Class 10 › Science › Chapter 2
Acids, Bases and Salts
Indicators, reactions of acids and bases, what they have in common, the pH scale and its importance, and the preparation and uses of important salts.
1. Acid-Base Indicators
Indicator: a dye (or mixture of dyes) that shows by a change in colour whether a substance is acidic or basic. Natural: litmus (from lichen), turmeric, red cabbage, beetroot, petals of Hydrangea, Petunia and Geranium. Synthetic: phenolphthalein, methyl orange.
| Indicator | In acid | In base |
|---|---|---|
| Litmus | Blue turns red | Red turns blue |
| Phenolphthalein | Colourless | Pink |
| Methyl orange | Red | Yellow |
| Turmeric | Stays yellow | Reddish-brown (curry stain on cloth with soap) |
- Neutral litmus solution is purple.
- Olfactory indicators: substances whose smell changes in acid or base. Onion, vanilla essence and clove oil are tested this way (Act. 2.2); the smell is lost in a base but stays in an acid.
- Test with only red litmus: base turns it blue; acid and distilled water leave it red. Then add a drop of the unknown solution that stays red to the blue one: the acid turns it red again.
2. Reaction of Acids and Bases with Metals
Acid + MetalAcid + Metal → Salt + Hydrogen gas
Zn(s) + H2SO4(aq) → ZnSO4(aq) + H2(g)
Mg + 2HCl → MgCl2 + H2Same pattern for Al, Fe and other metals; do not forget to balance.
2NaOH(aq) + Zn(s) → Na2ZnO2(s) + H2(g)Base + metal: H2 again; the salt is sodium zincate (Act. 2.4). This is not possible with all metals.
- Test for H2: a burning candle near a gas-filled soap bubble burns the gas with a pop sound.
- The metal displaces hydrogen from the acid. The effect is the same with HCl, HNO3 and CH3COOH, but the rate of bubbling depends on acid strength.
- Why not store curd or sour substances in brass or copper vessels? The acid reacts with the metal, spoiling the food and forming harmful compounds.
3. Reaction with Metal Carbonates and Hydrogencarbonates
GeneralMetal carbonate / hydrogencarbonate + Acid → Salt + CO2 + Water
Na2CO3(s) + 2HCl(aq) → 2NaCl(aq) + H2O(l) + CO2(g)
NaHCO3(s) + HCl(aq) → NaCl(aq) + H2O(l) + CO2(g)
Test for CO2 (lime water)
Ca(OH)2(aq) + CO2(g) → CaCO3(s) + H2O(l): lime water turns milky (white precipitate).
Excess CO2: CaCO3(s) + H2O(l) + CO2(g) → Ca(HCO3)2(aq). The milkiness disappears because calcium hydrogencarbonate is soluble.
- Limestone, chalk and marble are forms of calcium carbonate.
- Unknown compound A + HCl gives effervescence, and the gas puts out a burning candle (CO2). With calcium chloride as a product: CaCO3 + 2HCl → CaCl2 + H2O + CO2.
4. Acids with Bases, Metal Oxides and Non-metal Oxides
Neutralisation
Base + Acid → Salt + Water
NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l). Each reduces the other's effect. In Act. 2.6 the pink phenolphthalein turns colourless on adding acid, and the pink returns on adding NaOH.
Metal oxide
Metal oxide + Acid → Salt + Water
CuO + 2HCl → CuCl2 + H2O. The solution turns blue-green as CuO dissolves. So metal oxides are basic oxides.
Non-metal oxide
Non-metal oxide + Base → Salt + Water
CO2 + Ca(OH)2 → CaCO3 + H2O. So non-metal oxides are acidic oxides.
5. What Do All Acids and All Bases Have in Common?
- Acids give H+(aq) ions in water; bases give OH−(aq) ions in water.
- Conductivity (Act. 2.8): HCl and H2SO4 solutions make the bulb glow (current carried by ions). Glucose and alcohol contain hydrogen but do not glow the bulb, so they are not acids. Distilled water has no ions and does not conduct; rain water has dissolved acidic gases and does.
- Water is needed (Act. 2.9): dry HCl gas does not change dry blue litmus; it changes wet litmus. H+ separates from HCl only in the presence of water.
HCl + H2O → H3O+ + Cl− | H+ + H2O → H3O+H+ cannot exist alone; show it as H+(aq) or hydronium ion H3O+.
NaOH(s) → Na+(aq) + OH−(aq)Likewise KOH gives K+ + OH−; Mg(OH)2 gives Mg2+ + 2OH−.
- Alkali: a base that dissolves in water (NaOH, KOH, Ca(OH)2). All alkalis are bases but not all bases are alkalis (Mg(OH)2 is not).
- Neutralisation in terms of ions: H+(aq) + OH−(aq) → H2O(l).
- Basic solutions also contain a tiny amount of H+, but OH− ions are far more numerous, which is why they are basic.
6. Dissolving Acids and Bases in Water: Dilution
- Dissolving an acid or base in water is a highly exothermic process (Act. 2.10).
- Always add acid to water slowly with constant stirring, never water to concentrated acid. The heat can make the mixture splash and cause burns, and may crack the glass container.
- Dilution: mixing an acid or base with water decreases the concentration of H3O+ or OH− ions per unit volume.
- Adding more base to NaOH solution increases OH− concentration; diluting an acid decreases H3O+ concentration.
- Look out for the warning sign on bottles of concentrated acids and NaOH pellets.
7. How Strong are Acid and Base Solutions? The pH Scale
pH: a number that shows the acidic or basic nature of a solution; the p stands for potenz (German for power). The scale runs from 0 (very acidic) to 14 (very alkaline). Measured with universal indicator (a mixture of indicators) or pH paper.
pH < 7
Acidic
Higher H3O+ concentration means lower pH.
pH = 7
Neutral
Pure water.
pH > 7
Basic
Higher OH− concentration means higher pH.
| Substance | Approx. pH |
|---|---|
| Gastric juice | 1.2 |
| Lemon juice | 2.2 |
| Pure water / blood | 7 / 7.4 |
| Milk of magnesia | 10 |
| Sodium hydroxide solution | 14 |
- Strong acid / base: produces many H+ / OH− ions (HCl). Weak acid / base: produces few (CH3COOH). At the same concentration, HCl gives more H+ than acetic acid, so Mg fizzes more vigorously with HCl.
- Exercise-type reasoning: if solutions with pH 4, 1, 11, 7 and 9 are given, pH 1 is strongly acidic, 4 weakly acidic, 7 neutral, 9 weakly alkaline and 11 strongly alkaline. Hydrogen-ion concentration is highest at pH 1.
8. Importance of pH in Everyday Life
Body
Narrow pH range
Our body works at pH 7.0 to 7.8. Living organisms survive only in a narrow pH range.
Acid rain
pH below 5.6
Flows into rivers and lowers river pH, making survival of aquatic life difficult.
Digestion
Antacids
The stomach makes HCl. Excess acid causes pain; antacids (mild bases such as Mg(OH)2, milk of magnesia) neutralise it.
Teeth
Decay below pH 5.5
Enamel (calcium hydroxyapatite) is corroded by acids made by bacteria from sugar. Basic toothpaste neutralises the acid.
Stings
Chemical warfare
Bee sting and nettle leaves inject methanoic acid. Treat with a mild base: baking soda for a bee sting, dock leaf for a nettle sting.
Soil
Plants need a pH range
A farmer treats acidic soil with a base such as quick lime, slaked lime or chalk.
| Natural source | Acid | Natural source | Acid |
|---|---|---|---|
| Vinegar | Acetic acid | Sour milk (curd) | Lactic acid |
| Orange, lemon | Citric acid | Ant sting, nettle sting | Methanoic acid |
| Tamarind | Tartaric acid | Tomato | Oxalic acid |
9. Family and pH of Salts
- Salt family: salts with the same positive or negative radical. NaCl and Na2SO4 are sodium salts; NaCl and KCl are chloride salts.
| Made from | Nature | pH | Example |
|---|---|---|---|
| Strong acid + strong base | Neutral | 7 | NaCl, KNO3 |
| Strong acid + weak base | Acidic | < 7 | AlCl3, CuSO4 |
| Strong base + weak acid | Basic | > 7 | CH3COONa, Na2CO3, NaHCO3 |
10. Chemicals from Common Salt (NaCl)
- Sources: seawater, and rock salt (brown crystals from dried seas, mined like coal). Common salt is a neutral salt, made from HCl and NaOH.
Sodium hydroxide: chlor-alkali process
Electricity passed through brine (aqueous NaCl): 2NaCl(aq) + 2H2O(l) → 2NaOH(aq) + Cl2(g) + H2(g).
Cl2 at the anode, H2 at the cathode, NaOH near the cathode. All three are useful: Cl2 (water treatment, bleaching powder, PVC), H2 (fuels, ammonia), NaOH (soaps, detergents, paper, degreasing).
Bleaching powder
Ca(OCl)2 (representation)
Dry slaked lime + chlorine: 2Ca(OH)2 + 2Cl2 → Ca(ClO)2 + CaCl2 + 2H2O.
Uses: bleaching cotton, linen and wood pulp; oxidising agent in industry; disinfecting drinking water.
Baking soda
NaHCO3, mild basic salt
NaCl + H2O + CO2 + NH3 → NH4Cl + NaHCO3.
On heating: 2NaHCO3 → Na2CO3 + H2O + CO2.
Uses: baking powder (baking soda + tartaric acid; CO2 makes cakes rise), antacids, soda-acid fire extinguishers.
Washing soda
Na2CO3·10H2O
Recrystallise sodium carbonate: Na2CO3 + 10H2O → Na2CO3·10H2O. A basic salt.
Uses: glass, soap and paper industries; making borax; domestic cleaning; removing permanent hardness of water.
11. Water of Crystallisation and Plaster of Paris
Water of crystallisation: the fixed number of water molecules present in one formula unit of a salt.
- Copper sulphate, CuSO4·5H2O (blue): on heating it loses water, turns white and water droplets appear. Adding water restores the blue colour (Act. 2.15).
- Na2CO3·10H2O is not wet; the water is part of the crystal.
- Gypsum: CaSO4·2H2O.
CaSO4·2H2O → CaSO4·½H2O + 1½H2O 373 KGypsum on heating gives Plaster of Paris (calcium sulphate hemihydrate).
CaSO4·½H2O + 1½H2O → CaSO4·2H2OWith water it sets into a hard mass of gypsum.
- The ½ appears because two formula units of CaSO4 share one water molecule.
- Uses: plaster for fractured bones, toys, decoration, making surfaces smooth.
- Store in a moisture-proof container: moisture would set the powder into hard gypsum.
12. Quick Revision
| Reaction | Products | Test or note |
|---|---|---|
| Acid + metal | Salt + H2 | Burning candle: pop sound |
| Base + (Zn) metal | Salt + H2 | Not for all metals |
| Acid + metal carbonate / hydrogencarbonate | Salt + CO2 + H2O | Lime water turns milky |
| Acid + base | Salt + H2O | Neutralisation |
| Acid + metal oxide | Salt + H2O | Metal oxides are basic |
| Base + non-metal oxide | Salt + H2O | Non-metal oxides are acidic |
| Compound | Name / formula | Key fact |
|---|---|---|
| Bleaching powder | Ca(ClO)2 (calcium oxychloride) | Cl2 on dry slaked lime |
| Baking soda | NaHCO3 | Mild base, antacid |
| Washing soda | Na2CO3·10H2O | Softens hard water |
| Plaster of Paris | CaSO4·½H2O | Sets to gypsum with water |
Common mistakes:
• Adding water to concentrated acid instead of acid to water.
• Saying pH rises with H+ concentration; higher H+ means lower pH.
• Writing H+ alone; it exists as H+(aq) or H3O+.
• Calling every hydrogen-containing compound (glucose, alcohol) an acid.
• Forgetting that dry HCl gas shows no acidic behaviour without water.
• Mixing up bleaching powder (Cl2 + slaked lime) with baking soda and washing soda.
• Writing ½ water of crystallisation incorrectly, or forgetting the 10H2O in washing soda.
• Adding water to concentrated acid instead of acid to water.
• Saying pH rises with H+ concentration; higher H+ means lower pH.
• Writing H+ alone; it exists as H+(aq) or H3O+.
• Calling every hydrogen-containing compound (glucose, alcohol) an acid.
• Forgetting that dry HCl gas shows no acidic behaviour without water.
• Mixing up bleaching powder (Cl2 + slaked lime) with baking soda and washing soda.
• Writing ½ water of crystallisation incorrectly, or forgetting the 10H2O in washing soda.