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Acids, Bases and Salts: Class 10 Science Practice Questions

30 original exam-pattern questions with full answers, matched to the current CBSE Class 10 paper design, including case-based questions. Attempt each question before opening the answer — or start a free 14-day trial ↓ for the full bank.

Q1Case-based4 marks

During a water quality testing drive organised by the Jal Jeevan Mission at a rural primary school in Sitapur, Uttar Pradesh, science teacher Ms. Rekha demonstrated indicator tests on three water samples collected from different sources — a hand pump (Sample A), a nearby pond (Sample B), and a factory drain outlet (Sample C). She tested each sample with litmus solution, phenolphthalein, and methyl orange. Sample A turned litmus purple and gave no colour change with phenolphthalein; Sample B turned red litmus blue and turned phenolphthalein pink; Sample C turned blue litmus red, turned methyl orange to red, and a strip of turmeric paper dipped in it turned reddish-brown. The pH values recorded for Samples A, B, and C were 7, 10, and 3 respectively.

During a water quality testing drive organised by the Jal Jeevan Mission at a rural primary school in Sitapur, Uttar Pradesh, science teacher Ms. Rekha demonstrated indicator tests on three water samples collected from different sources — a hand pump (Sample A), a nearby pond (Sample B), and a factory drain outlet (Sample C). She tested each sample with litmus solution, phenolphthalein, and methyl orange. Sample A turned litmus purple and gave no colour change with phenolphthalein; Sample B turned red litmus blue and turned phenolphthalein pink; Sample C turned blue litmus red, turned methyl orange to red, and a strip of turmeric paper dipped in it turned reddish-brown. The pH values recorded for Samples A, B, and C were 7, 10, and 3 respectively.

Show answer
(a) Sample C turned methyl orange red. This indicates that Sample C is acidic in nature (pH = 3, which is less than 7).

(b) Sample B (pH = 10) turned phenolphthalein pink and turned red litmus blue, confirming it is basic (alkaline) in nature. A common laboratory chemical that would produce these results is sodium hydroxide (NaOH).

(c) Litmus is a natural indicator that shows three distinct behaviours depending on the nature of the solution: it turns red in acidic solutions (blue litmus turns red — Sample C, pH = 3), remains purple with no colour change in neutral solutions (Sample A, pH = 7), and turns blue in basic solutions (red litmus turns blue — Sample B, pH = 10). Therefore, litmus can effectively distinguish between all three types of solutions — acidic, neutral, and basic.

OR

(c) Sample A (hand pump water, pH = 7) is the safest for drinking among the three samples. It gave no colour change with phenolphthalein, turned litmus purple, and has a pH of 7, indicating it is neutral in nature. Safe drinking water should have a pH close to 7. Sample B is alkaline (pH = 10) and Sample C is strongly acidic (pH = 3), likely contaminated by factory effluents — both are unsuitable for drinking as they deviate significantly from neutral pH and may contain harmful dissolved chemicals.
Q2Case-based4 marks

Riya and her mother were making jam at home. Her mother added lemon juice to the jam mixture and stirred it well. Later, Riya noticed that the jam turned slightly pink when she accidentally spilled a few drops of it on her blue dress made of natural fabric. Curious, she tested the jam with litmus paper and turmeric solution at home. Her science teacher later explained that lemon juice contains citric acid and that the dress fabric acted like a natural indicator.

Riya and her mother were making jam at home. Her mother added lemon juice to the jam mixture and stirred it well. Later, Riya noticed that the jam turned slightly pink when she accidentally spilled a few drops of it on her blue dress made of natural fabric. Curious, she tested the jam with litmus paper and turmeric solution at home. Her science teacher later explained that lemon juice contains citric acid and that the dress fabric acted like a natural indicator.

Based on this situation, answer the following questions:
(a) What colour would red litmus paper show when dipped into lemon juice? Give one reason.
(b) What colour would the turmeric solution (yellow) turn when a few drops of lemon juice are added to it?
(c) Riya's teacher told her that the jam turned her dress pink because the dye in the blue fabric is a natural indicator. Name any TWO other natural indicators used in laboratories and state the colour each one shows in an acidic solution.

Show answer
(a) Red litmus paper will remain RED (show no change in colour) when dipped into lemon juice. [1 mark]
Reason: Lemon juice contains citric acid. Acids do not change the colour of red litmus paper — red litmus stays red in acidic solutions. (Only a base/alkali turns red litmus blue.)

(b) Turmeric solution will remain YELLOW when lemon juice is added to it. [1 mark]
Turmeric is a natural indicator that turns red/pink only in basic (alkaline) solutions. Since lemon juice is acidic, it does not change the yellow colour of turmeric solution.

(c) Two natural indicators and their colour in acidic solution: [1 + 1 = 2 marks]

(i) Litmus (extracted from lichen) — turns RED in acidic solution.

(ii) China rose (Hibiscus) solution — turns DARK PINK / MAGENTA in acidic solution.

[Any other correct natural indicator with correct colour in acid is acceptable, e.g., Red cabbage extract turns red/pink in acid.]

Note to examiner: Award 1 mark for each correctly named natural indicator paired with the correct colour it shows in an acidic solution. Names alone without colours, or colours alone without names, earn ½ mark each.
Q3Case-based4 marks

Rahul's grandmother often complains of acidity after meals. One day, Rahul noticed that she was chewing a small piece of a white tablet labelled 'Milk of Magnesia'. His science teacher later told him that this tablet contains magnesium hydroxide [Mg(OH)₂]. Rahul also found that his mother uses baking soda [NaHCO₃] dissolved in water to relieve her own acidity. Meanwhile, Rahul's younger sister Priya had a bee sting on her hand and was crying in pain. Rahul recalled from his science class that a bee sting is acidic in nature.

Read the following passage carefully and answer the questions that follow:

Rahul's grandmother often complains of acidity after meals. One day, Rahul noticed that she was chewing a small piece of a white tablet labelled 'Milk of Magnesia'. His science teacher later told him that this tablet contains magnesium hydroxide [Mg(OH)₂]. Rahul also found that his mother uses baking soda [NaHCO₃] dissolved in water to relieve her own acidity. Meanwhile, Rahul's younger sister Priya had a bee sting on her hand and was crying in pain. Rahul recalled from his science class that a bee sting is acidic in nature.

(a) Name the acid naturally present in the human stomach that helps in digestion. What happens when this acid is produced in excess?

(b) Both Mg(OH)₂ and NaHCO₃ are used to treat acidity. What type of substances are they? Name the chemical process by which they provide relief.

(c) Rahul suggested applying baking soda solution to Priya's bee sting. Is this suggestion correct? Justify your answer with a chemical reason. Also name any one natural substance that can be applied instead.

Show answer
(a) The acid present in the human stomach is hydrochloric acid (HCl). It activates the enzyme pepsin and helps in digestion of proteins. When HCl is produced in excess, it causes acidity — a condition marked by a burning sensation in the stomach and food pipe (heartburn / acid reflux). [1 mark]

(b) Mg(OH)₂ and NaHCO₃ are both bases (alkalis). Mg(OH)₂ is a mild base and NaHCO₃ is a mild base / salt of weak acid and strong base. They provide relief from acidity by the process of neutralisation — the base reacts with the excess hydrochloric acid in the stomach to form salt and water, reducing the acidic nature of the stomach contents.

Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O
NaHCO₃ + HCl → NaCl + H₂O + CO₂↑ [1 mark]

(c) Yes, Rahul's suggestion is correct. A bee sting injects formic acid (methanoic acid, HCOOH), which is acidic in nature (pH < 7). Baking soda solution (NaHCO₃) is a mild base (pH > 7). When it is applied to the bee sting, it neutralises the formic acid injected by the bee, thereby relieving the pain and reducing irritation. This is an application of the neutralisation reaction between an acid and a base.

One natural substance that can be applied: Baking soda paste / calamine solution (zinc carbonate) / toothpaste (any one acceptable). [2 marks]

[Total: 4 marks]
Q4Case-based4 marks

During a quality control workshop organised at a traditional pickle and preserve manufacturing unit in Amritsar, Punjab, food technologist Ms. Harleen introduced a batch of trainees to the use of chemical indicators for testing the acidity or alkalinity of various food samples. She arranged five solutions labelled A to E and tested each with litmus, phenolphthalein, methyl orange, and turmeric indicator. Solution A turned red litmus blue and made phenolphthalein turn pink; Solution B turned blue litmus red and changed methyl orange to red; Solution C showed no colour change with any indicator; Solution D turned turmeric indicator from yellow to red-brown; and Solution E turned methyl orange to orange-yellow. Ms. Harleen noted that the pH of Solution B was measured as 3, while the pH of Solution A was measured as 11.

During a quality control workshop organised at a traditional pickle and preserve manufacturing unit in Amritsar, Punjab, food technologist Ms. Harleen introduced a batch of trainees to the use of chemical indicators for testing the acidity or alkalinity of various food samples. She arranged five solutions labelled A to E and tested each with litmus, phenolphthalein, methyl orange, and turmeric indicator. Solution A turned red litmus blue and made phenolphthalein turn pink; Solution B turned blue litmus red and changed methyl orange to red; Solution C showed no colour change with any indicator; Solution D turned turmeric indicator from yellow to red-brown; and Solution E turned methyl orange to orange-yellow. Ms. Harleen noted that the pH of Solution B was measured as 3, while the pH of Solution A was measured as 11.

Show answer
(a)

Solution A is basic (alkaline) in nature. It turned red litmus blue and made phenolphthalein turn pink, both of which are characteristic responses in alkaline solutions. Phenolphthalein remains colourless in a neutral solution.

---

(b)

Solution B is acidic in nature. It turned blue litmus red and changed methyl orange to red, confirming its acidic character (pH = 3). In a basic (alkaline) solution, methyl orange shows a yellow colour.

---

(c)

(i) Solution B has pH = 3 and Solution A has pH = 11. Since pH = 3 is farther from 7 on the acidic side, Solution B is a stronger acid. Since pH = 11 is farther from 7 on the basic side, Solution A is a stronger base.

(ii) When tested with universal indicator:
Solution B (pH = 3) would show a red/orange-red colour (strongly acidic range); Solution A (pH = 11) would show a dark blue/violet colour (strongly basic range).

OR

Solution C showed no colour change with any indicator — litmus, phenolphthalein, methyl orange, and turmeric all remained unchanged. This indicates that Solution C is neutral, and its pH value is most likely 7.
Q5Case-based4 marks

During a traditional pickle-making workshop organised at a community kitchen in Amritsar, Punjab, food science instructor Ms. Gurpreet demonstrated how natural and synthetic indicators can identify whether a solution is acidic or basic. She prepared four solutions: (A) fresh lemon juice, (B) baking soda solution, (C) distilled water, and (D) vinegar. She tested each solution using litmus paper, phenolphthalein, methyl orange, and a turmeric-soaked paper strip. The pH values recorded for the four solutions were 2, 9, 7, and 3 respectively.

During a traditional pickle-making workshop organised at a community kitchen in Amritsar, Punjab, food science instructor Ms. Gurpreet demonstrated how natural and synthetic indicators can identify whether a solution is acidic or basic. She prepared four solutions: (A) fresh lemon juice, (B) baking soda solution, (C) distilled water, and (D) vinegar. She tested each solution using litmus paper, phenolphthalein, methyl orange, and a turmeric-soaked paper strip. The pH values recorded for the four solutions were 2, 9, 7, and 3 respectively.

Show answer
(a)

The solution with the lowest pH is Solution (A) — fresh lemon juice — with pH = 2. It is the most acidic solution among the four.

(b)

Turmeric paper remains yellow in neutral and acidic solutions and turns red/brick-red in basic solutions. Solution (B) is baking soda solution with pH = 9, which is basic. Therefore, turmeric paper turns red (brick-red) when dipped in Solution (B).

(c)

Solution (A) has pH = 2 — it is strongly acidic. Phenolphthalein is colourless in acidic solutions, so Solution (A) shows no colour (remains colourless) with phenolphthalein.

Solution (B) has pH = 9 — it is basic. Phenolphthalein turns pink in basic solutions, so Solution (B) shows a pink colour with phenolphthalein.

OR

Universal indicator shows a range of colours corresponding to different pH values. Solution (A) at pH 2 gives a red colour — strongly acidic. Solution (B) at pH 9 gives a blue/violet colour — basic. Solution (C) at pH 7 gives a green colour — neutral. Solution (D) at pH 3 gives an orange colour — weakly acidic.

Universal indicator is preferred over litmus because litmus only indicates whether a solution is acidic or basic, whereas universal indicator shows a distinct colour for each pH value, enabling precise determination of the degree of acidity or basicity.
Q6Case-based4 marks

A student, Priya, notices that her grandmother applies a paste of baking soda on a bee sting to get relief from pain. She also recalls that when her younger brother was stung by a wasp, her grandmother applied a few drops of vinegar instead. Priya is curious and decides to investigate the properties of baking soda solution and vinegar using a universal indicator and litmus paper in her school laboratory.

A student, Priya, notices that her grandmother applies a paste of baking soda on a bee sting to get relief from pain. She also recalls that when her younger brother was stung by a wasp, her grandmother applied a few drops of vinegar instead. Priya is curious and decides to investigate the properties of baking soda solution and vinegar using a universal indicator and litmus paper in her school laboratory.

(a) Identify the nature (acidic/basic/alkaline) of baking soda solution and state the approximate pH range it belongs to.

(b) Priya tests vinegar with blue litmus paper. State the observation she records and give one reason for it.

(c) Explain why baking soda paste provides relief from a bee sting but vinegar is used for a wasp sting. Write the balanced chemical equation for the reaction of baking soda with the acid present in a bee sting (formic acid / methanoic acid, HCOOH).

Show answer
(a) Baking soda (Sodium hydrogen carbonate, NaHCO₃) solution is basic (alkaline) in nature. Its pH is approximately 8–9 (greater than 7). [1 mark]

(b) Observation: Blue litmus paper turns red when dipped in vinegar. [½ mark]
Reason: Vinegar contains ethanoic acid (acetic acid, CH₃COOH), which is acidic in nature. Acids donate H⁺ ions in aqueous solution, and these H⁺ ions cause blue litmus to turn red. [½ mark]

(c) A bee sting injects an acid (formic acid / methanoic acid, HCOOH) into the skin, which causes pain and irritation. Baking soda (NaHCO₃) is a mild base. Applying baking soda paste neutralises the injected acid, reducing pain — this is a neutralisation reaction. [1 mark]

A wasp sting, on the other hand, injects an alkaline substance into the skin. Vinegar (ethanoic acid) is a mild acid. Applying vinegar neutralises the alkali injected by the wasp sting, providing relief. [½ mark]

Balanced chemical equation for baking soda reacting with formic acid (methanoic acid):

NaHCO₃ + HCOOH → HCOONa + H₂O + CO₂↑

(Sodium hydrogen carbonate + Methanoic acid → Sodium methanoate + Water + Carbon dioxide)
[½ mark]
Q7Case-based4 marks

During a water quality testing workshop organised at the Agri-Science Research Centre in Coimbatore, Tamil Nadu, soil chemist Dr. Ramesh collected 4 water samples from different sources: a nearby river, a textile dyeing unit effluent, a limestone cave spring, and a rainwater pond. He used four indicators — litmus, phenolphthalein, methyl orange, and a homemade red cabbage indicator — to test each sample. The textile effluent turned litmus red, turned methyl orange to red, and showed no colour change with phenolphthalein. The limestone spring water turned litmus blue, turned phenolphthalein pink, and the red cabbage indicator changed to green. The river water showed no colour change with either litmus or phenolphthalein, while the rainwater pond sample turned methyl orange to orange-yellow and litmus to a faint red.

During a water quality testing workshop organised at the Agri-Science Research Centre in Coimbatore, Tamil Nadu, soil chemist Dr. Ramesh collected 4 water samples from different sources: a nearby river, a textile dyeing unit effluent, a limestone cave spring, and a rainwater pond. He used four indicators — litmus, phenolphthalein, methyl orange, and a homemade red cabbage indicator — to test each sample. The textile effluent turned litmus red, turned methyl orange to red, and showed no colour change with phenolphthalein. The limestone spring water turned litmus blue, turned phenolphthalein pink, and the red cabbage indicator changed to green. The river water showed no colour change with either litmus or phenolphthalein, while the rainwater pond sample turned methyl orange to orange-yellow and litmus to a faint red.

Show answer
(a)

The textile dyeing unit effluent is acidic in nature. Litmus turned red and methyl orange turned red in the effluent — both are characteristic colour changes in acidic solutions. Phenolphthalein shows no colour change (remains colourless) in acidic solutions, which is consistent with this observation.

(b)

The limestone spring water is basic (alkaline) in nature. The red cabbage indicator turned green in this sample. Red cabbage indicator turns green in basic/alkaline solutions, confirming that the limestone spring water is alkaline. This is further supported by phenolphthalein turning pink in the same sample.

(c)

The textile effluent (pH ≈ 4, acidic) and the limestone spring water (pH ≈ 9, basic/alkaline) would give the following results with methyl orange and phenolphthalein:

| S.No. | Sample | Methyl orange | Phenolphthalein |
|-------|------------------------|---------------|-----------------|
| (i) | Textile effluent | Red | Colourless (no change) |
| (ii) | Limestone spring water | Yellow | Pink |

The difference occurs because the two indicators have different pH ranges of colour change. Methyl orange is sensitive to acidic conditions and changes colour in the pH range 3.1–4.4 (red in acid, yellow in neutral/base), whereas phenolphthalein is sensitive to basic conditions and changes colour in the pH range 8.3–10.0 (colourless in acid/neutral, pink in base). Therefore, only phenolphthalein detects the basic nature of the limestone spring water, and only methyl orange clearly distinguishes the acidic effluent at its particular pH range.
Q8Case-based4 marks

Ria is helping her grandmother in the kitchen. She notices that when her grandmother makes rice using tap water, the cooking vessel develops a white crust inside over time. Her grandmother also complains that the soap she uses leaves a white, sticky residue on her hands and does not lather well. Curious about this, Ria tests the tap water at home using a universal indicator and finds it has a pH of about 8. She also tests a sample of water from a friend's house in another locality and finds its pH to be 7. Her friend's soap lathers freely and leaves no residue.

Ria's science teacher later explains that the tap water in Ria's area contains dissolved calcium and magnesium salts, and this is the cause of all the problems Ria observed.

Read the following passage and answer the questions that follow:

Ria is helping her grandmother in the kitchen. She notices that when her grandmother makes rice using tap water, the cooking vessel develops a white crust inside over time. Her grandmother also complains that the soap she uses leaves a white, sticky residue on her hands and does not lather well. Curious about this, Ria tests the tap water at home using a universal indicator and finds it has a pH of about 8. She also tests a sample of water from a friend's house in another locality and finds its pH to be 7. Her friend's soap lathers freely and leaves no residue.

Ria's science teacher later explains that the tap water in Ria's area contains dissolved calcium and magnesium salts, and this is the cause of all the problems Ria observed.

Show answer
(a) Ria's tap water is hard water.

It contains dissolved calcium and magnesium salts (such as calcium hydrogencarbonate and magnesium hydrogencarbonate). These salts make the water hard, which is why soap does not lather freely and a white residue (scum) is formed.

[1 mark]

(b) The white, sticky residue left on Ria's grandmother's hands is scum.

Soap (sodium stearate, C₁₇H₃₅COONa) reacts with the calcium and magnesium salts present in Ria's hard tap water to form insoluble calcium stearate and magnesium stearate. This insoluble product is the white sticky residue called scum. Since soap is consumed in forming scum, very little is available to produce lather, which is why cleaning action is poor.

[1 mark]

(c) The white crust that forms inside the cooking vessel is calcium carbonate (CaCO₃), also known as scale or limescale.

When hard water containing dissolved calcium hydrogencarbonate — Ca(HCO₃)₂ — is heated, it decomposes to form insoluble calcium carbonate, which deposits on the inner surface of the vessel as a hard white crust. The chemical equation for this decomposition is:

Ca(HCO₃)₂ →(Heat)→ CaCO₃↓ + H₂O + CO₂↑

[2 marks]

OR (c)

Ria's friend's water (pH = 7) is soft water, which does not contain dissolved calcium and magnesium salts. Soft water does not react with soap, so no scum forms and lather is produced freely. This explains why her friend's soap works well.

Ria can use a synthetic detergent instead of soap to clean effectively with her hard tap water, because detergents contain sulphonate (–SO₃Na) groups whose calcium and magnesium salts are soluble in water — so no scum forms and lather is produced even in hard water.

[2 marks]
Q9MCQ1 mark

Which of the following common salts is used as an antacid in medicines to provide relief from acidity and is also a key ingredient in baking powder?

Show answer
Option (B) is correct.

Explanation: Sodium hydrogen carbonate (NaHCO₃), commonly known as baking soda, is a mild alkali that neutralises excess hydrochloric acid present in the stomach — the reaction produces sodium chloride, water, and carbon dioxide — thereby providing relief from acidity, which is why it is used as an antacid in medicines. It is also a key ingredient in baking powder, where it is mixed with a mild edible acid such as tartaric acid; on heating, CO₂ gas is released, causing the dough to rise. Sodium carbonate (Na₂CO₃) is strongly alkaline and is used as washing soda, not as an antacid; calcium sulphate (CaSO₄) is Plaster of Paris; and sodium chloride (NaCl) is common salt — none of these serve both functions described in the question.
Q10MCQ1 mark

An aqueous solution of sodium carbonate (Na₂CO₃) turns red litmus blue. Which of the following best explains this observation?

Show answer
Option (b) is correct.

Explanation: Sodium carbonate (Na₂CO₃) is formed by the reaction of sodium hydroxide (NaOH), a strong base, with carbonic acid (H₂CO₃), a weak acid. When such a salt dissolves in water, the solution becomes basic (alkaline) in nature — its pH is greater than 7. A basic solution turns red litmus blue, which is exactly what is observed. Since the solution contains more OH⁻ ions than H⁺ ions, it cannot be acidic or neutral, ruling out options (a), (c), and (d).
Q11MCQ1 mark

The chemical name of the salt whose dilute solution is electrolysed in the chlor-alkali process to produce chlorine gas, hydrogen gas, and sodium hydroxide solution is:

Show answer
Option (A) is correct.

Explanation: The chlor-alkali process involves the electrolysis of a dilute aqueous solution of sodium chloride (NaCl), commonly known as brine. During electrolysis, chlorine gas is produced at the anode, hydrogen gas is produced at the cathode, and sodium hydroxide solution is formed in the electrolyte. Sodium carbonate, sodium bicarbonate, and calcium sulphate are entirely different salts and are not used in this process.
Q12MCQ1 mark

A student adds a few drops of universal indicator to a solution and observes that it turns green. What is the nature of the solution?

Show answer
Option (C) is correct.
Explanation: Universal indicator shows a range of colours corresponding to different pH values. A green colour indicates a pH of 7, which means the solution is neutral — it is neither acidic nor basic. Strongly acidic solutions turn the indicator red or orange, weakly acidic solutions turn it yellow, and basic solutions turn it blue or violet.
Q13MCQ1 mark

A farmer finds that the crop yield in his field is poor. On testing the soil, he finds its pH to be 4.5. Which of the following treatments should he apply to improve the soil condition for better crop growth?

Show answer
Option (A) is correct.

Explanation: The pH of the soil is 4.5, which is well below 7, indicating strongly acidic soil. Most crops grow best when soil pH is close to neutral (6–7). To raise the pH of acidic soil, a base must be added to neutralise the excess acid present. Slaked lime, Ca(OH)₂, is a mild, inexpensive base that reacts with the acids in the soil, neutralising them and raising the pH to a range suitable for crop growth. Adding vinegar (acetic acid) would lower the pH further, making the soil more acidic. Adding water does not significantly alter the pH of soil. Adding common salt (NaCl) is neutral and has no effect on soil pH.
Q14MCQ1 mark

A student tests four different solutions using a universal indicator and records the following pH values: Solution P = 2, Solution Q = 7, Solution R = 11, Solution S = 5. Which of the following correctly identifies the nature of each solution?

Show answer
Option (A) is correct.

Explanation: On the pH scale (range 0–14), pH < 7 indicates an acidic solution, pH = 7 indicates a neutral solution, and pH > 7 indicates a basic solution. The closer the pH is to 0, the more strongly acidic the solution; the closer to 14, the more strongly basic. Applying this: Solution P (pH 2) is strongly acidic, Solution Q (pH 7) is neutral, Solution R (pH 11) is basic, and Solution S (pH 5) is weakly acidic, which matches Option (A) exactly.
Q15MCQ1 mark

A farmer tests the soil of his field and finds its pH to be 4.5. Which of the following statements correctly describes this soil and the appropriate corrective measure?

Show answer
Option (A) is correct.

Explanation: On the pH scale, a value below 7 indicates an acidic nature, a value of 7 indicates neutral, and a value above 7 indicates basic nature. Since the soil has a pH of 4.5, it is strongly acidic and unsuitable for most crops. The standard corrective measure in agriculture is to add slaked lime — calcium hydroxide, Ca(OH)₂ — which is a base. It reacts with the excess acid present in the soil through a neutralisation reaction, raising the pH toward the neutral range and restoring soil fertility. Options (B) and (C) are incorrect because pH 4.5 is neither basic nor neutral. Option (D) is incorrect because baking soda (NaHCO₃) is a mild base that would raise the pH, not lower it, and it is not the standard agricultural remedy for acidic soil.
Q16MCQ1 mark

A student tests four different solutions using a pH meter and records the following pH values: Solution P = 2, Solution Q = 7, Solution R = 10, Solution S = 13. Which of the following correctly identifies the nature of all four solutions?

Show answer
Option (A) is correct.

Explanation: On the pH scale (0–14), pH < 7 indicates an acidic solution, pH = 7 indicates a neutral solution, and pH > 7 indicates a basic solution. The closer the pH is to 0, the more strongly acidic the solution; the closer it is to 14, the more strongly basic it is. Solution P (pH 2) is close to 0 and is therefore strongly acidic. Solution Q (pH 7) is exactly neutral. Solution R (pH 10) is moderately above 7 and is therefore weakly basic. Solution S (pH 13) is close to 14 and is therefore strongly basic. This matches Option (A) exactly.
Q17MCQ1 mark

Select the correct chemical formula of baking soda from the following:

Show answer
Option (A) is correct.

Explanation: Baking soda is the common name for sodium hydrogen carbonate (IUPAC: sodium hydrogen carbonate), whose chemical formula is NaHCO₃. The remaining options are distinct compounds: Na₂CO₃·10H₂O is washing soda (sodium carbonate decahydrate), CaSO₄·½H₂O is Plaster of Paris, and NaOH is sodium hydroxide — a strong base, not the salt used as baking soda.
Q18MCQ1 mark

Consider the following salts: NaCl, NaHCO₃, Na₂CO₃·10H₂O, CaSO₄·½H₂O. Which one among these is used as a preservative in food industry and is also the raw material for the chlor-alkali process as well as the Solvay process for manufacturing soda ash?

Show answer
Option (A) is correct.

Explanation: NaCl (common salt / sodium chloride) serves as a food preservative by inhibiting microbial growth and is the primary raw material for two major industrial processes. In the chlor-alkali process, concentrated brine (aqueous NaCl) is electrolysed to produce chlorine gas, hydrogen gas, and sodium hydroxide. In the Solvay (ammonia-soda) process, NaCl and CaCO₃ are the starting raw materials used to manufacture Na₂CO₃ (soda ash). NaHCO₃ is a leavening agent and antacid, not a food preservative or feedstock for these processes. Na₂CO₃·10H₂O is the product of the Solvay process, not its raw material. CaSO₄·½H₂O (Plaster of Paris) is used in making casts and has no role in food preservation or either industrial process mentioned.
Q19MCQ1 mark

Which of the following represents a correct grouping of salts that are ALL prepared, directly or indirectly, from common salt (NaCl) as the starting raw material?

Show answer
Option (A) is correct.

Explanation: Common salt (NaCl) is the starting raw material for the chlor-alkali process, in which electrolysis of aqueous NaCl produces NaOH, Cl₂, and H₂ directly. NaOH is thus a direct product of NaCl. NaHCO₃ (baking soda, sodium hydrogen carbonate) is produced by the Solvay process, which uses NaCl and CO₂ as key starting materials. Na₂CO₃ (washing soda, sodium carbonate decahydrate) is obtained by heating NaHCO₃, which is itself derived from NaCl — making Na₂CO₃ an indirect product of NaCl. All three compounds in Option (A) therefore trace their preparation back to NaCl as the starting raw material. The compounds in the remaining options include CaSO₄·½H₂O (Plaster of Paris) and CaSO₄·2H₂O (gypsum), which are calcium sulphate compounds not derived from NaCl, and Ca(OCl)Cl (bleaching powder), which although it uses Cl₂ obtained from NaCl, is made by reacting Cl₂ with Ca(OH)₂ and is primarily a calcium-based product rather than an NaCl-derived salt. Na₂SO₄ is also not a product of NaCl processing. Thus only Option (A) correctly groups salts that are all prepared, directly or indirectly, from common salt.
Q20MCQ1 mark

A white crystalline salt, when dissolved in water, gives an alkaline solution and is commonly used by bakers to make cakes and bread rise. The same salt, when heated, releases a gas that turns lime water milky. Which of the following correctly identifies this salt?

Show answer
Option (A) is correct.

Explanation: Sodium hydrogen carbonate (NaHCO₃), commonly known as baking soda, is a white crystalline salt that dissolves in water to give a mildly alkaline solution (pH slightly above 7). It is widely used as a leavening agent in baking because, on heating, it decomposes according to: 2NaHCO₃ →(Heat)→ Na₂CO₃ + H₂O + CO₂↑. The CO₂ released causes dough to rise. This same CO₂ gas turns lime water milky because Ca(OH)₂ + CO₂ → CaCO₃↓ + H₂O. All three properties stated in the question — alkaline solution in water, use in baking, and release of a gas that turns lime water milky on heating — match NaHCO₃ exactly. NaCl is neutral in solution and releases no gas on heating; Na₂CO₃·10H₂O is used for washing, not baking; Plaster of Paris sets with water rather than releasing CO₂.
Q21MCQ1 mark

A substance is commonly added to antacid tablets to neutralise excess acid in the stomach. The same substance, when heated, produces a gas that turns lime water milky. Which of the following salts is this substance?

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Option (A) is correct.

Explanation: Sodium hydrogen carbonate (NaHCO₃) is a mild alkali used in antacid tablets because it neutralises excess hydrochloric acid in the stomach: NaHCO₃ + HCl → NaCl + H₂O + CO₂. When heated, it undergoes thermal decomposition — 2NaHCO₃ →(Heat)→ Na₂CO₃ + H₂O + CO₂↑ — and the CO₂ produced turns lime water milky (Ca(OH)₂ + CO₂ → CaCO₃↓ + H₂O), confirming its identity. Na₂CO₃·10H₂O (washing soda) is strongly alkaline and not used in antacids; CaSO₄·½H₂O (Plaster of Paris) and NaCl are neither alkaline nor do they produce CO₂ on heating, so options (B), (C), and (D) are eliminated.
Q22MCQ1 mark

Which of the following salts is used in the manufacture of glass, soap, and paper, and is also employed to soften hard water in laundry?

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Option (A) is correct.

Explanation: Sodium carbonate (Na₂CO₃), commonly known as washing soda, is a strongly alkaline salt used in the manufacture of glass, soap, and paper. It also softens hard water in laundry by reacting with the dissolved calcium and magnesium salts responsible for hardness, precipitating them as insoluble carbonates and thereby removing them from solution. Sodium chloride (NaCl) is common salt and has no such industrial or water-softening role. Sodium bicarbonate (NaHCO₃) is baking soda, used as an antacid and in baking, not in glass or soap manufacture. Calcium sulphate (CaSO₄) is gypsum or Plaster of Paris and serves none of these functions.
Q23MCQ1 mark

A student tested four different household liquids using universal indicator and recorded their pH values as follows: lemon juice (pH 2), baking soda solution (pH 9), distilled water (pH 7), and black coffee (pH 5). Which of the following correctly arranges these liquids in increasing order of their hydrogen ion concentration?

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Option (A) is correct.

Explanation: Hydrogen ion concentration [H⁺] is inversely related to pH, given by [H⁺] = 10⁻ᵖᴴ. A lower pH means a higher [H⁺], and a higher pH means a lower [H⁺]. Therefore, arranging in increasing order of [H⁺] is equivalent to arranging in increasing order of pH values. The pH values recorded are: baking soda solution (pH 9), distilled water (pH 7), black coffee (pH 5), and lemon juice (pH 2). Arranging these from lowest to highest [H⁺] gives: baking soda solution < distilled water < black coffee < lemon juice, which corresponds exactly to Option (A).
Q24MCQ1 mark

Consider the following four compounds: NaCl, Na₂CO₃·10H₂O, CaSO₄·2H₂O, CuSO₄·5H₂O. Among these, which compound contains the HIGHEST number of water molecules of crystallisation per formula unit AND is also commonly used as a cleaning agent in households to remove grease and stains?

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Option (A) is correct.

Explanation: Water of crystallisation is the fixed number of water molecules chemically combined within one formula unit of a salt. Among the given compounds, NaCl contains no water of crystallisation, CaSO₄·2H₂O contains 2, CuSO₄·5H₂O contains 5, and Na₂CO₃·10H₂O contains 10 water molecules per formula unit — the highest among all four. Na₂CO₃·10H₂O is washing soda, which is commonly used as a cleaning agent in households to remove grease and stains, satisfying both conditions stated in the question.
Q25MCQ1 mark

Plaster of Paris is obtained by heating gypsum at a specific temperature. Which of the following correctly represents the chemical formula of Plaster of Paris?

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Option (A) is correct.

Explanation: Plaster of Paris is obtained by heating gypsum (CaSO₄·2H₂O) at approximately 100–120°C, during which it loses three-quarters of its water of crystallisation to form calcium sulphate hemihydrate. The reaction is: CaSO₄·2H₂O →(373 K)→ CaSO₄·½H₂O + 1½H₂O. Option (B) is gypsum itself, option (C) does not correspond to any known hydrate of calcium sulphate, and option (D) is anhydrous calcium sulphate (dead burnt plaster), formed on heating to a much higher temperature.
Q26MCQ1 mark

Select the correct chemical formula of washing soda from the following:

Show answer
Option (A) is correct.

Explanation: Washing soda is the common name for sodium carbonate decahydrate, whose chemical formula is Na₂CO₃·10H₂O. It contains 10 molecules of water of crystallisation chemically combined within one formula unit of the salt. Option (B), NaHCO₃, is baking soda (sodium hydrogen carbonate). Option (C), NaCl·2H₂O, is not a recognised compound of this type. Option (D), Na₂SO₄·10H₂O, is Glauber's salt, not washing soda.
Q27MCQ1 mark

Consider the following four salts and their water of crystallisation: NaCl, Na₂CO₃·10H₂O, CaSO₄·2H₂O, CuSO₄·5H₂O. Arranged in increasing order of the number of water molecules of crystallisation per formula unit, the correct sequence is:

Show answer
Option (A) is correct.

Explanation: Water of crystallisation refers to the fixed number of water molecules chemically combined within one formula unit of a salt. NaCl has no water of crystallisation (0 molecules), CaSO₄·2H₂O (Gypsum) has 2, CuSO₄·5H₂O (Blue vitriol) has 5, and Na₂CO₃·10H₂O (Washing soda) has 10. Arranging in increasing order: 0 < 2 < 5 < 10, giving the sequence NaCl < CaSO₄·2H₂O < CuSO₄·5H₂O < Na₂CO₃·10H₂O.
Q28MCQ1 mark

Consider the following salts: NaCl, NaHCO₃, Na₂CO₃·10H₂O, CaSO₄·2H₂O. Among these, which salt loses its water of crystallisation on exposure to air (effloresces) and is therefore stored in airtight containers?

Show answer
Option (A) is correct.

Explanation: Efflorescence is the property by which a hydrated salt spontaneously loses its water of crystallisation to the surrounding dry air, leaving behind an anhydrous or less hydrated residue. Of the four salts listed, only Na₂CO₃·10H₂O (washing soda / sodium carbonate decahydrate) and CaSO₄·2H₂O (gypsum) contain water of crystallisation; NaCl and NaHCO₃ do not contain any. CaSO₄·2H₂O does not effloresce under normal atmospheric conditions, whereas Na₂CO₃·10H₂O readily loses all 10 molecules of water of crystallisation on exposure to dry air, forming anhydrous Na₂CO₃ (a white powder). Because of this tendency to effloresce, washing soda must be stored in airtight containers.
Q29MCQ1 mark

A student dissolves a small amount of sodium hydrogen carbonate in water and tests the solution with universal indicator paper — the paper turns blue-green. She then adds a few drops of dilute hydrochloric acid to this solution and observes brisk effervescence. After the effervescence stops completely, she tests the resulting solution again with universal indicator paper. Which of the following best explains the colour of the universal indicator in the final test?

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Option (d) is correct. Explanation: The reaction between sodium hydrogen carbonate and dilute hydrochloric acid is: NaHCO₃ + HCl → NaCl + H₂O + CO₂↑. The brisk effervescence confirms CO₂ is produced. The question states only a few drops of dilute HCl were added — there is no indication of excess HCl. Once the effervescence stops completely, the CO₂ that has not escaped remains dissolved in water as carbonic acid: CO₂ + H₂O ⇌ H₂CO₃. Carbonic acid is a weak acid (partially dissociates), so the final solution is weakly acidic — universal indicator turns yellow (pH approximately 5–6). NaCl is a salt of a strong acid (HCl) and a strong base (NaOH); it does not hydrolyse and does not affect pH. Option (a) is incorrect because NaCl is a neutral salt, not a basic salt — it cannot maintain alkalinity. Option (b) is incorrect because the amounts of NaHCO₃ and HCl used are not stated to be stoichiometrically equal; equal molar proportions cannot be assumed from the experimental description. Option (c) is incorrect because carbonic acid (H₂CO₃) is a weak acid, not a strong acid — it produces only mild acidity, not the orange-red colour associated with strongly acidic solutions (pH 2–3).
Q30MCQ1 mark

Which of the following salts is used by doctors to make plaster casts for setting fractured bones?

Show answer
Option (A) is correct.

Explanation: Plaster of Paris is calcium sulphate hemihydrate (CaSO₄·½H₂O). When mixed with water, it undergoes a setting reaction and converts to calcium sulphate dihydrate (gypsum), forming a hard rigid mass. This property of setting into a hard solid makes it suitable for preparing plaster casts used in orthopaedics to immobilise fractured bones during healing. The remaining options — washing soda (Na₂CO₃·10H₂O), baking soda (NaHCO₃), and common salt (NaCl) — do not possess this setting property and are not used for making fracture casts.

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