CLASS 10 SCIENCE · CHAPTER 2 · STUDY NOTES
Acids, Bases and Salts
A comprehensive guide to the chemical properties of acids and bases, the pH scale, and the industrial importance of salts derived from common salt.
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Identifying Acids and Bases: Indicators
An indicator gives evidence about a solution’s acidic or basic nature. Learn to distinguish a colour change from no change.SEE THE CHEMISTRY
Let the indicator reveal the solution.
Predict: will every indicator show the same colour in a base?
Litmus changes red in acid and blue in base. Neutral litmus solution is purple.
- Indicator
- A substance whose colour or odour changes in different acidic or basic conditions.
| Indicator | Color in Acid | Color in Base |
|---|---|---|
| Blue Litmus | Red | No Change |
| Red Litmus | No Change | Blue |
| Phenolphthalein | Colorless | Pink |
| Methyl Orange | Red | Yellow |
| Turmeric | Yellow | Reddish-Brown |
Litmus solution is purple in a neutral sample, red in an acid and blue in a base. Red and blue litmus papers already have a starting colour, so no change alone does not identify a neutral solution.
Phenolphthalein is colourless in acidic or neutral samples and pink in sufficiently basic samples. Turmeric is yellow in acid and reddish brown in base. Methyl orange is red in sufficiently acidic samples and yellow in neutral or basic samples.
Go deeper: Why use more than one observation?
An indicator changes over a pH interval; it is not a universal red–blue switch. A colourless phenolphthalein sample could be acidic or neutral.
Litmus is obtained from lichens. Onion, vanilla and clove illustrate olfactory indicators: odour behaviour can also change. These are supervised observations, not a reason to smell unknown chemicals.
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Question
Sample X leaves phenolphthalein colourless and turns blue litmus red. Is X neutral? Explain.
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How Acids and Bases React with Metals
Recognise the products of a suitable metal–acid reaction and distinguish gas formation from gas identification.SEE THE CHEMISTRY
Watch zinc release hydrogen.
Predict: which observation identifies the gas, rather than simply showing a reaction?
Zn + 2HCl → ZnCl₂ + H₂
A reactive metal displaces hydrogen from dilute acid. A teacher tests the gas: hydrogen burns with a pop. Not every metal–acid pair produces hydrogen.
- Metal–acid displacement
- A suitable metal replaces hydrogen in a dilute acid, forming a salt and hydrogen gas.
Zinc with dilute hydrochloric acid forms zinc chloride and hydrogen. A teacher identifies hydrogen by its characteristic pop when ignited; bubbles alone only show that gas formed.
Zinc can also react with heated sodium hydroxide to produce sodium zincate and hydrogen. This is not a rule for every metal or base.
Reaction of Zinc with Hydrochloric Acid
Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g)Zinc metal reacts with hydrochloric acid to form zinc chloride (a salt) and hydrogen gas. The zinc atoms displace the hydrogen atoms from the acid molecules.
Reaction of Zinc with Sodium Hydroxide
2NaOH(aq) + Zn(s) → Na₂ZnO₂(s) + H₂(g)Sodium hydroxide reacts with zinc to form a complex salt called sodium zincate and hydrogen gas. This reaction requires heating.
Go deeper: Why is this not a universal metal rule?
Metal reactivity and the acid matter. Copper does not release hydrogen from dilute HCl; oxidising acids can give other products.
The chapter writes zinc’s alkaline reaction in a simplified sodium-zincate form. Do not extend it to every metal. Acids in foods can attack unsuitable copper or brass containers and contaminate the food; this does not imply hydrogen must be produced.
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Question
Two gas samples both form bubbles. Only Y gives a pop in a teacher’s gas test. Which evidence supports hydrogen?
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Reactions with Carbonates and Hydrogencarbonates
Connect an acid–carbonate reaction to the changing appearance of lime water.SEE THE CHEMISTRY
Follow carbon dioxide into lime water.
Predict: will passing more CO₂ always make lime water milkier?
Ca(OH)₂ + CO₂ → CaCO₃↓ + H₂O
Carbon dioxide turns lime water milky because insoluble calcium carbonate forms. Gas bubbles alone do not identify the gas.
- Precipitate
- An insoluble solid that forms within a solution during a reaction.
An acid with a carbonate or hydrogencarbonate forms a salt, water and carbon dioxide. The identity of the metal determines the salt.
Passing CO₂ through lime water forms insoluble calcium carbonate, seen as milkiness. With excess CO₂, soluble calcium hydrogencarbonate forms and the milkiness disappears.
Go deeper: What does clearing the liquid mean?
The calcium has not vanished: its chemical form changes from an insoluble solid to dissolved hydrogencarbonate. Gas formation and precipitation are separate reactions.
Limestone, marble and chalk contain calcium carbonate. Acids can therefore react with these materials. Baking powder uses an acid–hydrogencarbonate reaction to create CO₂ pockets in dough.
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Question
A learner says, “The white solid dissolved because more gas shook it away.” Correct the explanation.
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Neutralisation and the Nature of Oxides
Explain neutralisation at ion level and use oxide reactions to identify acidic or basic behaviour.SEE THE CHEMISTRY
Hydronium and hydroxide become water.
Compare: does an oxide react like an acid or like a base?
H₃O⁺ + OH⁻ → 2H₂O
The acid and base remove each other’s excess acidic or basic ions. Na⁺ and Cl⁻ remain in solution; salt crystals do not suddenly appear.
- Neutralisation
- An acid–base reaction; the chapter’s examples form a salt and water.
Hydronium reacts with hydroxide to form water. In dilute HCl and NaOH, sodium and chloride remain dissolved as spectator ions.
Black copper oxide reacts with HCl to give blue-green copper chloride solution and water: it behaves as a basic oxide. CO₂ reacts with lime water as an acidic oxide.
Neutralisation Reaction
NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)Sodium hydroxide (base) reacts with hydrochloric acid (acid) to produce sodium chloride (salt) and water. The acidic and basic properties are neutralized.
Go deeper: Does neutralisation always finish at pH 7?
A neutralisation reaction can occur while one reactant remains in excess. A neutral final solution requires the appropriate reacting amounts and, for the simple pH-7 case at 25°C, a strong acid and strong base.
Many metallic oxides are basic and many non-metallic oxides acidic, but there are exceptions. Copper oxide is a compound, not copper metal: its reaction with HCl produces water rather than hydrogen.
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Question
After NaOH reacts with HCl, sodium chloride crystals are not visible. Has a salt still formed?
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The Role of Ions in Acids and Bases
Identify which particles carry charge in a solution and why dissolved glucose behaves differently from an acid.SEE THE CHEMISTRY
Ions carry current through the solution.
Predict: will every dissolved substance make the bulb glow?
In water, HCl gives H₃O⁺ and Cl⁻. Both kinds of mobile ion carry charge through the liquid; electrons carry charge in the metal wires.
- Alkali
- A base that dissolves in water and produces hydroxide ions in solution.
HCl in water produces H₃O⁺ and Cl⁻. Mobile positive and negative ions carry charge through the solution; electrons carry charge in the metal wires.
Glucose dissolves mainly as uncharged molecules. Under comparable conditions it does not make this bulb glow, even though glucose contains hydrogen atoms.
Go deeper: Dissolving is not the same as making ions
Dissolving distributes a substance in water; ionisation creates charged species from molecules. Dissociation separates ions already present in an ionic compound such as NaOH.
Pure water has a very small ion concentration, so its conductivity is very low, not absolutely zero. Dissolved salts or gases can increase conductivity. A salt solution can conduct without being acidic.
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Question
A salt solution makes the bulb glow. A learner calls it an acid. Is that conclusion justified?
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Dilution and Safety Precautions
Separate water’s role in forming hydrated ions from dilution and acid strength.SEE THE CHEMISTRY
Same ions. More water. Lower concentration.
Compare: water changes concentration; does it change an acid’s strength?
The same acid ions are spread through a larger volume. Dilution changes concentration; it does not turn a strong acid into a weak acid.
- Dilution
- A decrease in concentration caused by adding solvent, usually water in this chapter.
Dry HCl does not turn dry blue litmus red. Water on the paper lets HCl form hydrated hydrogen ions, which change the indicator.
Adding water spreads the acid through a larger volume and lowers its concentration. A strong acid ionises almost completely; a weak acid only partially ionises. Dilution does not change which acid it is.
Remember the order: acid into water. Concentrated acids and their dilution are handled by the teacher with laboratory protection.
Go deeper: Concentration, strength and heat are different ideas
In the fixed-ion model, tripling the volume divides concentration by three. It represents a fully ionised acid; weak-acid equilibria can change the proportion ionised during dilution.
Concentrated acid dilution releases heat. In the teacher-led demonstration, acid is added slowly to water with stirring so heat spreads through the larger water volume. Adding water to concentrated acid can cause dangerous local heating and splashing.
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Question
An ideal fully ionised acid occupies 40 mL. After adding water its volume is 160 mL. What fraction of the original concentration remains?
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