Acids, Bases and Salts — Part 2: Acids on salts and preparing salts: visual lesson, ICSE Class 10 Chemistry (Science Paper 2)

Lesson

Fizz, smell and precipitate

Part 2 — dilute acids release tell-tale gases from certain salts, and the solubility of a salt decides how we make it.

In this lesson you will

Write equations for dilute acids reacting with carbonates, hydrogen carbonates, sulphites, hydrogen sulphites and sulphides
Identify the gas evolved in each case with a test
Choose a method to prepare a given salt from its solubility
Write equations for preparing salts by each method

Table of three classes of salt, the gas each gives with dilute acid and a clue to the gas
Salt treated with dilute acid	Gas	Clue
Carbonate / hydrogen carbonate	CO₂	Lime water turns milky
Sulphite / hydrogen sulphite	SO₂	Smell of burning sulphur
Sulphide	H₂S	Smell of rotten eggs

Concept

Carbonates and hydrogen carbonates

Dilute acids decompose carbonates and hydrogen carbonates, giving a salt, water and carbon dioxide with brisk effervescence. Test for CO₂: it turns lime water milky (CaCO₃ precipitate); it has no effect on acidified potassium dichromate.

\ce{Na2CO3 + 2HCl -> 2NaCl + H2O + CO2}

\ce{KHCO3 + HNO3 -> KNO3 + H2O + CO2}

\ce{Ca(OH)2 + CO2 -> CaCO3 + H2O}

Table of three carbonate and hydrogen carbonate reactions with dilute acids and their products
Reactants	Products
Na₂CO₃ + dil. HCl	NaCl, H₂O, CO₂
KHCO₃ + dil. HNO₃	KNO₃, H₂O, CO₂
ZnCO₃ + dil. H₂SO₄	ZnSO₄, H₂O, CO₂

Concept

Sulphites, hydrogen sulphites and sulphides

Sulphites and hydrogen sulphites + dilute acid → salt + water + sulphur dioxide (choking smell of burning sulphur). Test for SO₂: turns acidified potassium dichromate paper from orange to green (and acidified KMnO₄ from pink to colourless); it also turns lime water milky. Sulphides + dilute acid → salt + hydrogen sulphide (smell of rotten eggs). Test for H₂S: turns lead acetate paper silvery black (PbS).

\ce{Na2SO3 + H2SO4 -> Na2SO4 + H2O + SO2}

\ce{NaHSO3 + HCl -> NaCl + H2O + SO2}

\ce{ZnS + H2SO4 -> ZnSO4 + H2S}

Table of the smells and tests for sulphur dioxide, hydrogen sulphide and carbon dioxide
Gas	Smell	Test
SO₂	Burning sulphur (choking)	Acidified K₂Cr₂O₇: orange → green
H₂S	Rotten eggs	Lead acetate paper: turns black
CO₂	None	Lime water: milky

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