Why Sulphuric Acid Can Make Two Sodium Salts but Hydrochloric Acid Makes One
Classify acids by concentration, strength and basicity and bases by solubility, strength and acidity, write the reactions of dilute acids with carbonates, bicarbonates, sulphites, bisulphites and sulphides, and confirm carbon dioxide, sulphur dioxide and hydrogen sulphide with the right test.
Why can sulphuric acid form two different sodium salts?
Add sodium hydroxide to hydrochloric acid in any proportion you like and only one salt can form — sodium chloride. Do the same with sulphuric acid and there are two possibilities. With half the alkali you get sodium hydrogen sulphate; with the full amount you get sodium sulphate.
The difference is the number of hydrogen atoms each acid can give up. A molecule of hydrochloric acid has one replaceable hydrogen; a molecule of sulphuric acid has two, and they can be replaced one at a time. That number is called the basicity of the acid, and it is one of three ways of classifying acids.
Bases are classified in three matching ways, and one of those — the number of hydroxyl ions a base provides — is the mirror image of basicity.
The second half of this part is about gases. Dilute acids release a gas from five kinds of salt, and the gas tells you which kind of salt you started with:
- Carbonates and hydrogen carbonates give carbon dioxide
- Sulphites and hydrogen sulphites give sulphur dioxide
- Sulphides give hydrogen sulphide
All of these happen in ordinary life. Baking soda stirred into a batter made with sour curd releases carbon dioxide that makes the batter rise. Marble flooring loses its shine where lemon juice or vinegar is spilt and left, because marble is a carbonate. The rotten-egg smell near some hot springs is hydrogen sulphide.
Each gas has a confirmatory test, and the tests are chosen so that no two gases can be confused — including the one case where the most familiar test, lime water, gives the same result for two different gases.
This page covers the second part of the ICSE Class 10 Chemistry chapter on acids, bases and salts: classifying acids and bases, the action of dilute acids on five kinds of salt, and the tests for the gases evolved.
The difference is the number of hydrogen atoms each acid can give up. A molecule of hydrochloric acid has one replaceable hydrogen; a molecule of sulphuric acid has two, and they can be replaced one at a time. That number is called the basicity of the acid, and it is one of three ways of classifying acids.
Bases are classified in three matching ways, and one of those — the number of hydroxyl ions a base provides — is the mirror image of basicity.
The second half of this part is about gases. Dilute acids release a gas from five kinds of salt, and the gas tells you which kind of salt you started with:
- Carbonates and hydrogen carbonates give carbon dioxide
- Sulphites and hydrogen sulphites give sulphur dioxide
- Sulphides give hydrogen sulphide
All of these happen in ordinary life. Baking soda stirred into a batter made with sour curd releases carbon dioxide that makes the batter rise. Marble flooring loses its shine where lemon juice or vinegar is spilt and left, because marble is a carbonate. The rotten-egg smell near some hot springs is hydrogen sulphide.
Each gas has a confirmatory test, and the tests are chosen so that no two gases can be confused — including the one case where the most familiar test, lime water, gives the same result for two different gases.
This page covers the second part of the ICSE Class 10 Chemistry chapter on acids, bases and salts: classifying acids and bases, the action of dilute acids on five kinds of salt, and the tests for the gases evolved.
How are acids classified by concentration, strength and basicity?
Concentration says how much acid is in the water, strength says how completely the acid ionises, and basicity says how many hydronium ions one molecule can produce.
1. By concentration.
- Concentrated acid — a large proportion of acid and little water
- Dilute acid — a small proportion of acid in a large amount of water
A safety rule follows directly. When diluting a concentrated acid, always add the acid slowly to water, never water to the acid. The mixing releases a great deal of heat, and a little water added to a lot of acid can boil and spatter.
2. By strength.
- Strong acid — ionises almost completely in aqueous solution, giving a high concentration of hydronium ions: hydrochloric, nitric and sulphuric acids
- Weak acid — ionises only partially, so most molecules stay un-ionised: acetic, carbonic, formic and sulphurous acids, and hydrogen sulphide
The single arrow for nitric acid and the double arrow for acetic acid are the whole difference, written in symbols.
Strength and concentration are independent. Vinegar is a dilute solution of a weak acid; the acid in a vehicle battery is a fairly concentrated solution of a strong acid. A dilute strong acid and a concentrated weak acid are both perfectly possible — mixing up the two words is the commonest error in this topic.
3. By basicity. The basicity of an acid is the number of hydronium ions that one molecule of the acid produces in aqueous solution — the number of replaceable hydrogen atoms in the molecule.
- Monobasic — basicity : hydrochloric acid , nitric acid , acetic acid
- Dibasic — basicity : sulphuric acid , carbonic acid , sulphurous acid
- Tribasic — basicity : phosphoric acid
Basicity decides how many series of salts an acid forms.
- A monobasic acid forms one — only normal salts, such as
- A dibasic acid forms two — and
- A tribasic acid forms three — , and
Worked example 1 — the boundary case. Acetic acid, , contains four hydrogen atoms. What is its basicity?
**Only the hydrogen of the group ionises; the three hydrogens bonded to carbon do not. Basicity . Basicity counts replaceable hydrogen, not total hydrogen.
Worked example 2. How many sodium salts can carbonic acid form, and what are they?
Basicity , so two**: sodium hydrogen carbonate — the baking soda in your kitchen — and sodium carbonate , washing soda.
An everyday example of strength. The citric acid in lemon juice is a weak acid, which is part of why it is safe to eat. Hydrochloric acid of the same concentration would be far more corrosive, because it ionises completely and produces many more hydronium ions.
1. By concentration.
- Concentrated acid — a large proportion of acid and little water
- Dilute acid — a small proportion of acid in a large amount of water
A safety rule follows directly. When diluting a concentrated acid, always add the acid slowly to water, never water to the acid. The mixing releases a great deal of heat, and a little water added to a lot of acid can boil and spatter.
2. By strength.
- Strong acid — ionises almost completely in aqueous solution, giving a high concentration of hydronium ions: hydrochloric, nitric and sulphuric acids
- Weak acid — ionises only partially, so most molecules stay un-ionised: acetic, carbonic, formic and sulphurous acids, and hydrogen sulphide
The single arrow for nitric acid and the double arrow for acetic acid are the whole difference, written in symbols.
Strength and concentration are independent. Vinegar is a dilute solution of a weak acid; the acid in a vehicle battery is a fairly concentrated solution of a strong acid. A dilute strong acid and a concentrated weak acid are both perfectly possible — mixing up the two words is the commonest error in this topic.
3. By basicity. The basicity of an acid is the number of hydronium ions that one molecule of the acid produces in aqueous solution — the number of replaceable hydrogen atoms in the molecule.
- Monobasic — basicity : hydrochloric acid , nitric acid , acetic acid
- Dibasic — basicity : sulphuric acid , carbonic acid , sulphurous acid
- Tribasic — basicity : phosphoric acid
Basicity decides how many series of salts an acid forms.
- A monobasic acid forms one — only normal salts, such as
- A dibasic acid forms two — and
- A tribasic acid forms three — , and
Worked example 1 — the boundary case. Acetic acid, , contains four hydrogen atoms. What is its basicity?
**Only the hydrogen of the group ionises; the three hydrogens bonded to carbon do not. Basicity . Basicity counts replaceable hydrogen, not total hydrogen.
Worked example 2. How many sodium salts can carbonic acid form, and what are they?
Basicity , so two**: sodium hydrogen carbonate — the baking soda in your kitchen — and sodium carbonate , washing soda.
An everyday example of strength. The citric acid in lemon juice is a weak acid, which is part of why it is safe to eat. Hydrochloric acid of the same concentration would be far more corrosive, because it ionises completely and produces many more hydronium ions.
How are bases classified by solubility, strength and acidity?
Solubility separates alkalis from other bases, strength says how completely an alkali produces hydroxyl ions, and acidity says how many hydroxyl ions one molecule of a base provides.
1. By solubility.
- Soluble bases, or alkalis: sodium hydroxide , potassium hydroxide , ammonium hydroxide , and calcium hydroxide , which is only slightly soluble
- Insoluble bases: copper(II) oxide , zinc oxide , copper(II) hydroxide , iron(III) hydroxide
2. By strength.
- Strong alkali — dissociates almost completely in water, giving a high concentration of hydroxyl ions: sodium hydroxide and potassium hydroxide
- Weak alkali — ionises only partially: ammonium hydroxide
A boundary case involving solubility. Lime water is a dilute alkali, not because calcium hydroxide fails to dissociate, but because very little of it dissolves in the first place. Solubility limits how many hydroxyl ions can be present, which is a different question from how completely the dissolved part dissociates.
3. By acidity. The acidity of a base is the number of hydroxyl ions that one molecule of the base produces — equivalently, the number of hydronium ions of an acid that one molecule can neutralise.
- Monoacidic — acidity : , ,
- Diacidic — acidity : , ,
- Triacidic — acidity : ,
A rule that makes neutralisation equations quick to balance. In complete neutralisation, the hydronium ions supplied must equal the hydroxyl ions supplied:
Worked example 1. Sulphuric acid, basicity , is neutralised by sodium hydroxide, acidity . How many molecules of alkali per molecule of acid?
So two, which is the in the equation for sodium sulphate.
Worked example 2. Aluminium hydroxide, acidity , is neutralised by phosphoric acid, basicity .
One molecule of each:
Worked example 3 — partial neutralisation of a base. Copper(II) hydroxide is diacidic. If only one of its hydroxyl groups is neutralised, a salt that still contains a hydroxyl group forms:
That is a basic salt, the counterpart of the acid salt — both are classified in Part 3.
An everyday note on the word. In daily speech, acidity means a burning stomach. In chemistry, the acidity of a base is simply a count of its hydroxyl ions — so calcium hydroxide, the slaked lime in whitewash, has an acidity of two while being thoroughly alkaline.
1. By solubility.
- Soluble bases, or alkalis: sodium hydroxide , potassium hydroxide , ammonium hydroxide , and calcium hydroxide , which is only slightly soluble
- Insoluble bases: copper(II) oxide , zinc oxide , copper(II) hydroxide , iron(III) hydroxide
2. By strength.
- Strong alkali — dissociates almost completely in water, giving a high concentration of hydroxyl ions: sodium hydroxide and potassium hydroxide
- Weak alkali — ionises only partially: ammonium hydroxide
A boundary case involving solubility. Lime water is a dilute alkali, not because calcium hydroxide fails to dissociate, but because very little of it dissolves in the first place. Solubility limits how many hydroxyl ions can be present, which is a different question from how completely the dissolved part dissociates.
3. By acidity. The acidity of a base is the number of hydroxyl ions that one molecule of the base produces — equivalently, the number of hydronium ions of an acid that one molecule can neutralise.
- Monoacidic — acidity : , ,
- Diacidic — acidity : , ,
- Triacidic — acidity : ,
A rule that makes neutralisation equations quick to balance. In complete neutralisation, the hydronium ions supplied must equal the hydroxyl ions supplied:
Worked example 1. Sulphuric acid, basicity , is neutralised by sodium hydroxide, acidity . How many molecules of alkali per molecule of acid?
So two, which is the in the equation for sodium sulphate.
Worked example 2. Aluminium hydroxide, acidity , is neutralised by phosphoric acid, basicity .
One molecule of each:
Worked example 3 — partial neutralisation of a base. Copper(II) hydroxide is diacidic. If only one of its hydroxyl groups is neutralised, a salt that still contains a hydroxyl group forms:
That is a basic salt, the counterpart of the acid salt — both are classified in Part 3.
An everyday note on the word. In daily speech, acidity means a burning stomach. In chemistry, the acidity of a base is simply a count of its hydroxyl ions — so calcium hydroxide, the slaked lime in whitewash, has an acidity of two while being thoroughly alkaline.
What happens when dilute acids act on carbonates, bicarbonates, sulphites, bisulphites and sulphides?
Carbonates and hydrogen carbonates give a salt, water and carbon dioxide in the cold; sulphites and hydrogen sulphites give a salt, water and sulphur dioxide on warming; sulphides give a salt and hydrogen sulphide.
1. Carbonates — no heating needed.
2. Hydrogen carbonates (bicarbonates) — no heating needed.
3. Sulphites — warm gently.
4. Hydrogen sulphites (bisulphites) — warm gently.
5. Sulphides — no heating needed.
Why sulphites need warming. Sulphur dioxide is very soluble in water, so in the cold much of it stays dissolved in the reaction mixture. Gentle warming drives it out so that it can be collected and tested. Carbon dioxide and hydrogen sulphide escape readily without heat.
Worked example — checking a balance. Is balanced?
- Potassium:
- Hydrogen: on the left; on the right
- Carbon:
- Sulphur:
- Oxygen: on the left; on the right
Balanced.
The boundary case that decides which acid to use. Marble chips, calcium carbonate, react steadily with dilute hydrochloric acid. With dilute sulphuric acid the reaction starts and then almost stops:
Calcium sulphate is only sparingly soluble, so it forms a coating on the marble that keeps the acid away. That is why carbon dioxide is prepared in the laboratory from marble and dilute hydrochloric acid, whose product, calcium chloride, dissolves. Lead carbonate behaves in the same way with hydrochloric acid, because lead chloride is only sparingly soluble in cold water.
An everyday example from the kitchen. Baking soda added to a batter made with sour curd or buttermilk reacts with the lactic acid in it. A hydrogen carbonate plus an acid gives carbon dioxide, and the bubbles trapped in the batter make it light and spongy — the same reaction as the second equation above, with a weak food acid in place of hydrochloric acid.
One more distinction worth knowing. Sodium hydrogen carbonate also gives carbon dioxide when heated on its own, with no acid at all, while sodium carbonate does not:
Heating the dry solid is therefore one way to tell the two sodium salts apart.
1. Carbonates — no heating needed.
2. Hydrogen carbonates (bicarbonates) — no heating needed.
3. Sulphites — warm gently.
4. Hydrogen sulphites (bisulphites) — warm gently.
5. Sulphides — no heating needed.
Why sulphites need warming. Sulphur dioxide is very soluble in water, so in the cold much of it stays dissolved in the reaction mixture. Gentle warming drives it out so that it can be collected and tested. Carbon dioxide and hydrogen sulphide escape readily without heat.
Worked example — checking a balance. Is balanced?
- Potassium:
- Hydrogen: on the left; on the right
- Carbon:
- Sulphur:
- Oxygen: on the left; on the right
Balanced.
The boundary case that decides which acid to use. Marble chips, calcium carbonate, react steadily with dilute hydrochloric acid. With dilute sulphuric acid the reaction starts and then almost stops:
Calcium sulphate is only sparingly soluble, so it forms a coating on the marble that keeps the acid away. That is why carbon dioxide is prepared in the laboratory from marble and dilute hydrochloric acid, whose product, calcium chloride, dissolves. Lead carbonate behaves in the same way with hydrochloric acid, because lead chloride is only sparingly soluble in cold water.
An everyday example from the kitchen. Baking soda added to a batter made with sour curd or buttermilk reacts with the lactic acid in it. A hydrogen carbonate plus an acid gives carbon dioxide, and the bubbles trapped in the batter make it light and spongy — the same reaction as the second equation above, with a weak food acid in place of hydrochloric acid.
One more distinction worth knowing. Sodium hydrogen carbonate also gives carbon dioxide when heated on its own, with no acid at all, while sodium carbonate does not:
Heating the dry solid is therefore one way to tell the two sodium salts apart.
Which gas is given off, and how do you confirm carbon dioxide, sulphur dioxide and hydrogen sulphide?
Carbon dioxide turns lime water milky but leaves acidified potassium dichromate unchanged; sulphur dioxide turns acidified potassium dichromate green; hydrogen sulphide smells of rotten eggs and blackens lead acetate paper.
1. Carbon dioxide.
- Colourless and odourless
- Turns lime water milky, because insoluble calcium carbonate forms:
- With excess gas, the milkiness disappears, as soluble calcium hydrogen carbonate forms:
- Has no effect on acidified potassium dichromate or on acidified potassium permanganate
2. Sulphur dioxide.
- Colourless, with the suffocating smell of burning sulphur
- Turns orange acidified potassium dichromate paper green:
- Decolourises pink acidified potassium permanganate:
- Also turns lime water milky, forming calcium sulphite:
3. Hydrogen sulphide.
- Colourless, with the smell of rotten eggs
- Turns moist lead acetate paper silvery black, as black lead sulphide forms:
The trap built into these tests. Lime water turns milky with both carbon dioxide and sulphur dioxide. A milky result alone does not prove carbon dioxide. The dichromate test separates them: green means sulphur dioxide, no change means carbon dioxide.
Worked check — is the dichromate equation balanced?
- Potassium ; chromium
- Sulphur: on the left; on the right
- Oxygen: on the left; on the right
- Hydrogen:
Balanced.
Worked example 1. A white solid gives a brisk effervescence with dilute hydrochloric acid in the cold. The gas turns lime water milky and does not affect acidified potassium dichromate.
The gas is carbon dioxide, so the solid is a carbonate or a hydrogen carbonate.
Worked example 2. A white solid gives a gas only on warming with dilute hydrochloric acid. The gas turns lime water milky and turns acidified potassium dichromate paper green.
The gas is sulphur dioxide, so the solid is a sulphite or a hydrogen sulphite. The need for warming is a second clue pointing the same way.
Worked example 3. A dark solid gives, in the cold, a gas with a smell of rotten eggs that turns lead acetate paper black.
The gas is hydrogen sulphide, so the solid is a sulphide — iron(II) sulphide is the usual laboratory example.
Worked example 4 — telling the two carbon dioxide sources apart. Both sodium carbonate and sodium hydrogen carbonate give carbon dioxide with dilute acid. Heat each dry solid in a test tube: only the hydrogen carbonate releases a gas that turns lime water milky.
An everyday link. The sharp smell of a freshly struck matchstick includes a little sulphur dioxide. Smell is never accepted as proof in an answer, though — the dichromate colour change is, because it responds to sulphur dioxide and not to the carbon dioxide present in every breath of air.
1. Carbon dioxide.
- Colourless and odourless
- Turns lime water milky, because insoluble calcium carbonate forms:
- With excess gas, the milkiness disappears, as soluble calcium hydrogen carbonate forms:
- Has no effect on acidified potassium dichromate or on acidified potassium permanganate
2. Sulphur dioxide.
- Colourless, with the suffocating smell of burning sulphur
- Turns orange acidified potassium dichromate paper green:
- Decolourises pink acidified potassium permanganate:
- Also turns lime water milky, forming calcium sulphite:
3. Hydrogen sulphide.
- Colourless, with the smell of rotten eggs
- Turns moist lead acetate paper silvery black, as black lead sulphide forms:
The trap built into these tests. Lime water turns milky with both carbon dioxide and sulphur dioxide. A milky result alone does not prove carbon dioxide. The dichromate test separates them: green means sulphur dioxide, no change means carbon dioxide.
Worked check — is the dichromate equation balanced?
- Potassium ; chromium
- Sulphur: on the left; on the right
- Oxygen: on the left; on the right
- Hydrogen:
Balanced.
Worked example 1. A white solid gives a brisk effervescence with dilute hydrochloric acid in the cold. The gas turns lime water milky and does not affect acidified potassium dichromate.
The gas is carbon dioxide, so the solid is a carbonate or a hydrogen carbonate.
Worked example 2. A white solid gives a gas only on warming with dilute hydrochloric acid. The gas turns lime water milky and turns acidified potassium dichromate paper green.
The gas is sulphur dioxide, so the solid is a sulphite or a hydrogen sulphite. The need for warming is a second clue pointing the same way.
Worked example 3. A dark solid gives, in the cold, a gas with a smell of rotten eggs that turns lead acetate paper black.
The gas is hydrogen sulphide, so the solid is a sulphide — iron(II) sulphide is the usual laboratory example.
Worked example 4 — telling the two carbon dioxide sources apart. Both sodium carbonate and sodium hydrogen carbonate give carbon dioxide with dilute acid. Heat each dry solid in a test tube: only the hydrogen carbonate releases a gas that turns lime water milky.
An everyday link. The sharp smell of a freshly struck matchstick includes a little sulphur dioxide. Smell is never accepted as proof in an answer, though — the dichromate colour change is, because it responds to sulphur dioxide and not to the carbon dioxide present in every breath of air.
Exam tip
What earns full marks on classification and gas-test questions?
Give the definition before the example, write the full balanced equation with state of heating, and name the test, the observation and the conclusion for every gas.
- Define basicity as the number of hydronium ions per molecule, and acidity as the number of hydroxyl ions per molecule
- Separate strength from concentration in every answer — use strong and weak for ionisation, concentrated and dilute for amount
- Use a double arrow for weak acids and ammonium hydroxide
- Count replaceable hydrogen, not total hydrogen — acetic acid is monobasic
- State whether heating is needed: none for carbonates, bicarbonates and sulphides; gentle warming for sulphites and bisulphites
- Balance every equation, including the coefficients of water and gas
- Give three parts for each gas test: reagent, colour change, gas identified
- Never use lime water alone to identify carbon dioxide — add the dichromate result
- Describe the paper as moist for the lead acetate test
- Explain why marble and dilute sulphuric acid stop reacting if asked about preparing carbon dioxide
The misconception to name. A concentrated acid is not the same as a strong acid. Concentrated acetic acid is still a weak acid, because most of its molecules remain un-ionised; dilute hydrochloric acid is still a strong acid, because what is present ionises almost completely. Using the two words interchangeably loses marks in definition questions.
A second trap. Writing the sulphite reaction without mentioning warming, or writing the carbonate reaction with heating. The heating condition is part of the expected answer, and it is also a clue in identification questions.
- Define basicity as the number of hydronium ions per molecule, and acidity as the number of hydroxyl ions per molecule
- Separate strength from concentration in every answer — use strong and weak for ionisation, concentrated and dilute for amount
- Use a double arrow for weak acids and ammonium hydroxide
- Count replaceable hydrogen, not total hydrogen — acetic acid is monobasic
- State whether heating is needed: none for carbonates, bicarbonates and sulphides; gentle warming for sulphites and bisulphites
- Balance every equation, including the coefficients of water and gas
- Give three parts for each gas test: reagent, colour change, gas identified
- Never use lime water alone to identify carbon dioxide — add the dichromate result
- Describe the paper as moist for the lead acetate test
- Explain why marble and dilute sulphuric acid stop reacting if asked about preparing carbon dioxide
The misconception to name. A concentrated acid is not the same as a strong acid. Concentrated acetic acid is still a weak acid, because most of its molecules remain un-ionised; dilute hydrochloric acid is still a strong acid, because what is present ionises almost completely. Using the two words interchangeably loses marks in definition questions.
A second trap. Writing the sulphite reaction without mentioning warming, or writing the carbonate reaction with heating. The heating condition is part of the expected answer, and it is also a clue in identification questions.
Did you know
Why does a drop of dilute acid tell a geologist whether a rock is limestone?
Geologists working in the field often carry a small dropper bottle of dilute hydrochloric acid. Put one drop on a rock and watch. If it fizzes at once, the rock contains calcium carbonate — limestone, chalk or marble. If nothing happens, it is something else, such as granite, made of silicate minerals.
The fizz is carbon dioxide, from exactly the reaction in this lesson:
A single drop of acid identifies a whole class of rock because only carbonates release a gas so readily from a cold, dilute acid.
The test has a subtle refinement that experienced geologists use. A rock called dolomite, a carbonate of calcium and magnesium, fizzes only weakly on a solid surface but fizzes clearly when a little of it is scratched into powder first. The larger surface area of the powder speeds up the reaction enough to see it — a neat demonstration that a reaction's speed depends on how much surface the acid can reach.
The same reaction happens in your home, more slowly. Marble floors and kitchen counters lose their shine where lemon juice, vinegar or tamarind water is spilt and left to stand. The weak acids in these foods react with the calcium carbonate of the marble, eating away the polished surface and leaving a dull patch. That is why marble surfaces are wiped quickly after acidic spills, and why acidic cleaners are never used on them.
And rainwater that has picked up acidic gases from the air slowly does the same to carbonate stone in buildings and statues, softening carved detail over a long time. Sulphuric acid in such rain produces calcium sulphate on the stone surface, which washes away far more easily than the original carbonate.
One reaction, then, links a geologist's dropper, a stained kitchen counter and a weathered carved stone — three different speeds of the same chemistry of an acid meeting a carbonate.
The fizz is carbon dioxide, from exactly the reaction in this lesson:
A single drop of acid identifies a whole class of rock because only carbonates release a gas so readily from a cold, dilute acid.
The test has a subtle refinement that experienced geologists use. A rock called dolomite, a carbonate of calcium and magnesium, fizzes only weakly on a solid surface but fizzes clearly when a little of it is scratched into powder first. The larger surface area of the powder speeds up the reaction enough to see it — a neat demonstration that a reaction's speed depends on how much surface the acid can reach.
The same reaction happens in your home, more slowly. Marble floors and kitchen counters lose their shine where lemon juice, vinegar or tamarind water is spilt and left to stand. The weak acids in these foods react with the calcium carbonate of the marble, eating away the polished surface and leaving a dull patch. That is why marble surfaces are wiped quickly after acidic spills, and why acidic cleaners are never used on them.
And rainwater that has picked up acidic gases from the air slowly does the same to carbonate stone in buildings and statues, softening carved detail over a long time. Sulphuric acid in such rain produces calcium sulphate on the stone surface, which washes away far more easily than the original carbonate.
One reaction, then, links a geologist's dropper, a stained kitchen counter and a weathered carved stone — three different speeds of the same chemistry of an acid meeting a carbonate.
Exam relevance
How do acid strength, basicity and gas tests carry into JEE and NEET?
This is foundation work for Class 11 Equilibrium and Class 12 The p-Block Elements, both examined in JEE Main and NEET Chemistry, and for the qualitative analysis section of JEE Main practical chemistry.
Where strong and weak lead. Class 11 turns ionises partially into a number — the degree of ionisation and the ionisation constant of a weak acid or base. Comparing acid strengths, calculating the pH of a weak acid, and explaining why acetic acid is weaker than hydrochloric acid are standard questions in both exams, and they rest on the single-arrow versus double-arrow distinction on this page.
Where basicity leads. Class 12 studies the oxoacids of phosphorus and sulphur, where basicity is decided by how many hydrogen atoms are attached to oxygen rather than directly to the central atom. Phosphoric acid is tribasic, phosphorous acid is dibasic and hypophosphorous acid is monobasic, even though all three contain three hydrogen atoms — a classic question in both JEE Main and NEET that is exactly the acetic acid argument applied to phosphorus.
Where the neutralisation rule leads. The relation between basicity, acidity and the number of molecules reacting becomes the basis of titration calculations in Class 11. Getting the factor of two for sulphuric acid right is the step most often missed in numericals on neutralisation.
Where the gas tests lead. JEE Main practical chemistry includes the chemical principles of qualitative salt analysis, and the tests for the carbonate and sulphide anions are part of it. The reactions on this page — dilute acid, gas evolved, confirmatory test — are exactly those principles. Class 12 p-block chemistry also asks for the reducing action of sulphur dioxide, which is what turns dichromate green.
Question types to expect. At this level: classification with examples, balanced equations with conditions, and gas identification. In competitive papers: ionisation constants and pH, basicity of oxoacids, titration numericals, and salt-analysis reasoning, often as match-the-column or assertion-reason items.
The single trap that costs marks. Counting all hydrogen atoms when finding basicity. Phosphorous acid has three hydrogen atoms but a basicity of two, because one hydrogen is bonded directly to phosphorus and cannot ionise. Both exams set this deliberately.
A second trap. Treating lime water as proof of carbon dioxide. Sulphur dioxide also turns it milky, and a salt-analysis question built around that confusion is a standard way to separate careful candidates from quick ones.
Board versus competitive emphasis. The ICSE paper marks definitions, balanced equations with heating conditions and the three-part gas test; a competitive paper marks a basicity value, a pH or an identification. The transferable habit is asking which hydrogen atoms can actually leave — the same question decides basicity here and in every oxoacid you meet later.
Where strong and weak lead. Class 11 turns ionises partially into a number — the degree of ionisation and the ionisation constant of a weak acid or base. Comparing acid strengths, calculating the pH of a weak acid, and explaining why acetic acid is weaker than hydrochloric acid are standard questions in both exams, and they rest on the single-arrow versus double-arrow distinction on this page.
Where basicity leads. Class 12 studies the oxoacids of phosphorus and sulphur, where basicity is decided by how many hydrogen atoms are attached to oxygen rather than directly to the central atom. Phosphoric acid is tribasic, phosphorous acid is dibasic and hypophosphorous acid is monobasic, even though all three contain three hydrogen atoms — a classic question in both JEE Main and NEET that is exactly the acetic acid argument applied to phosphorus.
Where the neutralisation rule leads. The relation between basicity, acidity and the number of molecules reacting becomes the basis of titration calculations in Class 11. Getting the factor of two for sulphuric acid right is the step most often missed in numericals on neutralisation.
Where the gas tests lead. JEE Main practical chemistry includes the chemical principles of qualitative salt analysis, and the tests for the carbonate and sulphide anions are part of it. The reactions on this page — dilute acid, gas evolved, confirmatory test — are exactly those principles. Class 12 p-block chemistry also asks for the reducing action of sulphur dioxide, which is what turns dichromate green.
Question types to expect. At this level: classification with examples, balanced equations with conditions, and gas identification. In competitive papers: ionisation constants and pH, basicity of oxoacids, titration numericals, and salt-analysis reasoning, often as match-the-column or assertion-reason items.
The single trap that costs marks. Counting all hydrogen atoms when finding basicity. Phosphorous acid has three hydrogen atoms but a basicity of two, because one hydrogen is bonded directly to phosphorus and cannot ionise. Both exams set this deliberately.
A second trap. Treating lime water as proof of carbon dioxide. Sulphur dioxide also turns it milky, and a salt-analysis question built around that confusion is a standard way to separate careful candidates from quick ones.
Board versus competitive emphasis. The ICSE paper marks definitions, balanced equations with heating conditions and the three-part gas test; a competitive paper marks a basicity value, a pH or an identification. The transferable habit is asking which hydrogen atoms can actually leave — the same question decides basicity here and in every oxoacid you meet later.
Key takeaways
What must you be able to do from this part?
Three ways to classify acids, three to classify bases, five reactions and three gas tests.
- Concentrated or dilute describes the amount of acid in water; strong or weak describes how completely it ionises
- Strong acids: hydrochloric, nitric, sulphuric. Weak acids: acetic, carbonic, formic, sulphurous, hydrogen sulphide
- Always add acid to water, never water to acid
- Basicity is the number of hydronium ions per molecule: monobasic , , ; dibasic , , ; tribasic
- Acetic acid is monobasic despite four hydrogen atoms — only replaceable hydrogen counts
- A dibasic acid forms two series of salts: and
- Alkalis are soluble bases; copper(II) oxide and iron(III) hydroxide are insoluble bases
- Strong alkalis: sodium and potassium hydroxide. Weak alkali: ammonium hydroxide
- Acidity of a base is the number of hydroxyl ions per molecule: monoacidic , diacidic , triacidic
- Acid molecules times basicity equals base molecules times acidity in complete neutralisation
- Carbonates and bicarbonates + dilute acid, in the cold: salt + water +
- Sulphites and bisulphites + dilute acid, on warming: salt + water +
- Sulphides + dilute acid, in the cold: salt +
- Marble and dilute sulphuric acid stop reacting because calcium sulphate coats the marble
- **: lime water milky, clears in excess, no effect on dichromate
- : smell of burning sulphur, acidified dichromate orange to green, permanganate decolourised, lime water milky too
- : rotten-egg smell, moist lead acetate paper turns black
- Sodium hydrogen carbonate gives on heating alone**; sodium carbonate does not
The sharpest self-test is five unlabelled white powders. Decide, for each of a carbonate, a bicarbonate, a sulphite, a bisulphite and a sulphide, what you would add, whether you would heat, what gas would appear and how you would prove it — then find the one pair your plan still cannot separate.
- Concentrated or dilute describes the amount of acid in water; strong or weak describes how completely it ionises
- Strong acids: hydrochloric, nitric, sulphuric. Weak acids: acetic, carbonic, formic, sulphurous, hydrogen sulphide
- Always add acid to water, never water to acid
- Basicity is the number of hydronium ions per molecule: monobasic , , ; dibasic , , ; tribasic
- Acetic acid is monobasic despite four hydrogen atoms — only replaceable hydrogen counts
- A dibasic acid forms two series of salts: and
- Alkalis are soluble bases; copper(II) oxide and iron(III) hydroxide are insoluble bases
- Strong alkalis: sodium and potassium hydroxide. Weak alkali: ammonium hydroxide
- Acidity of a base is the number of hydroxyl ions per molecule: monoacidic , diacidic , triacidic
- Acid molecules times basicity equals base molecules times acidity in complete neutralisation
- Carbonates and bicarbonates + dilute acid, in the cold: salt + water +
- Sulphites and bisulphites + dilute acid, on warming: salt + water +
- Sulphides + dilute acid, in the cold: salt +
- Marble and dilute sulphuric acid stop reacting because calcium sulphate coats the marble
- **: lime water milky, clears in excess, no effect on dichromate
- : smell of burning sulphur, acidified dichromate orange to green, permanganate decolourised, lime water milky too
- : rotten-egg smell, moist lead acetate paper turns black
- Sodium hydrogen carbonate gives on heating alone**; sodium carbonate does not
The sharpest self-test is five unlabelled white powders. Decide, for each of a carbonate, a bicarbonate, a sulphite, a bisulphite and a sulphide, what you would add, whether you would heat, what gas would appear and how you would prove it — then find the one pair your plan still cannot separate.