Acids, Bases and Salts Class 10: Notes and Important Questions

Acids, Bases and Salts Class 10 explains how to recognise acids and bases, how they react, how the pH scale measures them, and how everyday salts like baking soda, washing soda, bleaching powder and Plaster of Paris are made. This guide follows the chapter in order, lists the key equations, and ends with important questions, model answers and common mistakes.

Indicators: telling acids and bases apart

Acids taste sour and bases taste bitter, but tasting chemicals is dangerous. So we use indicators, substances that change colour or smell in an acidic or basic medium.

• Natural indicators: litmus (purple, extracted from lichens), turmeric, red cabbage, and petals of Hydrangea and Petunia.
• Synthetic indicators: methyl orange and phenolphthalein.
• Olfactory indicators change smell. Onion and vanilla essence lose their smell in a base like sodium hydroxide but keep it in an acid. These help students with visual impairments.

Reactions with metals

Acid + reactive metal → salt + hydrogen gas. The metal displaces hydrogen from the acid.

Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g)

Test for hydrogen: pass the gas through soap solution and bring a burning candle near the bubbles. Hydrogen burns with a pop sound.

Some bases also react with certain metals, such as zinc and aluminium, on heating:

2NaOH(aq) + Zn(s) → Na₂ZnO₂(s) + H₂(g) (sodium zincate)

This is why curd and pickles, which contain acids, should not be kept in brass or copper vessels: the acid can react with the metal and form toxic salts that cause food poisoning.

Reactions with carbonates and hydrogencarbonates

Acid + metal carbonate or hydrogencarbonate → salt + water + carbon dioxide. For example, sodium carbonate with hydrochloric acid gives sodium chloride, water and CO₂.

Lime water test for CO₂:

1. Dilute HCl is added to sodium carbonate.
2. The CO₂ produced passes through a delivery tube into lime water (calcium hydroxide solution).
3. Lime water turns milky because white, insoluble calcium carbonate (CaCO₃) forms.
4. With excess CO₂, the milkiness disappears because soluble calcium hydrogencarbonate, Ca(HCO₃)₂, forms.

Limestone, chalk and marble are all forms of calcium carbonate, so they react with acids in the same way. That is why acid rain damages marble monuments.

Neutralisation and oxides

Acid + base → salt + water. H⁺ ions from the acid combine with OH⁻ ions from the base to form water.

NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)

Antacids use this idea to treat acidity in the stomach.

• Metallic oxides are basic. Black copper oxide reacts with hydrochloric acid to give a blue-green solution of copper(II) chloride and water, just as a base would.
• Non-metallic oxides are acidic. Carbon dioxide reacts with calcium hydroxide (a base) to give salt and water, just as an acid would.

Ions: what makes an acid or a base

• Acids produce hydrogen ions (H⁺) in water. H⁺ cannot exist alone; it joins water to form hydronium ions (H₃O⁺).
• Bases produce hydroxide ions (OH⁻) in water. A base that dissolves in water is an alkali, such as NaOH and KOH.

Because of these free ions, solutions of acids and bases conduct electricity. In a bulb-and-battery test, hydrochloric acid lights the bulb but glucose does not. Glucose and alcohol contain hydrogen but do not release H⁺ ions in water, so they do not show acidic character.

Distilled water does not conduct electricity because it lacks free ions, while rain water does because dissolved gases like CO₂ form ions.

Dry HCl gas does not change dry blue litmus. H⁺ ions separate from HCl only in the presence of water, so wet litmus turns red.

Dilution and safety

Dissolving a concentrated acid or base in water is highly exothermic (it releases a lot of heat), especially for concentrated sulphuric and nitric acid. If water is poured into concentrated acid, the mixture can splash out and cause burns, and the glass may crack.

Always add acid to water, slowly, with constant stirring, so the heat spreads through the larger volume of water. In school, concentrated acids are handled by the teacher with laboratory protection. Dilution lowers the number of H₃O⁺ or OH⁻ ions per unit volume.

The pH scale

pH runs from 0 to 14. pH 7 is neutral (pure water), below 7 is acidic, and above 7 is basic. The "p" stands for potenz, German for power. The higher the hydronium ion concentration, the lower the pH. Universal indicator, a mixture of dyes, shows different colours at different pH values.

Strength depends on how much a substance ionises. Strong acids like HCl and H₂SO₄ ionise completely. Weak acids like acetic acid ionise only partly, so they give fewer H⁺ ions even at the same concentration.

pH in everyday life

• Our body works within a narrow pH range of 7.0 to 7.8.
• Acid rain: rain water with pH below 5.6. It lowers the pH of rivers and makes life hard for aquatic organisms.
• Soil: plants need a suitable pH; farmers add lime (a base) to acidic soil.
• Indigestion: the stomach makes hydrochloric acid; too much causes pain. A mild base such as milk of magnesia (magnesium hydroxide) neutralises the excess.
• Tooth decay starts when mouth pH falls below 5.5. Bacteria make acids from sugar, which corrode enamel (calcium hydroxyapatite). Cleaning the mouth after eating and using toothpaste, which is generally basic, helps.
• Stings: a bee sting is an acidic example, and nettle hairs contain methanoic acid. The idea here is neutralisation.

Salts, their families and their pH

Salts are ionic compounds formed by neutralisation. Salts sharing the same positive or negative radical form a family: NaCl and Na₂SO₄ are sodium salts; NaCl and KCl are chloride salts.

Common salt comes from seawater or is mined as rock salt: large brown crystals (brown because of impurities) left by ancient seas that dried up. It is the raw material for many chemicals.

Chemicals from common salt

Chlor-alkali process: electricity is passed through brine (concentrated NaCl solution).

2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g)

Chlorine forms at the anode, hydrogen at the cathode, and sodium hydroxide solution near the cathode.

Bleaching powder: chlorine reacts with dry slaked lime. NCERT writes it as Ca(ClO)₂, noting its real composition is complex.

2Ca(OH)₂ + 2Cl₂ → Ca(ClO)₂ + CaCl₂ + 2H₂O

Uses: bleaching cotton, linen and wood pulp; disinfecting drinking water; as an oxidising agent in industry.

Baking soda (NaHCO₃): NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃. It is a mild, non-corrosive base. On heating, 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂, and the CO₂ makes cakes soft and spongy. Baking powder is baking soda plus a mild edible acid like tartaric acid, which avoids a bitter taste. Baking soda is also used in antacids and soda-acid fire extinguishers.

Washing soda (Na₂CO₃.10H₂O): made by heating baking soda to get sodium carbonate, then recrystallising it. Used in glass, soap and paper industries, for cleaning, and to remove permanent hardness of water.

Water of crystallisation and Plaster of Paris

Water of crystallisation is the fixed number of water molecules in one formula unit of a salt. Blue copper sulphate (CuSO₄.5H₂O) turns white when heated as it loses water, and turns blue again when water is added.

Plaster of Paris: gypsum (CaSO₄.2H₂O) heated carefully to 373 K (100°C) becomes CaSO₄.1/2H₂O, a hemihydrate (two formula units share one water molecule). Mixed with water it sets back into hard gypsum:

CaSO₄.1/2H₂O + 1 1/2 H₂O → CaSO₄.2H₂O

It supports fractured bones and is used for toys, decorative items and smoothing walls.

Remember this

• Acid + metal → salt + H₂; acid + carbonate → salt + water + CO₂; acid + base → salt + water.
• Acids give H⁺ (as H₃O⁺) in water; bases give OH⁻.
• pH below 7 acidic, 7 neutral, above 7 basic; lower pH means more H⁺.
• Always add acid to water.
• Baking soda NaHCO₃, washing soda Na₂CO₃.10H₂O, Plaster of Paris CaSO₄.1/2H₂O, gypsum CaSO₄.2H₂O.

Important questions with answers

1. You have red litmus and three tubes: water, an acid and a base. Identify them. Dip red litmus in all three; the one turning it blue is the base. Dip this blue litmus in the other two; the one turning it red is the acid. The remaining tube is distilled water.

2. Why should curd not be stored in copper or brass vessels? Curd contains an acid that can react with the metal and form toxic salts, causing food poisoning.

3. What happens when excess CO₂ is passed through lime water? It first turns milky as calcium carbonate forms. With excess CO₂, soluble calcium hydrogencarbonate forms and the milkiness disappears.

4. Why are metallic oxides called basic oxides? They react with acids to give salt and water, which is how bases behave.

5. Why do glucose and alcohol not show acidic character? They do not release H⁺ ions in water.

6. Why add acid to water and not water to acid? Adding water to concentrated acid releases a lot of heat at once, which can splash acid or break the glass. Adding acid slowly to water spreads the heat safely.

7. Solution A has pH 6 and B has pH 8. Which has more H⁺ ions? A, because a lower pH means a higher hydrogen ion concentration.

8. Will an ammonium chloride solution be acidic, basic or neutral? Acidic (pH < 7), because it comes from a strong acid and a weak base.

9. Why must Plaster of Paris be stored in a moisture-proof container? It absorbs moisture and sets into hard gypsum, becoming useless.

Common mistakes to avoid

• Saying phenolphthalein turns pink in acid. It turns pink in base.
• Thinking every salt is neutral. Check the parent acid and base.
• Writing that a lower pH means fewer H⁺ ions. It is the opposite.
• Confusing baking soda (NaHCO₃) with washing soda (Na₂CO₃.10H₂O).
• Mixing up strength (how much it ionises) with concentration.

When you want to revise these reactions with guided questions, study this chapter with Joy on Learnijoy.