CBSE Class 10 Science Acids Bases and Salts VBQs Set 07

This page collects 18 value based, case based and real-life questions from Class 10 Science Chapter 2, Acids, Bases and Salts, with complete answers and teacher's notes. They link the chapter to bleaching powder and water safety, the chlor-alkali industry, plaster for fractures, tooth care, stings, indigestion and everyday indicators.

Real Life and Value Based Questions

Question 1. A compound X which is prepared from gypsum has the property of hardening when mixed with proper quantity of water. Identify X and write its chemical formula. State the difference in chemical composition between baking soda and baking powder.
Answer:
X is Plaster of Paris, CaSO₄·½H₂O, made by heating gypsum at 373 K.
Baking soda is pure sodium hydrogencarbonate, NaHCO₃. Baking powder is a mixture of baking soda and a mild edible acid such as tartaric acid.
In simple words: POP hardens with water; baking powder is baking soda plus an acid.

Teacher's Note:
1. Tartaric acid is the extra ingredient.
2. Plaster of Paris has half a molecule of water: CaSO₄·½H₂O.

Question 2. (i) How does antacid help to relieve indigestion in stomach? Name one antacid.
(ii) A farmer treats the soil with CaCO₃. What is the nature of soil? Why does the farmer treat the soil with CaCO₃?
Answer:
(i) Indigestion is caused by excess hydrochloric acid in the stomach. An antacid is a mild base that neutralises it. Example: milk of magnesia.
(ii) The soil is acidic. CaCO₃ is basic, so it neutralises the soil acid and brings the pH near neutral for good plant growth.
In simple words: Bases are used to neutralise unwanted acids.

Teacher's Note:
1. Two parts, two clear answers.
2. Antacid: magnesium hydroxide.

Question 3. The pH of the mouth of a person is lower than 5.5. What changes will occur in his mouth? How can these changes be controlled? Write any two measures.
Answer:
When the pH of the mouth falls below 5.5, tooth decay starts, because the acid corrodes the enamel, which is made of calcium phosphate.
Measures: (i) clean the mouth after eating food; (ii) use a toothpaste, which is generally basic and neutralises the excess acid.
In simple words: Low pH in the mouth corrodes enamel; clean and use a basic toothpaste.

Teacher's Note:
1. Mention the value 5.5.
2. Two measures are required.

Question 4. What is tooth enamel chemically? State the condition when it starts corroding. What happens when food particles left in mouth after eating degrades? Why do doctors suggest to use tooth paste to prevent tooth decay?
Answer:
Tooth enamel is calcium phosphate, the hardest material in the body.
It starts corroding when the pH in the mouth falls below 5.5.
Food particles left in the mouth are degraded by bacteria, producing acids that lower the pH and attack the enamel.
Toothpastes are generally basic, so they neutralise the excess acid in the mouth and prevent tooth decay.
In simple words: Basic toothpaste neutralises mouth acid and protects the enamel.

Teacher's Note:
1. Remember the number 5.5.
2. Brush after eating sweets.

Question 5. Account for the following:
(i) Antacid tablets are used by a person suffering from acidity.
(ii) Tooth paste is used for cleaning teeth.
(iii) Which acid is present in sting of ant?
Answer:
(i) Antacid tablets are basic and neutralise the excess acid in the stomach.
(ii) Toothpaste is basic and neutralises the acids produced by bacteria in the mouth, preventing tooth decay.
(iii) Formic acid (methanoic acid).
In simple words: Bases are used to cancel unwanted acids.

Teacher's Note:
1. Link each use to neutralisation.
2. Name formic acid.

Question 6. Name the acid produced in our stomach. What happens if there is an excess of acid in the stomach? How can it be cured? Name the antacid.
Answer:
Hydrochloric acid is produced in the stomach; it helps in the digestion of food.
On overeating, excess acid is produced, which causes pain and a burning sensation (indigestion).
It is cured by antacids, which are bases that neutralise the excess acid. Example: magnesium hydroxide (milk of magnesia).
In simple words: Stomach acid is HCl; antacids are mild bases.

Teacher's Note:
1. Name HCl and one antacid.
2. Antacids neutralise the acid.

Question 7. (i) Identify the acid and the base whose combination forms the common salt that you use in your food. Write its formula and chemical name of this salt. Name the source from where it is obtained.
(ii) What is rock salt? Mention its colour and the reason due to which it has this colour.
Answer:
(i) The acid is hydrochloric acid and the base is sodium hydroxide: HCl + NaOH → NaCl + H₂O. The salt is sodium chloride, NaCl. It is obtained from sea water by evaporation and from underground deposits of rock salt.
(ii) Rock salt is the large crystalline form of common salt found in underground deposits that are mined. It is often brown because of impurities.
In simple words: Common salt is made from HCl and NaOH; rock salt is mined and brown from impurities.

Teacher's Note:
1. Pure rock salt crystals can be colourless.
2. Salts are formed by the reaction of an acid with a base.

Question 8. (a) A liquid has a pH less than 7 which represents an acidic solution.
(i) State the nature of solution, if its pH increases from 7 to 14.
(ii) Mention the ion whose concentration increases with the increase in pH value.
(iii) Suggest a method that is generally used for measuring pH value.
(b) Give reason for the following:
(i) Tooth decay starts when the pH of the mouth is lower than 5.5.
(ii) Antacids are used for treatment of indigestion.
Answer:
(a)(i) The solution becomes basic (alkaline), and more strongly basic as the pH approaches 14.
(ii) The concentration of hydroxide ions, OH⁻, increases.
(iii) A pH paper or universal indicator is used for measuring the pH; the colour is matched with a chart.
(b)(i) Bacteria in the mouth form acids from leftover sugar; below pH 5.5 the acid corrodes tooth enamel, causing decay.
(ii) During indigestion the stomach has excess acid. Antacids contain bases like magnesium hydroxide, which neutralise it and give relief.
In simple words: pH above 7 means more OH⁻ ions.

Teacher's Note:
1. Use universal indicator to measure pH.
2. Link each answer to neutralisation.

Question 9. (A) Suggest one remedial measure each to counteract the change in pH in human beings in following cases:
(i) Production of too much acid in stomach during indigestion.
(ii) Stung by a honey bee/nettle leaves.
(B) Fresh milk has a pH of 6. When it changes into curd, will its pH increase or decrease? Why?
Answer:
(A)(i) Take an antacid such as milk of magnesia, Mg(OH)₂, which neutralises the excess acid.
(ii) Apply a mild base such as baking soda on the stung area to neutralise the formic acid.
(B) The pH will decrease, because lactic acid is formed when milk changes into curd, making it more acidic.
In simple words: Bases relieve acidic problems; curd has lower pH than milk.

Teacher's Note:
1. Name the base used each time.
2. Lower pH means more acidic.

Case Based Questions

Question 10. The flow chart shows an important compound used in Chemistry. X + Chlorine gives a white powder having strong smell of chlorine, Y. Y + H₂SO₄ gives Z + Chlorine + Water. (The letters do not represent chemical symbols of substances.)
(A) Identify X, Y and Z.
(B) Write chemical equations between:
(i) X and chlorine (ii) white powder Y and H₂SO₄ (C) What happens if Y is exposed to air for a long time? List any two important uses of Y.
Answer:
(A) X is dry slaked lime, Ca(OH)₂. Y is bleaching powder, CaOCl₂. Z is calcium sulphate, CaSO₄.
(B) (i) Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O. (ii) CaOCl₂ + H₂SO₄ → CaSO₄ + Cl₂ + H₂O.
(C) On long exposure to air, bleaching powder reacts with the carbon dioxide of air and gives out chlorine (its smell is noticed): CaOCl₂ + CO₂ → CaCO₃ + Cl₂. Uses: disinfecting drinking water; bleaching cotton and linen in the textile industry.
In simple words: Slaked lime plus chlorine gives bleaching powder, which gives chlorine back with acid.

Teacher's Note:
1. Identify the three substances before writing the equations.
2. Store bleaching powder in an airtight container.

Question 11. Pawan read about an industrial process for the electrolysis of sodium chloride solutions. It is the technology used to produce chlorine and sodium hydroxide which are commodity chemicals required by industry. Usually, the process is conducted on a brine solution (an aqueous solution of NaCl), in which NaOH, hydrogen, and chlorine are formed.
(A) Name the process referred to in the passage.
(B) Where are the products of the above process formed?
(C) How can you say that oxides of metals are basic in nature whereas oxides of non-metals are acidic in nature?
Answer:
(A) The chlor-alkali process (chlor for chlorine and alkali for sodium hydroxide).
(B) Chlorine gas is given off at the anode, hydrogen gas at the cathode, and sodium hydroxide solution is formed near the cathode.
(C) Metal oxides react with water to form bases (CaO + H₂O → Ca(OH)₂) and with acids to form salt and water (CaO + 2HCl → CaCl₂ + H₂O), so they are basic. Non-metal oxides dissolve in water to form acids (CO₂ + H₂O → H₂CO₃) and react with bases to form salt and water (CO₂ + Ca(OH)₂ → CaCO₃ + H₂O), so they are acidic.
In simple words: Electrolysis of brine gives chlorine, hydrogen and NaOH; metal oxides are basic and non-metal oxides are acidic.

Teacher's Note:
1. Name the electrode for each gas.
2. Give one equation each for metal and non-metal oxides.

Question 12. Rajesh was given a substance and asked to identify it. He conducted three tests on the substance and recorded the results below. (P) It releases carbon dioxide, water and a sodium salt on heating with water. (Q) It turns universal indicator greenish-blue. (R) It can be prepared from ammonia as a raw material.
(A) Which substance was Rajesh given?
(B) Give ONE use of the substance based on the properties mentioned in P and Q.
(C) Rajesh later read that recrystallisation of the sodium salt formed in P gives another basic salt that is used in manufacture of borax. Identify the sodium salt formed in P.
(D) State the reason of exhibiting basic character only in sodium hydroxide solution and not in ethanol solution.
Answer:
(A) Sodium hydrogencarbonate (baking soda), NaHCO₃: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂.
(B) It is used as an antacid, because it is mildly basic (greenish-blue with universal indicator), and as an ingredient of baking powder.
(C) The sodium salt formed in P is sodium carbonate, Na₂CO₃. Its recrystallisation gives washing soda: Na₂CO₃ + 10H₂O → Na₂CO₃·10H₂O.
(D) Sodium hydroxide ionises in water to give OH⁻ ions (NaOH → Na⁺ + OH⁻), which give basic character and let the solution conduct electricity. Ethanol does not ionise to give OH⁻ ions, so it is neutral and does not conduct.
In simple words: Baking soda is mildly basic and gives sodium carbonate on heating.

Teacher's Note:
1. Basic character means OH⁻ ions in water.
2. Baking soda is NaHCO₃ and washing soda is Na₂CO₃·10H₂O.

Question 13. A white powder is used by doctors to support fractured bones.
(A) Write the name and chemical formula of the powder.
(B) How is this prepared?
(C) When this white powder is mixed with water, a hard solid mass is obtained. Write a balanced chemical equation for the change.
(D) Give one more use of this powder.
Answer:
(A) Plaster of Paris (calcium sulphate hemihydrate), CaSO₄·½H₂O.
(B) It is made by heating gypsum, CaSO₄·2H₂O, at 373 K in a kiln: CaSO₄·2H₂O → CaSO₄·½H₂O + 1½H₂O.
(C) CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O. Gypsum sets as a hard solid mass.
(D) It is used for making toys, statues and decorative materials, and for smoothening surfaces.
In simple words: POP sets into hard gypsum when mixed with water.

Teacher's Note:
1. Heat gypsum only up to 373 K; stronger heating gives dead burnt plaster.
2. Write the equation with correct water numbers.

Question 14. Study the table related to colour change with indicators and answer the questions that follow. Solution P: pink with phenolphthalein, yellow with methyl orange. Solution Q: colourless with phenolphthalein, orange with methyl orange. Solution R: colourless with phenolphthalein, red with methyl orange.
(i) Name the solution which is acidic.
(ii) Arrange the given solutions in increasing order of their pH value.
(iii) What is the name of solution P and Q?
(iv) When solution P is added to the china rose indicator, what is the colour of solution P?
Answer:
(i) Solution R is acidic (methyl orange is red and phenolphthalein is colourless).
(ii) R < Q < P (strongly acidic, weakly acidic, basic).
(iii) P is a basic solution, for example sodium hydroxide solution. Q turns methyl orange orange and phenolphthalein stays colourless, so it is a weakly acidic solution, for example acetic acid (vinegar).
(iv) China rose indicator turns green in a basic solution, so P gives a green colour.
In simple words: Pink phenolphthalein means base; red methyl orange means acid.

Teacher's Note:
1. Learn colour changes of indicators.
2. Learn the colour of each indicator in acid and in base.

Question 15. The acids are sour in taste while bases are bitter in taste. Tasting a substance is not a good way of finding out if it is an acid or a base. Acids and bases can be better distinguished with the help of indicators. Indicators are substances that undergo a change of colour with a change of acidic, neutral or basic medium. Many of these indicators are derived from natural substances such as extracts from flower petals and berries. Litmus, a purple dye, is extracted from the lichen plant. Some indicators are prepared artificially, for example methyl orange and phenolphthalein. Litmus: red in acid, blue in alkali. Methyl orange: pinkish red in acid, yellow in alkali. Phenolphthalein: colourless in acid, pink in alkali.
(i) Give two examples each of natural and artificial indicators.
(ii) An aqueous solution turns red litmus solution blue. Excess addition of which solution would reverse the change: ammonium hydroxide solution or hydrochloric acid?
(iii) What will be the change in colour when a few drops of phenolphthalein is added to a solution having pH 8.5?
(iv) What is universal indicator?
Answer:
(i) Natural indicators: litmus and turmeric (red cabbage extract is another). Artificial indicators: methyl orange and phenolphthalein.
(ii) Hydrochloric acid. Excess acid neutralises the base and then makes the solution acidic, so the colour changes back to red; ammonium hydroxide is itself a base and would only keep the litmus blue.
(iii) Phenolphthalein turns pink, as pH 8.5 is basic.
(iv) A universal indicator is a mixture of dyes that changes colour gradually over a range of pH and is used to find the approximate pH of a solution.
In simple words: Indicators change colour in acid and base.

Teacher's Note:
1. Phenolphthalein turns pink above pH 8.2.
2. Name two of each type.

Question 16. Study the table of pH limits: Saliva 6.5-7.5; Lemon juice 2.2-2.4; Tomato juice 4.0-4.4; Coffee 4.5-5.5.
(i) What is the change on litmus paper when drops of tomato juice are dropped on it?
(ii) What is the nature of saliva given in table?
(iii) What is the effect of acid over pH paper?
(iv) What is the effect of base on litmus paper?
Answer:
(i) Tomato juice has pH 4.0 to 4.4, so it is acidic; blue litmus paper turns red.
(ii) Saliva has pH 6.5 to 7.5, so it is nearly neutral (very slightly acidic to slightly basic).
(iii) Acids turn pH paper towards orange or red (a low pH colour).
(iv) A base turns red litmus paper blue.
In simple words: pH below 7 is acidic, around 7 is neutral.

Teacher's Note:
1. Read the pH range of each item.
2. Remember that pH below 7 is acidic, 7 is neutral and above 7 is basic.

Question 17. Baking soda is used in small amounts for making bread and cakes. It helps to make these soft and spongy. An aqueous solution of baking soda turns red litmus blue. It is also used in soda acid fire extinguisher. Use this information to answer the following questions.
(i) Write the equation for the reaction between baking soda and acid.
(ii) How does it help in extinguishing fire?
(iii) What is the reaction involved when it is heated?
(iv) Is the pH value of baking soda solution lower than or higher than 7?
Answer:
(i) 2NaHCO₃(s) + H₂SO₄(aq) → Na₂SO₄(aq) + 2H₂O(l) + 2CO₂(g)
(ii) The carbon dioxide produced by the reaction of baking soda and the acid in a soda-acid fire extinguisher covers the fire and cuts off oxygen, thus extinguishing it.
(iii) 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂
(iv) Higher than 7, as the solution is alkaline.
In simple words: Baking soda reacts with acid and heat to give CO₂.

Teacher's Note:
1. Baking soda is mildly basic.
2. CO₂ does not support burning.

Question 18. Study the table: Substance A has pH 6.5 (greenish yellow with universal indicator), B has pH 10.5 (navy blue), C has pH 0 (dark red), D has pH 8.5 (greenish blue), E has pH 4.0 (orange). Answer the questions giving examples in each case.
(i) Which substance may be used as a preservative?
(ii) Which substance may be used in antacids?
(iii) Which substance may be used in cakes to make them fluffy?
(iv) Which substance is a strong acid?
Answer:
(i) Substance E (pH 4.0, weak acid), for example vinegar (acetic acid).
(ii) Substance B (pH 10.5, base), for example milk of magnesia.
(iii) Substance D (pH 8.5, mild base), for example baking soda.
(iv) Substance C (pH 0), for example concentrated hydrochloric acid.
In simple words: The pH value tells which substance is suitable for which use.

Teacher's Note:
1. Match the pH to acid, mild base or strong base.
2. Baking soda has pH about 8.5.