ChemistryICSE Class 10 · Study of Acids, Bases and Salts

Types of Salts and Action of Dilute Acids on Salts (ICSE Class 10)

Updated 17 October 2026 · Fact-checked

A salt is formed when the hydrogen of an acid is wholly or partly replaced by a metal or ammonium ion. A normal salt (NaCl) has no replaceable hydrogen or hydroxyl; an acid salt (NaHSO₄) still has replaceable hydrogen; a basic salt (Pb(OH)Cl) still has hydroxyl; a complex salt (K₄[Fe(CN)₆]) contains a complex ion. Dilute acids release CO₂ from carbonates and hydrogen carbonates, SO₂ from sulphites and hydrogen sulphites, and H₂S from sulphides.

Understand Types of Salts and Action of Dilute Acids on Salts

1. Types of salts.

Normal salt: formed when all the replaceable hydrogen of an acid is replaced by a metal (or ammonium). It has no replaceable H and no OH. Examples: NaCl, Na₂SO₄, CaCO₃, KNO₃, (NH₄)₂SO4.
Acid salt: formed when only part of the hydrogen of a polybasic acid is replaced. It still contains replaceable hydrogen. Its solution may be acidic (NaHSO₄, NaHSO₃), but NaHCO₃ solution is weakly alkaline. Examples: sodium hydrogen sulphate NaHSO₄, sodium hydrogen carbonate NaHCO₃, sodium hydrogen sulphite NaHSO₃. Preparation: $N a O H + H_{2} S O_{4} \to N a H S O_{4} + H_{2} O$ (excess acid).
Basic salt: formed when the hydroxyl groups of a base are partly replaced by the acid radical. It still contains the OH group. Examples: basic lead chloride Pb(OH)Cl, basic magnesium chloride Mg(OH)Cl, basic copper carbonate.
Complex salt: a salt in which a metal is joined to several molecules or ions in a complex ion that stays together in solution. Examples: tetraamminecopper(II) sulphate $[C u (N H_{3})_{4}] S O_{4}$ and potassium hexacyanoferrate(II) $K_{4} [F e (C N)_{6}]$ .

A normal salt is not always neutral. Na₂CO₃ gives an alkaline solution; NaHSO₄ gives an acidic solution.

2. Action of dilute acids on salts. A dilute acid decomposes the salts of weaker (or volatile) acids and releases a gas. Heating is needed if the reaction is slow.

Salt	Gas released	Test for the gas
Carbonate or hydrogen carbonate	carbon dioxide, CO₂	turns lime water milky
Sulphite or hydrogen sulphite	sulphur dioxide, SO₂	pungent smell of burning sulphur; turns acidified potassium dichromate from orange to green
Sulphide	hydrogen sulphide, H₂S	rotten-egg smell; turns lead acetate paper black

3. Equations.

$N a_{2} C O_{3} + 2 H C l \to 2 N a C l + H_{2} O + C O_{2}$
$C a C O_{3} + 2 H C l \to C a C l_{2} + H_{2} O + C O_{2}$
$N a H C O_{3} + H C l \to N a C l + H_{2} O + C O_{2}$
$N a_{2} S O_{3} + H_{2} S O_{4} \to N a_{2} S O_{4} + H_{2} O + S O_{2}$
$N a H S O_{3} + H C l \to N a C l + H_{2} O + S O_{2}$
$Z n S + H_{2} S O_{4} \to Z n S O_{4} + H_{2} S$
$F e S + 2 H C l \to F e C l_{2} + H_{2} S$

4. Gas volume calculations. At S.T.P., 1 mole of any gas occupies 22.4 dm³. Convert the mass of salt to moles, use the equation to find moles of gas, then multiply by 22.4 dm³.
For example, 10.6 g of Na₂CO₃ (molar mass 106 g/mol) is 0.1 mol. It gives 0.1 mol of CO₂, which is 0.1 x 22.4 = 2.24 dm³ at S.T.P.

Types of Salts and Action of Dilute Acids on Salts: formulas and key facts

Salts at a glance: Normal no replaceable H or OH (NaCl)
Acid replaceable H left (NaHSO₄)
Basic OH left (Pb(OH)Cl)
Complex contains a complex ion ([Cu(NH₃)₄]SO₄)
Carbonate with acid: $N a_{2} C O_{3} + 2 H C l \to 2 N a C l + H_{2} O + C O_{2}$
Sulphite with acid: $N a_{2} S O_{3} + H_{2} S O_{4} \to N a_{2} S O_{4} + H_{2} O + S O_{2}$
Sulphide with acid: $Z n S + H_{2} S O_{4} \to Z n S O_{4} + H_{2} S$
Gas tests: CO₂ lime water milky
SO₂ acidified K₂Cr2O7 orange to green
H₂S lead acetate paper black

Normal	no replaceable H or OH (NaCl)
Acid	replaceable H left (NaHSO₄)
Basic	OH left (Pb(OH)Cl)
Complex	contains a complex ion ([Cu(NH₃)₄]SO₄)

CO₂	lime water milky
SO₂	acidified K₂Cr2O7 orange to green
H₂S	lead acetate paper black

How to solve Types of Salts and Action of Dilute Acids on Salts questions

Follow this routine for equations and for identifying an unknown salt.

1Identify the negative radical: carbonate (CO₃), hydrogen carbonate (HCO₃), sulphite (SO₃), hydrogen sulphite (HSO₃) or sulphide (S).
2Write salt + dilute acid giving a new salt, water (if the radical has oxygen) and the gas.
3Balance the metal, the radicals and then hydrogen/oxygen.
4State the observation (effervescence, smell) and the test for the gas, with the colour change.
5For mass or volume, convert grams to moles, use the equation mole ratio, then use 22.4 dm³ per mole at S.T.P.

Types of Salts and Action of Dilute Acids on Salts in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Why some antacid tablets fizz and eggs smell when a metal sulphide meets acid

Suppose an antacid tablet that contains sodium hydrogen carbonate is dropped into a glass of dilute hydrochloric acid and fizzes, while a different sample containing iron(II) sulphide gives a rotten-egg smell on adding the same acid.

Name the gas in each case and write one equation.

Try it first, then show the solutionSolution

The hydrogen carbonate releases carbon dioxide: $N a H C O_{3} + H C l \to N a C l + H_{2} O + C O_{2}$ .
The sulphide releases hydrogen sulphide, with a rotten-egg smell: $F e S + 2 H C l \to F e C l_{2} + H_{2} S$ .

Answer: CO₂ from the hydrogen carbonate; H₂S from the sulphide.

Memory hook: Fizz means CO₂; rotten eggs mean H₂S.

Tricks to remember Types of Salts and Action of Dilute Acids on Salts

Short memory aids. Say each one aloud once, then use it on a question the same day.

N-A-B-C
Normal: no H left. Acid: H left. Basic: OH left. Complex: a complex ion present.
Use it when: Classifying the four types of salt.
Gas by smell
CO₂ has no smell, SO₂ smells of burning sulphur, H₂S smells of rotten eggs.
Use it when: Identifying the gas from the salt.
Orange to green, white to milky, to black
SO₂: dichromate orange to green. CO₂: lime water to milky. H₂S: lead acetate to black.
Use it when: Tests in one line.

Types of Salts and Action of Dilute Acids on Salts: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Classify · 2 marks · Short answerBoard level

Classify as normal, acid, basic or complex: (a) NaHCO₃, (b) KNO₃, (c) Pb(OH)Cl, (d) K₄[Fe(CN)₆].

Try it first, then show the solutionSolution

(a) NaHCO₃ still has replaceable hydrogen: acid salt.
(b) KNO₃: normal salt.
(c) Pb(OH)Cl still has a hydroxyl: basic salt.
(d) K₄[Fe(CN)₆] contains a complex ion: complex salt.

Answer: (a) acid (b) normal (c) basic (d) complex.

Example 2: Gas and test · 3 marks · Short answerBoard level

A dilute acid is added to sodium sulphite and warmed. Name the gas, give the equation and state one test for it.

Try it first, then show the solutionSolution

Gas: sulphur dioxide.
$N a_{2} S O_{3} + H_{2} S O_{4} \to N a_{2} S O_{4} + H_{2} O + S O_{2}$ .
Test: acidified potassium dichromate paper turns from orange to green.

Answer: SO₂; as above; orange to green.

Example 3: Volume of carbon dioxide · 3 marks · Short answerHarder board level

Calculate the volume of carbon dioxide at S.T.P. obtained when 20 g of calcium carbonate reacts with excess dilute hydrochloric acid. (Ca = 40, C = 12, O = 16; 1 mole of gas = 22.4 dm³)

Try it first, then show the solutionSolution

$C a C O_{3} + 2 H C l \to C a C l_{2} + H_{2} O + C O_{2}$ ; molar mass of CaCO₃ = 40 + 12 + 48 = 100 g/mol.
Moles of CaCO₃ = 20 / 100 = 0.2 mol, which gives 0.2 mol CO₂.
Volume = 0.2 x 22.4 = 4.48 dm³.

Answer: 4.48 dm³.

Example 4: Mass of sulphide · 3 marks · Short answerHarder board level

What mass of iron(II) sulphide is needed to give 2.24 dm³ of hydrogen sulphide at S.T.P. with dilute hydrochloric acid? (Fe = 56, S = 32)

Try it first, then show the solutionSolution

$F e S + 2 H C l \to F e C l_{2} + H_{2} S$ ; moles of H₂S = 2.24 / 22.4 = 0.1 mol.
Moles of FeS = 0.1 mol; molar mass = 56 + 32 = 88 g/mol.
Mass = 0.1 x 88 = 8.8 g.

Answer: 8.8 g.

Example 5: Identify the unknown · 5 marks · Long answerHOTS / competency

A white solid X effervesces with dilute hydrochloric acid to give a colourless, odourless gas that turns lime water milky; with excess gas the milkiness disappears. (a) Name the gas. (b) Give two possible identities of X. (c) State what the milkiness is. (d) Write the equation for one X of your choice. (e) How would you show X is not a sulphite?

Try it first, then show the solutionSolution

(a) Carbon dioxide.
(b) A carbonate or a hydrogen carbonate, such as sodium carbonate or sodium hydrogen carbonate.
(c) Milkiness is calcium carbonate.
(d) $N a_{2} C O_{3} + 2 H C l \to 2 N a C l + H_{2} O + C O_{2}$ .
(e) The gas has no smell and does not change acidified dichromate from orange to green.

Answer: CO₂; carbonate or hydrogen carbonate; calcium carbonate; equation; no SO₂ test.

Where students lose marks in Types of Salts and Action of Dilute Acids on Salts

The slips examiners see most often, and what to write instead.

Mistake: Writing the carbonate reaction without water
The equation is unbalanced when H₂O is omitted.
Wrong working
Na₂CO₃ + 2HCl -> 2NaCl + CO₂
Right working
Na₂CO₃ + 2HCl -> 2NaCl + H₂O + CO₂
Do this: Balance hydrogen and oxygen after metals and radicals.
Usually costs: 1 mark.
Mistake: Mixing up the gas tests
SO₂ and CO₂ are both colourless; candidates give lime water for SO₂ (which also turns lime water milky).
Do this: Use acidified potassium dichromate for SO₂ and lime water for CO₂.
Usually costs: 1 mark.
Mistake: Calling NaHSO₄ a normal salt
The remaining hydrogen is overlooked.
Do this: If a replaceable H is left, it is an acid salt.
Usually costs: 1 mark.
Mistake: Forgetting the molar mass step in gas volume problems
Candidates use grams in place of moles with 22.4 dm³.
Do this: Convert to moles first.
Usually costs: 1 to 2 marks.

Exam tips

Always state both the observation and the test with its colour change.
Learn two examples of each type of salt, one from the syllabus-friendly list above.
For gas volume numericals, write moles, mole ratio and 22.4 dm³ in separate lines.
Mention heating when the question says "warm" or "heated" for sulphites and sulphides.

Practice questions on Types of Salts and Action of Dilute Acids on Salts

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
Which of the following is an acid salt?
(a) Na₂SO₄
(b) NaHSO₄
(c) Pb(OH)Cl
(d) NaCl
Hint
Look for replaceable hydrogen.
Answer
(b) NaHSO₄.
Q2 · 1 mark · MCQBoard level
The gas that turns lead acetate paper black is:
(a) CO₂
(b) SO₂
(c) H₂S
(d) NH₃
Hint
Rotten-egg smell.
Answer
(c) H₂S.
Q3 · 2 marks · Short answerBoard level
Write the equation for the reaction of dilute hydrochloric acid with sodium hydrogen carbonate and state what you observe.
Hint
A gas is evolved.
Answer
$N a H C O_{3} + H C l \to N a C l + H_{2} O + C O_{2}$ ; brisk effervescence of a colourless gas that turns lime water milky.
Q4 · 3 marks · Short answerHarder board level
Define an acid salt and a basic salt with one example each, and write one equation to prepare the acid salt you named.
Hint
Partial replacement.
Answer
Acid salt: formed by partial replacement of the hydrogen of an acid, e.g. NaHSO₄, from $N a O H + H_{2} S O_{4} \to N a H S O_{4} + H_{2} O$ . Basic salt: formed by partial replacement of the OH of a base by an acid radical, e.g. Pb(OH)Cl.
Q5 · 3 marks · Short answerHarder board level
Find the volume of sulphur dioxide at S.T.P. produced when 12.6 g of sodium sulphite reacts with excess dilute sulphuric acid. (Na = 23, S = 32, O = 16)
Hint
Molar mass 126.
Answer
Molar mass of Na₂SO₃ = 46 + 32 + 48 = 126 g/mol; moles = 12.6 / 126 = 0.1; SO₂ = 0.1 mol = 0.1 x 22.4 = 2.24 dm³.
Q6 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Sodium carbonate is a normal salt but its aqueous solution is alkaline.
Reason (R): It is formed from a strong alkali and a weak acid.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Normal does not mean neutral.
Answer
(a).
Q7 · 4 marks · Long answerHarder board level
When dilute sulphuric acid is added to a solid P, a gas Q with a smell of rotten eggs is evolved. Q turns lead acetate paper black. (a) Name P and Q. (b) Write the equation (take P as zinc sulphide). (c) What mass of P is needed to give 1.12 dm³ of Q at S.T.P.? (Zn = 65, S = 32)
Hint
Mole ratio is 1 : 1.
Answer
(a) P: a sulphide such as zinc sulphide; Q: hydrogen sulphide. (b) $Z n S + H_{2} S O_{4} \to Z n S O_{4} + H_{2} S$ . (c) Moles of Q = 1.12 / 22.4 = 0.05; mass of ZnS = 0.05 x 97 = 4.85 g.
Q8 · 4 marks · Long answerHOTS / competency
A student tests a salt solution with litmus and finds that the solution of the salt NaHSO₄ turns blue litmus red. (a) Is NaHSO₄ a normal salt? (b) Why does it turn blue litmus red? (c) Write the ionisation of NaHSO₄. (d) How does this differ from Na₂SO₄?
Hint
Remaining hydrogen.
Answer
(a) No, it is an acid salt. (b) It still has a replaceable hydrogen, which gives hydronium ions in water. (c) $N a H S O_{4} \to N a^{+} + H^{+} + S O_{4}^{2 -}$ . (d) Na₂SO₄ has no replaceable hydrogen and is neutral to litmus.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Formula of sodium hydrogen carbonate?Show answer
NaHCO₃.
2.Which gas is released when dilute sulphuric acid is added to zinc sulphide?Show answer
Hydrogen sulphide.
3.Test for sulphur dioxide?Show answer
Acidified potassium dichromate turns from orange to green.
4.Name one basic salt.Show answer
Basic lead chloride, Pb(OH)Cl.
5.What volume does 1 mole of gas occupy at S.T.P.?Show answer
22.4 dm³.

Types of Salts and Action of Dilute Acids on Salts: frequently asked questions

What is the difference between an acid salt and a normal salt?

An acid salt still has replaceable hydrogen; a normal salt has none.

Do all carbonates react with dilute acids?

With dilute hydrochloric acid, carbonates release carbon dioxide; gentle warming may be needed if the reaction is slow.

How do you tell SO₂ from CO₂?

SO₂ has a pungent smell and turns acidified potassium dichromate green; CO₂ is odourless and does not.

What is a complex salt?

A salt that contains a complex ion, in which a metal ion is bound to several ions or molecules.