CBSE Class 10 Science Chapter 2 Notes — Acids, Bases and Salts

Acids — Definition and Properties

What are Acids?

Acids are substances that produce hydrogen ions (H⁺) when dissolved in water.

More precisely, these H⁺ ions combine with water to form hydronium ions (H₃O⁺).

Arrhenius Definition: An acid is a substance that dissociates in water to give H⁺ ions.

Properties of Acids

• Sour taste (e.g., lemon juice is citric acid, vinegar is acetic acid)
• Turn blue litmus red
• React with metals to produce hydrogen gas
• React with metal carbonates to produce CO₂
• pH less than 7

Important Acids and Their Sources

Bases — Definition and Properties

What are Bases?

Bases are substances that produce hydroxide ions (OH⁻) when dissolved in water. Bases that dissolve in water are called alkalis.

Properties of Bases

• Bitter taste
• Soapy or slippery feel
• Turn red litmus blue
• React with acids (neutralisation)
• pH greater than 7

How Acids and Bases React

Acid + Metal

Acid + Metal → Salt + Hydrogen gas

2HCl + Zn → ZnCl₂ + H₂↑

Test for hydrogen gas: A burning splinter held near the mouth of the test tube producing hydrogen gives a 'pop' sound.

Acid + Metal Carbonate or Metal Bicarbonate

Acid + Metal carbonate → Salt + Water + Carbon dioxide

HCl + Na₂CO₃ → 2NaCl + H₂O + CO₂↑ HCl + NaHCO₃ → NaCl + H₂O + CO₂↑

Test for CO₂: Pass the gas through lime water — it turns milky.

Acid + Base (Neutralisation)

HCl + NaOH → NaCl + H₂O (releases heat)

Base + Non-metallic Oxide

Non-metallic oxides are acidic in nature. Ca(OH)₂ + CO₂ → CaCO₃ + H₂O (lime water turns milky)

Why Acids Conduct Electricity in Water

Acids and bases conduct electricity because they dissociate into ions in water.

• HCl → H⁺ + Cl⁻ (in water)
• NaOH → Na⁺ + OH⁻ (in water)

Key conclusion: Acids and bases show acidic/basic properties only in the presence of water. Dry HCl gas does NOT change litmus because no H⁺ ions are formed without water.

Indicators and the pH Scale

Common Indicators

pH Scale

• pH stands for "potenz Hydrogen" (power of hydrogen)
• Range: 0 to 14
• pH = 7 → Neutral (pure water)
• pH < 7 → Acidic (lower the pH, stronger the acid)
• pH > 7 → Basic/Alkaline (higher the pH, stronger the base)

pH of Common Substances

pH in Everyday Life

• Tooth decay: Bacteria produce acids when they act on food sugars. When pH in the mouth falls below 5.5, enamel corrodes. Toothpastes are slightly basic (pH 8–9) to neutralise the acid.
• Soil pH: Farmers add lime (CaO) or slaked lime (Ca(OH)₂) to neutralise acidic soil.
• Bee/ant sting: Formic acid. Treat by applying baking soda (basic) to neutralise.
• Wasp sting: Alkaline. Treat by applying vinegar (acidic) to neutralise.

Salts — Types and Formation

A salt is formed when an acid and a base react together (neutralisation). Salts can be:

Family of Salts

Salts related to the same acid form a "family":

• HCl forms chlorides: NaCl, KCl, ZnCl₂
• H₂SO₄ forms sulphates: Na₂SO₄, CuSO₄
• HNO₃ forms nitrates: NaNO₃, Ca(NO₃)₂

Important Salts in Detail

1. Common Salt — Sodium Chloride (NaCl)

• Obtained from sea water by evaporation, or from rock salt mines
• Used in food, preserving fish/meat, making chemicals

Chemical reactions of NaCl:

Chlor-alkali process (electrolysis of brine): 2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g)

• Chlorine gas → bleaching powder, PVC, disinfectants
• Hydrogen gas → fuel, margarine production
• Sodium hydroxide → soap, paper, dyes

2. Baking Soda — Sodium Hydrogen Carbonate (NaHCO₃)

• Mild, non-corrosive base
• Used in baking (releases CO₂ which makes bread/cakes rise)
• Used in antacids (to neutralise excess acid in stomach)
• Used in fire extinguishers

Heating baking soda: 2NaHCO₃ →(Δ) Na₂CO₃ + H₂O + CO₂

3. Washing Soda — Sodium Carbonate (Na₂CO₃·10H₂O)

• Obtained by heating baking soda, then treating with water
• Used in glass, soap, paper making
• Used as cleaning agent (removes grease)
• Acts as a water softener (removes hardness)

Na₂CO₃(s) + 10H₂O → Na₂CO₃·10H₂O (washing soda)

When washing soda is exposed to air, it loses water of crystallisation: Na₂CO₃·10H₂O → Na₂CO₃·H₂O + 9H₂O (this is called efflorescence)

4. Bleaching Powder — Ca(OCl)Cl

• Made by passing chlorine over slaked lime (Ca(OH)₂)
• Ca(OH)₂ + Cl₂ → Ca(OCl)Cl + H₂O
• Used for disinfecting water, bleaching cotton/linen, making chloroform

5. Baking Powder

• A mixture of baking soda (NaHCO₃) and a mild acid like tartaric acid
• When heated in baking, CO₂ is released — this makes the dough rise
• The tartaric acid neutralises the Na₂CO₃ that would otherwise make the product taste soapy

6. Plaster of Paris — CaSO₄·½H₂O

2CaSO₄·2H₂O →(373 K) 2CaSO₄·½H₂O + 3H₂O

CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O

• Made by heating gypsum (CaSO₄·2H₂O) at 373 K:
• When mixed with water, it sets to hard gypsum:
• Used in setting fractured bones, making statues, in construction

Important: Plaster of Paris must be stored in a moisture-free container, else it absorbs water and becomes useless (hardens before use).

Water of Crystallisation

Many salts contain water molecules as part of their crystal structure. This water is called water of crystallisation.

When a hydrated salt is heated, it loses water → becomes anhydrous (e.g., blue CuSO₄·5H₂O → white anhydrous CuSO₄). Adding water restores the blue colour.

Board Exam Tips

• 8–10 marks typically come from this chapter in CBSE boards — it's one of the highest-scoring chapters
• Know ALL the common salts: NaCl, NaHCO₃, Na₂CO₃, Ca(OCl)Cl, CaSO₄·½H₂O — their names, formulae, and uses
• Always state the pH range when describing acidic, basic, or neutral solutions
• "Dry HCl does not turn blue litmus red" — this is a frequently asked 2-mark question; always explain WHY (no H⁺ ions without water)
• For the chlor-alkali process, mention all THREE products: NaOH, Cl₂, and H₂
• The water of crystallisation table is memorise-worthy

📚 Lakhmir Singh Science Class 10 — Chapter 2 Detailed Solutions

📝 Oswaal CBSE Question Bank — Chapter-wise Previous Year Papers

NCERT Exercise Solutions

Intext Questions

Page 18, Q1: You have been provided with three test tubes. One contains distilled water, and the other two contain an acidic solution and a basic solution. If you are given only red litmus paper, how will you identify the contents?

• Dip red litmus paper in each test tube.
• The one that turns blue → basic solution
• The other two (remain red) → distilled water and acidic solution
• To distinguish water from acid: test with a pH indicator or taste (water is neutral pH 7, acid has pH < 7). Alternatively, test with blue litmus — it turns red in acid but not in water.

Page 22, Q1: Why does dry HCl gas not change the colour of a dry litmus paper?

Page 25, Q2: Fresh milk has a pH of 6. How do you think the pH will change as it turns to curd? Will curd be more acidic or less acidic?

Page 28, Q1: What effect does the concentration of H⁺ ions have on the nature of the solution?

A concentrated acid has very high H⁺ ion concentration and a very low pH.

• Higher H⁺ concentration → lower pH → more acidic
• Lower H⁺ concentration → higher pH → more basic
• Equal H⁺ and OH⁻ concentrations → pH 7 → neutral

Chapter-End Exercise

Q1. A student needs to test whether a given solution is acidic or basic. Describe the procedure using litmus paper.

• Place a drop of the solution on a piece of blue litmus paper and a separate piece of red litmus paper.
• If blue litmus turns red → solution is acidic
• If red litmus turns blue → solution is basic
• If neither litmus paper changes colour → solution is neutral

Q2. What happens when an acid reacts with a metal? Give an example.

Example: Zn + H₂SO₄ → ZnSO₄ + H₂↑ (Zinc + Dilute sulphuric acid → Zinc sulphate + Hydrogen gas)

The hydrogen gas evolved can be tested by holding a burning splint near the test tube — it burns with a 'pop' sound.

Q3. What happens when an acid reacts with a metal carbonate or bicarbonate?

Metal carbonate + Acid → Salt + H₂O + CO₂↑ Example: Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂↑

Metal bicarbonate + Acid → Salt + H₂O + CO₂↑ Example: NaHCO₃ + HCl → NaCl + H₂O + CO₂↑

The CO₂ gas produced turns lime water milky: Ca(OH)₂ + CO₂ → CaCO₃↓ + H₂O (white precipitate)

Q4. What is neutralisation reaction? Give an example.

Acid + Base → Salt + Water

Example: NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l) (Sodium hydroxide + Hydrochloric acid → Sodium chloride + Water)

Q5. What is a strong acid? Give two examples.

Examples: 1. Hydrochloric acid (HCl): HCl → H⁺ + Cl⁻ 2. Sulphuric acid (H₂SO₄): H₂SO₄ → 2H⁺ + SO₄²⁻

Weak acids (like acetic acid, carbonic acid) only partially dissociate.

Q6. What is the pH scale? What does a pH of 7 mean?

• pH 0–6: acidic (lower = more acidic)
• pH 7: neutral (equal H⁺ and OH⁻ ions — e.g., pure water)
• pH 8–14: basic (higher = more basic)

A pH of 7 means the solution is neutral — neither acidic nor basic.

Q7. How is baking powder different from baking soda?

Baking powder avoids the bitter taste that Na₂CO₃ would give because the tartaric acid neutralises it.

Q8. What is the common name and chemical name of the compound with formula NaHCO₃?

• Common name: Baking soda
• Chemical name: Sodium hydrogen carbonate (or sodium bicarbonate)