NCERT Textbook Questions & Answers + Important Extra Questions
Section A: In-Text (Page-wise) Questions
Why is it necessary to clean the magnesium ribbon before burning it in air?
Answer
Over time, magnesium develops a layer of basic magnesium carbonate on its surface due to reaction with atmospheric moisture and carbon dioxide. This coating prevents smooth burning. Cleaning with sandpaper removes this layer and exposes fresh metal, allowing it to burn freely in air with a bright white flame.
Q2. Write the balanced chemical equations for the following reactions
(i) Hydrogen + Chlorine → Hydrogen chloride
(ii) Sodium + Water → Sodium hydroxide + Hydrogen
(iii) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride
Ans.
(i) H₂(g) + Cl₂(g) → 2HCl(g)
(ii) 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)
(iii) 3BaCl₂(aq) + Al₂(SO₄)₃(aq) → 3BaSO₄(s)↓ + 2AlCl₃(aq)
Write balanced chemical equations with state symbols for the following reactions:
(i) Aqueous solutions of barium chloride and sodium sulphate react to give insoluble barium sulphate and a solution of sodium chloride.
(ii) Sodium hydroxide solution reacts with hydrochloric acid solution to produce sodium chloride solution and water.
Answer
(i) BaCl₂(aq) + Na₂SO₄(aq)
⇒ BaSO₄(s)↓ + 2NaCl(aq)
(ii) NaOH(aq) + HCl(aq)
⇒ NaCl(aq) + H₂O(l)
A substance 'X' is used for whitewashing.
(i) Name the substance 'X' and write its formula.
(ii) Write its reaction when dissolved in water.
Answer
(i) The substance X is Calcium oxide (commonly called quick lime). Its chemical formula is CaO.
(ii) When calcium oxide is mixed with water, it forms calcium hydroxide (slaked lime) and releases a large amount of heat:
CaO(s) + H₂O(l) → Ca(OH)₂(aq) + Heat
The calcium hydroxide solution is then applied on walls as whitewash.
Show experimentally that during electrolysis of water, one gas is produced in double the volume of the other gas. Identify these gases.
Answer
When water is electrolysed (by passing electric current through acidulated water), hydrogen gas collects at the cathode and oxygen gas collects at the anode. The volume of hydrogen collected is exactly twice the volume of oxygen. This is because water (H₂O) contains two atoms of hydrogen for every one atom of oxygen.
Balanced equation: 2H₂O(l)
⇒ electricity 2H₂(g) + O₂(g)
The ratio of volumes H₂ : O₂ = 2 : 1
Q6. When solutions of lead(II) nitrate and potassium iodide are mixed
(i) What is the colour of the precipitate formed? Name the compound.
(ii) Write the balanced equation.
(iii) What type of reaction is this?
Ans.
(i) A bright yellow precipitate of lead(II) iodide (PbI₂) is formed.
(ii) Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s)↓ + 2KNO₃(aq)
(iii) This is a double displacement reaction as well as a precipitation reaction (since an insoluble product separates out as a solid).
Why does the colour of copper sulphate solution change when an iron nail is immersed in it?
Answer
Iron is more reactive than copper. When an iron nail is placed in blue copper sulphate solution, iron displaces copper from the solution. The blue colour fades and turns green (due to formation of iron sulphate), while a reddish-brown coating of copper deposits on the iron nail.
Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)
This is a displacement reaction.
Provide an example of a double displacement reaction other than the one given in the textbook activity.
Answer
When sodium sulphate reacts with barium chloride:
Na₂SO₄(aq) + BaCl₂(aq)
⇒ BaSO₄(s)↓ + 2NaCl(aq)
Both reactants exchange their ions to form two new products, making it a double displacement reaction.
Identify the substances that are oxidised and the substances that are reduced in the following reactions:
Answer
(i) 4Na(s) + O₂(g)
⇒ 2Na₂O(s) — Sodium is oxidised (gains oxygen); Oxygen is the oxidising agent.
(ii) CuO(s) + H₂(g)
⇒ Cu(s) + H₂O(l) — Hydrogen is oxidised (gains oxygen); Copper oxide is reduced (loses oxygen).
When magnesium ribbon burns in air, is it being oxidised or reduced? Explain.
Answer
Magnesium is oxidised because it combines with oxygen to form magnesium oxide:
2Mg(s) + O₂(g) → 2MgO(s)
Since magnesium gains oxygen, it undergoes oxidation.
Section B: NCERT Exercise Questions
Which of the following statements about the reaction below are incorrect?
Answer
2PbO(s) + C(s) → 2Pb(s) + CO₂(g)
Ans. (a) Lead is being reduced — Correct (PbO loses oxygen to become Pb).
(b) Carbon dioxide is being oxidised — Incorrect (CO₂ is a product, not being oxidised).
(c) Carbon is being oxidised — Correct (C gains oxygen to form CO₂).
(d) Lead oxide is being reduced — Correct (PbO loses oxygen).
So statements (a) and (b) together as given in the options need checking. The incorrect statement is (b).
Fe₂O₃ + 2Al → Al₂O₃ + 2Fe. The above reaction is an example of:
Answer
(d) Displacement reaction. Aluminium displaces iron from iron(III) oxide because Al is more reactive than Fe. This is also known as a thermite reaction.
What happens when dilute hydrochloric acid is added to iron filings? Tick the correct answer.
Answer
(a) Hydrogen gas and iron chloride are produced.
Fe(s) + 2HCl(aq) → FeCl₂(aq) + H₂(g)↑
What is a balanced chemical equation? Why should chemical equations be balanced?
Answer
A balanced chemical equation is one in which the total number of atoms of each element is the same on both sides (reactant and product). Chemical equations must be balanced to satisfy the Law of Conservation of Mass, which states that matter can neither be created nor destroyed in a chemical reaction. The total mass of the reactants must equal the total mass of the products.
Balance the following chemical equations
Answer
(a) 3H₂(g) + N₂(g) → 2NH₃(g)
(b) 2H₂S(g) + 3O₂(g)
⇒ 2H₂O(l) + 2SO₂(g)
(c) 3BaCl₂(aq) + Al₂(SO₄)₃(aq)
⇒ 3BaSO₄(s) + 2AlCl₃(aq)
(d) 2K(s) + 2H₂O(l) → 2KOH(aq) + H₂(g)↑
Balance the following equations
Answer
(a) 2HNO₃(aq) + Ca(OH)₂(aq)
⇒ Ca(NO₃)₂(aq) + 2H₂O(l)
(b) 2NaOH(aq) + H₂SO₄(aq)
⇒ Na₂SO₄(aq) + 2H₂O(l)
(c) NaCl(aq) + AgNO₃(aq)
⇒ AgCl(s)↓ + NaNO₃(aq)
(d) BaCl₂(aq) + H₂SO₄(aq)
⇒ BaSO₄(s)↓ + 2HCl(aq)
Write the balanced chemical equations for the following reactions
Answer
(a) Ca(OH)₂(aq) + CO₂(g)
⇒ CaCO₃(s)↓ + H₂O(l)
(b) Pb(s) + CuCl₂(aq)
⇒ PbCl₂(aq) + Cu(s)
(c) BaCl₂(aq) + Na₂SO₄(aq)
⇒ BaSO₄(s)↓ + 2NaCl(aq)
(d) Zn(s) + 2AgNO₃(aq)
⇒ Zn(NO₃)₂(aq) + 2Ag(s)
Write balanced chemical equations for the following and identify the type of reaction:
Answer
(a) 2KBr(aq) + BaI₂(aq)
⇒ 2KI(aq) + BaBr₂(aq) — Double displacement reaction
(b) ZnCO₃(s)
⇒ heat ZnO(s) + CO₂(g) — Decomposition reaction (thermal)
(c) H₂(g) + Cl₂(g)
⇒ 2HCl(g) — Combination reaction
(d) Mg(s) + 2HCl(aq)
⇒ MgCl₂(aq) + H₂(g)↑ — Displacement reaction
What do you mean by exothermic and endothermic reactions? Give examples.
Answer
Exothermic reactions are those that release energy (usually as heat) along with the products.
Example: Combustion of natural gas — CH₄(g) + 2O₂(g)
⇒ CO₂(g) + 2H₂O(l) + Heat energy
Another example: Respiration — C₆H₁₂O₆ + 6O₂
⇒ 6CO₂ + 6H₂O + Energy
Endothermic reactions are those that absorb energy from the surroundings.
Example: Decomposition of calcium carbonate — CaCO₃(s)
⇒ heat CaO(s) + CO₂(g)
Another example: Photosynthesis — 6CO₂ + 6H₂O
⇒ sunlight C₆H₁₂O₆ + 6O₂
Why is respiration considered an exothermic reaction? Explain.
Answer
During respiration, the food (glucose) we eat is broken down using inhaled oxygen. This process releases energy that our body uses for various life functions. Since energy is given out, it is an exothermic process.
C₆H₁₂O₆(aq) + 6O₂(g)
⇒ 6CO₂(g) + 6H₂O(l) + Energy
This energy keeps our body warm and powers all cellular activities.
Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.
Answer
In a combination reaction, two or more substances combine to form a single product:
2H₂(g) + O₂(g) → 2H₂O(l)
In a decomposition reaction, a single compound splits into two or more simpler substances:
2H₂O(l) →electricity 2H₂(g) + O₂(g)
Since decomposition breaks apart what combination puts together, they are considered opposite processes.
Write one equation each for decomposition reactions where energy is supplied in the form of heat, light, and electricity.
Answer
Heat (Thermal decomposition)
2Pb(NO₃)₂(s) →heat 2PbO(s) + 4NO₂(g) + O₂(g)
Light (Photolytic decomposition)
2AgCl(s) →sunlight 2Ag(s) + Cl₂(g)
Electricity (Electrolytic decomposition)
2H₂O(l) →electricity 2H₂(g) + O₂(g)
What is the difference between displacement and double displacement reactions? Write equations to illustrate your answer.
Answer
Displacement reaction: A more reactive element takes the place of a less reactive element in a compound.
Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s)
Here, zinc displaces copper because zinc is more reactive.
Double displacement reaction: Two compounds exchange their ions to form two new compounds.
AgNO₃(aq) + NaCl(aq)
⇒ AgCl(s)↓ + NaNO₃(aq)
Here, both compounds swap their positive and negative ions with each other.
In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write the equation for the reaction.
Answer
Cu(s) + 2AgNO₃(aq)
⇒ Cu(NO₃)₂(aq) + 2Ag(s)
Copper, being more reactive than silver, displaces silver from silver nitrate solution. The deposited silver is then recovered.
What do you mean by a precipitation reaction? Explain by giving examples.
Answer
A precipitation reaction is a type of chemical reaction in which two soluble reactants combine to produce an insoluble product called a precipitate that settles out of the solution.
Example 1: Na₂SO₄(aq) + BaCl₂(aq)
⇒ BaSO₄(s)↓ + 2NaCl(aq)
White precipitate of barium sulphate is formed.
Example 2: Pb(NO₃)₂(aq) + 2KI(aq)
⇒ PbI₂(s)↓ + 2KNO₃(aq)
Yellow precipitate of lead iodide settles out of the solution.
Explain the terms 'oxidation' and 'reduction' in terms of gain and loss of oxygen, with two examples each.
Answer
Oxidation — Gain of oxygen by a substance:
(i) 2Cu(s) + O₂(g)
⇒ heat 2CuO(s) — Copper is oxidised to copper oxide
(ii) 2Mg(s) + O₂(g)
⇒ 2MgO(s) — Magnesium is oxidised to magnesium oxide
Reduction — Loss of oxygen from a substance:
(i) CuO(s) + H₂(g)
⇒ heat Cu(s) + H₂O(l) — Copper oxide is reduced to copper
(ii) Fe₂O₃(s) + 3CO(g)
⇒ 2Fe(s) + 3CO₂(g) — Iron oxide is reduced to iron
A shiny brown-coloured element 'X' on heating in air becomes a black coloured compound. Name the element 'X' and the black compound formed.
Answer
The element X is Copper (Cu). The black compound formed on heating is Copper(II) oxide (CuO).
2Cu(s) + O₂(g) →heat 2CuO(s)
This is an example of oxidation as copper gains oxygen.
Why do we apply paint on iron articles?
Answer
Iron articles rust when exposed to moist air (containing oxygen and water). Painting creates a protective barrier that prevents direct contact between the iron surface and atmospheric moisture and oxygen. Without this contact, the rusting process (formation of iron oxide) cannot occur, thereby prolonging the life of iron articles.
Oil and fat-containing food items are flushed with nitrogen. Why?
Answer
Fats and oils get oxidised when exposed to atmospheric oxygen, leading to a condition called rancidity. Rancid food develops a foul smell and bad taste. By flushing the packaging with nitrogen (an inert gas), oxygen is displaced. In the absence of oxygen, oxidation cannot take place, and the food stays fresh for a longer period. This is an example of using an antioxidant strategy.
Q20. Explain the following terms with one example each
(a) Corrosion (b) Rancidity
Ans.
(a) Corrosion: It is the slow and gradual deterioration of metals due to chemical reaction with air, moisture, or chemicals in the environment.
Example: Iron left in damp air develops a reddish-brown flaky coating called rust (Fe₂O₃·xH₂O). Similarly, silver jewellery turns black due to formation of silver sulphide (Ag₂S), and copper vessels develop a green layer of basic copper carbonate [CuCO₃·Cu(OH)₂].
(b) Rancidity: When fats and oils in food get oxidised by atmospheric oxygen over time, the food develops an unpleasant smell and taste. This condition is called rancidity.
Example: Chips left in an open packet turn stale and develop an off-flavour after some days due to oxidation of the oil/fat used in frying them.
Prevention methods: Adding antioxidants like BHA/BHT, storing in airtight containers, flushing with nitrogen gas, and refrigeration.
Section C: Important Extra Questions for Board Exam
What is a chemical reaction? List four observations that help us determine whether a chemical reaction has taken place.
Answer
A chemical reaction is a process in which one or more substances (reactants) undergo a chemical change to form entirely new substances (products) with different properties.
Four signs that indicate a chemical reaction has occurred:
(i) Change in colour (e.g., blue CuSO₄ turns green when Fe nail is dipped)
(ii) Evolution of a gas (e.g., bubbles when Zn reacts with HCl)
(iii) Change in temperature (e.g., heat released when CaO is added to water)
(iv) Formation of a precipitate (e.g., white BaSO₄ forms when BaCl₂ reacts with Na₂SO₄)
What is a redox reaction? Explain with an example.
Answer
A reaction in which both oxidation and reduction occur simultaneously is called a redox (reduction-oxidation) reaction.
Example: CuO(s) + H₂(g)
⇒ heat Cu(s) + H₂O(l)
Here, CuO is reduced to Cu (loses oxygen), and H₂ is oxidised to H₂O (gains oxygen). Since both processes happen in the same reaction, it is a redox reaction.
Differentiate between combination and decomposition reactions with examples.
Answer
Combination reaction: Two or more reactants join together to form a single product.
CaO(s) + H₂O(l) → Ca(OH)₂(aq)
Decomposition reaction: A single reactant breaks down into two or more products.
CaCO₃(s) →heat CaO(s) + CO₂(g)
Key difference: Combination builds up a compound; decomposition breaks it apart. They are reverse processes of each other.
A white salt on heating decomposes to give brown fumes and a yellow residue. Name the salt and write the equation.
Answer
The salt is Lead nitrate [Pb(NO₃)₂].
2Pb(NO₃)₂(s)
⇒ heat 2PbO(s) + 4NO₂(g) + O₂(g)
The brown fumes are nitrogen dioxide (NO₂) and the yellow residue is lead oxide (PbO). This is a thermal decomposition reaction.
Why is photographic film kept in a dark-coloured bottle? Explain with a chemical equation.
Answer
Photographic film contains silver halides (like silver bromide, AgBr) which decompose when exposed to light:
2AgBr(s) →sunlight 2Ag(s) + Br₂(g)
If the film is exposed to light before use, the silver halide decomposes prematurely, ruining the film. Dark bottles prevent this unwanted photolytic decomposition.