Acids, Bases and Salts — Class 10 Science
What CBSE examines here (2026-27). This chapter is unchanged by rationalisation — every subsection, from indicators through to Plaster of Paris, is still in the current NCERT text, across five in-text question sets (19 questions) and one end-of-chapter Exercise (15 questions, not 20 — a stale count in older manifests). One correction below: bleaching powder's formula is written Ca(ClO)₂ in the current book (the older "Ca(OCl)Cl" form some guides use is not what NCERT's own exercise question names).
"Acids sting; bases are slippery. From your stomach acid to soap lather, chemistry is in everything."
1. About the Chapter
This chapter covers:
- Acids: properties, examples
- Bases: properties, examples
- Salts: formation and common types
- pH scale: acidity-basicity measurement
- Neutralisation: acid + base = salt + water
- Indicators: how we detect acid/base
Why Important
- Stomach acid, lemon, vinegar — daily acids
- Soap, antacid, ash — daily bases
- Common salt (NaCl) used everywhere
- Industrial applications
2. Acids
Definition
An acid is a substance that gives H⁺ ions (hydrogen ions) in solution.
Properties
- Sour taste (lemon, vinegar)
- Turn blue litmus RED
- React with metals to produce HYDROGEN gas
- React with bases to form salt + water
- Conduct electricity in solution (because of H⁺ ions)
- pH < 7
Common Acids
Strong Acids (fully ionise in water):
- HCl — Hydrochloric acid (stomach acid; muriatic acid)
- H₂SO₄ — Sulphuric acid (battery acid; very corrosive)
- HNO₃ — Nitric acid (used in fertilisers)
Weak Acids (partially ionise):
- CH₃COOH — Acetic acid (vinegar)
- C₆H₈O₇ — Citric acid (lemon, orange)
- H₂CO₃ — Carbonic acid (soda water)
- HCOOH — Formic acid (ant sting)
Reactions
With Metals: Acid + Metal → Salt + Hydrogen
- Zn + 2HCl → ZnCl₂ + H₂↑
With Carbonates: Acid + Carbonate → Salt + Water + CO₂
- 2HCl + Na₂CO₃ → 2NaCl + H₂O + CO₂↑
With Bases (Neutralisation): Acid + Base → Salt + Water
- HCl + NaOH → NaCl + H₂O
3. Bases
Definition
A base is a substance that gives OH⁻ ions (hydroxide ions) in solution.
(Alkalis are bases that are SOLUBLE in water.)
Properties
- Bitter taste (don't taste! Just for definition)
- Slippery/soapy feel
- Turn red litmus BLUE
- Conduct electricity in solution
- pH > 7
Common Bases
Strong Bases (alkalis):
- NaOH — Sodium hydroxide (caustic soda)
- KOH — Potassium hydroxide
- Ca(OH)₂ — Calcium hydroxide (slaked lime)
Weak Bases:
- NH₄OH — Ammonium hydroxide
- Mg(OH)₂ — Milk of magnesia (antacid)
Common Examples
- Soap (basic)
- Toothpaste (mildly basic)
- Antacids (Mg(OH)₂)
- Ash (basic)
4. Indicators
What Are Indicators?
Substances that CHANGE COLOUR in acids and bases.
Common Indicators
| Indicator | In Acid | In Base |
|---|---|---|
| Litmus | Red | Blue |
| Methyl orange | Red/pink | Yellow |
| Phenolphthalein | Colourless | Pink |
| Turmeric | Yellow | Red |
| China rose | Pink | Green |
Olfactory Indicators
Smell changes in acids/bases (e.g., onion, vanilla, clove).
Universal Indicator
Gives a RANGE of colours corresponding to different pH values.
5. The pH Scale
What is pH?
pH = 'Power of Hydrogen' — measure of acidity/basicity.
Scale: 0 to 14
- pH = 0: very strong acid
- pH = 7: NEUTRAL (pure water)
- pH = 14: very strong base
Examples
| Substance | pH (approx) |
|---|---|
| Battery acid (H₂SO₄) | 0.5 |
| Lemon juice | 2.4 |
| Vinegar | 3 |
| Tomato juice | 4 |
| Black coffee | 5 |
| Milk | 6.5 |
| Pure water | 7 |
| Blood | 7.4 |
| Sea water | 8 |
| Baking soda | 9 |
| Milk of magnesia | 10 |
| Ammonia | 11 |
| Lime water | 12 |
| NaOH | 14 |
Importance of pH
In Body:
- Stomach acid: pH ~1-3 (digestion)
- Blood: pH ~7.4 (slightly basic; tightly regulated)
In Plants:
- Most prefer pH 6-7
- Acidic soil → add lime
- Basic soil → add organic compost
In Daily Life:
- Tooth enamel: dissolves below pH 5.5
- Sugar produces acid in mouth → enamel decay
- Solution: brush, fluoride
Acid Rain:
- pH < 5.6 (normal rain ~5.6)
- Caused by SO₂, NO₂ from pollution
- Damages buildings, crops
6. Salts
Definition
A salt is formed when an acid reacts with a base (neutralisation).
Acid + Base → Salt + Water
Types of Salts
- Acidic salt: pH < 7 (e.g., NH₄Cl)
- Basic salt: pH > 7 (e.g., Na₂CO₃)
- Neutral salt: pH = 7 (e.g., NaCl)
Family of Salts
Salts with same positive or negative ion belong to same family:
- Sodium family: NaCl, NaNO₃, NaCO₃
- Chloride family: NaCl, KCl, CaCl₂
7. Common Salts (Important)
Sodium Chloride (Common Salt) — NaCl
- Most common salt
- Obtained from sea water by evaporation
- Used in food, preserving, industry
- Indian states major producers: Tamil Nadu, Gujarat, Rajasthan
Sodium Hydroxide (Caustic Soda) — NaOH
- Made by electrolysis of brine (NaCl solution)
- Chlor-alkali process
- Used in soap, paper, detergents
Bleaching Powder — Ca(ClO)₂
- Made by passing Cl₂ over slaked lime
- Used in textile/paper bleaching, disinfecting water
Baking Soda — NaHCO₃
- Sodium bicarbonate
- Used in baking (releases CO₂ when heated)
- Antacid (neutralises stomach acid)
- Fire extinguisher
Washing Soda — Na₂CO₃·10H₂O
- Sodium carbonate decahydrate
- Used in cleaning, glass manufacturing
- Softens hard water
Plaster of Paris — CaSO₄·½H₂O
- Made by heating gypsum
- Used in casts for broken bones, decorations, walls
- Sets quickly with water
Gypsum — CaSO₄·2H₂O
- Source for plaster of Paris
- Used in cement, fertilisers
8. Acid Rain
What is It?
Rain with pH < 5.6 due to dissolved SO₂, NO₂ (from pollution).
Sources
- Burning fossil fuels (vehicles, factories)
- Volcanic eruptions
- Indian cities heavily affected (Delhi, Kolkata)
Effects
- Damages crops, forests
- Erodes buildings (especially marble — Taj Mahal at risk)
- Acidifies lakes (kills fish)
- Corrodes metal infrastructure
Solutions
- Reduce vehicle emissions
- Renewable energy
- Smoke stack scrubbers
- Reforestation
9. Worked Examples
Example 1: Identify
What turns blue litmus red?
- ACIDS turn blue litmus RED.
- Bases turn red litmus blue.
Example 2: Reaction
HCl + NaOH → ?
- Neutralisation: HCl + NaOH → NaCl + H₂O
- Salt: sodium chloride; water
Example 3: pH
A solution has pH = 3. Is it acidic or basic?
- pH < 7 → ACIDIC
- Specifically, moderately strong acid.
Example 4: Identification
Substance turns turmeric paper red. What is it?
- Turmeric red in BASE.
- So the substance is BASIC.
10. Common Mistakes
-
Confusing acid/base properties
- Acids: sour, blue litmus → red.
- Bases: bitter/soapy, red litmus → blue.
-
pH scale confusion
- pH < 7: acid. pH > 7: base. pH = 7: neutral.
-
Bleaching powder formula
- Ca(ClO)₂ (calcium hypochlorite chloride).
-
Plaster of Paris vs gypsum
- Gypsum: CaSO₄·2H₂O
- Plaster of Paris: CaSO₄·½H₂O (less water)
-
Acids react with all metals
- Most active metals, NOT noble metals (gold, platinum).
11. Indian Context
Chemical Industry
India is major producer of:
- Soda ash (washing soda)
- Caustic soda
- Common salt (top 3 globally)
- Fertilisers (heavy acid use)
Indian Heroes
- P.C. Ray (1861-1944) — founder of Indian chemistry; established Bengal Chemicals
- G.N. Ramachandran — biochemistry
12. Conclusion
Acids, Bases, and Salts are EVERYWHERE in our lives:
- Acids: stomach, food preservatives, fertilisers
- Bases: soap, antacids, cleaning
- Salts: cooking, industry, medicine
Master:
- Properties of acids and bases
- pH scale
- Common indicators
- Neutralisation reactions
- Common salts and their uses
This is a HIGH-MARK chapter for board exam. Practice 15+ problems.
Chemistry: from your tongue to industrial plants — all linked.
