(C) Change in shape

According to the textbook, the indicators of a chemical reaction are change in state, change in colour, evolution of a gas, and change in temperature — not change in shape.

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

The textbook explicitly lists four observations that indicate a chemical reaction: change in state, change in colour, evolution of a gas, and change in temperature. "Change in shape" is a physical change (e.g., cutting, bending) and does NOT indicate a new substance has formed. Examiners expect students to recall this list precisely — note it is "change in state," not "change in shape."

The white powder is magnesium oxide (MgO). It is formed by the reaction of magnesium and oxygen.

Source: Chemical Reactions and Equations, Section 1.1; Metals and Non-metals, Section 3.2.1

The examiner expects the name/formula of the product and both reactants named clearly. The word equation is: Magnesium + Oxygen → Magnesium oxide. Mentioning "air" in the question is acceptable since oxygen is the reactive component in air; the product remains magnesium oxide.

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 direct reference to the Law of Conservation of Mass — always name it.
• The second point (equal atoms on both sides) is the practical consequence and earns the second mark.
• Do not confuse why we balance (the law) with how we balance (hit-and-trial method) — the question asks only for the reason.

A skeletal chemical equation has unequal numbers of atoms on both sides, while a balanced chemical equation has equal numbers of atoms of each element on both sides, satisfying the law of conservation of mass.

The examiner looks for the key distinction: skeletal = unbalanced (formulae written but atoms not equalised); balanced = atoms of each element are equal on LHS and RHS. Mentioning the law of conservation of mass adds accuracy and is rewarded.

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 law name and a brief reason. "Mass is neither created nor destroyed" is the key phrase from the textbook. Do not just write the law name without the reason — that risks losing the mark.

No, it is not a complete and balanced chemical equation. It is only a skeletal chemical equation.

Two ways it is incomplete/incorrect:

1. Unbalanced: LHS has 2 oxygen atoms (O₂) but RHS has only 1 (MgO), violating the law of conservation of mass.
2. Physical states missing: The symbols (s), (g), etc., for reactants and products are not mentioned, making the equation incomplete.

Source: Chapter 1, Sections 1.1.1 and 1.1.2

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Examiners look for two distinct points: (1) the balancing issue — atoms of Mg and O are unequal on both sides, and (2) the missing physical state symbols (s/g/l/aq). The textbook explicitly calls Mg + O₂ → MgO a "skeletal chemical equation." A balanced form would be 2Mg + O₂ → 2MgO. Always name both errors clearly for full marks.

Writing H₂O₄ is incorrect because it changes the chemical formula of water, implying a compound that does not exist. While balancing an equation, we can only change the coefficients (numbers in front of the formula), not the subscripts. The correct way is to write 4H₂O, which means four molecules of water, keeping the formula of each molecule unchanged.

Source: Chapter 1, Section 1.1.2 Balanced Chemical Equations

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• Examiners expect two clear points: (1) H₂O₄ is a non-existent/wrong compound — changing subscripts alters the identity of the substance; (2) only coefficients may be changed during balancing.
• The textbook explicitly states: "we cannot alter the formulae of the compounds… we can put coefficient '4' as 4H₂O and not H₂O₄." Quote or paraphrase this for full marks.
• Avoid lengthy explanations — two crisp points are enough for 2 marks.

(i) This is a displacement reaction. Iron, being more reactive than copper, displaces copper from copper sulphate solution.

(ii) Copper is deposited on the iron nails (they turn brownish). Iron displaces copper from CuSO₄ solution because iron is more reactive than copper. The reaction is:

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

As Cu²⁺ ions are used up, the blue colour of the solution fades.

Source: Chapter 1, Section 1.2.3 — Displacement Reaction

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• (i) must name the reaction type clearly — "displacement reaction" is the exact term used in the textbook.
• (ii) Two points are needed for full marks: what is deposited (copper/brownish layer) and why (iron is more reactive, so it displaces copper). Writing the balanced equation strengthens the answer and is expected in board exams.
• The fading of blue colour is due to conversion of CuSO₄ → FeSO₄ (which is pale green/colourless) — mention this to show complete understanding.

A reaction in which two or more substances combine to form a single new substance is called a combination reaction.

Example: Calcium oxide (quick lime) reacts with water to form calcium hydroxide (slaked lime):

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

This is used in everyday whitewashing of walls.

Source: Chapter 1 – Chemical Reactions and Equations, Section 1.2.1

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• 1 mark for the correct definition — must include "two or more substances" combining into "one/single product."
• 1 mark for the example — the equation (even in word form) with reactants and product named earns full credit. The CaO + H₂O → Ca(OH)₂ reaction is the textbook illustration, so use it confidently.
• Avoid writing a long explanation; the examiner checks for the definition keyword and a valid equation/example.

An exothermic energy change occurs — heat is released. It can be detected by touching the beaker, which feels hot.

Source: Chapter 1, Section 1.2.1 – Combination Reaction

The examiner looks for two things in one line: (1) the correct term exothermic and (2) the method of detection (touching the beaker/rise in temperature). Both must appear for full credit on a 1-mark question. Avoid writing "endothermic" — the textbook explicitly states "releasing a large amount of heat."

(i) The brown gas evolved is nitrogen dioxide (NO₂).

(ii) This is a thermal decomposition reaction, as the decomposition is caused by heat.

The reaction is:
$$2\text{Pb(NO}_3)_2(s) \xrightarrow{\text{Heat}} 2\text{PbO}(s) + 4\text{NO}_2(g) + \text{O}_2(g)$$

Source: Chapter 1, Section 1.2.2 Decomposition Reaction (Activity 1.6)

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• (i) is straightforward — the textbook explicitly states "These fumes are of nitrogen dioxide (NO₂)." Write the name and formula for full credit.
• (ii) The key term examiners look for is thermal decomposition (not just "decomposition"), because the energy source is heat. Writing the balanced equation earns the second mark and demonstrates understanding of the reaction type.

The three forms of energy that cause decomposition reactions are heat, light, and electricity.

1. Heat (Thermal decomposition):

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

1. Light (Photolytic decomposition):

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

1. Electricity (Electrolytic decomposition):

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

Source: Chapter 1 – Chemical Reactions and Equations, Section 1.2 (Types of Chemical Reactions)

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• The examiner expects all three forms named + one correct equation each — that's 1 mark per pair.
• Use the standard NCERT examples: lead nitrate (heat), silver chloride (light), water (electricity). These are the safest choices.
• Write state symbols and the energy type above the arrow — examiners notice and reward this detail.
• Do not confuse electrolytic decomposition of water with any combustion reaction.

Type of Reaction: Displacement reaction.

Iron (Fe) is more reactive than copper (Cu), so it displaces copper from copper sulphate solution. The reaction is:

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

Why the blue colour fades: Copper sulphate (CuSO₄) gives the solution its blue colour. As iron displaces copper, CuSO₄ is consumed and replaced by iron sulphate (FeSO₄), which is pale green. Since the concentration of CuSO₄ decreases, the blue colour of the solution gradually fades. Copper metal is 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 (displacement reaction).
• 1 mark for the balanced chemical equation.
• 1 mark for explaining the fading of blue colour — examiners want you to link it to the consumption of CuSO₄ and formation of FeSO₄. Just saying "copper is displaced" is insufficient; you must explain what happens to the blue-coloured ion.
• Always write the equation with state symbols — it signals precision to the examiner.

The reaction involved is a double displacement reaction (also called a precipitation reaction).

Reason: The sulphate ions (SO₄²⁻) from sodium sulphate and barium ions (Ba²⁺) from barium chloride exchange (displace) each other to form an insoluble white precipitate of barium sulphate.

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

Source: Chapter 1, Section 1.2.4 Double Displacement Reaction

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• Name of reaction (1 mark): Double displacement reaction. You may also call it a precipitation reaction — both are acceptable.
• Reason (1 mark): Examiners want you to mention exchange of ions between the reactants and/or the formation of an insoluble precipitate (BaSO₄). Either point earns the mark; stating both makes your answer complete.
• Writing the balanced equation supports your answer but is not always compulsory for 2 marks — include it to be safe.
• Remember: in double displacement, two compounds swap their ionic partners; neither element changes its oxidation state (unlike displacement or redox reactions).

Copper(II) oxide is reduced in this reaction.

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

Reason: A substance is said to be reduced when it loses oxygen during a reaction. In this reaction, copper(II) oxide (CuO) loses its oxygen to hydrogen and is converted to copper (Cu). Therefore, copper(II) oxide is reduced. (Simultaneously, hydrogen gains oxygen and is oxidised, making this a redox reaction.)

Source: Chapter 1, Section 1.2.5 Oxidation and Reduction

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• The examiner expects the term "reduced" clearly stated — this is worth 1 mark.
• The definition (loses oxygen = reduction) must be written — 1 mark.
• Applying the definition to this specific reaction (CuO losing oxygen to become Cu) — 1 mark.
• Mentioning the redox nature or hydrogen's oxidation is a bonus but keep it brief.
• Always write the chemical equation when explaining a reaction; it shows understanding and saves words.

Chips manufacturers flush bags with nitrogen gas to prevent oxidation (rancidity) of the chips. Nitrogen is inert and does not react with the food, keeping the chips fresh and crispy for longer.

Source: Chapter 4, Carbon and its Compounds

The passage mentions that saturated fats/oils can turn rancid due to oxidation. Nitrogen is used as an inert packaging gas to displace oxygen, preventing the chips (which contain fats/oils) from getting oxidised. Examiners expect the key words: inert, prevent oxidation/rancidity. One sentence with these two ideas earns full marks.