Science Class- 10
Class 10 - Chapter 2: Acids, Bases and Salts
Chapter: Acids, Bases and Salts – Solutions
1. A solution turns red litmus blue, its pH is likely to be
Answer: (d) 10
A solution that turns red litmus blue is basic in nature. Bases have pH greater than 7. Therefore, pH 10 is the correct answer.
2. A solution reacts with crushed egg-shells to give a gas that turns lime-water milky. The solution contains
Answer: (b) HCl
Egg-shells contain calcium carbonate (CaCO3).
CaCO3 + 2HCl → CaCl2 + H2O + CO2
The CO2 produced turns lime-water milky.
3. 10 mL of NaOH solution is completely neutralised by 8 mL of HCl solution. If 20 mL of the same NaOH solution is taken, the amount of HCl required will be
Answer: (d) 16 mL
10 mL NaOH requires 8 mL HCl.
20 mL NaOH requires:
= (20 × 8) / 10
= 16 mL
4. Which type of medicine is used for treating indigestion?
Answer: (c) Antacid
Antacids neutralise excess acid present in the stomach and provide relief from indigestion.
5. Write word equations and balanced chemical equations.
(a) Dilute sulphuric acid reacts with zinc granules.
Word Equation:
Zinc + Sulphuric acid → Zinc sulphate + Hydrogen
Balanced Equation:
Zn + H2SO4 → ZnSO4 + H2
(b) Dilute hydrochloric acid reacts with magnesium ribbon.
Word Equation:
Magnesium + Hydrochloric acid → Magnesium chloride + Hydrogen
Balanced Equation:
Mg + 2HCl → MgCl2 + H2
(c) Dilute sulphuric acid reacts with aluminium powder.
Word Equation:
Aluminium + Sulphuric acid → Aluminium sulphate + Hydrogen
Balanced Equation:
2Al + 3H2SO4 → Al2(SO4)3 + 3H2
(d) Dilute hydrochloric acid reacts with iron filings.
Word Equation:
Iron + Hydrochloric acid → Iron chloride + Hydrogen
Balanced Equation:
Fe + 2HCl → FeCl2 + H2
6. Compounds such as alcohols and glucose also contain hydrogen but are not categorised as acids. Describe an activity to prove it.
Take separate aqueous solutions of glucose, alcohol and hydrochloric acid.
Set up a circuit using a bulb and electrodes.
- HCl solution conducts electricity and the bulb glows.
- Glucose solution does not conduct electricity.
- Alcohol solution does not conduct electricity.
This shows that acids produce H+ ions in water, whereas glucose and alcohol do not. Therefore, they are not acids.
7. Why does distilled water not conduct electricity, whereas rain water does?
Distilled water is almost free from dissolved salts and ions, so it does not conduct electricity.
Rain water contains dissolved gases and mineral salts that produce ions. Therefore, rain water conducts electricity.
8. Why does dry HCl gas not show acidic behaviour in the absence of water?
Acids show acidic behaviour only when they release H+ ions.
Dry HCl gas does not ionise in the absence of water. Hence, it does not produce H+ ions and does not show acidic behaviour.
9. Five solutions A, B, C, D and E have pH values 4, 1, 11, 7 and 9 respectively.
A = 4, B = 1, C = 11, D = 7, E = 9
(a) Neutral?
Answer: D (pH = 7)
(b) Strongly alkaline?
Answer: C (pH = 11)
(c) Strongly acidic?
Answer: B (pH = 1)
(d) Weakly acidic?
Answer: A (pH = 4)
(e) Weakly alkaline?
Answer: E (pH = 9)
(f) Increasing order of hydrogen-ion concentration
C < E < D < A < B
(pH increases → H+ concentration decreases)
10. Equal lengths of magnesium ribbons are taken in test tubes A and B. HCl is added to A and CH3COOH is added to B.
Fizzing will occur more vigorously in test tube A.
Hydrochloric acid is a strong acid and ionises completely in water, producing more H+ ions.
Acetic acid is a weak acid and ionises only partially.
Therefore, magnesium reacts faster with HCl and produces hydrogen gas more rapidly.
11. Fresh milk has a pH of 6. How will the pH change as it turns into curd?
As milk changes into curd, lactic acid is produced.
The acidity increases and the pH decreases below 6.
12. A milkman adds a very small amount of baking soda to fresh milk.
(a) Why does he shift the pH from 6 to slightly alkaline?
Baking soda is basic. It increases the pH of milk and makes it slightly alkaline.
(b) Why does this milk take longer to set as curd?
Curd formation requires an acidic medium.
Since the milk is slightly alkaline, more lactic acid must be produced before curd formation begins. Hence, curd setting takes longer.
13. Plaster of Paris should be stored in a moisture-proof container. Explain why.
Plaster of Paris reacts with moisture and changes into gypsum.
CaSO4·½H2O + 1½H2O → CaSO4·2H2O
This causes it to harden and lose its usefulness. Therefore, it should be stored in moisture-proof containers.
14. What is a neutralisation reaction? Give two examples.
A reaction between an acid and a base to form salt and water is called a neutralisation reaction.
Example 1:
HCl + NaOH → NaCl + H2O
Example 2:
H2SO4 + 2KOH → K2SO4 + 2H2O
15. Give two important uses of washing soda and baking soda.
(a) Washing Soda (Na2CO3·10H2O)
- Used for cleaning and washing clothes.
- Used for removing permanent hardness of water.
(b) Baking Soda (NaHCO3)
- Used in baking powder to make cakes and bread soft and fluffy.
- Used as an antacid to relieve acidity and indigestion.