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 |
Try it: find the pH of six samples
Six unlabelled test tubes, the same six samples as the board practical. Test them with universal indicator, read each pH off the chart, confirm with litmus, phenolphthalein and methyl orange — then work out which tube holds which.
Find the pH of six samples
Procedure · 5 steps
- Choose universal indicator, then tap test tube A to add a few drops.
- Test all six tubes with universal indicator.
- Match each colour with the pH chart and write down the pH.
- Confirm with litmus, phenolphthalein or methyl orange which tubes are acidic and which are basic.
- Name the sample in each tube.
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.
