NCERT Solutions Class 10 Science Chapter 2 – Acids, Bases and Salts
Complete NCERT Solutions for Class 10 Science Chapter 2 – Acids, Bases and Salts. This chapter covers properties of acids and bases, pH scale, salts, and their chemical reactions with detailed explanations.
Chapter Overview
| Topic | Weightage |
|---|---|
| Properties of Acids and Bases | 3-4 marks |
| Chemical Reactions | 3-4 marks |
| pH Scale and Indicators | 2-3 marks |
| Salts and their Properties | 3-4 marks |
| Total Chapter Weightage | 8-10 marks |
Key Concepts
1. Acids
- Definition: Substances that produce H⁺ ions in water
- Properties: Sour taste, turn blue litmus red, conduct electricity
- Examples: HCl, H₂SO₄, HNO₃, CH₃COOH
2. Bases
- Definition: Substances that produce OH⁻ ions in water
- Properties: Bitter taste, soapy feel, turn red litmus blue
- Examples: NaOH, KOH, Ca(OH)₂, NH₄OH
3. pH Scale
- Ranges from 0 to 14
- pH < 7: Acidic
- pH = 7: Neutral
- pH > 7: Basic/Alkaline
Important Chemical Reactions
Acid + Metal → Salt + Hydrogen
2HCl + Zn → ZnCl₂ + H₂↑
H₂SO₄ + Fe → FeSO₄ + H₂↑
Acid + Metal Carbonate → Salt + CO₂ + Water
2HCl + Na₂CO₃ → 2NaCl + H₂O + CO₂↑
2HCl + CaCO₃ → CaCl₂ + H₂O + CO₂↑
Acid + Base → Salt + Water (Neutralization)
HCl + NaOH → NaCl + H₂O
H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O
Acid + Metal Oxide → Salt + Water
2HCl + CuO → CuCl₂ + H₂O
NCERT Exercise Solutions
Question 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: Put red litmus paper in all three test tubes.
Step 2: The solution that turns red litmus to blue is the basic solution.
Step 3: Now dip the blue litmus (from step 2) in the remaining two solutions.
Step 4: The solution that turns blue litmus to red is the acidic solution.
Step 5: The remaining solution that shows no change is distilled water.
Question 2
Why should curd and sour substances not be kept in brass and copper vessels?
Curd and sour substances contain acids. These acids react with brass and copper metals to form toxic compounds (salts of copper) which are harmful for health. The reaction is:
Cu + 2CH₃COOH → Cu(CH₃COO)₂ + H₂↑
The copper acetate formed is poisonous.
Question 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?
Gas liberated: Hydrogen gas (H₂)
Example: Zn + H₂SO₄ → ZnSO₄ + H₂↑
Test: Bring a burning matchstick near the gas. Hydrogen burns with a pop sound (characteristic test for H₂).
Question 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 CO₂.
Since calcium chloride is formed, the metal compound A is calcium carbonate (CaCO₃).
Balanced equation:
CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂↑
Question 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., dissociate in water to produce hydrogen ions (H⁺):
HCl → H⁺ + Cl⁻
These H⁺ ions are responsible for acidic character.
Alcohol and glucose, though containing hydrogen, do not dissociate in water to produce H⁺ ions. Hence, they do not show acidic character.
Important Salts
Common Salt (NaCl)
- Source: Sea water, rock salt deposits
- Uses: Cooking, preservation, chemical industry
- Chemicals made: NaOH, Cl₂, Na₂CO₃, NaHCO₃
Baking Soda (NaHCO₃)
- Preparation: NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃
- Uses: Baking, antacid, fire extinguisher
Washing Soda (Na₂CO₃.10H₂O)
- Preparation: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂
- Uses: Cleaning, water softening, glass making
Plaster of Paris (CaSO₄.½H₂O)
- Preparation: CaSO₄.2H₂O → CaSO₄.½H₂O + 1½H₂O
- Uses: Plaster casts, statues, decoration
Important Points for Exam
- Acids turn blue litmus red; bases turn red litmus blue
- Universal indicator shows different colors for different pH values
- Our stomach contains HCl (gastric juice)
- Antacids neutralize excess stomach acid
- Tooth decay starts when mouth pH falls below 5.5
- Bees inject formic acid; wasps inject alkaline liquid
Key Concepts: Acids, Bases and Salts
Acids, Bases and Salts is a very important chapter for CBSE Class 10 boards. Expect 6–9 marks from this chapter alone, covering definitions, reactions, indicators, the pH scale, and common salts.
Definitions
- Acid: A substance that releases H+ ions (or H3O+ in water). Sour taste, turns blue litmus red, corrosive. Examples: HCl, H2SO4, HNO3, CH3COOH, citric acid, lactic acid.
- Base: A substance that releases OH- ions in water. Bitter taste, soapy feel, turns red litmus blue. Examples: NaOH, KOH, Ca(OH)2, NH4OH, Mg(OH)2.
- Alkali: A base that is soluble in water. All alkalis are bases, but not all bases are alkalis.
- Salt: Formed when an acid reacts with a base (neutralisation). Acids + Base → Salt + Water
Indicators
- Litmus: Natural indicator from lichen. Blue in base, red in acid.
- Phenolphthalein: Colourless in acid, pink/magenta in base.
- Methyl Orange: Red in acid, yellow in base.
- Turmeric: Yellow in acid/neutral, red/brown in base.
The pH Scale
pH measures the concentration of H+ ions in a solution. Scale: 0 to 14.
- pH less than 7 = Acidic
- pH = 7 = Neutral (pure water)
- pH greater than 7 = Basic (alkaline)
- Lower pH = stronger acid; Higher pH = stronger base
- Each step on the pH scale = 10× change in H+ concentration
pH in daily life: Human blood (7.35–7.45), stomach acid (1–2), lemon juice (2.5), baking soda (8.3), bleach (12–13).
Reactions of Acids and Bases
- Acid + Metal: Salt + Hydrogen gas. 2HCl + Zn → ZnCl2 + H2 (H2 burns with a pop sound)
- Acid + Metal Carbonate/Bicarbonate: Salt + CO2 + Water. HCl + Na2CO3 → NaCl + CO2 + H2O (CO2 turns lime water milky)
- Acid + Metal Oxide (Base): Salt + Water. HCl + CuO → CuCl2 + H2O (neutralisation)
- Acid + Base: Salt + Water. HCl + NaOH → NaCl + H2O (neutralisation — exothermic)
Important Salts and Their Uses
- Common Salt (NaCl): Cooking, preservation, manufacture of NaOH, Cl2, HCl (chlor-alkali process)
- Washing Soda (Na2CO3.10H2O): Cleaning, removing hardness of water, manufacture of glass and soap
- Baking Soda (NaHCO3): Antacid (neutralises excess stomach acid), baking (releases CO2 to make bread/cake rise), fire extinguisher
- Bleaching Powder (CaOCl2): Disinfectant for water, bleaching cotton and linen, disinfecting swimming pools
- Plaster of Paris (CaSO4.1/2H2O): Setting broken bones, moulds, casts, blackboard chalk
Important Board Exam Questions
- Why should acids never be added to water? Why not water to acid?
- A student tested a substance with litmus paper — red litmus turned blue, blue litmus remained blue. What type of substance is it?
- What is neutralisation? Write the ionic equation.
- Why is baking soda used in baking? What gas does it release?
- What is the difference between washing soda and baking soda?
- How is bleaching powder made? Write the reaction.
- What happens when Plaster of Paris is mixed with water?
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