rightarrow \text{MgO}$ is not a balanced chemical equation.

Reason: The number of atoms of each element is not equal on both sides. There are 2 oxygen atoms on the LHS but only 1 oxygen atom on the RHS. Hence, the mass is not conserved.

Law: A balanced chemical equation must satisfy the Law of Conservation of Mass, which states that mass can neither be created nor destroyed in a chemical reaction, so the total mass of reactants equals the total mass of products.

The balanced equation is: $2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}$

Source: Chapter 1, Sections 1.1.1 and 1.1.2

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• 1 mark for stating it is NOT balanced.
• 1 mark for the reason — unequal number of oxygen atoms on LHS (2) and RHS (1). Examiners want you to count atoms specifically.
• 1 mark for naming the Law of Conservation of Mass (full name required).
• Writing the correctly balanced equation is a good add-on and shows understanding, but do not skip naming the law — that is the direct mark-fetch here.

No, the equation Mg + O₂ → MgO is not balanced.

Reason: On the LHS there are 2 atoms of oxygen (in O₂), but on the RHS there is only 1 atom of oxygen (in MgO). Since the number of atoms of each element is not equal on both sides, it violates the law of conservation of mass.

Balancing the equation:

| Element | LHS | RHS |
|---------|-----|-----|
| Mg | 1 | 1 |
| O | 2 | 1 |

To balance oxygen, place coefficient 2 before MgO, then balance Mg:

$$2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}$$

This is the correctly balanced equation (Mg: 2 = 2; O: 2 = 2). ✓

Source: Chapter 1, Sections 1.1.1 and 1.1.2

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• 1 mark for stating the equation is not balanced.
• 1 mark for the correct reason (unequal oxygen atoms on LHS and RHS / law of conservation of mass).
• 1 mark for writing the correctly balanced equation: 2Mg + O₂ → 2MgO.

Key examiner tip: Always state which element is unbalanced and why it violates the law of conservation of mass. Never change the formula of a compound (e.g., do not write MgO₂); only add coefficients.

A chemical equation must be balanced due to the Law of Conservation of Mass, which states that mass can neither be created nor destroyed in a chemical reaction.

Examiners expect the name of the law clearly stated. You may add a brief reason — that the number of atoms of each element must be equal on both sides — to strengthen a 1-mark answer, but the law's name is the core requirement.

A skeletal chemical equation is an unbalanced chemical equation in which the number of atoms of each element is not equal on both sides of the arrow.

Example: $\text{Mg} + \text{O}_2 \rightarrow \text{MgO}$ is a skeletal chemical equation.

Source: Chapter 1, Section 1.1.1 Writing a Chemical Equation

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The examiner expects the definition along with the key idea that it is unbalanced. Quoting the textbook example (Eq. 1.2) strengthens the answer and is appropriate for 1 mark. Avoid writing a long explanation — one definition line plus one example is sufficient.

Changing subscripts in a chemical formula changes the identity of the substance itself. For example, writing H₂O₄ instead of H₂O gives a completely different compound. According to the law of conservation of mass, the number of atoms of each element must remain equal on both sides. To achieve this balance, we can only change coefficients (number of molecules), never subscripts.

Source: Chapter 1, Section 1.1.2 — Balanced Chemical Equations

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Examiners look for two key points (1 mark each):

1. Changing subscripts changes the chemical formula/identity of the substance.
2. Only coefficients may be changed to balance an equation (to satisfy the law of conservation of mass).

Avoid saying "it breaks the rules" — always link it to the law of conservation of mass and the identity of the compound. Keep the example brief but precise.

The notations (s), (l), (g), and (aq) represent the physical states of substances — solid, liquid, gas, and aqueous solution (dissolved in water) respectively.

Source: Chapter 1, Section 1.1 Chemical Equations

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Examiners award the mark for correctly identifying all four symbols with their meanings. Remember: (aq) means the substance is dissolved in water — this is the one students most often forget or confuse. Write all four in order for full credit.

H₂O(g) indicates that water is in the gaseous (steam) state, meaning the reaction occurs at a high temperature where water exists as vapour, not liquid.

The symbol (g) denotes gaseous state. CBSE expects students to know that physical state symbols — (s), (l), (g), (aq) — make a chemical equation more informative. The examiner awards the mark for correctly identifying (g) = gaseous state / steam / high-temperature condition.

Meaning of a Balanced Equation:
A chemical equation is said to be balanced when the number of atoms of each element is the same on both the reactant side (LHS) and the product side (RHS). This is required by the law of conservation of mass, which states that mass can neither be created nor destroyed in a chemical reaction.

Hit-and-Trial Method (Example):

Consider the skeletal equation:
$$\text{Fe} + \text{H}_2\text{O} \rightarrow \text{Fe}_3\text{O}_4 + \text{H}_2$$

Step 1: Draw boxes around each formula; do not change any formula while balancing.

Step 2: Count atoms on each side:

| Element | LHS | RHS |
|---------|-----|-----|
| Fe | 1 | 3 |
| O | 1 | 4 |
| H | 2 | 2 |

Step 3: Start with the compound having the most atoms — Fe₃O₄. Balance O first: put coefficient 4 before H₂O → 4 O on LHS.

Step 4: Now H = 8 on LHS; put coefficient 4 before H₂ → 8 H on RHS.

Step 5: Balance Fe: put coefficient 3 before Fe on LHS.

Balanced equation:
$$3\text{Fe} + 4\text{H}_2\text{O} \rightarrow \text{Fe}_3\text{O}_4 + 4\text{H}_2$$

Verify: Fe = 3, H = 8, O = 4 on both sides. ✓

This method of using smallest whole-number coefficients by trial is called the hit-and-trial method.

Source: Chapter 1, Section 1.1.2 — Balanced Chemical Equations

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• Key concept to state first: link balancing to the law of conservation of mass — examiners look for this explicitly.
• The example: The textbook uses Fe + H₂O → Fe₃O₄ + H₂ as the standard worked example; use it unless you choose the Mg + O₂ example, but the Fe one better illustrates the steps.
• Steps matter: CBSE expects you to show the step-by-step logic (identify unbalanced element → add coefficients → verify). A bare answer without steps loses method marks.
• Never change formulae — only add coefficients. This is a common mistake examiners watch for.
• Verification at the end is expected for full marks on a 5-mark question.

Type of Reaction: This is a displacement reaction.

Reason for colour change: Iron displaces copper from copper sulphate solution because iron is more reactive than copper. The reaction is:

$$\text{Fe}(s) + \text{CuSO}_4(aq) \rightarrow \text{FeSO}_4(aq) + \text{Cu}(s)$$

The iron nails turn brownish because a layer of copper metal is deposited on them. The blue colour of copper sulphate solution fades because copper sulphate is consumed and green iron sulphate (FeSO₄) is formed in its place.

Source: Chapter 1, Section 1.2.3 – Displacement Reaction

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• 1 mark for naming the reaction type (displacement reaction).
• 1 mark for the balanced chemical equation (examiners expect it here).
• 1 mark for correctly explaining both colour changes — the brownish deposit on the nail (copper) AND the fading blue colour (copper sulphate being replaced by FeSO₄).

Common mistake: Students explain only one colour change and lose a mark. Always address both — the nail and the solution.

The mixture feeling hot indicates that heat is released during the reaction. This tells us the reaction is exothermic in nature.

$$\text{CaO}(s) + \text{H}_2\text{O}(l) \rightarrow \text{Ca(OH)}_2(aq) + \text{Heat}$$

Reactions in which heat is released along with the formation of products are called exothermic reactions.

Source: Chapter 1, Section 1.2.1 Combination Reaction

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• 1 mark for identifying the reaction as exothermic and stating that heat is released.
• 1 mark for correctly using the term "exothermic reaction" and defining it.
• The equation is a bonus touch that shows understanding, but the definition is what earns marks.
• Remember: this reaction is both a combination reaction (CaO + H₂O → one product) and an exothermic reaction — the question asks specifically about the heat aspect, so focus on exothermic.

When walls are whitewashed, calcium hydroxide [Ca(OH)₂] is applied. Initially, it remains as a dull paste. Over two to three days, it reacts with carbon dioxide (CO₂) present in air to form calcium carbonate (CaCO₃):

$$\text{Ca(OH)}_2 + \text{CO}_2 \rightarrow \text{CaCO}_3 + \text{H}_2\text{O}$$

Calcium carbonate is a hard, shiny crystalline substance, which gives the wall its shiny appearance. This is an example of a combination reaction where CO₂ combines with Ca(OH)₂.

Source: Chapter 1, Chemical Reactions and Equations

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• The key chemistry here is Ca(OH)₂ + CO₂ → CaCO₃ + H₂O; examiners expect this equation.
• Mention that CaCO₃ is responsible for the shiny finish — that's the direct answer to "why shiny."
• Identifying it as a combination reaction can fetch an extra mark, so include it.
• Do not confuse with decomposition or displacement — this is strictly a combination/addition reaction involving atmospheric CO₂.

When ferrous sulphate (FeSO₄) crystals are heated, their colour changes from green to white (as water of crystallisation is lost), and then to a reddish-brown solid. The reaction is:

$$2\text{FeSO}_4(s) \xrightarrow{\Delta} \text{Fe}_2\text{O}_3(s) + \text{SO}_2(g) + \text{SO}_3(g)$$

The products formed are ferric oxide (Fe₂O₃), sulphur dioxide (SO₂), and sulphur trioxide (SO₃). A characteristic smell of burning sulphur is also observed.

Source: Chapter 1, Section 1.2.2 – Decomposition Reaction

• The question is worth 2 marks: 1 mark for colour change, 1 mark for products (equation or names).
• Mention both stages of colour change (green → white → reddish-brown) for full credit, since the textbook specifies crystals lose water first, then decompose.
• Writing the balanced equation counts as identifying products — examiners accept either the equation or naming all three products.
• The type of reaction (thermal decomposition) may earn bonus credit if space allows.

The three types of decomposition reactions based on energy used are:

1. Thermal decomposition (heat energy):

$$2\text{FeSO}_4(s) \xrightarrow{\Delta} \text{Fe}_2\text{O}_3(s) + \text{SO}_2(g) + \text{SO}_3(g)$$

1. Electrolytic decomposition (electrical energy):

$$2\text{H}_2\text{O}(l) \xrightarrow{\text{electricity}} 2\text{H}_2(g) + \text{O}_2(g)$$

1. Photolytic decomposition (light energy):

$$2\text{AgCl}(s) \xrightarrow{\text{sunlight}} 2\text{Ag}(s) + \text{Cl}_2(g)$$

Source: Chapter 1, Section 1.2 Types of Chemical Reactions

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• Examiners expect all three names correctly stated with the type of energy clearly identified — this earns 1 mark each.
• Each reaction must be balanced and include state symbols where possible for full credit.
• These three examples (FeSO₄, electrolysis of water, AgCl) are the standard textbook examples — use them exactly.
• The key term for light-based decomposition is photolytic (or photodecomposition); writing just "light decomposition" may lose a mark.

In decomposition reactions, energy is absorbed to break a single substance into two or more products. Since energy is absorbed (not released), they are called endothermic reactions.

The key link is: decomposition = breaking down of one substance → requires energy input → energy absorbed = endothermic. Examiners want both the definition of endothermic (energy absorbed) and its connection to decomposition in one line.

(i) The brown fumes are nitrogen dioxide (NO₂). The solid residue is lead oxide (PbO).

(ii) Balanced chemical equation:

$$2\text{Pb(NO}_3)_2(s) \xrightarrow{\Delta} 2\text{PbO}(s) + 4\text{NO}_2(g) + \text{O}_2(g)$$

This is a thermal decomposition reaction because a single reactant (lead nitrate) breaks down into simpler products (lead oxide, nitrogen dioxide, and oxygen) on the application of heat.

Source: Chapter 1, Section 1.2.2 — Decomposition Reaction, Activity 1.6

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• (i) is worth 1 mark — name both products correctly.
• (ii) carries 2 marks — 1 for the balanced equation and 1 for naming the reaction type with a reason. Many students lose the second mark by just naming "decomposition" without explaining why (single reactant → multiple simpler products). Always state the reason when asked.
• The condition arrow must show heat (Δ or "Heat") for full credit.

This is a displacement reaction.

The chemical equation is:

$$\text{Fe}(s) + \text{CuSO}_4(aq) \rightarrow \text{FeSO}_4(aq) + \text{Cu}(s)$$

Iron displaces copper from copper sulphate solution because iron is more reactive than copper. The Cu²⁺ ions responsible for the blue colour of copper sulphate are replaced by Fe²⁺ ions (forming colourless iron sulphate), so the blue colour fades. The displaced copper deposits on the nail as a brownish coating.

Source: Chapter 1, Section 1.2.3 – Displacement Reaction

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• 1 mark for naming the reaction type (displacement reaction).
• 1 mark for the balanced chemical equation.
• 1 mark for explaining why the blue colour fades — examiners want you to link the loss of Cu²⁺ ions to the formation of FeSO₄ (which is pale/colourless in dilute solution), not just say "iron takes the place of copper."
• Always write the equation with state symbols — it shows precision and is expected at board level.

This is a double displacement reaction (also called a precipitation reaction).

$$\text{Na}_2\text{SO}_4(aq) + \text{BaCl}_2(aq) \rightarrow \text{BaSO}_4(s) + 2\text{NaCl}(aq)$$

It is called a double displacement reaction because the ions of the two reactants exchange (displace) with each other — $\text{SO}_4^{2-}$ ions combine with $\text{Ba}^{2+}$ ions, and $\text{Na}^+$ ions combine with $\text{Cl}^-$ ions. Since one product (BaSO₄) is insoluble and forms a white precipitate, it is also called a precipitation reaction.

Source: Chapter 1, Section 1.2.4 – Double Displacement Reaction

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• 3 marks = ~3 scorable points: (1) name the reaction type, (2) give the balanced equation, (3) explain why it has that name (ion exchange). Make sure all three are present.
• Examiners expect the term double displacement and the reason: exchange of ions between reactants.
• Mentioning that it is also a precipitation reaction (white, insoluble BaSO₄ forms) can earn the third mark or serve as supporting detail — include it.
• Always write state symbols (aq), (s) in equations; they signal understanding and are expected in board answers.

When copper(II) oxide (CuO) is heated with hydrogen gas, copper(II) oxide is reduced.

The reaction is:

$$\text{CuO} + \text{H}_2 \xrightarrow{\text{Heat}} \text{Cu} + \text{H}_2\text{O}$$

Reason: Copper(II) oxide loses oxygen during this reaction (it goes from CuO to Cu). Since reduction is defined as the loss of oxygen, copper(II) oxide undergoes reduction. (Hydrogen gains oxygen and is oxidised.)

Source: Chapter 1, Section 1.2.5 — Oxidation and Reduction

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• The examiner wants you to name the substance (CuO) and give the reason in terms of gain/loss of oxygen — both are needed for full marks.
• Write the equation as supporting evidence; it makes the answer clear and earns the equation mark.
• The key definition to quote: "If a substance loses oxygen during a reaction, it is said to be reduced."
• Avoid saying "copper is reduced" — be precise: copper(II) oxide is reduced. Copper is the product after reduction.

Chips manufacturers flush bags with nitrogen gas to prevent oxidation of the chips (which causes them to become stale, rancid, or lose flavour).

Nitrogen helps because it is a chemically inert (non-reactive) gas — it does not react with the chips or the oils in them. By displacing oxygen inside the bag, nitrogen prevents the oxidation reaction that spoils the food.

This question tests knowledge of the chemical properties of nitrogen (inertness) and its practical application in food preservation. Key points examiners look for: (1) naming the problem — oxidation/spoilage/rancidity, and (2) explaining why nitrogen works — it is inert and displaces oxygen. These are the two marks. Don't just say "nitrogen is safe"; link it clearly to displacing oxygen and being non-reactive.