Class 10 Science Chapter 2 Acids, Bases and Salts
Welcome to Class 10 Science Chapter 2 Acids, Bases and Salts. This chapter is one of the most important topics in Chemistry and helps students understand the properties and uses of acids, bases, and salts in everyday life. It is an essential chapter for school examinations, board exams, and competitive tests.
In this chapter, students will learn about the characteristics of acids and bases, chemical indicators, pH scale, neutralization reactions, and the preparation and properties of different salts. The chapter also explains important compounds such as baking soda, washing soda, bleaching powder, and plaster of Paris, along with their uses in daily life and industry.
The concepts are presented in a simple and easy-to-understand manner, making learning more effective for students. Understanding this chapter helps build a strong foundation in Chemistry and enables students to apply scientific concepts to real-life situations.
Whether you are preparing for class tests, annual examinations, or board exams, Class 10 Science Chapter 2 Acids, Bases and Salts will help you strengthen your concepts, improve your problem-solving skills, and achieve better results in Science.
Class 10 Science Chapter 2 Acids, Bases and Salts
Selected Questions &Answers
Q. What is an olfactory indicator? Give one example. HSLC ’24
Ans: Substances whose smell changes in acidic and basic media are called olfactory indicators. For example, onion.
Q. What is a neutralization reaction?
Ans: The reaction between an acid and a base is called a neutralisation reaction.
Q. What happens when blue litmus is added to an acid?
Ans: An acid turns blue litmus red.
Q. Write the names of two natural indicators.
Ans: Turmeric and red cabbage leaves.
Q. Write the name of one synthetic indicator.
Ans: Methyl orange or phenolphthalein.
Q. Give an example of a neutralization reaction.
Ans: NaOH + HCl→NaCl + H₂O
Q. Why are curd and sour food substances not kept in brass and copper vessels?
Ans: Curd and sour food substances contain acids. Acids react with brass or copper vessels and can spoil both the food and the vessel.
Q. Why do acids show acidic nature?
Ans: Acids produce H+ions in aqueous solution. Therefore, they show an acidic nature.
Q. What is meant by dilution of an acid or a base?
Ans: When an acid or base is mixed with water, the concentration of ions per unit volume decreases. This process is called dilution of an acid or base.
Q. Why does an aqueous solution of an acid conduct electricity?
Ans: An aqueous solution of an acid releases H? ions. Therefore, it conducts electricity.
Q. How does the concentration of hydronium ions (H3O) change when an acid solution is diluted?
Ans: The concentration of hydronium ions (H3O) decreases.
Q. How does the concentration of hydroxide ions (OH) change when more base is dissolved in sodium hydroxide solution?
Ans: The concentration of hydroxide ions (OH) increases.
Q. Why does dry HCl not change the colour of dry litmus paper?
Ans: Dry HCl does not release H? ions. Therefore, it cannot change the colour of dry litmus paper.
Q. What are strong acids and strong bases?
Ans: If a solution contains a large amount of H⁺ ions, it is called a strong acid; if it contains a large amount of OH⁺ ions, it is called a strong base.
Q. When does tooth decay occur?
Ans: Tooth decay occurs when the pH value in the mouth falls below 5.5.
Q. Write one method to prevent tooth decay.
Ans: Tooth decay can be prevented by using toothpaste to clean teeth, as toothpaste is basic in nature.
Q. Why do we feel irritation and pain when a bee stings?
Ans: When a bee stings, it releases methanoic acid from its sting. Therefore, irritation and pain are felt.
Q. What can be used to get relief from irritation and pain caused by a bee sting?
Ans: Baking soda.
Q. What is the nature of salts formed from a strong acid and a weak base?
Ans: Acidic in nature.
Q. What is the nature of salts formed from a strong base and a weak acid?
Ans: Basic in nature.
Q. What is brine?
Ans: An aqueous solution of sodium chloride is called brine.
Q. Write two uses of hydrogen.
Ans: (a) As a fuel.
(b) In the manufacture of artificial butter (hydrogenation of oils).
Q. Write two uses of chlorine.
Ans: (a) In water purification.
(b) As a disinfectant.
Q. Write two uses of NaOH.
Ans: (a) For removing grease from metals.
(b) In the manufacture of soap and detergents.
Q. How is bleaching powder prepared?
Ans: Bleaching powder is prepared by passing chlorine over slaked lime.
Q. Write the reaction involved in the preparation of bleaching powder.
Ans: Ca(OH)2 + Cl₂ → CaOCl₂ + H₂O
Q. What are weak acids and weak bases?
Ans: If a solution contains a small amount of H⁺ions, it is called a weak acid; if it contains a small amount of OH⁺ions, it is called a weak base.
Q. When does acid rain occur?
Ans: Acid rain occurs when the pH of rainwater falls below 5.6.
Q. What is indigestion?
Ans: When excess acid is produced in the stomach, it causes a burning sensation and pain. This is called indigestion.
Q. Write one use of bleaching powder.
Ans: As an oxidising agent in chemical industries or for disinfecting drinking water.
Q. Write one use of baking soda.
Ans: For preparing baking powder or in soda-acid fire extinguishers.
Q. How is washing soda prepared?
Ans: Washing soda is prepared by recrystallisation of sodium carbonate.
Q. Write one use of washing soda.
Ans: It is used to remove the permanent hardness of water.
Q. Why are curd and sour food substances not kept in brass and copper vessels?
Ans: Curd and sour food substances contain acids. These acids react with brass and copper and spoil the food. Therefore, such foods are not kept in these vessels.
Q. You are given three test tubes. One contains distilled water, and the other two contain an acid solution and a basic solution, respectively. If you are given only red litmus paper, how will you identify the contents of each test tube? HSLC ’19
Ans: Take three red litmus papers.
Now, the test tube in which the litmus turns purple contains water.
The test tube in which the litmus turns blue contains a base.
The test tube in which there is no change contains an acid.
Q. A metallic compound reacts with dilute hydrochloric acid, producing effervescence. The gas evolved extinguishes a burning candle. If one of the compounds formed is calcium chloride, write the balanced equation.
Ans: CaCO₃+ 2HCl → CaCl₂+ CO₂+ H₂O
Q. Why do aqueous solutions of HCl and HNO3 show acidic character, whereas solutions of compounds like alcohol and glucose do not?
Ans: In aqueous solution, they ionize to produce hydrogen ions (H⁺). Therefore, they show acidic character. On the other hand, alcohol and glucose do not ionize to produce H? ions in aqueous solution, so they do not show acidic character.
Q. Why does an aqueous solution of an acid conduct electricity?
Ans: Acids ionize in aqueous solution to produce hydrogen ions. Therefore, they conduct electricity.
Q. Why does dry HCl gas not change the colour of dry litmus paper?
Ans: Dry HCl gas does not change the colour of dry litmus paper because it does not produce H+ions in the absence of water. Acids show their acidic nature only in aqueous solution. Since there is no water, HCl gas cannot ionize, so the litmus paper shows no colour change.
Q. Why is it advised to add acid to water while diluting, and not water to acid?
Ans: Mixing water with acid releases a large amount of heat, which may break the container and cause burns. Therefore, water is not added to acid; instead, acid is added to water.
Q. How does the concentration of hydronium ions change when an acid is diluted?
Ans: When acid is mixed with water, the concentration of H3O+ ions per unit volume decreases, making the acid dilute.
Q. How does the concentration of hydroxide ions change when more base is dissolved in the sodium hydroxide solution?
Ans: The concentration of hydroxide ions per unit volume increases.
Q. Under what soil conditions should a farmer apply quicklime (calcium oxide), slaked lime (calcium hydroxide), or chalk (calcium carbonate)?
Ans: These should be applied when the soil is too acidic to neutralise the acidity.
Q. What is the common name of the compound CaOCl₂?
Ans: Bleaching powder.
Q. What happens when sodium hydrogen carbonate is heated? Write the reaction.
Ans: On heating sodium hydrogen carbonate, sodium carbonate, carbon dioxide, and water are produced.
2NaHCO₃ → Na₂CO₃+ CO₂+ H₂O
Q. Write the reaction between plaster of Paris and water.
Ans: 2CaSO₄·½H₂O + H₂O → 2CaSO₄·2H₂O
Q. A solution turns red litmus blue. Its pH may be— HSLC ’16, ’18
(a) 1
(b) 4
(c) 5
(d) 10
Ans: (d) 10
Q. A solution reacts with powdered eggshell to produce a gas. The gasturns limewater milky. The solution contains— HSLC ’15, ’19
(a) NaCl
(b) HCl
(c) LiCl
(d) KCl
Ans: (b) HCl
Q. 10 mL of a NaOH solution completely neutralises 8 mL of an HCl solution. If 200 mL of the same NaOH solution is taken, the volume of HCl solution (same concentration) required will be—
(a) 4 mL
(b) 8 mL
(c) 12 mL
(d) 160 mL
Ans: (d) 160 mL
Q. Which type of medicine is used to treat indigestion? HSLC ’17
(a) Antibiotic
(b) Painkiller
(c) Antacid
(d) Antiseptic
Ans: (c) Antacid
Q. Write the word equations and balanced equations for the following reactions when—
(a) Dilute sulphuric acid reacts with zinc granules.
(b) Dilute hydrochloric acid reacts with magnesium ribbon.
(c) Dilute sulphuric acid reacts with aluminium powder.
(d) Dilute hydrochloric acid reacts with iron filings.
Ans:
(a) Sulphuric acid + Zinc → Zinc sulphate + Hydrogen
H₂SO₄ + Zn → ZnSO₄ + H₂
(b) Hydrochloric acid + Magnesium → Magnesium chloride + Hydrogen
2HCl + Mg → MgCl₂ + H₂
(c) Sulphuric acid + Aluminium → Aluminium sulphate + Hydrogen
3H₂SO₄ + 2Al → Al₂(SO₄)₃ + 3H₂
(d) Hydrochloric acid + Iron → Iron(II) chloride + Hydrogen
2HCl + Fe → FeCl₂ + H₂
Q. Compounds like alcohol and glucose also contain hydrogen, but they are not classified as acids. Describe an experiment to prove this.
Ans: Take a solution of glucose or alcohol in a beaker. Insert two nails into the solution using a cork. Connect the nails to a 6-volt battery, a bulb, and a switch. When the switch is turned on, the bulb does not glow. This shows that hydrogen ions (H⁺) are not produced in the solution. Therefore, alcohol and glucose are not classified as acids.
Q. Why does distilled water not conduct electricity, whereas rainwater does?
Ans: Rainwater contains H⁺ions, so it conducts electricity. On the other hand, distilled water does not contain sufficient H? ions, so it does not conduct electricity.
Q. Why do acids not show acidic behaviour in the absence of water?
Ans: In the absence of water, acids do not form hydronium ions (H₃O⁺).
Therefore, they do not show acidic behaviour.
Q. Five solutions A, B, C, D, and E have pH values 4, 1, 11, 7, and 9, respectively, when tested with a universal indicator. Identify which solution is—
(a) Neutral
(b) Strongly basic
(c) Strongly acidic
(d) Weakly acidic
(e) Weakly basic
Arrange the pH values in increasing order of hydrogen ion concentration.
Ans:
(a) D – Neutral
(b) C – Strongly basic
(c) B – Strongly acidic
(d) A – Weakly acidic
(e) E – Weakly basic
Increasing order of hydrogen ion concentration: 11 < 9 < 7 < 4 < 1
Q. Two test tubes, A and B, contain equal lengths of magnesium ribbon. Hydrochloric acid (HCl) is added to test tube A, and acetic acid is added to test tube B. Both acids have the same concentration and volume. In which test tube will effervescence occur more rapidly, and why?
Ans: Effervescence will occur more rapidly in test tube A. This is because HCl is a stronger acid than CH3COOH, so the reaction between HCl and magnesium is faster.
Q. A milkman adds a small amount of baking soda to fresh milk.
(a) Why does he make the pH of the milk slightly basic from about 6?
(b) Why does this milk take longer to turn into curd?
Ans:
(a) Adding baking soda makes the milk slightly basic, which prevents it from spoiling quickly.
(b) Lactic acid formed in milk reacts with baking soda and gets neutralised. Therefore, more lactic acid is required for curd formation, so it takes more time for the milk to turn into curd.
Q. What is a neutralisation reaction? Give two examples. Also, write two important uses of each of washing soda and baking soda.
Ans: When an acid reacts with a base to produce salt and water, the reaction is called a neutralisation reaction.
Examples:
(a) HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)
(b) 2NaOH(aq) + H₂SO₄(aq) → Na₂SO₄(aq) + 2H₂O(l)
Uses of washing soda:
(a) Used to remove permanent hardness of water.
(b) Used in the manufacture of glass, soap, and detergents.
Uses of baking soda:
(a) Used in baking powder.
(b) Used in soda-acid fire extinguishers.
Q. What is an alkali?
Ans: Substances that dissolve in water to produce bases are called alkalis.
Q. Write the names of two oxides that produce bases when they react with water.
Ans:
(a) Magnesium oxide
(b) Calcium oxide
Q. What is a litmus solution? What is its colour in neutral condition?
Ans: Litmus is a natural dye extracted from lichens.
In neutral condition, its colour is purple.
Q. Which type of medicine is used to treat indigestion? HSLC ’17
(a) Antibiotic
(b) Painkiller
(c) Antacid
(d) Antiseptic
Ans: (c) Antacid
Q. Why are food storage containers coated with tin and not with zinc? HSLC ’18
(a) Zinc is costlier than tin
(b) Zinc has a higher melting point than tin
(c) Zinc is more reactive than tin
(d) Zinc is less reactive than tin
Ans: (c) Zinc is more reactive than tin
Q. What is the common name of the compound CaOCl2?
Ans: Bleaching powder.
Q. What is pH?
Ans: pH is defined as the negative logarithm of hydronium ion (H3O+)concentration.
Q. What is a neutralisation reaction? Give two examples. HSLC ’19
Ans: When an acid reacts with a base to form a salt and water, the reaction is called a neutralisation reaction.
Examples:
NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)
2KOH(aq) + H₂SO₄(aq) → K₂SO₄(aq) + 2H₂O(l)
Q. What is the chemical name of baking soda? How is it prepared? Write the chemical equation. HSLC ’18
Ans: The chemical name of baking soda is sodium hydrogen carbonate(NaHCO₃). It is prepared by passing CO₂ through ammoniated brine.
NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃
Q. You are given two solutions, A and B. The pH of A is 3, and the pH of B is 4. In which solution is the concentration of hydrogen ions higher? Which solution is acidic and which is basic? HSLC ’17
Ans: Solution A has a higher concentration of hydrogen ions. Both A and B are acidic since their pH values are less than 7.
Q. You are given two solutions, A and B. The pH of A is 6, and the pH of B is 4. In which solution is the concentration of hydrogen ions higher? Which solution is acidic and which is basic? HSLC ’15, ’20
Ans: Solution B has a higher concentration of hydrogen ions.
Both A and B are acidic.
Q. Name the sodium compound used to convert hard water into soft water, HSLC’16
Ans: Sodium carbonate (Na₂CO₃).
Q. Why should plaster of Paris be stored in a moisture-proof container? Explain.
Ans: Plaster of Paris reacts with moisture to form gypsum again. As a result, it becomes unfit for use.
Multiple Choice Questions (MCQ)
Q. 10 mL of a NaOH solution completely neutralises 8 mL of an HCl solution. If 20 mL of the same NaOH solution is taken, the volume of HCl solution required will be—
(a) 4 mL
(b) 8 mL
(c) 16 mL
(d) 12 mL
Ans: (c) 16 mL
Q. Which type of medicine is used to treat indigestion?
(a) Antibiotic
(b) Painkiller
(c) Antacid
(d) Antiseptic
Ans: (c) Antacid
Q. When an acid reacts with a metal carbonate or metal hydrogencarbonate, the gas usually evolved is—
(a) Carbon dioxide gas
(b) Hydrogen gas
(c) Nitrogen gas
(d) Chlorine gas
Ans: (a) Carbon dioxide gas
Q. The acid present in an ant’s sting is—
(a) Methanoic acid
(b) Tartaric acid
(c) Citric acid
(d) Acetic acid
Ans: (a) Methanoic acid
Q. The acid present in tamarind is—
(a) Methanoic acid
(b) Tartaric acid
(c) Citric acid
(d) Acetic acid
Ans: (b) Tartaric acid
Q. The acid present in curd is—
(a) Methanoic acid
(b) Lactic acid
(c) Citric acid
(d) Oxalic acid
Ans: (b) Lactic acid
Q. A solution reacts with powdered eggshell to produce a gas which turns limewater milky. The solution is—
(a) NaCl
(b) HCl
(c) LiCl
(d) KCl
Ans: (b) HCl
Q. Which of the following is not a natural indicator?
(a) Turmeric
(b) Red cabbage leaves
(c) Phenolphthalein
(d) Hibiscus petals
Ans: (c) Phenolphthalein
Q. The pH of pure water is—
(a) 7
(b) 5
(c) 10
(d) 1
Ans: (a) 7
Q. The colour of litmus in neutral condition is—
(a) Yellow
(b) Red
(c) Purple
(d) Pink
Ans: (c) Purple
Q. When dilute hydrochloric acid is added to copper oxide, the colour of the solution becomes—
(a) Reddish green
(b) Blue-green
(c) Orange-green
(d) Purple-green
Ans: (b) Blue-green
Q. The cation present in acids is—
(a) H⁺
(b) OH⁺
(c) Cl⁺
(d) Na⁺
Ans: (a) H⁺
Q. A solution turns red litmus blue. Its pH may be—
(a) 1
(b) 4
(c) 5
(d) 10
Ans: (d) 10
Q. The pH range of our body is—
(a) 5.0 to 6.0
(b) 5.5 to 5.6
(c) 8.1 to 8.7
(d) 7.0 to 7.8
Ans: (d) 7.0 to 7.8
Q. The acid present in lemon is—
(a) Methanoic acid
(b) Tartaric acid
(c) Citric acid
(d) Acetic acid
Ans: (c) Citric acid
Q. Which of the following is an antacid?
(a) Magnesium chloride
(b) Magnesium hydroxide
(c) Magnesium oxide
(d) Magnesium sulphate
Ans: (b) Magnesium hydroxide
