Acid Bases and Salts Class 10 Notes ICSE — Rachna Chapter 3
These acid bases and salts class 10 notes icse walk through Chapter 3, Study of Acids, Bases and Salts, in the Rachna Sagar Class 10 Chemistry book. The chapter opens with the physical and chemical properties of bases. It then explains the pH scale, salt classification and preparation, solubility rules, water of crystallisation and acid rain. Keep this guide open beside your textbook.
Download Study of Acids Bases and Salts Class 10 ICSE Chemistry Chapter 3 PDF
Download the Study of Acids Bases and Salts Class 10 ICSE Chemistry chapter PDF from the Rachna Sagar textbook to revise offline or print for practice. It is the full chapter as printed in the book — 33 pages, 3 sections, 33 questions.
What is inside Chapter 3
33 pages · 3 sections · 33 exercise questions
| Section | Title |
|---|---|
| 31 | The physical properties of bases are |
| 32 | The chemical properties of bases are |
| 50 | Solubility Table of Salts |
- IMPORTANT QUESTIONS: 28 questions
- ANSWERS: 5 questions
31 The physical properties of bases are
Bases have a bitter taste. They feel slippery to touch. Those properties belong to the base itself, before any reaction takes place.
Soluble bases also give characteristic colour changes with indicators. These colour changes are the quickest way to recognise a base in the laboratory.
| Name of indicator | Colour change |
|---|---|
| Red litmus | Blue |
| Phenolphthalein | Pink |
| Methyl orange | Yellow |
So a solution that turns red litmus blue, phenolphthalein pink and methyl orange yellow is behaving like a soluble base.
32 The chemical properties of bases are
Chemical properties describe what a base does when it meets another substance. This part of the ICSE chapter is the largest because it ties bases to salts. Each reaction follows one pattern: the base supplies its negative part, and the other reactant supplies its positive part. For a quick refresher on ions, revisit the chemical bonding chapter before you continue.
Reaction with ammonium salts
Alkalis displace ammonia gas from ammonium salts. The reaction needs heat and produces the salt of the alkali, ammonia and water.
2\mathrm{NH_4Cl} + \mathrm{Ca(OH)_2} \xrightarrow{\Delta} \mathrm{CaCl_2} + 2\mathrm{NH_3} + 2\mathrm{H_2O}
\mathrm{NH_4Cl} + \mathrm{NaOH} \xrightarrow{\Delta} \mathrm{NaCl} + \mathrm{NH_3} + \mathrm{H_2O}
Ammonia is a colourless gas with a pungent, irritating smell. It turns Nessler’s reagent brown. It also gives dense white fumes with concentrated hydrochloric acid.
Absorption of carbon dioxide
Alkalis readily absorb carbon dioxide from the atmosphere. When potassium hydroxide or sodium hydroxide is left exposed to air, it slowly changes into its carbonate.
2\mathrm{KOH} + \mathrm{CO_2} \longrightarrow \mathrm{K_2CO_3} + \mathrm{H_2O}
2\mathrm{NaOH} + \mathrm{CO_2} \longrightarrow \mathrm{Na_2CO_3} + \mathrm{H_2O}
This is why alkali solutions are stored in closed bottles. Carbon dioxide is an acidic gas, so it combines with the alkali instead of escaping.
Neutralisation with acids
Bases react with acids to form salt and water. This reaction is called neutralisation. The base may be soluble or insoluble.
2\mathrm{NaOH} + \mathrm{H_2SO_4} \longrightarrow \mathrm{Na_2SO_4} + 2\mathrm{H_2O}
\mathrm{KOH} + \mathrm{HNO_3} \longrightarrow \mathrm{KNO_3} + \mathrm{H_2O}
\mathrm{Cu(OH)_2} + 2\mathrm{HNO_3} \longrightarrow \mathrm{Cu(NO_3)_2} + 2\mathrm{H_2O}
The first equation shows how sodium sulphate forms. The last shows that even an insoluble base like copper hydroxide neutralises an acid.
Precipitating insoluble bases
Alkalis precipitate different coloured insoluble bases from their soluble salts. The colour of the precipitate helps identify the metal.
| Salt solution | Alkali used | Precipitate and colour |
|---|---|---|
| Zinc chloride | Potassium hydroxide | Zinc hydroxide — white |
| Iron(II) chloride | Ammonium hydroxide | Iron(II) hydroxide — dirty green |
| Iron(III) chloride | Sodium hydroxide | Iron(III) hydroxide — reddish brown |
| Copper sulphate | Sodium hydroxide | Copper hydroxide — pale blue |
\mathrm{FeCl_3} + 3\mathrm{NaOH} \longrightarrow \mathrm{Fe(OH)_3}\downarrow + 3\mathrm{NaCl}
Insoluble bases also decompose on heating. They give the corresponding metallic oxide and water. Copper hydroxide, for example, turns into black copper oxide.
\mathrm{Cu(OH)_2} \xrightarrow{\Delta} \mathrm{CuO} + \mathrm{H_2O}
The pH scale
The pH scale tells whether a solution is acidic, alkaline or neutral. Pure water has pH 7. Acidic solutions have pH less than 7, and alkaline solutions have pH greater than 7.
A universal indicator is a mixture of several organic dyes. It does more than show acidity or alkalinity; it also predicts the strength of the solution.
In this ICSE chapter, salt solutions are often judged by pH. Ammonium chloride solution is acidic, sodium acetate solution is alkaline, and sodium chloride solution is neutral.
Definition and classification of salts
A salt is a chemical compound that, on dissolving in water, gives positive particles other than hydrogen ions and negative particles other than hydroxyl ions. Sodium chloride, for example, splits into sodium ions and chloride ions.
\mathrm{NaCl} \rightleftharpoons \mathrm{Na^+} + \mathrm{Cl^-}
| Salt type | How it forms | Examples |
|---|---|---|
| Normal salt | Complete replacement of the replaceable hydrogen ions of an acid | \mathrm{NaCl}, \mathrm{KNO_3}, \mathrm{CuSO_4}, \mathrm{Na_3PO_4} |
| Acid salt | Partial replacement of the hydrogen ions, so it still contains hydrogen | \mathrm{NaHSO_4}, \mathrm{KHSO_4}, \mathrm{NaH_2PO_4}, \mathrm{NaHSO_3} |
| Basic salt | Partial replacement of the hydroxyl ions of a diacidic or triacidic base | \mathrm{Pb(OH)Cl}, \mathrm{Cu(OH)Cl}, \mathrm{Mg(OH)Cl}, \mathrm{Pb(OH)NO_3} |
| Double salt | Two simple salts crystallise together from a saturated solution | Potash alum, Mohr’s salt, chrome alum |
| Mixed salt | Contains more than one acid radical or basic radical | \mathrm{CaOCl_2}, \mathrm{NaKCO_3} |
| Complex salt | Forms one complex ion and one simple ion on dissolving in water | \mathrm{Na[Ag(CN)_2]}, tetraammine copper(II) sulphate |
Preparation of soluble salts
Soluble salts are made by several different routes. The method depends on the starting materials available.
- By synthesis: heating the elements together.
2\mathrm{Fe} + 3\mathrm{Cl_2} \longrightarrow 2\mathrm{FeCl_3}
- By simple displacement: an active metal reacting with a dilute acid.
\mathrm{Zn} + \mathrm{H_2SO_4}\text{(dil.)} \longrightarrow \mathrm{ZnSO_4} + \mathrm{H_2}\uparrow
- By neutralisation: a base reacting with an acid.
\mathrm{NH_4OH} + \mathrm{HCl}\text{(dil.)} \longrightarrow \mathrm{NH_4Cl} + \mathrm{H_2O}
- From carbonates, sulphites or sulphides: reacting them with dilute acids.
\mathrm{CaCO_3} + 2\mathrm{HCl}\text{(dil.)} \longrightarrow \mathrm{CaCl_2} + \mathrm{H_2O} + \mathrm{CO_2}
- From chlorides or nitrates: heating them with concentrated sulphuric acid.
\mathrm{NaCl} + \mathrm{H_2SO_4}\text{(conc.)} \xrightarrow{\text{below }200^\circ\mathrm{C}} \mathrm{NaHSO_4} + \mathrm{HCl}
- From metals and alkali: zinc, lead and aluminium react with boiling alkali.
\mathrm{Zn} + 2\mathrm{NaOH} \xrightarrow{\text{boiling}} \mathrm{Na_2ZnO_2} + \mathrm{H_2}
Preparation of insoluble salts
Insoluble salts are prepared by synthesis or by double decomposition. In double decomposition, two soluble salts exchange partners and one product precipitates.
\mathrm{Fe} + \mathrm{S} \xrightarrow{\Delta} \mathrm{FeS}
\mathrm{Pb(NO_3)_2} + 2\mathrm{NaCl} \longrightarrow \mathrm{PbCl_2}\downarrow + 2\mathrm{NaNO_3}
\mathrm{BaCl_2} + \mathrm{Na_2SO_4} \longrightarrow \mathrm{BaSO_4}\downarrow + 2\mathrm{NaCl}
An insoluble salt cannot be made directly from another insoluble salt. It must first be converted into a soluble salt, usually with dilute nitric acid.
Worked pattern with original numbers: convert a 10.0 g sample of barium carbonate into barium sulphate. First dissolve the carbonate in dilute nitric acid.
\mathrm{BaCO_3} + 2\mathrm{HNO_3}\text{(dil.)} \longrightarrow \mathrm{Ba(NO_3)_2} + \mathrm{H_2O} + \mathrm{CO_2}
Then precipitate the sulphate from the soluble nitrate.
\mathrm{Ba(NO_3)_2} + \mathrm{Na_2SO_4} \longrightarrow \mathrm{BaSO_4}\downarrow + 2\mathrm{NaNO_3}
The final product is a white precipitate of barium sulphate, insoluble in all mineral acids.
Laboratory preparations
The chapter describes four common laboratory preparations.
- Iron(III) chloride: dry chlorine is passed over heated iron. The product is deliquescent, so anhydrous calcium chloride keeps moisture out.
- Copper sulphate: copper oxide, copper hydroxide or copper carbonate reacts with dilute sulphuric acid. The hot solution is filtered, evaporated and cooled to give blue \mathrm{CuSO_4 \cdot 5H_2O} crystals.
- Zinc sulphate: zinc reacts with dilute sulphuric acid. When effervescence stops, the excess zinc is filtered off and the solution is evaporated to get crystals.
- Sodium sulphate: titration first finds the exact acid-alkali ratio. Mixing them in that ratio and evaporating gives \mathrm{Na_2SO_4 \cdot 10H_2O}.
Exam pattern: the end-of-chapter questions ask for a named method of salt preparation and a balanced equation. Anion identification appears as gas tests and precipitate colours.
Properties of salts
| Property | What happens | Examples |
|---|---|---|
| Efflorescence | A hydrated salt loses water of crystallisation and crumbles to powder | \mathrm{Na_2CO_3 \cdot 10H_2O}, \mathrm{Na_2SO_4 \cdot 10H_2O}, \mathrm{MgSO_4 \cdot 7H_2O} |
| Deliquescence | A salt absorbs moisture and turns into a saturated solution | Ferric chloride, magnesium chloride |
| Hygroscopy | A substance absorbs moisture and becomes wet | Anhydrous calcium chloride, calcium oxide, concentrated sulphuric acid |
| Hydrolysis | A salt reacts with water to reform the parent acid or base | Ammonium chloride is acidic; sodium acetate is alkaline |
Hygroscopic substances are used as drying agents. Deliquescent salts go further and dissolve in the moisture they absorb.
Ammonium chloride is a salt of a strong acid and a weak base. Its hydrolysis leaves hydrogen ions, so the solution is acidic.
\mathrm{NH_4^+} + \mathrm{H_2O} \longrightarrow \mathrm{NH_4OH} + \mathrm{H^+}
Sodium acetate is a salt of a weak acid and a strong base. Its hydrolysis leaves hydroxyl ions, so the solution is alkaline.
\mathrm{CH_3COO^-} + \mathrm{H_2O} \longrightarrow \mathrm{CH_3COOH} + \mathrm{OH^-}
Salts of a strong acid and a strong base do not hydrolyse. Their solutions remain neutral.
Water of crystallisation
Water of crystallisation is the definite number of water molecules that enters loose chemical combination when a salt crystallises from its saturated solution.
Hydrated salts lose this water on heating. They lose their crystalline shape, and the colour often fades.
\mathrm{CuSO_4 \cdot 5H_2O} \xrightarrow{\Delta} \mathrm{CuSO_4} + 5\mathrm{H_2O}
Blue, crystalline hydrated copper sulphate becomes white, amorphous anhydrous copper sulphate.
Amphoteric metals, oxides and hydroxides
Zinc and aluminium displace hydrogen from acids as well as from alkalies. Substances that react with both are called amphoteric.
\mathrm{Zn} + \mathrm{H_2SO_4}\text{(dil.)} \longrightarrow \mathrm{ZnSO_4} + \mathrm{H_2}\uparrow
\mathrm{Zn} + 2\mathrm{NaOH} \xrightarrow{\text{heat}} \mathrm{Na_2ZnO_2} + \mathrm{H_2}\uparrow
2\mathrm{Al} + 2\mathrm{NaOH} + 2\mathrm{H_2O} \xrightarrow{\text{heat}} 2\mathrm{NaAlO_2} + 3\mathrm{H_2}\uparrow
The oxides and hydroxides of zinc and aluminium behave the same way. They react with acids and with alkalies to give salt and water only.
\mathrm{ZnO} + 2\mathrm{NaOH}\text{(dil.)} \xrightarrow{\Delta} \mathrm{Na_2ZnO_2} + \mathrm{H_2O}
\mathrm{Al_2O_3} + 2\mathrm{NaOH}\text{(dil.)} \xrightarrow{\Delta} 2\mathrm{NaAlO_2} + \mathrm{H_2O}
\mathrm{Zn(OH)_2} + 2\mathrm{NaOH}\text{(dil.)} \xrightarrow{\Delta} \mathrm{Na_2ZnO_2} + 2\mathrm{H_2O}
That is why \mathrm{ZnO}, \mathrm{Al_2O_3}, \mathrm{Zn(OH)_2} and \mathrm{Al(OH)_3} are called amphoteric.
50 Solubility Table of Salts
The solubility table tells you which salts dissolve in water and which precipitate. This predicts whether a double decomposition reaction will work.
| Rule | Exceptions |
|---|---|
| All salts of sodium, potassium and ammonium are soluble | None |
| All nitrates and nitrites are soluble | None |
| All sulphates are soluble | \mathrm{CaSO_4}, \mathrm{PbSO_4}, \mathrm{BaSO_4} |
| All chlorides are soluble | \mathrm{Hg_2Cl_2}, \mathrm{AgCl}, \mathrm{PbCl_2} — lead chloride dissolves in hot water |
| All sulphites are insoluble | \mathrm{Na_2SO_3}, \mathrm{K_2SO_3}, (\mathrm{NH_4})_2\mathrm{SO_3} |
| All sulphides are insoluble | \mathrm{Na_2S}, \mathrm{K_2S}, (\mathrm{NH_4})_2\mathrm{S} |
| All carbonates are insoluble | \mathrm{Na_2CO_3}, \mathrm{K_2CO_3}, (\mathrm{NH_4})_2\mathrm{CO_3} |
Mnemonic: for the insoluble groups, the sodium, potassium and ammonium salts are the SPA exceptions that stay soluble. For sulphates, remember Ca, Pb and Ba. For chlorides, remember Hg, Ag and Pb.
Common mistakes to avoid:
- Forgetting that \mathrm{CaSO_4}, \mathrm{PbSO_4} and \mathrm{BaSO_4} are insoluble sulphates.
- Forgetting that \mathrm{PbCl_2} dissolves in hot water but not in cold water.
- Confusing deliquescence with hygroscopy. Deliquescent salts form a saturated solution; hygroscopic substances only become wet.
- Trying to convert one insoluble salt into another in a single step. Always go through a soluble nitrate first.
For the official syllabus scope of this chapter, check the CISCE website.
Equation for the acid rain
Oxides of sulphur from oil refineries and oxides of nitrogen from automobile exhausts produce acid rain.
Sulphur dioxide combines with oxygen and water to form sulphuric acid.
2\mathrm{SO_2} + 2\mathrm{H_2O} + \mathrm{O_2} \longrightarrow 2\mathrm{H_2SO_4}
Nitrogen dioxide forms nitric acid through a chain of reactions.
\mathrm{N_2} + \mathrm{O_2} \xrightarrow{\text{thunder and lightning}} 2\mathrm{NO}
2\mathrm{NO} + \mathrm{O_2} \longrightarrow 2\mathrm{NO_2}
4\mathrm{NO_2} + 2\mathrm{H_2O} + \mathrm{O_2} \longrightarrow 4\mathrm{HNO_3}
What to practise from Acid Bases and Salts Class 10 ICSE
The exercise blocks in this book chapter are listed below. Attempt them after you finish the three sections above. The Class 10 Chemistry hub lists the other chapters in this book.
- IMPORTANT QUESTIONS — 28 questions
- ANSWERS — 5 questions
Acid Bases and Salts Class 10 ICSE Notes: Frequently Asked Questions
How are acid bases and salts class 10 notes icse organised in this chapter?
The chapter starts with the physical and chemical properties of bases. It then moves to the pH scale, salt classification, preparation methods, salt properties, water of crystallisation and amphoteric behaviour. A solubility table and the acid rain equations close the theory.
What is the difference between an acid salt and a normal salt?
A normal salt forms when all the replaceable hydrogen ions of an acid are replaced by a metal or basic radical. An acid salt forms when only some hydrogen ions are replaced, so it still contains hydrogen and its aqueous solution is acidic.
Why does hydrated copper sulphate turn white on heating?
Hydrated copper sulphate, \mathrm{CuSO_4 \cdot 5H_2O}, loses its five water molecules on heating. The blue crystalline solid becomes white anhydrous copper sulphate, and the crystals crumble to an amorphous powder.
How is lead sulphate prepared from lead carbonate in two steps?
First, react lead carbonate with dilute nitric acid to make soluble lead nitrate. Then add dilute sulphuric acid or a soluble sulphate to precipitate lead sulphate. Direct addition of acid to lead carbonate fails because the insoluble sulphate coats the carbonate and stops the reaction.
Which salts are insoluble in water according to the solubility table?
Sulphites, sulphides and carbonates are generally insoluble except their sodium, potassium and ammonium salts. Sulphates are soluble except calcium, lead and barium sulphates. Chlorides are soluble except mercurous chloride, silver chloride and lead chloride.
Reference: ICSE Class 10 Chemistry textbook material, chapter 3: Study of Acids Bases and Salts — CISCE-aligned.
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