Acids, Bases and Salts - NCERT Solutions

Text Questions
Exercise Questions
Key Concepts
Activities

Text Questions

1. You have been provided with three test tubes. One of them contains distilled water and the other two contain an acidic solution and a basic solution, respectively. If you are given only red litmus paper, how will you identify the contents of each test tube?

Step 1: Dip the red litmus paper in each test tube one by one.

Step 2: The test tube in which the red litmus paper turns blue contains the basic solution.

Step 3: Now use the same blue litmus paper (obtained from the basic solution) to test the remaining two solutions.

Step 4: The solution that turns the blue litmus paper back to red is the acidic solution.

Step 5: The solution that does not change the color of either red or blue litmus paper is distilled water (neutral).

2. Why should curd and sour substances not be kept in brass and copper vessels?

Curd and other sour substances contain acids. When these acids come in contact with brass and copper vessels, they react with the metals to form toxic compounds. These compounds can contaminate the food and make it unfit for consumption, potentially causing health issues.

Chemical reaction: Acid + Metal → Salt + Hydrogen gas

For example: 2CH3COOH + Cu → (CH3COO)2Cu + H2

3. Which gas is usually liberated when an acid reacts with a metal? Illustrate with an example. How will you test for the presence of this gas?

When an acid reacts with a metal, hydrogen gas is usually liberated.

Example: Zinc reacts with dilute sulphuric acid to form zinc sulphate and hydrogen gas.

Zn(s) + H2SO4(aq) → ZnSO4(aq) + H2(g)

Test for hydrogen gas:

  1. Collect the gas in a soap bubble.
  2. Bring a burning candle near the gas-filled bubble.
  3. If the gas burns with a 'pop' sound, it confirms the presence of hydrogen gas.
4. Metal compound A reacts with dilute hydrochloric acid to produce effervescence. The gas evolved extinguishes a burning candle. Write a balanced chemical equation for the reaction if one of the compounds formed is calcium chloride.

The gas that extinguishes a burning candle is carbon dioxide (CO2).

Since one of the products is calcium chloride, the metal compound A must be calcium carbonate.

Balanced chemical equation:

CaCO3(s) + 2HCl(aq) → CaCl2(aq) + H2O(l) + CO2(g)

5. Why do HCl, HNO3, etc., show acidic characters in aqueous solutions while solutions of compounds like alcohol and glucose do not show acidic character?

HCl, HNO3, etc., show acidic character in aqueous solutions because they dissociate in water to produce H+ ions:

HCl(aq) → H+(aq) + Cl-(aq)

HNO3(aq) → H+(aq) + NO3-(aq)

Compounds like alcohol and glucose do not dissociate in water to produce H+ ions. They exist as molecules in solution and hence do not show acidic character.

6. Why does an aqueous solution of an acid conduct electricity?

An aqueous solution of an acid conducts electricity because acids dissociate in water to produce ions (H+ and the corresponding anion). These ions are responsible for the conduction of electric current through the solution.

For example: HCl(aq) → H+(aq) + Cl-(aq)

7. Why does dry HCl gas not change the colour of the dry litmus paper?

Dry HCl gas does not change the color of dry litmus paper because:

  1. HCl gas does not produce H+ ions in the absence of water.
  2. For acidic behavior, H+ ions are required, which are produced only when HCl dissolves in water: HCl + H2O → H3O+ + Cl-
  3. In the dry state, HCl exists as molecules and not as ions.
8. While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?

It is recommended to add acid to water (and not water to acid) because:

  1. The process of dissolving an acid in water is highly exothermic (releases a lot of heat).
  2. If water is added to a concentrated acid, the heat generated may cause the mixture to splash out and cause burns.
  3. When acid is added slowly to water with constant stirring, the heat is distributed evenly and the risk of splashing is minimized.
9. How is the concentration of hydronium ions (H3O+) affected when a solution of an acid is diluted?

When a solution of an acid is diluted:

  1. The concentration of hydronium ions (H3O+) decreases.
  2. This is because the same number of H3O+ ions are now present in a larger volume of solution.
  3. As a result, the acidity of the solution decreases and the pH value increases (moves closer to 7).
10. How is the concentration of hydroxide ions (OH-) affected when excess base is dissolved in a solution of sodium hydroxide?

When excess base is dissolved in a solution of sodium hydroxide:

  1. The concentration of hydroxide ions (OH-) increases.
  2. This is because more base molecules dissociate to produce OH- ions.
  3. As a result, the basicity of the solution increases and the pH value increases (moves further away from 7 towards 14).
11. You have two solutions, A and B. The pH of solution A is 6 and pH of solution B is 8. Which solution has more hydrogen ion concentration? Which of this is acidic and which one is basic?

pH = -log[H+], so lower pH means higher hydrogen ion concentration.

Solution A (pH = 6) has higher hydrogen ion concentration than solution B (pH = 8).

Solution A (pH = 6) is acidic (pH < 7).

Solution B (pH = 8) is basic (pH > 7).

12. What effect does the concentration of H+(aq) ions have on the nature of the solution?

The concentration of H+(aq) ions determines the nature of the solution:

  • High concentration of H+ ions → Acidic solution (pH < 7)
  • Low concentration of H+ ions → Basic solution (pH > 7)
  • Equal concentration of H+ and OH- ions → Neutral solution (pH = 7)
13. Do basic solutions also have H+(aq) ions? If yes, then why are these basic?

Yes, basic solutions also have H+(aq) ions, but in very low concentration.

Basic solutions are basic because:

  1. They have a high concentration of OH- ions.
  2. The concentration of OH- ions is much higher than the concentration of H+ ions.
  3. In any aqueous solution, the product of H+ and OH- ion concentrations is constant (10-14 at 25°C).
14. Under what soil condition do you think a farmer would treat the soil of his fields with quick lime (calcium oxide) or slaked lime (calcium hydroxide) or chalk (calcium carbonate)?

A farmer would treat the soil with these substances when the soil is too acidic. Acidic soil is not suitable for the growth of many plants.

These basic substances neutralize the excess acid in the soil:

  • CaO + 2H+ → Ca2+ + H2O
  • Ca(OH)2 + 2H+ → Ca2+ + 2H2O
  • CaCO3 + 2H+ → Ca2+ + H2O + CO2
15. What is the common name of the compound Ca(OCl)Cl?

The common name of Ca(OCl)Cl is bleaching powder.

16. Name the substance which on treatment with chlorine yields bleaching powder.

Dry slaked lime [Ca(OH)2] on treatment with chlorine yields bleaching powder.

Chemical equation: Ca(OH)2 + Cl2 → Ca(OCl)Cl + H2O

17. Name the sodium compound which is used for softening hard water.

Sodium carbonate (Na2CO3.10H2O), commonly known as washing soda, is used for softening hard water.

18. What will happen if a solution of sodium hydrocarbonate is heated? Give the equation of the reaction involved.

When sodium hydrogencarbonate (baking soda) is heated, it decomposes to give sodium carbonate, water and carbon dioxide.

Chemical equation: 2NaHCO3(s) → Na2CO3(s) + H2O(g) + CO2(g)

19. Write an equation to show the reaction between Plaster of Paris and water.

Plaster of Paris (CaSO4.½H2O) reacts with water to form gypsum (CaSO4.2H2O).

Chemical equation: CaSO4.½H2O + 1½H2O → CaSO4.2H2O

Exercise Questions

1. A solution turns red litmus blue, its pH is likely to be (a) 1 (b) 4 (c) 5 (d) 10

Answer: (d) 10

Explanation: A solution that turns red litmus blue is basic. Basic solutions have pH greater than 7. Among the given options, only pH = 10 is greater than 7.

2. A solution reacts with crushed egg-shells to give a gas that turns lime-water milky. The solution contains (a) NaCl (b) HCl (c) LiCl (d) KCl

Answer: (b) HCl

Explanation: Egg-shells contain calcium carbonate. When calcium carbonate reacts with an acid, it produces carbon dioxide gas which turns lime water milky. Among the given options, only HCl is an acid.

Reaction: CaCO3 + 2HCl → CaCl2 + H2O + CO2

3. 10 mL of a solution of NaOH is found to be completely neutralised by 8 mL of a given solution of HCl. If we take 20 mL of the same solution of NaOH, the amount HCl solution (the same solution as before) required to neutralise it will be (a) 4 mL (b) 8 mL (c) 12 mL (d) 16 mL

Answer: (d) 16 mL

Explanation: According to the law of chemical equivalence, the amount of acid required to neutralize a base is directly proportional to the amount of base.

If 10 mL NaOH requires 8 mL HCl, then 20 mL NaOH will require (20/10) × 8 = 16 mL HCl.

4. Which one of the following types of medicines is used for treating indigestion? (a) Antibiotic (b) Analgesic (c) Antacid (d) Antiseptic

Answer: (c) Antacid

Explanation: Indigestion is caused by excess acid in the stomach. Antacids are basic substances that neutralize the excess acid and provide relief.

5. Write word equations and then balanced equations for the reaction taking place when:

(a) dilute sulphuric acid reacts with zinc granules.

Word equation: Sulphuric acid + Zinc → Zinc sulphate + Hydrogen

Balanced equation: H2SO4(aq) + Zn(s) → ZnSO4(aq) + H2(g)

(b) dilute hydrochloric acid reacts with magnesium ribbon.

Word equation: Hydrochloric acid + Magnesium → Magnesium chloride + Hydrogen

Balanced equation: 2HCl(aq) + Mg(s) → MgCl2(aq) + H2(g)

(c) dilute sulphuric acid reacts with aluminium powder.

Word equation: Sulphuric acid + Aluminium → Aluminium sulphate + Hydrogen

Balanced equation: 3H2SO4(aq) + 2Al(s) → Al2(SO4)3(aq) + 3H2(g)

(d) dilute hydrochloric acid reacts with iron filings.

Word equation: Hydrochloric acid + Iron → Ferrous chloride + Hydrogen

Balanced equation: 2HCl(aq) + Fe(s) → FeCl2(aq) + H2(g)

6. Compounds such as alcohols and glucose also contain hydrogen but are not categorised as acids. Describe an Activity to prove it.

Activity to show that alcohols and glucose do not behave as acids:

  1. Take solutions of alcohol (ethanol) and glucose in separate beakers.
  2. Fix two nails on a cork and place the cork in a 100 mL beaker.
  3. Connect the nails to the two terminals of a 6-volt battery through a bulb and a switch.
  4. Pour the alcohol solution into the beaker and switch on the current.
  5. Observe that the bulb does not glow.
  6. Repeat the experiment with glucose solution. The bulb does not glow in this case either.
  7. Now repeat with dilute HCl solution. The bulb glows in this case.

Conclusion: Alcohol and glucose solutions do not conduct electricity, which means they do not produce H+ ions in solution. Hence, they are not acids.

7. Why does distilled water not conduct electricity, whereas rain water does?

Distilled water does not conduct electricity because it does not contain any ions (H+ or OH-). It consists only of water molecules.

Rain water conducts electricity because it contains dissolved carbon dioxide from the air, which forms carbonic acid. This carbonic acid dissociates to produce H+ and HCO3- ions that can conduct electricity.

CO2 + H2O → H2CO3 → H+ + HCO3-

8. Why do acids not show acidic behaviour in the absence of water?

Acids do not show acidic behavior in the absence of water because:

  1. Acidic behavior is due to the presence of H+ ions.
  2. In the absence of water, acids do not dissociate to produce H+ ions.
  3. For example, dry HCl gas does not produce H+ ions and hence does not show acidic properties.
  4. When dissolved in water, HCl dissociates as: HCl + H2O → H3O+ + Cl-
9. Five solutions A,B,C,D and E when tested with universal indicator showed pH as 4,1,11,7 and 9, respectively. Which solution is (a) neutral? (b) strongly alkaline? (c) strongly acidic? (d) weakly acidic? (e) weakly alkaline?

(a) Neutral: Solution D (pH = 7)

(b) Strongly alkaline: Solution C (pH = 11)

(c) Strongly acidic: Solution B (pH = 1)

(d) Weakly acidic: Solution A (pH = 4)

(e) Weakly alkaline: Solution E (pH = 9)

10. Equal lengths of magnesium ribbons are taken in test tubes A and B. Hydrochloric acid (HCl) is added to test tube A, while acetic acid (CH3COOH) is added to test tube B. Amount and concentration taken for both the acids are same. In which test tube will the fizzing occur more vigorously and why?

Fizzing will occur more vigorously in test tube A (with HCl).

Reason:

  1. HCl is a strong acid and completely dissociates in water to produce more H+ ions.
  2. Acetic acid is a weak acid and only partially dissociates in water, producing fewer H+ ions.
  3. The reaction rate depends on the concentration of H+ ions.
  4. Since HCl produces more H+ ions, the reaction with magnesium is faster and more vigorous.
11. Fresh milk has a pH of 6. How do you think the pH will change as it turns into curd? Explain your answer.

When milk turns into curd, its pH decreases (becomes more acidic).

Explanation:

  1. Fresh milk has pH around 6 (slightly acidic).
  2. When milk turns into curd, lactose (milk sugar) is converted into lactic acid by the action of bacteria.
  3. This increases the acidity of the milk, decreasing its pH to around 4-5.
12. A milkman adds a very small amount of baking soda to fresh milk. (a) Why does he shift the pH of the fresh milk from 6 to slightly alkaline? (b) Why does this milk take a long time to set as curd?

(a) The milkman adds baking soda (sodium hydrogencarbonate, NaHCO3) to fresh milk to make it slightly alkaline because:

  • Baking soda is a basic substance.
  • It neutralizes the slight acidity of fresh milk, making it slightly alkaline.

(b) This milk takes a long time to set as curd because:

  • The bacteria (Lactobacillus) that convert milk to curd work best in slightly acidic medium.
  • When the milk is made alkaline by adding baking soda, the bacteria cannot work efficiently.
  • This delays the formation of lactic acid and hence the setting of curd.
13. Plaster of Paris should be stored in a moisture-proof container. Explain why?

Plaster of Paris should be stored in a moisture-proof container because:

  1. Plaster of Paris (CaSO4.½H2O) has a strong tendency to absorb moisture from the atmosphere.
  2. When it comes in contact with moisture, it sets into a hard mass (gypsum) by combining with water.
  3. Chemical reaction: CaSO4.½H2O + 1½H2O → CaSO4.2H2O
  4. Once set, it cannot be used for molding purposes.
14. What is a neutralisation reaction? Give two examples.

A neutralization reaction is a chemical reaction between an acid and a base to form salt and water.

General equation: Acid + Base → Salt + Water

Examples:

  1. NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)
  2. 2KOH(aq) + H2SO4(aq) → K2SO4(aq) + 2H2O(l)
15. Give two important uses of washing soda and baking soda.

Uses of washing soda (Na2CO3.10H2O):

  1. It is used in glass, soap and paper industries.
  2. It is used for removing permanent hardness of water.

Uses of baking soda (NaHCO3):

  1. It is used in baking powder for making cakes and bread soft and spongy.
  2. It is used as an antacid to neutralize excess acid in the stomach.

Key Concepts

Acids

  • Sour in taste
  • Turn blue litmus red
  • Produce H+ ions in aqueous solution
  • pH less than 7
  • Examples: HCl, H2SO4, CH3COOH

Bases

  • Bitter in taste
  • Turn red litmus blue
  • Soapy to touch
  • Produce OH- ions in aqueous solution
  • pH greater than 7
  • Examples: NaOH, KOH, Ca(OH)2

Salts

  • Formed by neutralization reaction between acid and base
  • pH depends on the strength of acid and base used
  • Examples: NaCl, Na2CO3, CaSO4

pH Scale

  • Measures hydrogen ion concentration
  • Range: 0 to 14
  • pH = 7: Neutral
  • pH < 7: Acidic
  • pH > 7: Basic

Indicators

  • Substances that change color in acidic or basic medium
  • Natural indicators: Litmus, turmeric, red cabbage
  • Synthetic indicators: Phenolphthalein, methyl orange
  • Olfactory indicators: Onion, vanilla, clove oil

Important Chemical Compounds

  • Bleaching powder: Ca(OCl)Cl - Used for bleaching and disinfecting
  • Baking soda: NaHCO3 - Used in baking and as antacid
  • Washing soda: Na2CO3.10H2O - Used in glass and soap industries
  • Plaster of Paris: CaSO4.½H2O - Used for supporting fractured bones

Water of Crystallization

  • Fixed number of water molecules present in one formula unit of a salt
  • Example: CuSO4.5H2O (blue vitriol) has 5 water molecules
  • On heating, water of crystallization is lost and the salt becomes anhydrous

Activities

Activity 2.1: Testing acids and bases with indicators

Procedure:

  1. Collect solutions of HCl, H2SO4, HNO3, CH3COOH, NaOH, Ca(OH)2, KOH, Mg(OH)2, NH4OH.
  2. Put a drop of each solution on a watch-glass.
  3. Test with red litmus, blue litmus, phenolphthalein and methyl orange.
  4. Record observations in a table.

Observations:

Solution Red Litmus Blue Litmus Phenolphthalein Methyl Orange
HCl No change Red Colorless Red
NaOH Blue No change Pink Yellow
Activity 2.3: Reaction of acids with metals

Procedure:

  1. Take 5 mL of dilute H2SO4 in a test tube.
  2. Add a few pieces of zinc granules.
  3. Observe the surface of zinc granules - bubbles of hydrogen gas are formed.
  4. Pass the gas through soap solution - bubbles are formed.
  5. Take a burning candle near a gas-filled bubble - the gas burns with a pop sound.
  6. Repeat with HCl, HNO3 and CH3COOH.

Conclusion: Acids react with metals to produce hydrogen gas.

Activity 2.5: Reaction of metal carbonates with acids

Procedure:

  1. Take 0.5 g of Na2CO3 in test tube A and 0.5 g of NaHCO3 in test tube B.
  2. Add 2 mL of dilute HCl to both test tubes.
  3. Observe effervescence due to CO2 gas.
  4. Pass the gas through lime water - it turns milky due to formation of CaCO3.

Conclusion: Metal carbonates and hydrogencarbonates react with acids to produce CO2 gas.

Activity 2.6: Neutralization reaction

Procedure:

  1. Take 2 mL of dilute NaOH in a test tube and add 2 drops of phenolphthalein.
  2. Solution turns pink (basic).
  3. Add dilute HCl drop by drop with shaking.
  4. Pink color disappears when solution becomes neutral.
  5. Add a few drops of NaOH - pink color reappears.

Conclusion: Acid and base neutralize each other's effect.

Activity 2.8: Testing electrical conductivity

Procedure:

  1. Fix two nails on a cork and place in a beaker.
  2. Connect nails to a 6V battery through a bulb and switch.
  3. Pour dilute HCl in the beaker and switch on - bulb glows.
  4. Repeat with H2SO4 - bulb glows.
  5. Repeat with glucose and alcohol solutions - bulb does not glow.

Conclusion: Acids conduct electricity due to presence of ions, while glucose and alcohol do not.

Activity 2.11: Testing pH of common substances

Procedure:

  1. Collect samples of saliva (before and after meal), lemon juice, aerated drink, carrot juice, coffee, tomato juice, tap water, 1M NaOH, 1M HCl.
  2. Test each with pH paper.
  3. Record the color and approximate pH value.
  4. Classify as acidic, basic or neutral.

Observations:

Substance pH Nature
Lemon juice 2-3 Acidic
Tomato juice 4-5 Acidic
Tap water 7 Neutral
1M NaOH 14 Basic
Activity 2.15: Water of crystallization

Procedure:

  1. Heat a few crystals of copper sulphate in a dry boiling tube.
  2. Observe the color change from blue to white.
  3. Notice water droplets in the boiling tube.
  4. Add 2-3 drops of water on the heated sample - blue color is restored.

Conclusion: Copper sulphate crystals contain water of crystallization (5H2O) which is lost on heating.