1.1.2 Balanced Chemical Equations

The law of conservation of mass states that mass can neither be created nor destroyed in a chemical reaction. Therefore, every element must have the same number of atoms before and after the reaction.

Activity 1.3 can be written as:

Zinc + Sulphuric acid → Zinc sulphate + Hydrogen

Zn + H₂SO₄ → ZnSO₄ + H₂ (1.3)

Equation (1.3) is already balanced: Zn = 1, H = 2, S = 1 and O = 4 on each side.

Now consider the skeletal equation:

Fe + H₂O → Fe₃O₄ + H₂ (1.4)

Balancing by the hit-and-trial method

Step I: Draw boxes around every formula. Do not change anything inside a box.

Fe + H₂O → Fe₃O₄ + H₂ (1.5)

Step II: Count the atoms.

Step III: Start with the compound containing the most atoms. Balance oxygen by placing 4 before water:

Fe + 4H₂O → Fe₃O₄ + H₂ (1.6)

Step IV: Balance hydrogen by placing 4 before hydrogen:

Fe + 4H₂O → Fe₃O₄ + 4H₂ (1.7)

Step V: Balance iron by placing 3 before iron:

3Fe + 4H₂O → Fe₃O₄ + 4H₂ (1.8)

Step VI: Check every element. Fe: 3 = 3, H: 8 = 8, O: 4 = 4. Use the smallest whole-number coefficients:

3Fe + 4H₂O → Fe₃O₄ + 4H₂ (1.9)

Step VII: Writing symbols of physical states

3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g) (1.10)

Here (s) means solid, (l) liquid, (g) gas and (aq) an aqueous solution. In equation (1.10), (g) with water shows that water is used as steam.

Temperature, pressure and catalysts are written above or below the arrow when required:

CO(g) + 2H₂(g) →[340 atm] CH₃OH(l) (1.11)

In sunlight and in the presence of chlorophyll:

6CO₂(aq) + 12H₂O(l) →[Sunlight, chlorophyll] C₆H₁₂O₆(aq) + 6O₂(aq) + 6H₂O(l) (1.12)

Textbook check: Using these steps, can you balance Eq. (1.2) given in the text earlier?

4Al + 3O₂ → 2Al₂O₃

1.2 Types of Chemical Reactions

Atoms do not change into atoms of another element, disappear or appear from nowhere during a chemical reaction. Existing bonds break and new bonds form, producing new substances.

1.2.1 Combination Reaction

Activity 1.4 Quicklime and water

1. Take a small amount of calcium oxide, or quicklime, in a beaker.
2. Slowly add water.
3. Touch the outside of the beaker carefully.
4. Record the temperature change.

Observation: The beaker becomes hot. Calcium oxide reacts vigorously with water to form calcium hydroxide, or slaked lime, and releases heat.

1. 1
2. 2
3. 3
4. 4
5. 5

1. 1Beaker
2. 2Hand checking temperature
3. 3Quicklime
4. 4Water
5. 5Slaked lime mixture

Figure 1.3 Formation of slaked lime by the reaction of calcium oxide with water.

CaO(s) + H₂O(l) → Ca(OH)₂(aq) + Heat (1.13)

Two reactants form a single product. This is a combination reaction.

Do You Know? Slaked lime solution is used for whitewashing. It slowly reacts with carbon dioxide in air. Calcium carbonate forms after two to three days and gives the wall a shiny finish. Marble also has the formula CaCO₃.

Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l) (1.14)

Other combination reactions are:

C(s) + O₂(g) → CO₂(g) (1.15)

2H₂(g) + O₂(g) → 2H₂O(l) (1.16)

Activity 1.4 also releases a large amount of heat. A reaction that releases heat along with its products is exothermic.

Burning natural gas:

CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g) (1.17)

Respiration is exothermic because glucose reacts with oxygen in our cells and releases usable energy:

C₆H₁₂O₆(aq) + 6O₂(aq) → 6CO₂(aq) + 6H₂O(l) + Energy (1.18)

Decomposition of vegetable matter into compost is also exothermic.

Source prompt: Identify the reaction in Activity 1.1. Magnesium and oxygen form one product while heat is given out, so it is an exothermic combination reaction.

1.2.2 Decomposition Reaction

Activity 1.5 Heating ferrous sulphate crystals

Caution: Keep the mouth of the boiling tube pointed away from yourself and others. If the odour is checked, gently waft the gas towards the nose; do not smell it directly.

1. Take about 2 g ferrous sulphate crystals in a dry boiling tube.
2. Note their green colour.
3. Heat the tube over a burner or spirit lamp as shown in Figure 1.4.
4. Observe the colour after heating and the characteristic odour of burning sulphur.

Observation and explanation: Green FeSO₄·7H₂O crystals first lose water on heating. The remaining ferrous sulphate then decomposes. Brown ferric oxide remains, while sulphur dioxide and sulphur trioxide escape as gases.

1. 1
2. 2
3. 3
4. 4
5. 5
6. 6

1. 1Student
2. 2Boiling tube
3. 3Ferrous sulphate crystals
4. 4Test-tube holder
5. 5Bunsen burner
6. 6Wafted gases

Figure 1.4 Correct way of heating a boiling tube containing ferrous sulphate crystals and of smelling the odour.

2FeSO₄(s) →[Heat] Fe₂O₃(s) + SO₂(g) + SO₃(g) (1.19)

One reactant forms two or more simpler products. This is a decomposition reaction.

Calcium carbonate decomposes on heating. This industrial reaction is used in the manufacture of cement:

CaCO₃(s) →[Heat] CaO(s) + CO₂(g) (1.20)

A decomposition reaction caused by heat is called thermal decomposition.

Activity 1.6 Heating lead nitrate

1. Take about 2 g lead nitrate powder in a boiling tube.
2. Hold the boiling tube with a pair of tongs.
3. Heat it over a flame as shown in Figure 1.5.
4. Record the colour change and fumes.

Observation: Brown fumes of nitrogen dioxide are emitted. Lead oxide and oxygen are also formed.

1. 1
2. 2
3. 3
4. 4
5. 5
6. 6

1. 1Hand
2. 2Tongs
3. 3Boiling tube
4. 4Lead nitrate
5. 5Bunsen burner
6. 6Nitrogen dioxide fumes

Figure 1.5 Heating of lead nitrate and emission of nitrogen dioxide.

2Pb(NO₃)₂(s) →[Heat] 2PbO(s) + 4NO₂(g) + O₂(g) (1.21)

Activity 1.7 Electrolysis of water

Caution: The gas test must be performed carefully by the teacher.

1. Drill two holes at the base of a plastic mug and fit rubber stoppers in them.
2. Insert carbon electrodes through the stoppers and connect them to a 6 V battery.
3. Fill the mug with water so that the electrodes are immersed. Add a few drops of dilute sulphuric acid.
4. Fill two test tubes with water and invert one over each electrode.
5. Switch on the current. Bubbles displace water from both test tubes.
6. Compare the gas volumes. Remove the filled tubes carefully.
7. Let the teacher bring a burning candle near each tube and identify the gases from their behaviour.

Observation: Hydrogen forms at the cathode. Oxygen forms at the anode. The volume of hydrogen is about twice the volume of oxygen because water contains hydrogen and oxygen in a 2:1 ratio.

1. 1
2. 2
3. 3
4. 4
5. 5
6. 6
7. 7
8. 8
9. 9

1. 1Plastic mug
2. 2Acidified water
3. 3Graphite electrodes
4. 4Rubber stoppers
5. 5Inverted test tubes
6. 6Switch
7. 76 V battery
8. 8Hydrogen
9. 9Oxygen

Figure 1.6 Electrolysis of water.

2H₂O(l) →[Electricity] 2H₂(g) + O₂(g)

NCERT leaves this activity equation unnumbered. It must not be labelled (1.21).

Board extension: Electrolysis details

Pure water conducts electricity very poorly. A few drops of dilute sulphuric acid provide ions, so current can pass through the water more easily. In this experiment, the acid mainly helps conduction; the chapter equation still shows water decomposing.

Use this electrode map:

• Cathode: hydrogen gas, H₂
• Anode: oxygen gas, O₂
• Volume ratio, cathode:anode: 2:1
• Mass ratio, cathode:anode: 4:32 = 1:8

Why are the two ratios different? The balanced equation gives two molecules of H₂ for one molecule of O₂, so the volume ratio is 2:1. But H₂ is much lighter than O₂. Two moles of H₂ have a mass of 4 g, while one mole of O₂ has a mass of 32 g.

[TIP: First check whether the question asks for volume or mass. Volume is 2:1; mass is 1:8, written in cathode:anode order.]

Activity 1.8 Sunlight on silver chloride

1. Take about 2 g silver chloride in a china dish and note its white colour.
2. Place the dish in sunlight for some time.
3. Observe the colour after exposure.

Observation: White silver chloride turns grey because light decomposes it into silver and chlorine.

1. 1
2. 2
3. 3
4. 4

1. 1China dish
2. 2Silver chloride
3. 3Sunlight
4. 4Silver

Figure 1.7 Silver chloride turns grey in sunlight to form silver metal.

2AgCl(s) →[Sunlight] 2Ag(s) + Cl₂(g) (1.22)

Silver bromide behaves in the same way:

2AgBr(s) →[Sunlight] 2Ag(s) + Br₂(g) (1.23)

These light-sensitive reactions were used in black-and-white photography. This is called photochemical decomposition or photolytic decomposition.

Board application: Silver chloride is kept in a dark-coloured container when light exposure must be prevented. The container blocks the energy that would otherwise start its photolytic decomposition.

What form of energy is causing these decomposition reactions?

Heat, electricity and light supply the energy needed to break down reactants. A reaction that absorbs energy is endothermic.

Carry out the following activity

1. Take about 2 g barium hydroxide in a test tube.
2. Add 1 g ammonium chloride and mix with a glass rod.
3. Touch the bottom of the test tube with your palm.
4. Decide whether the change is exothermic or endothermic.

Observation: The test tube feels cold because the reaction absorbs energy. It is endothermic.

1.2.3 Displacement Reaction

Activity 1.9 Iron nails in copper sulphate

1. Clean three iron nails by rubbing them with sandpaper.
2. Mark two test tubes as A and B. Put about 10 mL copper sulphate solution in each.
3. Tie two nails with thread and immerse them in test tube B for about 20 minutes.
4. Keep the third nail aside for comparison.
5. Remove the immersed nails after 20 minutes.
6. Compare the blue colour in test tubes A and B.
7. Compare the immersed nails with the nail kept aside.

1. 1
2. 2
3. 3
4. 4
5. 5

1. 1Clean comparison nail
2. 2Blue copper sulphate control
3. 3Two immersed nails
4. 4Pale-green iron sulphate
5. 5Copper-coated nails

Figure 1.8(a)-(b) Iron nails and copper sulphate solutions compared before and after the experiment.

Observation: The immersed nails become brownish and the blue solution in test tube B fades. Iron is more reactive than copper, so iron displaces copper. Copper deposits on the nails and pale-green iron sulphate forms.

Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s) (1.24)

A reaction in which a more reactive element removes a less reactive element from its compound is a displacement reaction.

Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s) (1.25)

Pb(s) + CuCl₂(aq) → PbCl₂(aq) + Cu(s) (1.26)

Zinc and lead are more reactive than copper, so they displace copper from its compounds.

1.2.4 Double Displacement Reaction

Activity 1.10 Sodium sulphate and barium chloride

1. Take about 3 mL sodium sulphate solution in one test tube.
2. Take about 3 mL barium chloride solution in another test tube.
3. Mix the two solutions.
4. Record the substance formed.

1. 1
2. 2
3. 3
4. 4

1. 1Sodium sulphate solution
2. 2Barium chloride solution
3. 3Receiving test tube
4. 4Barium sulphate precipitate

Figure 1.9 Formation of barium sulphate and sodium chloride.

Observation: A white insoluble substance forms. An insoluble solid formed in a solution is a precipitate, and a reaction producing it is a precipitation reaction.

Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq) (1.27)

Ba²⁺ and SO₄²⁻ form insoluble BaSO₄; sodium chloride remains in solution. The reacting compounds exchange ions, so this is a double displacement reaction.

Recall Activity 1.2

Lead(II) nitrate solution was mixed with potassium iodide solution.

1. What was the colour of the precipitate? Name it.
2. Write the balanced chemical equation.
3. Is this also a double displacement reaction?

Answers:

1. The precipitate was yellow lead iodide, PbI₂.
2. Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) + 2KNO₃(aq)
3. Yes. The two compounds exchange ions and form insoluble lead iodide.

Fast distinction: Displacement means one element replaces another. Double displacement means two compounds exchange ions.

Board extension: Double displacement without a precipitate

This extension connects Chapter 1 classification with a current CBSE mixed-chapter pattern. It is not an additional NCERT section.

2NaOH(aq) + H₂SO₄(aq) → Na₂SO₄(aq) + 2H₂O(l)

• Sodium hydroxide is a base and sulphuric acid is an acid, so this is a neutralisation reaction.
• The ions exchange partners, so it is also a double displacement reaction.
• It is not a precipitation reaction because sodium sulphate is soluble and no insoluble solid forms.

Do not use “double displacement” and “precipitation” as if they always mean the same thing. First identify ion exchange; then check whether an insoluble solid actually appears.

Board extension: Non-metal displacement

Displacement is not limited to metals. A more reactive non-metal can also displace a less reactive non-metal from its compound.

Cl₂(g) + 2KI(aq) → 2KCl(aq) + I₂(s)

Chlorine displaces iodine from potassium iodide. In a case-based answer, state the displacement pattern and balance the iodine and potassium compounds with coefficient 2.

1.2.5 Oxidation and Reduction

Activity 1.11 Heating copper powder

1. Place about 1 g copper powder in a china dish.
2. Heat the dish as shown in Figure 1.10.
3. Observe the colour of the copper surface.

1. 1
2. 2
3. 3
4. 4
5. 5
6. 6

1. 1China dish
2. 2Copper powder
3. 3Copper oxide
4. 4Wire gauze
5. 5Tripod stand
6. 6Bunsen burner

Figure 1.10 Oxidation of copper to copper oxide.

What do you observe? Why has this black substance formed?

Observation: The reddish-brown copper surface becomes coated with black copper(II) oxide because oxygen is added to copper.

2Cu + O₂ →[Heat] 2CuO (1.28)

If hydrogen is passed over the heated copper oxide, the black coating turns brown again. Copper oxide loses oxygen and copper is obtained:

CuO + H₂ →[Heat] Cu + H₂O (1.29)

A substance that gains oxygen is oxidised. A substance that loses oxygen is reduced. In equation (1.29), hydrogen gains oxygen and is oxidised, while copper(II) oxide loses oxygen and is reduced. Because the two processes happen together, it is an oxidation-reduction or redox reaction.

CuO + H₂ →[Heat] Cu + H₂O (1.30) Oxidation: H₂ → H₂O Reduction: CuO → Cu

Other redox examples are:

ZnO + C → Zn + CO (1.31)

Carbon gains oxygen and is oxidised to carbon monoxide. Zinc oxide loses oxygen and is reduced to zinc.

MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂ (1.32)

Hydrochloric acid loses hydrogen and is oxidised to chlorine. Manganese dioxide loses oxygen and is reduced to manganese chloride; the removed oxygen combines with hydrogen to form water.

In NCERT terms:

• Oxidation: gain of oxygen or loss of hydrogen.
• Reduction: loss of oxygen or gain of hydrogen.

1.3 Have You Observed the Effects of Oxidation Reactions in Everyday Life?

1.3.1 Corrosion

New iron articles are shiny, but iron left exposed develops reddish-brown rust. When a metal is attacked by moisture, acids or other substances around it, the metal is said to corrode and the process is called corrosion.

• Iron forms reddish-brown rust.
• Silver develops a black coating.
• Copper develops a green coating.

Corrosion damages car bodies, bridges, iron railings, ships and other metal objects. Rusting causes serious economic loss because damaged iron must be repaired or replaced. Paint limits the contact of iron with air and moisture, so it slows rusting.

Board extension: Copper corrosion and material choice

This extension prepares you for a current CBSE application pattern. It is not an additional NCERT section.

• Copper exposed to moist air slowly develops a green coating of basic copper carbonate.
• This coating stays attached and helps shield the copper below from further attack.
• Iron rust is loose and flaky. When it falls away, fresh iron is exposed and corrosion can continue.
• Copper is malleable, so it can be made into thin sheets and shaped for roofs, domes and similar structures.

For a material-choice answer, do not write only “copper corrodes less.” Name the coating, compare what the two coatings do, and connect the chosen material property to its use.

1.3.2 Rancidity

When fats and oils are oxidised, their smell and taste change. This spoilage is called rancidity.

Rancidity can be slowed by:

• adding antioxidants to food containing fats and oils;
• keeping food in airtight containers;
• flushing packets with nitrogen so oxygen cannot easily reach the food.

Recall Activity 1.1

Magnesium burns with a dazzling flame in oxygen and changes into white magnesium oxide. Magnesium gains oxygen, so it is being oxidised.

What you have learnt

• A complete chemical equation represents reactants, products and physical states symbolically.
• An equation must be balanced so that every element has the same number of atoms on both sides.
• Combination reactions form one new substance from two or more substances.
• Decomposition reactions split one substance into two or more substances.
• Exothermic reactions release heat; endothermic reactions absorb energy.
• A displacement reaction occurs when one element displaces another from its compound.
• A double displacement reaction exchanges atoms or groups of atoms, called ions.
• Precipitation reactions produce insoluble salts.
• Oxidation is gain of oxygen or loss of hydrogen; reduction is loss of oxygen or gain of hydrogen.

How to use this lesson

Attempt every question on paper first. Reveal the answer only after writing your own. For equations, check formulae, coefficients, physical states and conditions separately.

Fe(s) + 2HCl(aq) → FeCl₂(aq) + H₂(g)

C₆H₁₂O₆(aq) + 6O₂(aq) → 6CO₂(aq) + 6H₂O(l) + Energy

CaCO₃(s) →[Heat] CaO(s) + CO₂(g)

C₆H₁₂O₆(aq) + 6O₂(aq) → 6CO₂(aq) + 6H₂O(l) + Energy

CaO(s) + H₂O(l) → Ca(OH)₂(aq) + Heat

CaCO₃(s) →[Heat] CaO(s) + CO₂(g)

Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s)

Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq)

Cu(s) + 2AgNO₃(aq) → Cu(NO₃)₂(aq) + 2Ag(s)

2Cu + O₂ →[Heat] 2CuO

Group Activity

1. Take four beakers and label them A, B, C and D.
2. Put 25 mL water in A, B and C, and copper sulphate solution in D.
3. Measure and record the temperature of every liquid.
4. Add two spatulas of potassium sulphate, ammonium nitrate, anhydrous copper sulphate and fine iron filings to A, B, C and D respectively. Stir each mixture.
5. Measure and record the final temperature.
6. Use the temperature changes to decide which processes are exothermic and which are endothermic.

Safety: Perform the activity under teacher supervision and avoid direct contact with the chemicals.

Revision ladder

1. Write correct formulae. 2. Balance coefficients. 3. Add states. 4. Add conditions. 5. Identify the pattern. 6. State the observation or reason.

Purpose and source boundary

The first six lessons follow the current NCERT textbook. This lesson is a separate exam-practice supplement. It does not add or renumber any textbook topic.

The previous-year questions below were checked against the original English board-paper pages on 26 July 2026. A question shows what CBSE asked in that historical paper. It does not prove what the current year's paper will contain.

The marking points are platform study guides. They are not official CBSE marking schemes themselves, but the answers for these nine questions were checked against CBSE's published marking-scheme value points for 2023-2026. The explanations sometimes add reasoning so that you understand why a point earns credit.

Your full-marks practice route

Use the chapter in this order. Do not jump straight to the answer check.

1. Finish the six NCERT lessons and the chapter quiz.
2. Use the balancing workbench until all four equations are completed without hints.
3. Write each PYQ answer in its practice box. Reveal the marking points only after writing.
4. Open every weak-skill repair link, revise that exact section, and start a fresh attempt.
5. Complete the five current-pattern challenges.
6. Attempt Diagnostic A under its 30-minute writing limit.
7. Attempt Diagnostic B as a separate transfer check.
8. Return for fresh D3 and D7 attempts when the retention card says they are due. A date alone never marks revision complete.

[REMEMBER: A high quiz score proves recognition. Full board confidence also needs complete written answers, balanced equations, timed work and later retention.]

The full-mark answer pattern

For an activity or reaction question, use O-I-E-C:

1. Observation: What was seen, smelled or felt?
2. Inference: What substance formed or what change occurred?
3. Equation: Write the balanced equation with states and conditions when known.
4. Classification: Name the reaction and justify the pattern.

For a reason question, use cause → process → result. For a multi-part question, copy the same (a), (b), (i) and (ii) labels into the answer.

Board extension: oxidising and reducing agents

This vocabulary appears in board questions and also connects Chapter 1 with later chemistry chapters.

• An oxidising agent causes another substance to be oxidised. The oxidising agent itself is reduced.
• A reducing agent causes another substance to be reduced. The reducing agent itself is oxidised.

In CuO + H₂ →[Heat] Cu + H₂O:

• H₂ gains oxygen, so H₂ is oxidised and acts as the reducing agent.
• CuO loses oxygen, so CuO is reduced and acts as the oxidising agent.

[TIP: Track what happens to each reactant first. Name the agent only after identifying which reactant is oxidised and which is reduced.]

A two-pass method for unfamiliar equations

Pass 1 - Use oxygen or hydrogen.

1. Find the reactant that gains oxygen or loses hydrogen. It is oxidised, so it is the reducing agent.
2. Find the reactant that loses oxygen or gains hydrogen. It is reduced, so it is the oxidising agent.

Pass 2 - Use simple oxidation numbers only when oxygen or hydrogen transfer is not obvious.

• An element by itself has oxidation number 0.
• Hydrogen is usually +1.
• Oxygen is usually -2.
• Fluorine is always -1.
• The oxidation numbers in a neutral compound add to 0.
• An increase means oxidation. A decrease means reduction.

The substance whose oxidation number increases is the reducing agent. The substance whose oxidation number decreases is the oxidising agent.

[TIP: Do not guess the agent from its name or from the side of the equation. Show the change first, then name the agent.]

Verified previous-year questions

PYQ 1 - Observation, reaction type and equation - 2 marks

Source: CBSE Science 2023, paper 31/2/1, Question 21(a), printed page 13.

What is observed when aqueous solutions of potassium iodide and lead nitrate are mixed together? Name the type of reaction and write the chemical equation for the reaction that occurs.

PYQ 2 - Photochemical decomposition - 3 marks

Source: CBSE Science 2023, paper 31/2/1, Question 27, printed page 17.

Silver chloride kept in a china dish turns grey in sunlight.

(a) Write the colour of silver chloride when it was kept in the china dish.

(b) Name the type of chemical reaction taking place and write the chemical equation for the reaction.

(c) State one use of the reaction. Name one more chemical which can be used for the same purpose.

PYQ 3 - Linked decomposition and combination reactions - 5 marks

Source: CBSE Science 2024, paper 31/4/1, Question 34(b), printed page 19.

(i) Define a decomposition reaction. How can we say that (I) electrolysis of water, and (II) blackening of silver bromide when exposed to sunlight, are decomposition reactions? Mention the type of energy involved in each case.

(ii) "The type of reactions in which (I) calcium oxide is formed, and (II) calcium hydroxide is formed are opposite reactions to each other." Justify this statement with the help of chemical equations.

PYQ 4 - Precipitation, ions and whitewashing chemistry - 5 marks

Source: CBSE Science 2024, paper 31/4/2, Question 34(b), printed page 19.

(i) Write the balanced chemical equation for:

Lead nitrate + Potassium iodide → Lead iodide + Potassium nitrate

Is this a double displacement reaction? Justify your answer. Name the compound precipitated and write the ions present in it.

(ii) Write the method of preparation of Ca(OH)₂. What happens when CO₂ is passed through it? Write the balanced chemical equation for the reaction involved.

PYQ 5 - Conservation of mass and unfamiliar balancing - 3 marks

Source: CBSE Science 2025, paper 31/4/1, Question 27(A), printed page 15.

Why do we balance a chemical equation? Name and state the law that suggests the balancing of a chemical equation. Balance:

Zn + H₃PO₄ → Zn₃(PO₄)₂ + H₂

PYQ 6 - Three observations with equations - 3 marks

Source: CBSE Science 2026, paper 31/1/3, Question 26, printed page 15.

What happens when:

(a) An iron nail is dipped in copper(II) sulphate solution?

(b) Potassium iodide solution is mixed with lead nitrate solution?

(c) Silver chloride is exposed to sunlight?

Write balanced chemical equations to support your answer.

PYQ 7 - Reducing agents in unfamiliar equations - 3 marks

Source: CBSE Science 2023, paper 31/1/1, Question 27, printed page 21.

(a) Identify the reducing agent in the following reactions:

(i) 4NH₃ + 5O₂ → 4NO + 6H₂O

(ii) H₂O + F₂ → HF + HOF

(iii) Fe₂O₃ + 3CO → 2Fe + 3CO₂

(iv) 2H₂ + O₂ → 2H₂O

(b) Define a redox reaction in terms of gain or loss of oxygen.

PYQ 8 - Electrolysis and photodecomposition - 3 marks

Source: CBSE Science 2023, paper 31/4/2, Question 33, printed page 17.

(i) While electrolysing water before passing the current some drops of an acid are added. Why? Name the gases liberated at cathode and anode. Write the relationship between the volume of gas collected at anode and the volume of gas collected at cathode.

(ii) What is observed when silver chloride is exposed to sunlight? Give the type of reaction involved.

PYQ 9 - Lead nitrate decomposition and gas mass ratio - 3 marks

Source: CBSE Science 2024, paper 31/1/3, Question 33, printed page 17.

(i) Define a decomposition reaction. Write chemical equation for the reaction that occurs when lead nitrate is heated strongly in a boiling tube.

(ii) In electrolytic decomposition of water two gases are liberated at the electrodes. Give the mass ratio of the gas liberated at the cathode and at the anode.

Latest official SQP-pattern challenges

As checked on 26 July 2026, CBSE's newest published main-subject Class X SQP is the 2025-26 paper. Its Chapter 1 questions combine more than one skill: balancing with redox reasoning, comparing displacement observations, diagnosing an electrolysis setup, evaluating corrosion in a material choice, and distinguishing double displacement from precipitation in a neutralisation. The five questions below are original platform questions built on those patterns. They are not previous-year questions and are not predictions.

Official references: [CBSE Class X 2025-26 SQP page](https://cbseacademic.nic.in/SQP_CLASSX_2025-26.html), [Science SQP](https://cbseacademic.nic.in/web_material/SQP/ClassX_2025_26/Science-SQP.pdf), and [Science marking scheme](https://cbseacademic.nic.in/web_material/SQP/ClassX_2025_26/Science-MS.pdf).

Current-pattern challenge 1 - Balance, classify and track redox - 2 marks

Balance both equations:

1. Fe₂O₃ + Al → Al₂O₃ + Fe
2. C₂H₆ + O₂ → CO₂ + H₂O

State whether each is a redox reaction. For the first equation, name one substance oxidised and one substance reduced.

Current-pattern challenge 2 - Compare two displacement setups - 2 marks

An iron nail is placed in blue copper sulphate solution. A copper strip is placed in pale-green iron sulphate solution.

1. State the observation in each test tube.
2. Write the balanced equation for the reaction that occurs.
3. Use relative reactivity to explain why the other test tube shows no reaction.

Current-pattern challenge 3 - Diagnose an electrolysis setup - 3 marks

A student connects two electrodes to a battery and dips them in distilled water. The bulb does not glow and no gas is collected.

1. Explain why the setup does not work.
2. Name one substance that can be added in a small amount to make the solution conduct.
3. If the corrected setup works, name the gas expected at each electrode.

Current-pattern challenge 4 - Evaluate a corrosion decision - 3 marks

An old building has thin copper roof sheets that have developed a green coating. The owner proposes replacing them with unpainted iron sheets because the copper has “started corroding.”

1. Name the green substance formed on copper. (1)
2. Should unpainted iron replace the copper merely to avoid corrosion? Compare what happens to the two corrosion coatings. (1)
3. State the property that allows copper to be made into thin shaped roof sheets. (1)

Current-pattern challenge 5 - Classify beyond the obvious - 3 marks

A student mixes aqueous sodium hydroxide with dilute sulphuric acid. Sodium sulphate and water form.

1. Write the balanced equation. (1)
2. Give two valid classifications for the reaction. (1)
3. Explain why it is not a precipitation reaction. (1)

Board-style format practice

Assertion-Reason 1

Assertion (A): The reaction of quicklime with water is exothermic.

Reason (R): Quicklime reacts with water to form calcium hydroxide and releases heat.

Choose:

1. Both A and R are true, and R correctly explains A.
2. Both A and R are true, but R does not correctly explain A.
3. A is true, but R is false.
4. A is false, but R is true.

Assertion-Reason 2

Assertion (A): Silver chloride is stored away from sunlight.

Reason (R): Light decomposes silver chloride into silver and chlorine.

Choose the correct relationship using the same four options.

Case-based question - 4 marks

A student performs three reactions:

• Test tube A: green ferrous sulphate crystals are heated.
• Test tube B: an iron nail is placed in blue copper sulphate solution.
• Dish C: white silver chloride is placed in sunlight.

(a) Name the reaction type in test tube A. (1)

(b) State one colour observation in test tube B. (1)

(c) Write the balanced equation for the change in dish C. (1)

(d) Name the form of energy responsible for the change in dish C. (1)

Attempt rules

• Time: 30 minutes writing + 8 minutes checking.
• Maximum marks: 30.
• Use blank paper and a pen.
• Keep notes, the chapter pack and answer reveals closed.
• Attempt every question before opening the answer check.
• The questions are original board-style practice. They are not predictions or previous-year questions.

Question paper

Section A - Objective questions - 9 marks

Choose the correct answer. Questions 7-9 are Assertion-Reason questions.

1. Which equation is correctly balanced? (1)

1. H₂ + O₂ → H₂O
2. 2H₂ + O₂ → 2H₂O
3. H₂ + 2O₂ → H₂O
4. 2H₂ + 2O₂ → H₂O

1. The brown fumes produced by heating lead nitrate are mainly: (1)

1. oxygen
2. nitrogen dioxide
3. sulphur dioxide
4. hydrogen

1. Which reaction is both double displacement and precipitation? (1)

1. CaO + H₂O → Ca(OH)₂
2. Fe + CuSO₄ → FeSO₄ + Cu
3. Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl
4. 2AgCl → 2Ag + Cl₂

1. During electrolysis of water, the volume ratio H₂:O₂ is: (1)

1. 1:1
2. 1:2
3. 2:1
4. 4:1

1. In CuO + H₂ → Cu + H₂O, the reducing agent is: (1)

1. CuO
2. H₂
3. Cu
4. H₂O

1. Nitrogen flushing delays rancidity because it: (1)

1. increases the packet temperature
2. adds water to the food
3. limits contact between fats and oxygen
4. converts fats into proteins

For Questions 7-9 choose:

1. Both A and R are true, and R correctly explains A.
2. Both A and R are true, but R does not correctly explain A.
3. A is true, but R is false.
4. A is false, but R is true.

1. Assertion: A balanced equation obeys conservation of mass. Reason: The number of atoms of each element is equal on both sides. (1)

1. Assertion: Silver chloride turns grey in sunlight. Reason: Silver chloride combines with chlorine to form a grey compound. (1)

1. Assertion: CaO + H₂O → Ca(OH)₂ is a combination reaction. Reason: Two reactants form one product. (1)

Section B - Very short answers - 6 marks

1. Why must a magnesium ribbon be cleaned before burning? Write the balanced equation for its burning. (2)

1. State two differences between displacement and double displacement reactions. (2)

1. Why is respiration exothermic? Write the equation used in this chapter. (2)

Section C - Short answers - 6 marks

1. Green ferrous sulphate crystals are heated strongly. State two observations, name the reaction type and write the balanced decomposition equation. (3)

1. A student mixes lead nitrate and potassium iodide solutions. Write the observation and balanced equation. Explain why the reaction has two valid classifications. (3)

Section D - Long answer - 5 marks

1. Compare thermal, electrolytic and photochemical decomposition. For each, name the energy supplied and write one balanced equation. Finish by explaining why all three are decomposition reactions. (5)

Section E - Case-based question - 4 marks

1. Three unlabeled stations show the following results:

• Station P becomes warm when water is added to a white solid.
• Station Q changes from a blue solution to a pale-green solution when an iron nail is added.
• Station R contains an oily food sample that develops an unpleasant smell after long exposure to air.

(a) Name the white solid at Station P. (1)

(b) Classify the reaction at Station P. (1)

(c) Write the balanced equation for Station Q. (1)

(d) Name the process at Station R and state one way to slow it. (1)

Final eight-minute check

Use this order:

1. Did I answer every subpart?
2. Did I balance every equation?
3. Did I add states and conditions where known?
4. Did I include the observation and the reason?
5. Did I label question numbers clearly?

Score profile

Also record the first failed skill: balancing, observation, equation, classification, reason, redox agent, or multi-part completion. The first failed skill is more useful than the total alone.

Attempt rules

• Time: 22 minutes writing + 5 minutes checking.
• Maximum marks: 20.
• Attempt this only after Diagnostic A.
• Use blank paper and a pen.
• Keep every lesson, revision sheet and answer reveal closed.
• The questions are original board-style practice. They are not predictions or previous-year questions.

Question paper

Section A - Objective questions - 5 marks

1. A few drops of dilute sulphuric acid are added before electrolysing water mainly to: (1)

1. supply oxygen
2. increase electrical conductivity
3. change hydrogen into oxygen
4. make the water boil

1. During electrolysis, 10 cm³ oxygen is collected at the anode. The expected volume of hydrogen at the cathode is: (1)

1. 5 cm³
2. 10 cm³
3. 20 cm³
4. 80 cm³

1. The cathode:anode mass ratio of gases formed during electrolysis of water is: (1)

1. 2:1
2. 1:2
3. 8:1
4. 1:8

1. In PbO + CO → Pb + CO₂, the reducing agent is: (1)

1. PbO
2. CO
3. Pb
4. CO₂

1. Photolytic decomposition is another name for decomposition caused by: (1)

1. heat
2. pressure
3. light
4. electricity

Section B - Reasoning questions - 6 marks

1. Water gives hydrogen and oxygen in a cathode:anode volume ratio of 2:1, but their mass ratio is 1:8. Explain both ratios. (2)

1. In 2CuO + C → 2Cu + CO₂, identify the oxidising agent and reducing agent. Explain using oxygen transfer. (2)

1. White silver bromide is placed in sunlight. State the observation, name the reaction type and write the balanced equation. (2)

Section C - Observation to equation - 3 marks

1. A student strongly heats a white metal nitrate in a boiling tube. Brown fumes are released, a yellow residue remains while hot, and a colourless gas that supports burning is also formed.

(a) Identify the metal nitrate and the brown gas. (1)

(b) Write the balanced equation with the condition. (1)

(c) Name the reaction type. (1)

Section D - Case-based transfer - 6 marks

1. A laboratory has three stations:

• Station P passes electricity through acidified water.
• Station Q heats lead(II) oxide with hydrogen: PbO + H₂ → Pb + H₂O.
• Station R places pale-yellow silver bromide in sunlight.

(a) At Station P, why is the water acidified? Name the gases at the cathode and anode. (2)

(b) At Station Q, identify the oxidising agent and reducing agent. Explain each choice using oxygen. (2)

(c) At Station R, state the colour change and write the balanced equation with its condition. (2)

Final five-minute check

1. Did I read volume and mass correctly?
2. Did I write ratios in the order asked?
3. Did I identify the substance that is oxidised before naming the reducing agent?
4. Did I add heat, light or electricity over the arrow?
5. Did every observation include a visible colour, gas or temperature change?

Score profile

Record the first failed skill: conductivity, electrode labels, volume ratio, mass ratio, observation, balancing, reaction condition, or redox agent.

Passing Diagnostic A does not cancel a failure here. A student is ready only when both papers are strong without opening the answers.