Science Class 10 PYQ on Acids, Bases and Salts (Brilliant Mastery)

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Crack the code of chemistry with past papers!

PYQ on Acids from NCERT Science Class 10 serve as a powerful roadmap for exam preparation. They highlight the recurring themes that examiners value, such as the properties of acids and bases, neutralization reactions, the role of indicators, and practical applications like common salts in daily life. By analyzing these questions, students can identify the depth of understanding required—whether it is recalling definitions, applying concepts to real‑world situations, or solving numerical problems related to pH and chemical equations.

PYQ on Acids also reveal the balance between theory and application, showing how conceptual clarity is tested alongside experimental knowledge. For instance, questions often demand explanations of everyday phenomena, like why toothpaste is basic or how salts are used in industry. Practicing these questions not only strengthens memory but also builds exam temperament, as students learn to manage time and structure answers effectively.

Moreover, revisiting PYQ on Acids reduces anxiety by familiarizing learners with the exam’s style and expectations. In essence, previous year questions act as a rehearsal stage, where students sharpen their skills, anticipate challenges, and transform preparation into performance. They are not just practice—they are the blueprint for success in mastering Acids, Bases and Salts. PYQ on Acids

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Section A

1. Select exothermic processes from the following:

(i) Dilution of acid
(iI) Burning of natural gas
(iII) Evaporation of water
(iV) Electrolysis of water
(A) (i) and (iI)
(B) (iI) and (iII)
(C) (iII) and (iV)
(D) (i) and (iV)

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2. When dilute hydrochloric acid is added separately to the metal’s aluminium, copper, iron, magnesium, and zinc, they show different levels of reactivity. Select the statement that accurately summarizes the findings.

(A) Aluminium reacts most vigorously, while iron does not react at all

(B) Magnesium reacts most vigorously, while iron does not react at all

(C) Magnesium reacts most vigorously, while copper does not react at all

(D) Zinc reacts most vigorously, while copper does not react at all

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3. Oxides of aluminium and zinc are:

(A) Acidic
(B) Amphoteric
(C) Basic
(D) Neutral

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4. Juice of tamarind turns blue litmus to red. It is because of the presence of an acid called:

(A) oxalic acid
(B) lactic acid
(C) methanoic acid
(D) tartaric acid

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5. Which one of the following oxides reacts with HCl as well as NaOH to form corresponding salt and water?

(A) CaO
(B) Al₂O₃
(C) Fe₃O₄
(D) K₂O

Year 2025

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Section B

1. Both A and R are factually correct, and R provides the precise logical explanation for A.
2. A is true and R is also true, but R does not logically justify or clarify A.
3. Assertion A holds true; however, the reason given in R is factually incorrect.
4. Assertion A is not valid, but the statement in Reason R is accurate.

1. Assertion (A): Sodium sulphate is a neutral salt.
Reason (R): Sodium sulphate is a neutral salt produced by the reaction of a strong acid and a strong base.

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Section C

1. (i) What is an alloy?
(iI) State the composition of solder and the property that makes it ideal for soldering electrical wires.

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2. During electrolytic refining of silver:
(i) What are the electrodes – cathode and anode made up of?
(iI) What is anode mud? Why is it called so?

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3. You have two solutions, X and Y. Solution X turns red, while solution Y turns blue when tested with universal indicator.
(a) In which solution is the hydrogen ion concentration more than the other? Give reason to justify your answer.
(b) If ‘X’ and ‘Y’ are mixed in proper amounts and the resultant mixture of ‘X’ and ‘Y’, with the universal indicator turns green, what would be the nature of the resultant mixture? Justify your answer.

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4. (a) Write the name and chemical formula of a sodium compound which is sometimes added for faster cooking. How is it produced from sodium chloride as one of the raw materials? Give chemical equation for the reaction involved.
(b) The compound mentioned in (a) above is also an ingredient of antacids. Why?

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5. (i) Give the relation between hydrogen ion concentration of an aqueous solution and its pH.
(iI) Concentrated acids should not be diluted by adding water to acid. Why?
(iII) Why do same molar concentrations of hydrochloric acid and acetic acid not produce same amounts of hydrogen ions?

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6. Give reason:
(i) Crystals of washing soda change to white powder on exposure to air.
(iI) Plaster of Paris should be stored in moisture-proof containers.
(iII) Baking soda can be used on bee sting area to get relief.
(iV) Distilled water does not conduct electricity.
(v) Farmers treat the soil of their fields with quick lime.

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7. What is chlor-alkali process? Write chemical equation for the reaction involved in this process. Name the electrode at which (i) chlorine gas, and (ii) hydrogen gas is given off.

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Answer Key

Section A

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Section B

1. Both A and R are factually correct, and R provides the precise logical explanation for A.

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Section C

1. (i) Alloy: A homogeneous mixture of two or more metals, or a metal and non‑metal, prepared to improve properties like strength, hardness, or resistance.

(iI) Solder composition: Lead (Pb) + Tin (Sn).

Property: Low melting point → makes it suitable for welding/joining electric wires without damaging them.

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2. Electrolytic Refining of Silver

(i) Electrodes:

Cathode: Pure silver (thin sheet).

Anode: Impure silver (block).

(iI) Anode mud:

Called “mud” because it collects as a residue/sludge beneath the anode.

Insoluble impurities (like gold, platinum, etc.) that settle down at the bottom of the electrolytic cell.

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3.

(a)
Universal indicator:

• Red color indicates strongly acidic solution (pH ~1–3).
• Blue color indicates basic solution (pH ~10–12).

Hydrogen ion concentration [H⁺] is higher in acidic solutions. So, solution X (red) has a higher [H⁺] than solution Y (blue).
(Higher acidity = higher [H⁺]; basic solution has low [H⁺])

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(b)
If mixing X (acid) and Y (base) in proper amounts gives a green color with universal indicator:

• Green typically indicates neutral pH (~7).
  So the resultant mixture is neutral in nature.

Reason: The acid and base have reacted completely in stoichiometric amounts to form salt and water (neutralization reaction), leaving no excess H⁺ or OH⁻.

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4.

(a)
The sodium compound sometimes added for faster cooking is sodium hydrogencarbonate (common name: baking soda).
Chemical formula: NaHCO₃.

Production from sodium chloride (NaCl):
It is produced via the Solvay process. Main raw materials: NaCl, NH₃, CO₂, and water.

Steps involved:

1. Brine (NaCl solution) is saturated with ammonia → ammoniated brine.
2. Carbon dioxide gas is passed through it, forming ammonium hydrogencarbonate, which then reacts with NaCl to form NaHCO₃ (less soluble, precipitates):

NH₃​+H₂​O+CO_2​→NH_4​HCO_3​

NH₄​HCO₃​+NaCl→NaHCO_3​↓+NH_4​Cl

Equation for NaHCO₃ precipitation (net ionic view): Na⁺+HCO3−→NaHCO₃↓

but more precisely, from brine and ammonium bicarbonate:

NaCl+NH₄HCO₃→NaHCO₃+NH₄Cl

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(b)
NaHCO₃ is an ingredient of antacids because:

• It is a mild base (alkaline) that neutralizes excess hydrochloric acid (HCl) in the stomach:

NaHCO₃+HCl→NaCl+H₂O+CO₂

• It provides quick relief from acidity and heartburn by increasing stomach pH.

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5.

(i) Relation between hydrogen ion concentration and pH

The pH is defined as the negative logarithm (base 10) of the hydrogen ion concentration:

pH=−log_⁡10[H⁺]

Where [H⁺] is in moles per liter (M).

• Higher [H⁺] → lower pH (acidic)
• Lower [H⁺]→ higher pH (basic/alkaline)

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(iI) Why concentrated acids should not be diluted by adding water to acid

When diluting concentrated acids, the process is highly exothermic.
If water is added to the concentrated acid, the large amount of heat released can cause violent boiling and splashing of the acid, which is dangerous and can cause acid burns.

The correct method is to add acid slowly to water with constant stirring so that the heat is dissipated gradually and safely.

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(iII) Why same molar concentrations of HCl and acetic acid do not produce same amounts of hydrogen ions

HCl is a strong acid — it dissociates completely in aqueous solution:

HCl→H⁺+Cl⁻

So 0.1 M HCl gives 0.1 M H⁺.

Acetic acid (CH₃COOH) is a weak acid — it dissociates partially in solution (only a small fraction of molecules release H⁺ ions):

CH₃COOH⇌H⁺+CH3COO⁻

So 0.1 M acetic acid gives much less than 0.1 M H⁺ (typically ~1% dissociated at 0.1 M, so [H⁺]≈0.001 M[H⁺]≈0.001 M or pH ~3).

Thus, despite having the same molar concentration, their degree of dissociation differs, leading to different [H⁺].

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6.

(i) Crystals of washing soda change to white powder on exposure to air.
Washing soda is sodium carbonate decahydrate (Na₂CO₃·10H₂O). On exposure to air, it loses water of crystallization and effloresces, forming anhydrous sodium carbonate (Na₂CO₃), which appears as white powder.

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(iI) Plaster of Paris should be stored in moisture-proof containers.
Plaster of Paris (CaSO₄·½H₂O) absorbs moisture from air and sets to form gypsum (CaSO₄·2H₂O), becoming a hard solid. Storing in moisture-proof containers prevents this premature setting.

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(iII) Baking soda can be used on bee sting area to get relief.
Bee sting venom contains formic acid (methanoic acid). Baking soda (sodium hydrogencarbonate, NaHCO₃) is a mild base that neutralizes the acid, reducing pain and inflammation.

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(iV) Distilled water does not conduct electricity.
Distilled water has very few ions (H⁺ and OH⁻ ions from self-ionization are extremely low in concentration: 10⁻⁷ M each). Electricity conduction in water requires mobile ions, which are negligible in pure distilled water.

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(v) Farmers treat the soil of their fields with quick lime.
Quick lime (CaO) is basic and neutralizes acidic soils, raising the pH to a level suitable for plant growth. It also provides calcium as a nutrient and improves soil structure.

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7. Chlor-alkali process
The chlor-alkali process is an industrial method for producing chlorine gas, hydrogen gas, and sodium hydroxide (NaOH) by the electrolysis of a concentrated aqueous sodium chloride (brine) solution.

Chemical equation for the reaction: 2NaCl(_aq)+2H2O(l)_{→Electrolysis}2NaOH(_aq)+Cl₂(g)+H₂(g)

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(i) Chlorine gas is given off at the anode (positive electrode).
(ii) Hydrogen gas is given off at the cathode (negative electrode).

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