CBSE Board-Style Questions with Detailed Explanations — 1 Mark, 2 Marks, 3 Marks & 4 Marks
This page covers Chapter 2 – Acids, Bases and Salts of Class 10 CBSE Science. This chapter is one of the most important chapters for board exams as well as competitive exams like NTSE and Olympiads. Questions from this chapter appear every year in board examinations and carry significant marks.
Here you will find 15 carefully crafted, original questions across all mark categories — exactly as expected in CBSE board exams. Each question is followed by a detailed, board-style explanation to help you understand not just the answer, but the concept behind it.
Key Topics Covered in This Chapter:
What happens to the colour of blue litmus paper when it is dipped in a dilute solution of hydrochloric acid (HCl)?
When blue litmus paper is dipped in dilute hydrochloric acid, it turns red.
Acids release hydrogen ions (H⁺) in solution. These H⁺ ions react with the litmus dye (a natural indicator made from lichens) and cause a colour change from blue to red. This is a characteristic property of all acidic solutions — they turn blue litmus red. This test is one of the simplest ways to identify an acidic substance in the laboratory.
What is the chemical formula of washing soda? State one of its uses.
The chemical formula of washing soda is Na₂CO₃ · 10H₂O (Sodium carbonate decahydrate).
Use: It is used as a cleaning agent in homes and laundry (dhobis) to remove grease stains.
The "10H₂O" part represents water of crystallisation — 10 water molecules that are chemically bonded within each formula unit of washing soda. On heating, these water molecules are removed and it becomes anhydrous sodium carbonate (Na₂CO₃).
A student adds a pinch of baking soda to lemon juice. He observes brisk effervescence. Name the gas responsible for this effervescence.
The gas responsible for the effervescence is Carbon dioxide (CO₂).
Lemon juice contains citric acid. When baking soda (NaHCO₃) reacts with an acid, it produces carbon dioxide gas, water, and a salt.
The CO₂ gas escapes rapidly, causing the bubbling or effervescence observed. This is the same principle used in baking — CO₂ makes the dough rise and become fluffy.
What is the pH of a neutral solution? What will happen to the pH of pure water if a few drops of sodium hydroxide solution are added to it?
The pH of a neutral solution is 7. On adding NaOH (a base) to pure water, the pH will increase above 7 (the solution becomes alkaline/basic).
NaOH releases OH⁻ ions in water. As OH⁻ concentration increases, the solution becomes more alkaline and pH rises above 7.
Distinguish between a strong acid and a weak acid with one example each. How does the concentration of H⁺ ions differ between them?
Strong Acid: A strong acid completely dissociates (ionises) into ions when dissolved in water, producing a high concentration of H⁺ ions. Example: Hydrochloric acid (HCl) → H⁺ + Cl⁻
Weak Acid: A weak acid only partially dissociates in water, producing a relatively low concentration of H⁺ ions. Example: Acetic acid (CH₃COOH)
For the same concentration, a strong acid has a much higher [H⁺] than a weak acid, which means a strong acid has a lower pH value. This also means strong acids are more corrosive than weak acids of the same concentration.
A solution turns red litmus blue. What is the nature of the solution? Write the chemical equation when this type of solution reacts with dilute sulphuric acid.
A solution that turns red litmus blue is basic (alkaline) in nature.
When sodium hydroxide (NaOH, a common base) reacts with dilute sulphuric acid:
When an acid reacts with a base, the products formed are a salt and water. This reaction is called neutralisation. The H⁺ ions from acid combine with OH⁻ ions from base to form water. The reaction is exothermic — heat is released.
What is plaster of Paris? Write the chemical equation for its preparation from gypsum and mention one of its important uses.
Plaster of Paris is calcium sulphate hemihydrate — CaSO₄ · ½H₂O. It is obtained by heating gypsum (CaSO₄ · 2H₂O) at 100°C.
Use: It is used by doctors to set fractured bones in the correct position (making casts/splints).
Plaster of Paris absorbs water quickly and re-converts back to gypsum, expanding slightly in the process. This is why it hardens and sets well when mixed with water, making it ideal for making casts, moulds, and in construction.
Why does dry hydrogen chloride gas not change the colour of dry blue litmus paper, but hydrochloric acid solution does? What does this tell you about acids?
Dry HCl gas does not change the colour of dry blue litmus because in the absence of water, HCl cannot ionise to produce H⁺ ions. When dissolved in water, HCl dissociates completely: HCl → H⁺ + Cl⁻. These H⁺ ions turn the litmus red.
This experiment proves that water is essential for an acid to show its acidic properties. It is the H⁺ (or H₃O⁺ — hydronium ion) produced in aqueous solution that gives an acid its characteristic properties. Without water, no ionisation occurs, hence no acidic behaviour.
In CBSE exams, this is a very commonly asked conceptual question — always mention "water is necessary for ionisation" as the key reason.
What are olfactory indicators? Name two substances that can be used as olfactory indicators and describe how they behave in acidic and basic solutions.
Olfactory indicators are substances whose smell (odour) changes in acidic or basic solutions. They are useful for people who are colour-blind, as they detect pH through smell rather than colour change.
Two examples and their behaviour:
Explain the chlor-alkali process. Write the products formed and mention one use of each product obtained.
When electricity is passed through an aqueous solution of sodium chloride (brine), it decomposes. This process is called the chlor-alkali process.
Note: Chlorine and hydrogen are kept separate to prevent them from mixing and reacting explosively.
What is water of crystallisation? Give one example with its chemical formula. Describe what happens when blue vitriol is heated and then exposed to moist air again.
Water of crystallisation is the fixed number of water molecules that are chemically bonded (not loosely held) within each formula unit of a crystalline salt. These water molecules are responsible for the shape and colour of the crystal.
Example: Blue vitriol (copper sulphate crystals) — CuSO₄ · 5H₂O
What is a universal indicator? How is the pH of a solution related to the concentration of hydrogen ions? Identify whether the following solutions are acidic, basic, or neutral: pH = 3, pH = 7, pH = 11.
A universal indicator is a mixture of several indicators (like methyl orange, bromothymol blue, thymol blue, phenolphthalein) that shows different colours at different pH values across the range 1–14. It can be used to determine the approximate pH of any solution.
Relationship: As the concentration of H⁺ ions increases, the pH decreases (more acidic). As H⁺ ion concentration decreases, pH increases (more basic).
Classification:
More H⁺ ions → Lower pH → More acidic. Fewer H⁺ ions (more OH⁻) → Higher pH → More basic. pH and [H⁺] are inversely related.
With the help of well-labelled diagrams or equations, explain what happens when (a) zinc metal reacts with dilute sulphuric acid, (b) sodium carbonate reacts with dilute hydrochloric acid, (c) copper oxide reacts with dilute hydrochloric acid, and (d) sodium hydroxide reacts with dilute nitric acid. Identify the type of reaction in each case.
Zinc dissolves in the acid, producing zinc sulphate (salt) and hydrogen gas. The hydrogen gas burns with a 'pop' sound when a lighted splint is brought near it. Type: Displacement reaction (metal displaces H₂ from acid).
Brisk effervescence is observed due to the release of CO₂ gas. The gas turns lime water milky. Type: Double displacement reaction.
The black copper oxide dissolves in the acid, and the solution turns blue-green due to the formation of copper chloride (CuCl₂). Type: Neutralisation-type reaction (acid + metal oxide).
The base and acid neutralise each other to form a salt (sodium nitrate) and water. Heat is released (exothermic). Type: Neutralisation reaction.
Explain the importance of pH in everyday life with four different examples from daily life or biology. How does the body regulate pH, and why is maintaining the correct pH critical for living organisms?
The stomach secretes hydrochloric acid (HCl), which maintains a pH of about 1.5–2. This acidic environment helps in digesting food and kills harmful bacteria. However, excess acid causes acidity (heartburn), which is treated with antacids — mild bases like Mg(OH)₂ (milk of magnesia) that neutralise excess acid.
Bacteria in the mouth break down sugar and release acids. When the pH in the mouth falls below 5.5, the acidic environment begins to dissolve tooth enamel (calcium phosphate), causing tooth decay. Toothpastes are slightly basic (pH > 7) to neutralise the mouth acid and protect teeth.
Plants grow best in soils with specific pH ranges (generally 6–7.5). If the soil is too acidic, farmers add lime (calcium hydroxide) to increase the pH and make it suitable for crops. Acidic soil cannot support most crops well.
Bee stings release an acidic substance (formic acid, pH ≈ 3.5) causing burning pain. Rubbing a mild base like baking soda on the sting neutralises the acid and provides relief. In contrast, wasp stings are alkaline, so a mild acid (like vinegar) is used as a remedy.
Human blood must be maintained at pH 7.35–7.45. Even a slight shift (acidosis or alkalosis) can be life-threatening as enzymes function optimally only in a very narrow pH range. Our body has buffer systems (bicarbonate buffer) and the respiratory system to maintain this pH.
Describe the preparation of sodium hydroxide (NaOH) by the chlor-alkali process and sodium carbonate (Na₂CO₃) by the Solvay process. Mention any two important industrial or household uses of each.
NaOH is manufactured by the chlor-alkali process. Electricity is passed through a concentrated aqueous solution of sodium chloride (brine) in a special electrolytic cell.
NaOH (caustic soda) collects near the cathode. Cl₂ is released at the anode and H₂ at the cathode.
Uses of NaOH:
Na₂CO₃ is obtained industrially by the Solvay process. In this process, carbon dioxide is passed through a concentrated solution of salt (brine) saturated with ammonia. The less soluble NaHCO₃ precipitates out and is then heated to form Na₂CO₃:
Na₂CO₃ · 10H₂O (washing soda) is obtained by recrystallising anhydrous Na₂CO₃ from water.
Uses of Na₂CO₃ (Washing Soda):