(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 — not change in shape.

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

The four recognised indicators of a chemical reaction (from the NCERT text) are: change in state, change in colour, evolution of a gas, and change in temperature. "Change in shape" is a physical change (e.g., cutting, moulding) and does not indicate a new substance has formed. Examiners expect you to recall this list exactly.

(i) The substance formed when magnesium ribbon burns in air is magnesium oxide (MgO).

$$\text{Magnesium} + \text{Oxygen} \rightarrow \text{Magnesium oxide}$$

(ii) This is a chemical change, because a new substance (magnesium oxide) is formed which has different properties from the original magnesium ribbon, and the change cannot be reversed.

Source: Chapter 1, Section 1.1 Chemical Equations

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• Part (i) is a direct recall fact — always name the product and write the word equation if space allows.
• Part (ii) needs a reason: the key indicator of a chemical change is formation of a new substance with different properties. Examiners also accept "irreversible change" as a valid reason. Avoid vague answers like "it looks different."

A chemical equation must be balanced to satisfy the law of conservation of mass, which states that mass can neither be created nor destroyed in a chemical reaction. This means the number of atoms of each element must be equal on both the reactant side (LHS) and the product side (RHS) of the equation.

Source: Chapter 1, Section 1.1.2 Balanced Chemical Equations

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• The examiner expects two key points: (1) mention of the law of conservation of mass, and (2) equal number of atoms on both sides.
• Do not just say "atoms are equal" — name the law; that's where the marks are.
• One sentence on the law + one sentence on what it implies (equal atoms) = full 2 marks.

The equation Mg + O₂ → MgO is only a skeletal chemical equation. It is unbalanced because the number of atoms of each element is not equal on both sides — there are 2 oxygen atoms on the LHS but only 1 on the RHS. According to the law of conservation of mass, a complete chemical equation must be balanced, with equal numbers of each type of atom on both sides.

The balanced equation is: 2Mg + O₂ → 2MgO

Source: Chapter 1, Section 1.1.1 and 1.1.2

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Examiners award marks for two key points:

1. Identifying it as a skeletal (unbalanced) equation and stating why it is unbalanced (atom count mismatch).
2. Linking to the law of conservation of mass — this is the scientific reason a balanced equation is required.

Writing the correct balanced equation earns the second mark. Do not just say "it needs balancing" — show the atom counts or state the law explicitly.

The Law of Conservation of Mass makes it necessary to balance a chemical equation, as mass can neither be created nor destroyed in a chemical reaction.

Source: Chapter 1, Section 1.1.2 Balanced Chemical Equations

Examiners expect the exact name of the law. You may add a one-line explanation (atoms of each element must be equal on both sides) to complete the answer. Do not write more than one sentence for a 1-mark question.

It is a skeletal chemical equation; it is missing balancing — the number of atoms of each element is not equal on both sides.

Examiners expect two things: the term "skeletal chemical equation" (unbalanced equation written using formulae) and the point that it needs to be balanced. The balanced equation is 4Fe + 3O₂ → 2Fe₂O₃. Both parts are needed for full credit.

Changing the formula of a compound (e.g., writing H₂O₄ instead of H₂O) would represent an entirely different substance that does not exist in the reaction. Each chemical formula is fixed and represents a specific compound with definite properties. To balance an equation, only coefficients (numbers placed in front of formulas) may be changed, so that the number of atoms of each element remains equal on both sides, satisfying the Law of Conservation of Mass.

Source: Chapter 1, Section 1.1.2 — Balanced Chemical Equations

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• Examiners expect two clear points: (1) altering the formula changes the identity/composition of the substance, and (2) only coefficients are allowed because the law of conservation of mass must be satisfied without changing what substances are present.
• Quoting or referencing the textbook example (H₂O → 4H₂O, not H₂O₄) shows you understand the rule directly from the source.
• Avoid writing a long paragraph; two crisp points earn full marks here.

Balanced equation:

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

Verification: LHS — Mg: 2, O: 2 | RHS — Mg: 2, O: 2 ✓

Ratio of combination: Magnesium and oxygen combine in the ratio 2 : 1 (in terms of number of molecules/moles).

Source: Chapter 1, Section 1.1.2 Balanced Chemical Equations

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• The skeletal equation Mg + O₂ → MgO is unbalanced (2 O on left, 1 O on right). Place coefficient 2 before both Mg and MgO to balance all atoms.
• Examiners award 1 mark for the correct balanced equation and 1 mark for stating the correct ratio (2 Mg : 1 O₂).
• Never change subscripts to balance; only add coefficients in front of formulae.

The state symbols represent the physical states of substances in a reaction:

• (s) – solid
• (l) – liquid
• (g) – gaseous
• (aq) – aqueous (substance dissolved in water)

Including these symbols makes a chemical equation more informative by showing the physical state of each reactant and product, giving a complete picture of the reaction.

Source: Chapter 1, Section 1.1.2 (Step VII – Writing Symbols of Physical States)

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• Examiners expect all four symbols defined clearly — losing even one definition can cost half a mark.
• The second part (why important) is often missed; the key phrase from the textbook is "more informative" — state symbols tell us how each substance is present in the reaction.
• Remember: (aq) specifically means dissolved in water, not just any solution.

(a) Why the equation is unbalanced:

The given equation $\text{Fe} + \text{H}_2\text{O} \rightarrow \text{Fe}_3\text{O}_4 + \text{H}_2$ is unbalanced because the number of atoms of each element is not equal on both sides, violating the Law of Conservation of Mass.

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

Since Fe and O atoms differ on LHS and RHS, the equation is a skeletal (unbalanced) chemical equation.

(b) Fully balanced equation with state symbols:

Using the hit-and-trial method — balance O first (coefficient 4 for H₂O), then H (coefficient 4 for H₂), then Fe (coefficient 3):

$$3\text{Fe}(s) + 4\text{H}_2\text{O}(g) \rightarrow \text{Fe}_3\text{O}_4(s) + 4\text{H}_2(g)$$

The state symbol (g) is used for H₂O because water is used as steam in this reaction.

Source: Chapter 1, Sections 1.1.1 and 1.1.2

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• (a) Examiners expect you to explicitly count atoms of each element on both sides and show the mismatch in a table. Simply saying "it's not balanced" without evidence will lose marks.
• (b) Always include state symbols when asked — (s) for Fe and Fe₃O₄, (g) for H₂O (steam), and (g) for H₂. Forgetting (g) for steam is a common error. The balanced coefficients 3, 4, 1, 4 are the key numbers to remember.
• The method used here is the hit-and-trial method — start with the compound having the most atoms (Fe₃O₄), balance O → H → Fe in that order.

This is a displacement reaction.

Copper ($\text{Cu}$) is the substance that gets displaced.

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

Iron displaces copper because iron is more reactive than copper. It removes copper from copper sulphate solution, forming iron sulphate; hence the blue colour of the solution fades.

Source: Chapter 1, Section 1.2.3 – Displacement Reaction

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• 1 mark for correctly naming the reaction type (displacement reaction).
• 1 mark for identifying copper as the displaced substance AND giving the reason (iron is more reactive than copper).
• Writing the balanced equation is a strong supporting point and shows clarity — examiners appreciate it even if not explicitly demanded.
• Do not say iron is displaced; iron does the displacing, copper is displaced.

Heat is released. This represents an exothermic reaction, as heat is evolved along with the formation of the product (slaked lime).

Source: Chapter 1, Section 1.2.1 – Combination Reaction

For a 1-mark question, examiners award the mark for correctly stating either that heat is released or that it is an exothermic reaction — ideally mention both in one line. The key term is exothermic; do not write "endothermic" (which is when energy is absorbed). The reaction CaO + H₂O → Ca(OH)₂ + Heat is the standard example of an exothermic combination reaction in this chapter.

Both reactions — C + O₂ → CO₂ and 2H₂ + O₂ → 2H₂O — represent combustion reactions, which are a type of oxidation (exothermic) reaction.

Common feature: In both reactions, a substance (carbon/hydrogen) combines with oxygen and releases heat and light energy. This gain of oxygen by the substance is called oxidation, making both reactions exothermic combustion reactions.

Source: Chapter 1 (Types of Chemical Reactions); Chapter 4, Section 4.3.1 (Combustion)

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• Examiners expect students to name the reaction type correctly — combustion/oxidation — for 1 mark.
• The second mark is for identifying the common feature: both involve combination with oxygen, releasing heat and light (exothermic).
• Avoid writing just "combination reaction" — combustion is the specific and more accurate term here; also note it is a type of oxidation.
• Mentioning "heat and light released" strengthens the answer.

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

The reaction is: $2\text{Pb(NO}_3)_2 \xrightarrow{\Delta} 2\text{PbO} + 4\text{NO}_2 + \text{O}_2$. Examiners expect both products named correctly. One mark is typically split: ½ for NO₂, ½ for PbO — so both must be stated.

Source: Chapter 1, Section 1.2.2 (Activity 1.6)

Decomposition reactions can be driven by three forms of energy:

1. Heat (Thermal decomposition)
2. Light (Photolytic decomposition)
3. Electricity (Electrolytic decomposition)

Example (Heat):

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

Ferrous sulphate decomposes on heating to give ferric oxide, sulphur dioxide and sulphur trioxide.

Source: Chapter 1 – Chemical Reactions and Equations

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• The question has two parts: name the energy forms (1 mark each, total 2 marks) and give one example (1 mark). Name all three forms clearly.
• Any valid equation for heat, light, or electricity is acceptable. Common choices: heat → FeSO₄ or CaCO₃; light → AgCl or AgBr; electricity → H₂O electrolysis.
• Write the equation with state symbols and the energy symbol (Δ or hν) above the arrow for full marks.
• Examiners award 1 mark for each correctly named energy form and 1 mark for a correct balanced equation with appropriate energy indicated.

This is a photochemical decomposition reaction. Silver chloride decomposes in the presence of sunlight to form silver metal and chlorine gas.

$$2\text{AgCl} \xrightarrow{\text{Sunlight}} 2\text{Ag} + \text{Cl}_2$$

The silver formed gives the white AgCl its grey colour.

Source: Chapter 1, Chemical Reactions and Equations

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• 1 mark for correctly identifying it as a photochemical decomposition reaction.
• 1 mark for the correct products: silver (Ag) and chlorine (Cl₂), ideally with the balanced equation.
• CBSE expects the word "decomposition" and the qualifier "photochemical" (sunlight-driven). Simply writing "decomposition" without the photochemical context may lose half a mark.
• The equation is a strong addition and shows clarity; always write it when you can within the word limit.

Endothermic means a reaction in which energy is absorbed from the surroundings.

In decomposition reactions, a single substance breaks down into two or more substances. Breaking the chemical bonds in the reactant requires energy. Since energy must be supplied (as heat, light, or electricity) to break these bonds, decomposition reactions are endothermic.

Source: Chemical Reactions and Equations, Chapter 1 — What you have learnt

• 1 mark for correctly defining endothermic (energy is absorbed).
• 1 mark for explaining that bond-breaking in the reactant requires an external energy input, linking it specifically to decomposition reactions.
• Examiners expect you to contrast with exothermic (energy released) implicitly by using precise language like "absorbed."
• Avoid vague phrases like "needs energy to work" — say bonds must be broken and energy is absorbed from surroundings.

This is a displacement reaction.

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

Reason for colour change: Iron is more reactive than copper, so it displaces copper from copper sulphate solution. The blue colour of copper sulphate (CuSO₄) gradually fades because it is converted into iron sulphate (FeSO₄), which is pale green/colourless. Copper gets deposited on the iron nail, turning it brownish.

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 the reason — iron displaces copper because it is more reactive; CuSO₄ (blue) is converted to FeSO₄ (colourless/pale green), causing the blue colour to fade.
• Always write the chemical equation in a displacement reaction answer — examiners expect it.
• Don't forget to mention why the colour fades (the product FeSO₄ is not blue).

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

When sodium sulphate solution is mixed with barium chloride solution, the ions exchange between the two reactants. The sulphate ions (SO₄²⁻) and barium ions (Ba²⁺) combine to form an insoluble white precipitate of barium sulphate (BaSO₄), while sodium chloride remains in solution.

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

The insoluble solid formed is called a precipitate, and any reaction that produces a precipitate is called a precipitation reaction. Since there is an exchange of ions between the reactants, it is a double displacement reaction.

Source: Chapter 1, Section 1.2.4 Double Displacement Reaction

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• 3 marks are typically split as: (1) correct name of reaction type, (1) reason/explanation of ion exchange, (1) correct balanced equation.
• Always write the balanced chemical equation with state symbols — examiners specifically look for (s), (aq) etc.
• Mention both names — "double displacement" AND "precipitation reaction" — to be safe, as either may be the expected answer.
• The key distinguishing feature of a double displacement reaction is exchange of ions between the two reactants.

The reaction is: $\text{CuO} + \text{H}_2 \xrightarrow{\text{Heat}} \text{Cu} + \text{H}_2\text{O}$

Hydrogen is oxidised — because it gains oxygen (to form water).

Copper(II) oxide is reduced — because it loses oxygen (to form copper).

This is a redox reaction, where oxidation and reduction occur simultaneously.

Source: Chapter 1, Section 1.2.5 — Oxidation and Reduction

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• Examiners award 1 mark for the equation, 1 mark for identifying the oxidised substance with reason, and 1 mark for identifying the reduced substance with reason.
• The key definitions to apply: gain of oxygen = oxidised; loss of oxygen = reduced.
• Always write the balanced equation first — it shows you clearly which substance gains and which loses oxygen.
• Do not confuse: CuO is reduced (it gives away oxygen), H₂ is oxidised (it receives oxygen). A common mistake is to reverse these.

A substance that gains oxygen during a chemical reaction is oxidised. (Oxidation is defined as the gain of oxygen or loss of hydrogen.)

The definition directly from Chapter 1 states: "Oxidation is the gain of oxygen or loss of hydrogen." Examiners expect the one-word answer oxidised plus its definition for full marks. Don't confuse oxidation with reduction (which is the loss of oxygen).

Chips manufacturers flush packets with nitrogen gas to prevent oxidation (rancidity) of the chips. Oxygen in air causes the chips to turn stale and spoil quickly. Nitrogen is used because it is a chemically inert (non-reactive) gas — it does not react with the food, keeping the chips fresh while also preventing the packet from being crushed.

This question draws on the concept of chemical reactivity of gases (from Chapter 4, Carbon and its Compounds context — inert gases). Key points examiners look for: (1) naming the problem — oxidation/rancidity, (2) explaining why nitrogen helps — it is inert/non-reactive. Avoid writing lengthy explanations; two clear points are enough for 2 marks.