In this chapter, you will learn
- —Understand the chemical properties of acids and bases with equations
- —Explain what happens when acids or bases react with metals, metal carbonates, and metal bicarbonates
- —Describe acid-base neutralisation and its applications
- —Understand the pH scale and the role of hydrogen ions in defining acidity
- —Explain the importance of pH in everyday life (digestion, soil, tooth decay, stings)
- —Learn the preparation, properties, and uses of common salts — NaOH, NaHCO₃, Na₂CO₃, bleaching powder, and plaster of Paris
- —Understand the concept of water of crystallisation
Acid-Base Indicators and Their Reactions
Indicators are substances that change colour (or smell) in the presence of acids or bases. They help us identify whether a solution is acidic or basic.
| Indicator | In Acid | In Base |
|---|---|---|
| Litmus (natural) | Turns red | Turns blue |
| Methyl orange | Red (pink) | Yellow |
| Phenolphthalein | Colourless | Pink |
| Turmeric | Yellow (no change) | Reddish-brown |
Olfactory indicators change smell in acids or bases. Examples: onion (loses smell in base), vanilla (loses smell in base), clove oil.
Key Points:
- Acids taste sour (Latin: acidus = sour) and turn blue litmus red
- Bases taste bitter, feel soapy, and turn red litmus blue
- Litmus is extracted from lichens
Exam Tip
Litmus colour changes are tested every year. Acid → red litmus stays red, blue litmus turns red. Base → red litmus turns blue, blue litmus stays blue. Phenolphthalein is colourless in acid, pink in base.
Common Mistake
Students say 'acid turns litmus red'. Be specific: acid turns BLUE litmus red. Red litmus remains red in acid (no change).
Chemical Properties of Acids
Acids show several characteristic chemical reactions:
1. Acids + Metals → Salt + Hydrogen gas
2HCl(aq) + Zn(s) → ZnCl₂(aq) + H₂(g)↑
H₂SO₄(aq) + Zn(s) → ZnSO₄(aq) + H₂(g)↑
Hydrogen gas burns with a pop sound (test with a burning matchstick)
2. Acids + Metal Carbonates → Salt + Water + CO₂
2HCl(aq) + Na₂CO₃(s) → 2NaCl(aq) + H₂O(l) + CO₂(g)↑
CO₂ turns lime water milky: Ca(OH)₂ + CO₂ → CaCO₃↓ + H₂O
3. Acids + Metal Bicarbonates → Salt + Water + CO₂
HCl(aq) + NaHCO₃(s) → NaCl(aq) + H₂O(l) + CO₂(g)↑
4. Acids + Bases → Salt + Water (Neutralisation)
NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)
This is an exothermic reaction
5. Acids + Metal Oxides → Salt + Water
CuO(s) + 2HCl(aq) → CuCl₂(aq) + H₂O(l)
Black CuO dissolves in HCl to give blue-green CuCl₂ solution
Common acids: HCl (hydrochloric), H₂SO₄ (sulphuric), HNO₃ (nitric), CH₃COOH (acetic/ethanoic), H₂CO₃ (carbonic)
Exam Tip
The reaction of acid + metal carbonate/bicarbonate giving CO₂ is tested very frequently. Remember the lime water test: CO₂ turns lime water milky (CaCO₃ precipitate).
Common Mistake
Students forget that metal oxides are basic in nature. That is why acids react with metal oxides — it is essentially an acid-base reaction.
Chemical Properties of Bases
Bases also show characteristic chemical reactions:
1. Bases + Metals → Salt + Hydrogen gas (with certain reactive metals)
2NaOH(aq) + Zn(s) → Na₂ZnO₂(aq) + H₂(g)↑
Zinc and aluminium react with bases (they are amphoteric metals)
2. Bases + Non-metal Oxides → Salt + Water
2NaOH(aq) + CO₂(g) → Na₂CO₃(aq) + H₂O(l)
Non-metal oxides (CO₂, SO₂) are acidic in nature, so they react with bases
3. Bases + Acids → Salt + Water (Neutralisation)
Ca(OH)₂(aq) + 2HCl(aq) → CaCl₂(aq) + 2H₂O(l)
Common bases: NaOH (sodium hydroxide), KOH (potassium hydroxide), Ca(OH)₂ (calcium hydroxide / slaked lime), Mg(OH)₂ (milk of magnesia), NH₄OH (ammonium hydroxide)
Key Distinctions:
- Base: A substance that reacts with acid to give salt and water (may or may not dissolve in water)
- Alkali: A base that dissolves in water to give OH⁻ ions. All alkalis are bases, but not all bases are alkalis
- Example: Cu(OH)₂ is a base (insoluble) but not an alkali. NaOH is both a base and an alkali (soluble)
Exam Tip
Remember: metal oxides are BASIC, non-metal oxides are ACIDIC. This explains why base + non-metal oxide works just like base + acid. The base–alkali distinction is commonly asked.
Common Mistake
Students think only acids react with metals. Some metals like zinc and aluminium also react with strong bases like NaOH to release hydrogen gas.
Acids and Bases in Water — Role of H⁺ and OH⁻ Ions
Acids produce hydrogen ions (H⁺) in water. Bases produce hydroxide ions (OH⁻) in water. These ions are responsible for their properties.
Acid in water: HCl → H⁺(aq) + Cl⁻(aq)
Base in water: NaOH → Na⁺(aq) + OH⁻(aq)
Why is water necessary?
- Acids do NOT show acidic behaviour without water (dry HCl gas does not change litmus colour)
- H⁺ ions cannot exist alone — they combine with H₂O to form hydronium ions: H₃O⁺
- HCl + H₂O → H₃O⁺ + Cl⁻
Dilution of acids and bases:
- Mixing acid or base with water is called dilution
- Dilution is highly exothermic — the solution heats up
- Always add acid to water, never water to acid — adding water to concentrated acid can cause violent splashing due to rapid heat generation
Strong vs Weak Acids/Bases:
| Type | Example | Ionisation |
|---|---|---|
| Strong acid | HCl, H₂SO₄, HNO₃ | Completely ionise in water |
| Weak acid | CH₃COOH, H₂CO₃ | Partially ionise in water |
| Strong base | NaOH, KOH, Ca(OH)₂ | Completely ionise in water |
| Weak base | NH₄OH | Partially ionise in water |
Exam Tip
NCERT favourite: 'Why does dry HCl gas not change the colour of dry litmus paper?' Answer: HCl needs water to produce H⁺ (H₃O⁺) ions which are responsible for acidic behaviour.
Common Mistake
Students add water to acid during dilution. The correct procedure is to ALWAYS add acid to water slowly with stirring. Adding water to concentrated acid causes violent spattering.
The pH Scale
The pH scale measures how acidic or basic a solution is, on a scale from 0 to 14.
pH = 0–6 → Acidic (lower pH = stronger acid)
pH = 7 → Neutral (pure water)
pH = 8–14 → Basic/Alkaline (higher pH = stronger base)
pH of common substances:
| Substance | Approx. pH | Nature |
|---|---|---|
| Gastric juice (HCl) | ~1.2 | Strongly acidic |
| Lemon juice | ~2.2 | Acidic |
| Pure water | 7 | Neutral |
| Blood | 7.35–7.45 | Slightly basic |
| Milk of magnesia | ~10 | Basic |
| NaOH (1M) | 14 | Strongly basic |
pH is measured using:
- Universal indicator — shows a range of colours for different pH values
- pH paper — dipped in solution, colour is matched with a chart
Note: pH stands for "potenz of Hydrogen" (German: potenz = power). It indicates the concentration of H⁺ (or H₃O⁺) ions in a solution.
Exam Tip
Memorise: pH < 7 = acid, pH = 7 = neutral, pH > 7 = base. Blood pH is 7.35–7.45. Gastric juice is ~1.2. These values are commonly asked in MCQs.
Common Mistake
Students think pH 0 means 'no acid'. pH 0 actually means VERY strong acid (highest H⁺ concentration). Lower pH = more acidic, not less.
Importance of pH in Everyday Life
The pH value plays a crucial role in many biological and chemical processes:
1. pH in our digestive system:
- Stomach produces HCl (pH ~1.2) for digestion
- Excess acid causes acidity/indigestion
- Remedy: Antacids like milk of magnesia (Mg(OH)₂) neutralise excess acid
2. pH and tooth decay:
- Tooth enamel (calcium phosphate) is the hardest substance in the body
- It starts corroding when mouth pH falls below 5.5
- Bacteria in the mouth produce acids from food (especially sugar)
- Prevention: Brushing with basic toothpaste neutralises the acid
3. pH of soil:
- Plants grow best in a specific pH range
- If soil is too acidic → add quicklime (CaO) or slaked lime (Ca(OH)₂)
- If soil is too basic → add organic matter / compost
4. pH and self-defence by animals/plants:
- Bee sting: injects formic acid (acidic) → apply baking soda (basic) for relief
- Wasp sting: injects basic substance → apply vinegar (acidic) for relief
- Nettle leaf sting: injects methanoic acid → rub dock leaves (basic) for relief
5. pH in industrial processes:
- Factories must neutralise acidic/basic waste before discharge into water bodies
- Acid rain (pH < 5.6) damages buildings, crops, and aquatic life
Exam Tip
Bee sting = acidic (apply baking soda). Wasp sting = basic (apply vinegar). Tooth decay starts below pH 5.5. These are very common 1-2 mark questions.
Common Mistake
Students mix up bee sting and wasp sting remedies. Remember: Bee = acid → apply Base (baking soda). Wasp = base → apply acid (Vinegar). B-B and W-V.
Salts — Formation, Families, and pH
A salt is an ionic compound formed by the neutralisation reaction between an acid and a base.
Acid + Base → Salt + Water
The salt contains the cation from the base and the anion from the acid.
Family of Salts:
| Acid | Salt family | Examples |
|---|---|---|
| HCl | Chlorides | NaCl, KCl, CaCl₂ |
| H₂SO₄ | Sulphates | Na₂SO₄, CuSO₄, ZnSO₄ |
| HNO₃ | Nitrates | NaNO₃, KNO₃, Ca(NO₃)₂ |
| H₂CO₃ | Carbonates | Na₂CO₃, CaCO₃, NaHCO₃ |
pH of salt solutions depends on the strength of acid and base:
- Strong acid + Strong base → Neutral salt (pH = 7). Example: NaCl
- Strong acid + Weak base → Acidic salt (pH < 7). Example: NH₄Cl
- Weak acid + Strong base → Basic salt (pH > 7). Example: Na₂CO₃
Exam Tip
Know which acid produces which salt family: HCl → chlorides, H₂SO₄ → sulphates, HNO₃ → nitrates. Also remember: strong acid + strong base = neutral salt (pH 7).
Common Mistake
Students think all salts are neutral. The pH of a salt solution depends on the relative strengths of the parent acid and base. Na₂CO₃ is basic, NH₄Cl is acidic.
Common Salt (NaCl) and Chemicals from Common Salt
Common salt (NaCl) is the raw material for making many important chemicals.
1. Sodium Hydroxide (NaOH) — Caustic Soda:
Chlor-alkali process: Electrolysis of brine (concentrated NaCl solution)
2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g)
At cathode: NaOH + H₂ | At anode: Cl₂
Uses of NaOH: soaps, detergents, paper, artificial fibres (rayon)
2. Bleaching Powder — CaOCl₂ (Calcium oxychloride):
Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O
Slaked lime + Chlorine → Bleaching powder
Uses: bleaching cotton/textiles, disinfecting drinking water, oxidising agent
3. Baking Soda — NaHCO₃ (Sodium hydrogen carbonate):
NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃
Uses: baking (releases CO₂ on heating → makes bread fluffy), antacid, fire extinguishers, soda-acid type
4. Washing Soda — Na₂CO₃·10H₂O (Sodium carbonate decahydrate):
From baking soda:
2NaHCO₃ → Na₂CO₃ + H₂O + CO₂ (on heating)
Na₂CO₃ + 10H₂O → Na₂CO₃·10H₂O (recrystallisation)
Uses: cleaning agent (laundry), softening hard water, glass/soap/paper manufacturing
5. Plaster of Paris — CaSO₄·½H₂O (Calcium sulphate hemihydrate):
From gypsum:
CaSO₄·2H₂O →373 K CaSO₄·½H₂O + 1½H₂O
Gypsum (heated at 373 K) → Plaster of Paris
Setting of Plaster of Paris:
CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O (gypsum — hard solid)
POP absorbs water and sets into a hard mass (gypsum) — used in casts, moulds, statues
Uses: fracture casts, moulds, statues, decorative items, fireproofing, chalk
Exam Tip
Plaster of Paris and its setting reaction is a board exam favourite. Also memorise: NaHCO₃ (baking soda) vs Na₂CO₃ (washing soda) — students often confuse these two.
Common Mistake
Students write the formula of washing soda as Na₂CO₃ (without water of crystallisation). The correct formula is Na₂CO₃·10H₂O. Similarly, plaster of Paris is CaSO₄·½H₂O, not CaSO₄.
Water of Crystallisation
Water of crystallisation is the fixed number of water molecules present in one formula unit of a salt in its crystalline form.
Important examples:
| Salt | Formula | Water molecules | Colour |
|---|---|---|---|
| Copper sulphate (blue vitriol) | CuSO₄·5H₂O | 5 | Blue |
| Ferrous sulphate (green vitriol) | FeSO₄·7H₂O | 7 | Green |
| Washing soda | Na₂CO₃·10H₂O | 10 | White |
| Gypsum | CaSO₄·2H₂O | 2 | White |
NCERT Activity — Heating copper sulphate crystals:
- Blue CuSO₄·5H₂O crystals are heated in a dry test tube
- The blue colour disappears → white powder (anhydrous CuSO₄) is formed
- Water droplets appear on the cooler parts of the test tube
- On adding water to the white powder, blue colour returns
CuSO₄·5H₂O →Heat CuSO₄ + 5H₂O
(Blue crystals → White powder + Water)
Key Point: Water of crystallisation gives the crystal its shape, colour, and structure. Removing it changes the physical appearance of the salt.
Exam Tip
CuSO₄·5H₂O (blue → white on heating) is the most asked activity. Also know: gypsum has 2H₂O, POP has ½H₂O, washing soda has 10H₂O.
Common Mistake
Students confuse anhydrous CuSO₄ (white) with hydrated CuSO₄·5H₂O (blue). The colour is due to water of crystallisation. Removing water removes the blue colour.