⚗️ Acids, Bases
and Salts
A complete visual guide — all concepts explained in simple language, with every question answered and key reactions illustrated.
1 Introduction & Indicators
You already know that acids taste sour (like lemon) and bases taste bitter (like baking soda). But tasting chemicals in the lab is dangerous — so scientists use indicators to test them safely.
When someone suffers from acidity after eating too much, we give them baking soda solution — because it is a base that neutralises the excess acid in the stomach.
What Are Indicators?
Indicators are substances that change colour in the presence of an acid or a base. They help us identify whether a solution is acidic, basic, or neutral.
🔴 Red Litmus
In Base → turns Blue
🔵 Blue Litmus
In Base → stays Blue
🌸 Phenolphthalein
In Base → turns Pink
🟠 Methyl Orange
In Base → turns Yellow
Fig. A – How common indicators respond to acids, neutral substances, and bases
Litmus is a purple dye extracted from lichen (a plant of division Thallophyta). In neutral conditions it is purple. Natural indicators also include red cabbage, turmeric, and petals of flowers like Hydrangea, Petunia, and Geranium.
Olfactory Indicators
Some substances change their smell (odour) — not colour — in acidic or basic conditions. These are called olfactory indicators.
Onion: Its smell disappears in a basic solution (NaOH) but remains in an acidic solution (HCl). So onion is a good olfactory indicator.
Vanilla: Its smell goes away in a basic solution — another useful olfactory indicator.
Clove oil: Its smell remains even in both acidic and basic solutions — so it is not a good olfactory indicator.
2 How Acids & Bases React with Metals
Acids + Metals
When a metal is added to an acid, hydrogen gas is released and a salt is formed. This hydrogen gas burns with a "pop" sound when a burning candle is brought near it — that's the test for hydrogen gas.
| Acid | Metal | Salt Formed | Gas |
|---|---|---|---|
| Sulphuric acid (H2SO4) | Zinc (Zn) | Zinc sulphate (ZnSO4) | H2↑ |
| Hydrochloric acid (HCl) | Magnesium (Mg) | Magnesium chloride (MgCl2) | H2↑ |
| Hydrochloric acid (HCl) | Iron (Fe) | Ferrous chloride (FeCl2) | H2↑ |
| Sulphuric acid (H2SO4) | Aluminium (Al) | Aluminium sulphate (Al2(SO4)3) | H2↑ |
The pop sound test confirms hydrogen gas: bring a burning candle near the gas — it burns with a distinctive "pop" sound. Hydrogen is highly flammable!
Bases + Metals
Not all metals react with bases. Zinc reacts with sodium hydroxide (NaOH), releasing hydrogen and forming sodium zincate:
(Sodium zincate)
Bases do NOT react with all metals. For example, copper and silver do not react with NaOH. Only reactive metals like zinc, aluminium, and sodium show this reaction.
3 Metal Carbonates & Hydrogencarbonates + Acids
When metal carbonates (like Na2CO3) or metal hydrogencarbonates (like NaHCO3) react with acids, they produce CO2 gas, water, and a salt. The CO2 gas turns lime water (calcium hydroxide solution) milky — this is the test for CO2.
Sodium Carbonate + HCl
→
2NaCl + H2O + CO2↑
Sodium Hydrogencarbonate + HCl
→
NaCl + H2O + CO2↑
CO2 Test (Lime Water)
→
CaCO3↓ + H2O
(White precipitate = milky)
Excess CO2 → Milky disappears
→
Ca(HCO3)2
(Soluble — milkiness clears)
Limestone, chalk, and marble are all different physical forms of calcium carbonate (CaCO3). All of them react with acids to release CO2.
4 Neutralisation Reaction
When an acid and a base react together, they cancel each other's effect. This produces a salt and water. This reaction is called Neutralisation.
Fig. B – Neutralisation: acid and base cancel each other to form salt and water
H+(aq) + OH–(aq) → H2O(l) [at the ion level]
Add phenolphthalein to NaOH → it turns pink. Now add HCl drop by drop → the pink fades to colourless (neutralisation complete). Add NaOH again → pink returns. This beautifully shows neutralisation is reversible.
5 Metallic & Non-metallic Oxides
Metallic Oxides + Acids (Basic Oxides)
Metal oxides react with acids just like bases do — they produce a salt and water. This is why metal oxides are called basic oxides.
Solution turns blue-green due to copper(II) chloride (CuCl₂)
Non-metallic Oxides + Bases (Acidic Oxides)
Non-metallic oxides (like CO2) react with bases just like acids do — producing a salt and water. This proves that non-metallic oxides are acidic in nature.
(Lime water turns milky — white precipitate of CaCO₃)
Non-metallic Oxides
React like ACIDS
→ Called Acidic oxides
Metallic Oxides
React like BASES
→ Called Basic oxides
6 What Do All Acids & Bases Have in Common?
All acids and bases may look different, but they share something at the particle level. When dissolved in water, acids produce H+ (hydrogen) ions and bases produce OH– (hydroxide) ions.
Why Water Is Necessary for Acidic/Basic Properties
If you pass electricity through HCl solution → the bulb glows (ions present). But through glucose or alcohol solution → the bulb does not glow (no ions). Even though glucose and alcohol contain hydrogen, they are not acids — they don't produce H+ ions in water.
H+ + H2O → H3O+ (hydronium ion) — H⁺ cannot exist alone!
KOH(s) →H₂O K+(aq) + OH–(aq)
Mg(OH)2(s) →H₂O Mg2+(aq) + 2OH–(aq)
All alkalis are bases, but not all bases are alkalis. An alkali is a base that dissolves in water. For example, NaOH is both a base and an alkali; but Cu(OH)2 is a base that does not dissolve in water, so it is NOT an alkali.
Diluting Acids or Bases — Safety!
Dissolving an acid or base in water is highly exothermic (releases lots of heat). Always add acid to water — NEVER water to acid! If water is added to concentrated acid, the heat released can cause the acid to splash and cause severe burns.
When acid or base is diluted, the concentration of H3O+/OH– ions per unit volume decreases. This process is called dilution.
7 The pH Scale
The pH scale (0 to 14) measures how acidic or basic a solution is. The "p" in pH stands for potenz (German for "power"). It measures the concentration of H+ ions in a solution.
Fig. C – The pH Scale (0 = most acidic, 7 = neutral, 14 = most basic/alkaline)
pH < 7
More H⁺ ions
Lower pH = Stronger acid
pH = 7
Equal H⁺ and OH⁻
e.g., Pure water
pH > 7
More OH⁻ ions
Higher pH = Stronger base
| Substance | Approx. pH | Nature |
|---|---|---|
| Gastric juice (stomach acid) | ~1.2 | Strongly Acidic |
| Lemon juice | ~2.2 | Strongly Acidic |
| Vinegar | ~3 | Acidic |
| Tomato juice | ~4 | Acidic |
| Coffee | ~5 | Weakly Acidic |
| Fresh milk | ~6 | Weakly Acidic |
| Pure water / Blood | 7.0 / 7.4 | Neutral |
| Milk of magnesia | ~10 | Basic |
| Soap solution | ~9–10 | Basic |
| 1M NaOH solution | ~14 | Strongly Basic |
Strong vs Weak Acids/Bases
Strong acids (like HCl) completely ionize in water, releasing many H+ ions → lower pH.
Weak acids (like CH3COOH — acetic acid) only partially ionize → fewer H+ ions → higher pH than strong acid of same concentration.
Same logic applies to bases: Strong bases (like NaOH) give more OH– ions than weak bases (like NH4OH).
8 Importance of pH in Everyday Life
🌱 Plants & Soil
🌧️ Acid Rain
🫁 Our Body
🦷 Tooth Decay
🐝 Bee Sting
🌿 Nettle Sting
Indigestion: The stomach naturally produces HCl for digestion. Sometimes it produces too much acid → pain. Antacids (like milk of magnesia — Mg(OH)2) are mild bases that neutralise this excess acid and provide relief. Baking soda (NaHCO3) also works as an antacid.
9 Salts — Family & pH
Salts are formed when an acid reacts with a base. The positive part of the salt comes from the base, and the negative part comes from the acid.
Family of Salts
Salts that share the same positive or negative ions belong to the same "family".
Sodium family: NaCl, Na2SO4, Na2CO3 (all have Na+)
Chloride family: NaCl, KCl, MgCl2 (all have Cl–)
Sulphate family: Na2SO4, CuSO4, K2SO4 (all have SO42–)
pH of Salts
| Salt Formed From | pH | Nature | Example |
|---|---|---|---|
| Strong acid + Strong base | = 7 | Neutral | NaCl (HCl + NaOH) |
| Strong acid + Weak base | < 7 | Acidic | NH4Cl (HCl + NH4OH) |
| Weak acid + Strong base | > 7 | Basic | Na2CO3 (H2CO3 + NaOH) |
Not all salts are neutral! Salts can be acidic (e.g., AlCl3, ZnSO4, CuSO4) or basic (e.g., Na2CO3, NaHCO3), depending on the strength of the acid and base that formed them.
10 Chemicals from Common Salt (NaCl)
Common salt (sodium chloride — NaCl) is a crucial raw material for industry. It is found dissolved in seawater and as solid rock salt in underground mines. From NaCl, we can make several important chemicals:
Fig. D – Important products manufactured from common salt (NaCl)
1. Sodium Hydroxide (NaOH) — Chlor-Alkali Process
When electricity is passed through brine (aqueous NaCl), it splits into three products: NaOH, Cl2, and H2. This is called the chlor-alkali process (chlor = chlorine, alkali = sodium hydroxide).
Cl₂ at anode | H₂ at cathode | NaOH near cathode
| Product | Where Produced | Main Uses |
|---|---|---|
| Chlorine (Cl₂) | At anode (+) | Water purification, PVC, pesticides, disinfectants, bleaching powder |
| Hydrogen (H₂) | At cathode (–) | Fuel, making ammonia for fertilisers, margarine production |
| Sodium Hydroxide (NaOH) | Near cathode (–) | De-greasing metals, making soaps & detergents, paper making |
2. Bleaching Powder — Ca(ClO)2
Chlorine gas is passed over dry slaked lime [Ca(OH)2] to make bleaching powder. Its actual composition is complex, but it is represented as Ca(ClO)2.
Uses of Bleaching Powder:
(i) Bleaching cotton, linen (textile industry) and wood pulp (paper industry)
(ii) As an oxidising agent in chemical industries
(iii) To disinfect drinking water — kills germs
3. Baking Soda — NaHCO3
Chemical name: Sodium hydrogencarbonate. It is a mild, non-corrosive basic salt. It is made from NaCl:
CO₂ makes bread/cake rise → soft and spongy!
Uses of Baking Soda (NaHCO₃):
(i) Baking powder = NaHCO3 + tartaric acid → CO2 makes dough rise
(ii) Antacid — neutralises excess stomach acid (HCl)
(iii) Used in soda-acid fire extinguishers — reacts with H2SO4 to produce CO2 that smothers fire
4. Washing Soda — Na2CO3·10H2O
Obtained by recrystallising sodium carbonate. Na2CO3 is made by heating NaHCO3. Adding 10 molecules of water gives washing soda.
(Washing soda — a basic salt)
Uses of Washing Soda (Na₂CO₃):
(i) Used in glass, soap, and paper industries
(ii) Manufacture of sodium compounds like borax
(iii) Domestic cleaning agent
(iv) Removes permanent hardness of water
Baking soda = NaHCO3 (sodium hydrogencarbonate) — mild base, used in kitchen and as antacid.
Washing soda = Na2CO3·10H2O (sodium carbonate decahydrate) — used for cleaning and industry.
11 Water of Crystallisation
Water of crystallisation is the fixed number of water molecules that are part of the crystal structure of a salt. These water molecules are not "free" — they are chemically attached to the salt formula.
| Salt | Formula | Water Molecules | Appearance |
|---|---|---|---|
| Copper sulphate (Blue vitriol) | CuSO4·5H2O | 5 | Blue crystals |
| Washing soda | Na2CO3·10H2O | 10 | Transparent crystals |
| Gypsum | CaSO4·2H2O | 2 | White crystals |
| Ferrous sulphate (Green vitriol) | FeSO4·7H2O | 7 | Green crystals |
Heat blue CuSO4·5H2O crystals → they turn white (anhydrous CuSO4) and water droplets form on the tube. This proves the crystals contained water even though they appeared dry. Add a few drops of water back → blue colour returns!
Crystals with water of crystallisation are called hydrated salts. When water is removed by heating, the salt becomes anhydrous (without water) and often changes colour. The water of crystallisation makes Na2CO3·10H2O look like it has 10 water molecules per formula unit — but the salt is NOT wet. The water is part of its crystal structure.
12 Plaster of Paris
Gypsum (CaSO4·2H2O) is heated at 373 K (100°C) to lose 1½ water molecules and form Plaster of Paris — calcium sulphate hemihydrate.
(Gypsum) (Plaster of Paris)
(Plaster of Paris + water → Gypsum — hard solid mass)
Why ½H₂O? It's not really half a molecule — rather, two formula units of CaSO4 share one water molecule, so each unit gets ½H2O in the formula.
🏥 Medical Use
🏺 Other Uses
⚠️ Store Carefully
13 All Questions & Answers
• The test tube that turns red litmus blue → contains the basic solution.
• The other two test tubes where red litmus stays red: you cannot directly distinguish acid from distilled water using only red litmus paper.
However, since we already know one of the remaining two is acid and the other is distilled water (from the question), we can label them accordingly. To formally distinguish acid from water, we'd need blue litmus or another indicator. With red litmus alone: base = identified; the other two = labelled by elimination.
Example: Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g)↑
Test for hydrogen gas: Bring a burning candle near the mouth of the test tube. Hydrogen gas burns with a characteristic "pop" sound. This confirms the presence of hydrogen.
Balanced equation:
CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)↑
2Ca(OH)₂ + 2Cl₂ → Ca(ClO)₂ + CaCl₂ + 2H₂O
2NaHCO₃ →Heat Na₂CO₃ + H₂O + CO₂↑
The CO₂ gas produced can be tested by passing it through lime water — it turns milky.
CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O
(Plaster of Paris) (Gypsum)
Turning red litmus blue means the solution is basic. Basic solutions have pH > 7. Among the options, only 10 is greater than 7.
Egg-shells are calcium carbonate (CaCO₃). The gas that turns lime water milky is CO₂. Only an acid reacts with carbonates to produce CO₂. Among the options, only HCl is an acid.
The ratio is fixed: 10 mL NaOH : 8 mL HCl. If NaOH doubles to 20 mL, HCl also doubles → 8 × 2 = 16 mL.
Indigestion is caused by excess stomach acid (HCl). Antacids are mild bases that neutralise this excess acid and provide relief. Examples: milk of magnesia, baking soda.
Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g)↑
(b) Magnesium + Hydrochloric acid → Magnesium chloride + Hydrogen
Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)↑
(c) Aluminium + Sulphuric acid → Aluminium sulphate + Hydrogen
2Al(s) + 3H₂SO₄(aq) → Al₂(SO₄)₃(aq) + 3H₂(g)↑
(d) Iron + Hydrochloric acid → Ferrous chloride + Hydrogen
Fe(s) + 2HCl(aq) → FeCl₂(aq) + H₂(g)↑
• Test glucose solution → bulb does NOT glow (no ions, no conductivity)
• Test alcohol solution → bulb does NOT glow
• Test HCl solution → bulb glows (ions present)
This proves glucose and alcohol do not produce H⁺ ions in water, even though they contain hydrogen. Hence, they are NOT acids.
Rain water dissolves CO₂ from the atmosphere as it falls: CO₂ + H₂O → H₂CO₃ (carbonic acid). This acid partly ionises to give H⁺ and HCO₃⁻ ions. These ions allow rain water to conduct electricity. Rain water also picks up other dust particles and gases, further increasing its ion content.
(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 order of H⁺ concentration (lower pH = higher H⁺):
C (pH 11) < E (pH 9) < D (pH 7) < A (pH 4) < B (pH 1)
HCl is a strong acid — it completely ionises in water, producing a high concentration of H⁺ ions. Acetic acid is a weak acid — it only partially ionises, so fewer H⁺ ions are available. More H⁺ ions react faster with magnesium to release hydrogen gas. Therefore, the reaction with HCl is much faster and more vigorous.
When milk turns into curd, bacteria (Lactobacillus) ferment the lactose (milk sugar) and produce lactic acid. This lactic acid increases the H⁺ ion concentration in the milk, lowering the pH below 6. The sour taste of curd is due to this lactic acid.
(b) Curd forms when the pH drops sufficiently (due to lactic acid production). Since the milk is now alkaline due to baking soda, the bacteria need more time to produce enough lactic acid to overcome the alkalinity and lower the pH to a level at which curd can set. Hence, it takes longer.
CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O (Gypsum)
If stored in a container that allows moisture (even from the air) to enter, the Plaster of Paris will absorb moisture and set into a hard mass of gypsum. Once it sets, it cannot be used again. Therefore, it must be stored in airtight, moisture-proof containers.
Base + Acid → Salt + Water
Example 1: NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)
(Sodium hydroxide + Hydrochloric acid → Sodium chloride + Water)
Example 2: Ca(OH)₂(aq) + H₂SO₄(aq) → CaSO₄(aq) + 2H₂O(l)
(Calcium hydroxide + Sulphuric acid → Calcium sulphate + Water)
(i) Used in glass, soap, and paper industries.
(ii) Used to remove permanent hardness of water and as a domestic cleaning agent.
Baking Soda (NaHCO₃):
(i) Used in baking powder — CO₂ produced makes bread/cake soft and spongy.
(ii) Used as an antacid — neutralises excess stomach acid and relieves indigestion.
📚 Chapter Summary — Quick Revision
- Acids are sour, turn blue litmus red, and produce H⁺ (H₃O⁺) ions in water.
- Bases are bitter, turn red litmus blue, and produce OH⁻ ions in water. Soluble bases are called alkalis.
- Indicators (litmus, phenolphthalein, methyl orange) show whether a substance is acidic or basic by changing colour.
- Olfactory indicators (onion, vanilla) detect acids/bases through change in smell.
- Acid + Metal → Salt + H₂ gas (burns with pop sound)
- Acid + Metal carbonate/hydrogencarbonate → Salt + CO₂ + Water (CO₂ turns lime water milky)
- Base + Acid → Salt + Water (Neutralisation Reaction)
- Metal oxide + Acid → Salt + Water (metal oxides = basic oxides)
- Non-metal oxide + Base → Salt + Water (non-metal oxides = acidic oxides)
- Acids/bases only show their properties when dissolved in water (H⁺ and OH⁻ ions need water).
- pH scale: 0–14. pH 7 = neutral, pH < 7 = acidic, pH > 7 = basic.
- Strong acid = more H⁺ ions = lower pH. Weak acid = fewer H⁺ ions = higher pH (closer to 7).
- Always add acid to water — NOT water to acid (dilution is exothermic and dangerous).
- Chlor-alkali process: Electrolysis of brine → NaOH + Cl₂ + H₂
- Bleaching powder = Ca(OH)₂ + Cl₂; used for bleaching and water purification.
- Baking soda (NaHCO₃) = antacid, baking, fire extinguisher.
- Washing soda (Na₂CO₃·10H₂O) = cleaning, glass industry, removes water hardness.
- Water of crystallisation = fixed water molecules in crystal structure (e.g., CuSO₄·5H₂O).
- Gypsum (CaSO₄·2H₂O) →373K Plaster of Paris (CaSO₄·½H₂O); sets hard when mixed with water.
- pH of body = 7.0–7.8. Acid rain = pH below 5.6. Tooth decay starts below pH 5.5.
+ Naturally Occurring Acids
| Natural Source | Acid Present |
|---|---|
| Vinegar | Acetic acid (Ethanoic acid) |
| Orange / Lemon | Citric acid |
| Tamarind | Tartaric acid |
| Tomato | Oxalic acid |
| Sour milk / Curd | Lactic acid |
| Ant sting / Nettle sting | Methanoic acid (Formic acid) |
| Stomach (gastric juice) | Hydrochloric acid (HCl) |
Class 10 Science · Chapter 2 · Acids, Bases and Salts
Based on NCERT Textbook (Reprint 2025-26) · Explained for school students
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