Class 10 Science · Chapter 2
Acids, Bases and Salts
Everything you need to know — explained simply, with all questions answered!
Chemical Reactions
pH Scale
Salts & Uses
Everyday Chemistry
What's Inside?
Introduction: What Are Acids & Bases?
Acids
- Taste sour
- Turn blue litmus red
- pH < 7
- Produce H⁺ ions in water
- Examples: HCl, H₂SO₄, HNO₃, CH₃COOH
Bases
- Taste bitter, feel soapy
- Turn red litmus blue
- pH > 7
- Produce OH⁻ ions in water
- Examples: NaOH, Ca(OH)₂, KOH, Mg(OH)₂
Imp — Indicators
- Natural indicators: Litmus, turmeric, red cabbage, Hydrangea, Petunia, Geranium
- Synthetic indicators: Methyl orange, Phenolphthalein
- Olfactory indicators: Onion, vanilla, clove oil — their smell changes in acid/base
- Litmus is a purple dye extracted from lichen (division Thallophyta). Purple when neutral.
| Indicator | In Acid | In Base |
|---|---|---|
| Red Litmus | Stays Red | Turns Blue |
| Blue Litmus | Turns Red | Stays Blue |
| Phenolphthalein | Colourless | Pink |
| Methyl Orange | Red/Orange | Yellow |
| Turmeric | Yellow | Reddish-brown |
| Onion (olfactory) | Retains smell | Loses smell |
Chemical Reactions of Acids & Bases
Acids + Metals
When an acid reacts with a metal, it produces hydrogen gas (H₂) and a salt.
General Equation
Acid + Metal → Salt + Hydrogen gas (H₂↑)
Zinc + Sulphuric Acid
H₂SO₄(aq) + Zn(s) → ZnSO₄(aq) + H₂(g)↑
Zinc + HCl
2HCl(aq) + Zn(s) → ZnCl₂(aq) + H₂(g)↑
Test for H₂ gas: Hold a burning candle near the gas. It burns with a 'pop' sound confirming hydrogen.
Bases + Metals (Selective)
Not all metals react with bases. Zinc reacts with NaOH to produce sodium zincate and hydrogen.
Zinc + Sodium Hydroxide
2NaOH(aq) + Zn(s) → Na₂ZnO₂(aq) + H₂(g)↑ (Sodium Zincate)
Acids + Metal Carbonates / Hydrogencarbonates
These reactions produce a salt, CO₂ gas, and water. CO₂ turns lime water milky.
General Equation
Metal Carbonate/Hydrogencarbonate + Acid → Salt + CO₂↑ + H₂O
Na₂CO₃ + 2HCl
Na₂CO₃(s) + 2HCl(aq) → 2NaCl(aq) + H₂O(l) + CO₂(g)↑
NaHCO₃ + HCl
NaHCO₃(s) + HCl(aq) → NaCl(aq) + H₂O(l) + CO₂(g)↑
CO₂ + Lime Water (turns milky)
Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s)↓ + H₂O(l)
Excess CO₂ — solution turns clear again
CaCO₃(s) + H₂O(l) + CO₂(g) → Ca(HCO₃)₂(aq) [soluble]
Neutralisation Reaction (Acid + Base)
When acid and base react, they cancel each other's effect forming salt and water. This is neutralisation.
General
Acid + Base → Salt + Water
HCl + NaOH
HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)
Ionic Equation
H⁺(aq) + OH⁻(aq) → H₂O(l)
Imp Point
Phenolphthalein is pink in NaOH. Adding HCl makes it colourless (neutralisation). Adding more NaOH brings the pink back!
Metal Oxides + Acids (Basic Oxides)
Metal oxides react with acids like bases do. So metallic oxides are basic oxides.
General
Metal Oxide + Acid → Salt + Water
CuO + HCl → blue-green solution
CuO(s) + 2HCl(aq) → CuCl₂(aq) + H₂O(l)
Non-Metallic Oxides + Bases (Acidic Oxides)
CO₂ reacts with Ca(OH)₂ like an acid does. So non-metallic oxides are acidic oxides.
CO₂ + Ca(OH)₂
CO₂(g) + Ca(OH)₂(aq) → CaCO₃(s) + H₂O(l)
What Do All Acids & Bases Have in Common?
Acids → H⁺ ions
All acids release H⁺ ions in water. They ONLY show acidic properties in water.
HCl + H₂O → H₃O⁺ + Cl⁻
H⁺ + H₂O → H₃O⁺ (Hydronium ion)
Bases → OH⁻ ions
All bases release OH⁻ ions in water. Bases that dissolve in water are Alkalis.
NaOH → Na⁺(aq) + OH⁻(aq)
KOH → K⁺(aq) + OH⁻(aq)
Mg(OH)₂ → Mg²⁺(aq) + 2OH⁻(aq)
Imp — Dilution Safety Rule
- Dissolving acid/base in water is highly exothermic.
- Always add acid to water — never water to acid (may splash/burn).
- Dilution decreases concentration of H₃O⁺ or OH⁻ ions per unit volume.
Why Don't Glucose & Alcohol Show Acidic Properties?
They contain hydrogen but do NOT produce H⁺ ions in water. In an electric current test, glucose/alcohol solutions do not glow the bulb — confirming no ions are present.
The pH Scale
"p" stands for potenz (German = "power"). Ranges from 0 (most acidic) to 14 (most basic).
Acidic (more H⁺)
Neutral pH=7
Alkaline (more OH⁻)
0–1
Battery acid / 1M HCl
2–3
Lemon / Vinegar
4–5
Tomato / Coffee
6
Milk / Saliva
7
Pure Water
8–9
Baking soda
10–11
Milk of Magnesia
12–14
Bleach / NaOH
Imp — pH in Everyday Life
- Our body works at pH 7.0–7.8. Life survives only in a narrow pH range.
- Acid rain: pH < 5.6. Harms aquatic life in rivers.
- Tooth decay: Starts below pH 5.5. Bacteria produce acids from sugar. Toothpaste (basic) neutralises it.
- Stomach: Produces HCl for digestion. Excess → indigestion. Antacid (Milk of Magnesia) neutralises excess acid.
- Soil: Acidic soil → add quicklime/slaked lime/chalk to neutralise and raise pH.
- Bee sting: Methanoic acid → apply baking soda (mild base) for relief.
- Nettle sting: Methanoic acid → dock leaf (basic) gives relief.
Strong vs. Weak Acids/Bases
Strong Acids (more H⁺): HCl, H₂SO₄, HNO₃
Weak Acids (fewer H⁺): CH₃COOH
Strong Bases (more OH⁻): NaOH, KOH
Weak Bases (fewer OH⁻): NH₄OH, Mg(OH)₂
More About Salts
Families of Salts
Salts with the same positive or negative ion belong to the same family.
Sodium family: NaCl, Na₂SO₄, NaNO₃, Na₂CO₃
Chloride family: NaCl, KCl, NH₄Cl, AlCl₃
Imp — pH of Salts
- 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
Chemicals from Common Salt (NaCl)
Chlor-Alkali Process
Electrolysis of brine (NaCl solution) gives 3 useful products:
Electrolysis of Brine
2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g)↑ + H₂(g)↑
Anode: Cl₂ | Cathode: H₂ | Near cathode: NaOH
🟡 Chlorine (Cl₂)
- Make bleaching powder
- PVC, disinfectants
🟢 NaOH
- Degreasing metals
- Soap & paper making
Hydrogen (H₂)
- Fuel, margarine
- Ammonia production
Bleaching Powder — Ca(ClO)₂
Made by passing Cl₂ over dry slaked lime Ca(OH)₂.
Manufacture
2Ca(OH)₂ + 2Cl₂ → Ca(ClO)₂ + CaCl₂ + 2H₂O
Uses of Bleaching Powder
- Bleaching cotton, linen, wood pulp in textile/paper industries
- Oxidising agent in chemical industries
- Making drinking water germ-free
Baking Soda — NaHCO₃
Chemical name: Sodium Hydrogencarbonate. Mild, non-corrosive basic salt.
On heating
2NaHCO₃ →(Heat)→ Na₂CO₃ + H₂O + CO₂↑
Baking Powder reaction
NaHCO₃ + H⁺ → CO₂↑ + H₂O + Sodium salt
(CO₂ makes bread/cake soft & spongy)
Uses of Baking Soda
- Making baking powder (NaHCO₃ + tartaric acid) — CO₂ makes cakes spongy
- Antacid — neutralises excess stomach acid
- Soda-acid fire extinguisher
Washing Soda — Na₂CO₃·10H₂O
Obtained by recrystallising sodium carbonate. It is a basic salt.
Making Washing Soda
Na₂CO₃ + 10H₂O → Na₂CO₃·10H₂O (Washing Soda)
Uses of Washing Soda
- Glass, soap, and paper industries
- Manufacture of sodium compounds (e.g., borax)
- Domestic cleaning agent
- Removing permanent hardness of water
Water of Crystallisation
Definition: The fixed number of water molecules present in one formula unit of a salt crystal.
Copper Sulphate
CuSO₄·5H₂O — Blue crystals
Heat → white (CuSO₄)
Add water → blue returns
Gypsum
CaSO₄·2H₂O
Used to make
Plaster of Paris
Washing Soda
Na₂CO₃·10H₂O
Contains 10 water molecules
Plaster of Paris — CaSO₄·½H₂O
Made by heating gypsum at 373 K. Two formula units of CaSO₄ share one water molecule.
Gypsum → Plaster of Paris
CaSO₄·2H₂O →(373K)→ CaSO₄·½H₂O + 1½H₂O
Plaster of Paris + Water → Gypsum (hardens)
CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O (hard solid)
Moisture-proof storage: Plaster of Paris absorbs moisture from air and converts back to gypsum — making it useless for setting plaster casts.
Uses of Plaster of Paris
- Setting fractured bones in the correct position
- Making toys and decorative materials
- Making surfaces smooth
In-Text Questions & Answers
Q1Three test tubes: distilled water, acid, base. Using only red litmus paper, how will you identify each?
Answer:
Dip red litmus in each tube:
• Base → red litmus turns blue
• Acid → red litmus stays red
• Distilled water → red litmus stays red
To tell acid from water: now dip the confirmed-blue litmus back in the remaining two — one is acid, one is water. The acid can also be identified by checking with the base-confirmed solution (no fizz with water, but acidic character observable).
Dip red litmus in each tube:
• Base → red litmus turns blue
• Acid → red litmus stays red
• Distilled water → red litmus stays red
To tell acid from water: now dip the confirmed-blue litmus back in the remaining two — one is acid, one is water. The acid can also be identified by checking with the base-confirmed solution (no fizz with water, but acidic character observable).
Q2Why should curd and sour substances not be kept in brass and copper vessels?
Answer: Curd/sour foods contain acids (lactic acid, citric acid). These acids react with brass (zinc + copper) and copper metal to form harmful metallic salts that are toxic. The food gets a bad metallic taste and spoils.
Q3Which gas is liberated when acid reacts with metal? How will you test for it?
Answer: Hydrogen gas (H₂) is liberated.
Test: Bring a burning candle near the mouth of the test tube. H₂ burns with a characteristic 'pop' sound.
Test: Bring a burning candle near the mouth of the test tube. H₂ burns with a characteristic 'pop' sound.
Zn + H₂SO₄ → ZnSO₄ + H₂↑
Q4Metal compound A reacts with HCl, gas extinguishes candle, one product is CaCl₂. Write balanced equation.
Answer: Gas extinguishes candle = CO₂. Product is CaCl₂ → compound A is CaCO₃ (Calcium Carbonate).
CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)↑
Q5Why do HCl/HNO₃ show acidic character but alcohol/glucose do not?
Answer: HCl and HNO₃ ionise in water to produce H⁺ ions — responsible for acidic behaviour. Alcohol and glucose contain hydrogen but do not ionise to release H⁺ ions in water, so they are not acidic.
Q6Why does an aqueous solution of acid conduct electricity?
Answer: Acids produce free ions (H⁺ and anions like Cl⁻) in water. These free ions move and carry electric current. Without ions, no electricity flows — which is why glucose/alcohol solutions don't conduct.
Q7Why does dry HCl gas not change the colour of dry litmus paper?
Answer: HCl produces H⁺ ions only in the presence of water. Without moisture, no ionisation occurs, so no H⁺ ions are formed and litmus shows no colour change.
Q8Why should acid be added to water, not water to acid?
Answer: This is an exothermic process. If water is added to concentrated acid, sudden intense heat is produced that can cause the mixture to splash and burn skin. Adding acid slowly to a large volume of water distributes the heat safely.
Q9How is H₃O⁺ concentration affected when an acid solution is diluted?
Answer: H₃O⁺ (hydronium ion) concentration decreases per unit volume. The solution becomes less acidic and the pH moves closer to 7 (increases).
Q10How is OH⁻ concentration affected when excess base is dissolved in NaOH solution?
Answer: Concentration of OH⁻ ions increases. The pH value rises (becomes more basic, further from 7).
Q11pH of solution A = 6, pH of solution B = 8. Which has more H⁺ ions? Which is acidic/basic?
Answer:
• Solution A (pH 6) has MORE H⁺ ions — lower pH = more acidic = more H⁺
• Solution A is acidic (pH < 7)
• Solution B is basic (pH > 7)
• Solution A (pH 6) has MORE H⁺ ions — lower pH = more acidic = more H⁺
• Solution A is acidic (pH < 7)
• Solution B is basic (pH > 7)
Q12Under what soil condition would a farmer treat soil with quicklime/slaked lime/chalk?
Answer: When the soil is too acidic (low pH). These are all bases that neutralise the excess acid in soil, raising pH to a level suitable for healthy plant growth.
Q13Common name of Ca(ClO)₂?
Answer: Bleaching Powder
Q14Name the substance which on treatment with chlorine yields bleaching powder.
Answer: Slaked Lime — Ca(OH)₂ (dry calcium hydroxide)
Q15Sodium compound used for softening hard water?
Answer: Washing Soda — Na₂CO₃·10H₂O
Q16What happens if sodium hydrogencarbonate solution is heated? Give equation.
Answer: It decomposes to give sodium carbonate, water, and CO₂ gas.
2NaHCO₃ →(Heat)→ Na₂CO₃ + H₂O + CO₂↑
Q17Equation for reaction between Plaster of Paris and water?
CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O (Gypsum — hard solid)
Exercise Questions — Solved
MCQs
1A solution turns red litmus blue. Its pH is likely: (a)1 (b)4 (c)5 (d)10
Answer: (d) 10 — Turns red litmus blue = it is a base. Bases have pH > 7. Only option 10 is > 7.
2Solution reacts with egg-shells giving gas that turns lime water milky. Contains: (a)NaCl (b)HCl (c)LiCl (d)KCl
Answer: (b) HCl — Egg shells = CaCO₃. Acid + CaCO₃ → CO₂ (turns lime water milky). Only HCl is an acid here. Others are neutral salts.
310 mL NaOH neutralised by 8 mL HCl. For 20 mL NaOH, HCl needed: (a)4 (b)8 (c)12 (d)16 mL
Answer: (d) 16 mL
Ratio: 10 mL NaOH → 8 mL HCl
∴ 20 mL NaOH → (20/10) × 8 = 16 mL HCl
Ratio: 10 mL NaOH → 8 mL HCl
∴ 20 mL NaOH → (20/10) × 8 = 16 mL HCl
4Medicine for treating indigestion: (a)Antibiotic (b)Analgesic (c)Antacid (d)Antiseptic
Answer: (c) Antacid — Indigestion = excess stomach acid. Antacids are mild bases that neutralise it.
Long Answer Questions
5Balanced equations for: (a) Dil. H₂SO₄ + Zn (b) Dil. HCl + Mg (c) Dil. H₂SO₄ + Al (d) Dil. HCl + Fe
(a) Sulphuric acid + Zinc → Zinc sulphate + Hydrogen
(b) Hydrochloric acid + Magnesium → Magnesium chloride + Hydrogen
(c) Sulphuric acid + Aluminium → Aluminium sulphate + Hydrogen
(d) Hydrochloric acid + Iron → Ferrous chloride + Hydrogen
H₂SO₄(aq) + Zn(s) → ZnSO₄(aq) + H₂(g)↑
2HCl(aq) + Mg(s) → MgCl₂(aq) + H₂(g)↑
3H₂SO₄(aq) + 2Al(s) → Al₂(SO₄)₃(aq) + 3H₂(g)↑
2HCl(aq) + Fe(s) → FeCl₂(aq) + H₂(g)↑
6Alcohol and glucose contain hydrogen but are not acids. Describe an activity to prove it.
Electric Conductivity Test:
1. Take HCl, H₂SO₄, glucose, and alcohol solutions separately in beakers.
2. Set up two electrodes connected to a 6V battery and bulb in each beaker. Switch on.
Observations:
• HCl and H₂SO₄ → bulb glows (ions carry current)
• Glucose and alcohol → bulb does NOT glow (no ions)
Conclusion: Glucose/alcohol do not produce H⁺ ions — they are NOT acids.
1. Take HCl, H₂SO₄, glucose, and alcohol solutions separately in beakers.
2. Set up two electrodes connected to a 6V battery and bulb in each beaker. Switch on.
Observations:
• HCl and H₂SO₄ → bulb glows (ions carry current)
• Glucose and alcohol → bulb does NOT glow (no ions)
Conclusion: Glucose/alcohol do not produce H⁺ ions — they are NOT acids.
7Why does distilled water not conduct electricity, whereas rain water does?
Answer: Distilled water is pure with no dissolved ions — cannot conduct electricity. Rain water dissolves CO₂ from air to form carbonic acid (H₂CO₃), which ionises to give H⁺ and HCO₃⁻ ions. These ions allow rain water to conduct electricity.
8Why do acids not show acidic behaviour in the absence of water?
Answer: Acids need water to ionise and release H⁺ ions. Without water, H⁺ ions cannot separate from the acid molecule. No free H⁺ ions = no acidic properties. (Proved by dry HCl — it doesn't change dry litmus.)
9pH values: A=4, B=1, C=11, D=7, E=9. Find (a) neutral (b) strongly alkaline (c) strongly acidic (d) weakly acidic (e) weakly alkaline. Arrange in increasing order of H⁺ concentration.
(a) Neutral → D (pH 7)
(b) Strongly alkaline → C (pH 11)
(c) Strongly acidic → B (pH 1)
(d) Weakly acidic → A (pH 4)
(e) Weakly alkaline → E (pH 9)
Increasing H⁺ concentration (higher pH = less H⁺):
C < E < D < A < B (pH 11 → 9 → 7 → 4 → 1)
(b) Strongly alkaline → C (pH 11)
(c) Strongly acidic → B (pH 1)
(d) Weakly acidic → A (pH 4)
(e) Weakly alkaline → E (pH 9)
Increasing H⁺ concentration (higher pH = less H⁺):
C < E < D < A < B (pH 11 → 9 → 7 → 4 → 1)
10HCl added to tube A, CH₃COOH to tube B (same amount, same concentration, Mg ribbon). Which fizzes more vigorously and why?
Answer: Test Tube A (HCl) fizzes more vigorously.
HCl is a strong acid — fully ionises to give many H⁺ ions. CH₃COOH is a weak acid — partially ionises, giving fewer H⁺ ions. More H⁺ ions = faster reaction with Mg = more H₂ gas = more vigorous fizzing.
HCl is a strong acid — fully ionises to give many H⁺ ions. CH₃COOH is a weak acid — partially ionises, giving fewer H⁺ ions. More H⁺ ions = faster reaction with Mg = more H₂ gas = more vigorous fizzing.
11Fresh milk has pH = 6. How does pH change as it turns into curd? Explain.
Answer: pH decreases (becomes more acidic, below 6).
Reason: Bacteria ferment milk, converting lactose into lactic acid. As lactic acid builds up, H⁺ ion concentration increases, lowering the pH. That's why curd tastes sour!
Reason: Bacteria ferment milk, converting lactose into lactic acid. As lactic acid builds up, H⁺ ion concentration increases, lowering the pH. That's why curd tastes sour!
12Milkman adds baking soda to fresh milk. (a) Why shift pH to slightly alkaline? (b) Why does this milk take longer to set as curd?
(a) Fresh milk (pH 6) is slightly acidic. Baking soda (basic) shifts pH slightly above 7. This prevents the milk from turning sour quickly — longer shelf life.
(b) Curd-forming bacteria need an acidic environment. Since the milk is now alkaline (due to baking soda), the bacteria must first neutralise the added base before the milk turns acidic enough to set as curd. This extra step takes more time.
(b) Curd-forming bacteria need an acidic environment. Since the milk is now alkaline (due to baking soda), the bacteria must first neutralise the added base before the milk turns acidic enough to set as curd. This extra step takes more time.
13Plaster of Paris should be stored in a moisture-proof container. Why?
Answer: POP absorbs moisture from air and converts back to gypsum (hard solid), which cannot set properly when needed later.Once it becomes gypsum, it is useless as plaster. Hence moisture-proof storage is essential.
CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O (Gypsum)
14What is a neutralisation reaction? Give two examples.
Answer: A reaction between an acid and a base that produces salt and water, cancelling each other's properties.
Example 1: HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)
Example 2: H₂SO₄(aq) + 2KOH(aq) → K₂SO₄(aq) + 2H₂O(l)
15Give two important uses of washing soda and baking soda.
Washing Soda (Na₂CO₃·10H₂O):
1. Used in glass, soap, and paper industries
2. Removes permanent hardness of water; used as domestic cleaning agent
Baking Soda (NaHCO₃):
1. Making baking powder → CO₂ makes cakes soft and spongy
2. Antacid — neutralises excess stomach acid; used in soda-acid fire extinguishers
1. Used in glass, soap, and paper industries
2. Removes permanent hardness of water; used as domestic cleaning agent
Baking Soda (NaHCO₃):
1. Making baking powder → CO₂ makes cakes soft and spongy
2. Antacid — neutralises excess stomach acid; used in soda-acid fire extinguishers
Quick Revision — What You Learnt
- Acid-base indicators show presence of acids/bases by colour or smell change.
- Acidic nature = H⁺(aq) ions. Basic nature = OH⁻(aq) ions.
- Acid + Metal → Salt + H₂↑. Acid + Metal Carbonate → Salt + CO₂↑ + H₂O.
- Acid + Base → Salt + Water (Neutralisation). Ionic: H⁺ + OH⁻ → H₂O.
- Metallic oxides are basic; non-metallic oxides are acidic.
- Acids conduct electricity in water due to ions. Dry HCl does not.
- pH scale 0–14: <7 acidic, =7 neutral, >7 basic.
- Body works at pH 7.0–7.8. Tooth decay below pH 5.5. Acid rain pH < 5.6.
- Always add acid TO water (exothermic — prevents splashing).
- NaCl → Chlor-alkali process → NaOH + Cl₂ + H₂.
- Bleaching powder = Ca(ClO)₂. Baking soda = NaHCO₃. Washing soda = Na₂CO₃·10H₂O.
- CuSO₄·5H₂O = blue crystals (water of crystallisation).
- Gypsum (CaSO₄·2H₂O) →(373K)→ Plaster of Paris (CaSO₄·½H₂O) + water → Gypsum again.
Naturally Occurring Acids
| Natural Source | Acid Present | Natural Source | Acid Present |
|---|---|---|---|
| Vinegar | Acetic acid (CH₃COOH) | Sour milk/Curd | Lactic acid |
| Orange | Citric acid | Lemon | Citric acid |
| Tamarind | Tartaric acid | Ant sting | Methanoic acid |
| Tomato | Oxalic acid | Nettle sting | Methanoic acid |
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