Reactions of Acids and Bases with Metals and Carbonates

Reaction of Acids with Metals

In the previous section, we learnt to identify acids and bases with indicators. Now let's study their chemical properties.

Activity 2.3 — Zinc and Dilute $H_2SO_4$

Take some dilute $H_2SO_4$ in a test tube and add granulated zinc.

Observations:

1. Bubbles appear around the zinc granules.
2. The test tube becomes warm (exothermic).
3. Zinc gradually dissolves.

Reaction:

$$Zn(s) + H_2SO_4(aq) \rightarrow ZnSO_4(aq) + H_2(g)\uparrow$$

Test for Hydrogen Gas

Pass the evolved gas through a soap solution — soap bubbles form (filled with hydrogen). Bring a burning candle close — the gas burns with a 'pop' sound.

With Other Acids

The same experiment with $HCl$, $HNO_3$, and $CH_3COOH$ (acetic acid) gives the same observations.

(i) Zinc + $HCl$: $$Zn(s) + 2HCl(aq) \rightarrow ZnCl_2(aq) + H_2(g)\uparrow$$

(ii) Magnesium + $H_2SO_4$: $$Mg(s) + H_2SO_4(aq) \rightarrow MgSO_4(aq) + H_2(g)\uparrow$$

(iii) Iron + $HCl$: $$Fe(s) + 2HCl(aq) \rightarrow FeCl_2(aq) + H_2(g)\uparrow$$

General Rule

Acid + Active Metal → Salt + Hydrogen Gas

$$\text{Acid} + \text{Metal} \rightarrow \text{Salt} + H_2\uparrow$$

Recall — this is a displacement reaction (from Chapter 1). The metal displaces the $H^+$ of the acid.

Which Metals Will React with Acids?

Metals above hydrogen in the activity series ($Zn, Mg, Fe, Al, ...$) can release $H_2$ from acids. Metals below hydrogen ($Cu, Ag, Au$) do not react with dilute acids.

[Board Important] Activity questions are asked nearly every year.

Reaction of Bases with Metals

Do all metals also react with bases? No — only some.

Activity 2.4 — Zinc and Sodium Hydroxide

Take 2 mL of $NaOH$ solution in a test tube. Add zinc pieces and heat.

Observations:

1. Bubbles of hydrogen gas are evolved.
2. Zinc gradually dissolves.
3. A new salt is formed — sodium zincate ($Na_2ZnO_2$).

Reaction:

$$Zn(s) + 2NaOH(aq) \rightarrow Na_2ZnO_2(aq) + H_2(g)\uparrow$$

(sodium zincate)

Special Note — Which Metals?

Only some metals react with bases:

• Zinc (Zn)
• Aluminium (Al)
• Some other 'amphoteric' metals

These metals are called 'amphoteric' — because they react with both acids and bases.

Example with aluminium:

$$2Al + 6NaOH \rightarrow 2Na_3AlO_3 + 3H_2\uparrow$$

(sodium aluminate)

General Rule

Base + Amphoteric Metal → Salt + Hydrogen Gas

$$\text{Base} + \text{Zn/Al} \rightarrow \text{Salt} + H_2\uparrow$$

Acid vs. Base — Reaction with Metal

Aspect	Acid + Metal	Base + Metal
Which metals?	Active (above H)	Only Zn, Al, etc.
Products	Metal salt + $H_2$	Metal zincate/aluminate + $H_2$
Example	$Zn + HCl$	$Zn + NaOH$

[Board Important] This comparison is often a 3-mark question.

Reaction of Acids with Metal Carbonates

Activity 2.5 — Sodium Carbonate and Hydrochloric Acid

Take some sodium carbonate ($Na_2CO_3$) in a test tube. Add dilute $HCl$.

Observations:

1. Brisk effervescence — a colourless, odourless gas is released.
2. This gas turns lime water milky (test it).

Reaction:

$$Na_2CO_3(s) + 2HCl(aq) \rightarrow 2NaCl(aq) + H_2O(l) + CO_2(g)\uparrow$$

General Rule

Acid + Metal Carbonate → Salt + Water + $CO_2$

$$\text{Acid} + \text{Carbonate} \rightarrow \text{Salt} + H_2O + CO_2\uparrow$$

Other Examples

(i) Calcium carbonate + $HCl$:

$$CaCO_3(s) + 2HCl(aq) \rightarrow CaCl_2(aq) + H_2O(l) + CO_2(g)\uparrow$$

This is the same reaction used to prepare $CO_2$ gas in the laboratory.

(ii) Magnesium carbonate + $HCl$:

$$MgCO_3 + 2HCl \rightarrow MgCl_2 + H_2O + CO_2$$

Test for $CO_2$ — Lime Water Test

$$Ca(OH)_2(aq) + CO_2(g) \rightarrow CaCO_3(s)\downarrow + H_2O(l)$$

(lime water turns milky)

If excess $CO_2$ is bubbled, the $CaCO_3$ precipitate dissolves (a bicarbonate forms):

$$CaCO_3(s) + H_2O(l) + CO_2(g) \rightarrow Ca(HCO_3)_2(aq)$$

(calcium bicarbonate — soluble)

Reaction of Acids with Metal Hydrogen Carbonates

Sodium Hydrogen Carbonate + $HCl$

$$NaHCO_3(s) + HCl(aq) \rightarrow NaCl(aq) + H_2O(l) + CO_2(g)\uparrow$$

(notice — water and $CO_2$ form, just like with carbonates.)

General Rule

Acid + Metal Hydrogen Carbonate → Salt + Water + $CO_2$

$$\text{Acid} + \text{Bicarbonate} \rightarrow \text{Salt} + H_2O + CO_2\uparrow$$

A Famous Example

In a fire extinguisher:

$NaHCO_3$ and dilute $H_2SO_4$ react —

$$2NaHCO_3 + H_2SO_4 \rightarrow Na_2SO_4 + 2H_2O + 2CO_2\uparrow$$

The $CO_2$ released helps extinguish the fire.

Ant Sting — An Example

When an ant bites, it injects formic acid ($HCOOH$). To relieve the pain, we apply baking soda ($NaHCO_3$) —

$$NaHCO_3 + HCOOH \rightarrow HCOONa + H_2O + CO_2$$

(the acid is neutralised, the burning sensation reduces)

Memorise in One Line

"Acid + Carbonate = Salt + Water + $CO_2$"

"Acid + Bicarbonate = Salt + Water + $CO_2$"

The outcomes are the same — only the proportions differ.

[Board Important] This reaction is asked every year.

Vigour of the Acid-Metal Reaction

Not all acids react with metals at the same rate. It depends on their 'strength'.

Strong vs. Weak Acids

Property	Strong acid	Weak acid
Ionisation in water	Complete (~ 100%)	Partial (< 5%)
Examples	$HCl, H_2SO_4, HNO_3$	$CH_3COOH$ (acetic)
$H^+$ concentration	High	Low
Reaction with metal	Vigorous	Slow
pH	1-3	4-6

Experimental Comparison

Dilute $HCl$ (strong acid) with Zn — vigorous bubbling.

Dilute $CH_3COOH$ (weak acid) with Zn — slow bubbling.

Both eventually produce $ZnCl_2$ or $Zn(CH_3COO)_2$ + $H_2$.

Summary of All Reactions in This Section

Reaction Type	General Form	Example
Acid + active metal	Acid + Zn → salt + $H_2$	$Zn + 2HCl \rightarrow ZnCl_2 + H_2$
Base + amphoteric metal	Base + Zn → salt + $H_2$	$Zn + 2NaOH \rightarrow Na_2ZnO_2 + H_2$
Acid + carbonate	Acid + $CO_3^{2-}$ → salt + water + $CO_2$	$Na_2CO_3 + 2HCl \rightarrow 2NaCl + H_2O + CO_2$
Acid + bicarbonate	Acid + $HCO_3^-$ → salt + water + $CO_2$	$NaHCO_3 + HCl \rightarrow NaCl + H_2O + CO_2$

Daily-Life Applications

1. Treating acidity: Baking soda + stomach $HCl$.
2. Treating ant stings: Baking soda + formic acid.
3. Fire extinguishers: $NaHCO_3$ + acid → $CO_2$ → puts out fires.
4. Lime water test: for identifying $CO_2$.
5. Food industry: how baking powder works.

🧠 Memory Capsule

A one-glance recap to revisit just before the board exam — every key reaction.

1. Acid + Metal → Salt + $H_2$

Strong acid and active metal (above H)

Must-memorise examples:

• $Zn + H_2SO_4 \rightarrow ZnSO_4 + H_2\uparrow$
• $Zn + 2HCl \rightarrow ZnCl_2 + H_2\uparrow$
• $Mg + 2HCl \rightarrow MgCl_2 + H_2\uparrow$
• $Fe + 2HCl \rightarrow FeCl_2 + H_2\uparrow$

2. Base + Amphoteric Metal → Salt + $H_2$

Only Zn, Al, etc.

Must-memorise example:

• $Zn + 2NaOH \rightarrow Na_2ZnO_2 + H_2\uparrow$
• (sodium zincate)

3. Acid + Carbonate → Salt + $H_2O$ + $CO_2$

Must-memorise examples:

• $Na_2CO_3 + 2HCl \rightarrow 2NaCl + H_2O + CO_2$
• $CaCO_3 + 2HCl \rightarrow CaCl_2 + H_2O + CO_2$ (lab preparation of $CO_2$)

4. Acid + Bicarbonate → Salt + $H_2O$ + $CO_2$

Must-memorise example:

• $NaHCO_3 + HCl \rightarrow NaCl + H_2O + CO_2$

5. Test for Hydrogen

Burns with a 'pop' sound.

6. Test for $CO_2$

Turns lime water milky: $$Ca(OH)_2 + CO_2 \rightarrow CaCO_3\downarrow + H_2O$$

7. Strong vs Weak Acids

Strong	Weak
$HCl, H_2SO_4, HNO_3$	$CH_3COOH$
pH 1-3	pH 4-6
Vigorous reaction with metals	Slow reaction

8. Board's 'Golden' Questions

1. What are the observations when Zn reacts with dilute $H_2SO_4$? Write the reaction.
2. State the general rule for acid + carbonate reactions.
3. Do bases always release $H_2$ from metals? Why?
4. What gas turns lime water milky? Write the reaction.
5. What do you apply on an ant sting and why?

The Bottom Line: Acid + active metal = salt + $H_2$; acid + carbonate = salt + water + $CO_2$.

Solved Examples

Example 1: NCERT — Zn + $H_2SO_4$ Experiment

Granulated zinc was placed in a test tube and dilute $H_2SO_4$ was added. (a) What are the observations? (b) Write the reaction. (c) How would you identify the gas evolved?

Solution:

(a) Observations:

1. Bubbles appear around the zinc granules.
2. The test tube becomes warm.
3. Zinc gradually dissolves.

(b) Reaction:

$$Zn(s) + H_2SO_4(aq) \rightarrow ZnSO_4(aq) + H_2(g)\uparrow$$

(c) Test for hydrogen:

• Pass the gas through a soap solution — soap bubbles will form.
• Bring a burning candle close — the gas burns with a 'pop' sound.
• This is the classic test for $H_2$.

[NCERT textbook — frequently asked]

Example 2: General Rule of Acid-Metal Reaction

What is the general rule for the reaction between an acid and a metal? Give three examples.

Solution:

General rule: An active metal reacts with an acid to give a salt and hydrogen gas.

$$\text{Acid} + \text{Active Metal} \rightarrow \text{Salt} + H_2\uparrow$$

Three examples:

(i) $\quad Zn(s) + 2HCl(aq) \rightarrow ZnCl_2(aq) + H_2(g)\uparrow$

(ii) $\quad Mg(s) + H_2SO_4(aq) \rightarrow MgSO_4(aq) + H_2(g)\uparrow$

(iii) $\quad Fe(s) + 2HCl(aq) \rightarrow FeCl_2(aq) + H_2(g)\uparrow$

Exception: $Cu, Ag, Au$ (below hydrogen) do not react with dilute acids.

[Board 3-mark question]

Example 3: Zinc + $NaOH$

What happens when zinc metal is heated with sodium hydroxide?

Solution:

Reaction:

$$Zn(s) + 2NaOH(aq) \rightarrow Na_2ZnO_2(aq) + H_2(g)\uparrow$$

(sodium zincate)

Observations:

1. Zinc gradually dissolves.
2. Hydrogen gas is evolved.
3. A colourless solution (sodium zincate) is formed.

Special note:

• This shows that zinc is an amphoteric metal.
• It reacts with both acids and bases.
• Other amphoteric metals — aluminium (Al), tin (Sn), lead (Pb).

Common feature with acid: in both cases hydrogen gas is released.

[Board Important]

Example 4: NCERT — Formation of $CO_2$

What are the observations when sodium carbonate is reacted with dilute $HCl$? Write the reaction.

Solution:

Observations:

1. Brisk effervescence — a colourless, odourless gas is evolved.
2. If the gas is passed through lime water, it turns milky.

Reaction:

$$Na_2CO_3(s) + 2HCl(aq) \rightarrow 2NaCl(aq) + H_2O(l) + CO_2(g)\uparrow$$

Test for $CO_2$:

$$Ca(OH)_2(aq) + CO_2(g) \rightarrow CaCO_3(s)\downarrow + H_2O(l)$$

A white precipitate of $CaCO_3$ forms — making lime water milky.

(If excess $CO_2$ is bubbled, the white precipitate dissolves — calcium bicarbonate is formed, which is soluble.)

[NCERT textbook question]

Example 5: General Acid-Carbonate Rule

What is the general rule for the reaction between an acid and a metal carbonate? Give two examples.

Solution:

General rule:

$$\text{Acid} + \text{Metal Carbonate} \rightarrow \text{Salt} + H_2O + CO_2\uparrow$$

Two examples:

(i) Calcium carbonate + $HCl$ (lab preparation of $CO_2$):

$$CaCO_3(s) + 2HCl(aq) \rightarrow CaCl_2(aq) + H_2O(l) + CO_2(g)\uparrow$$

(ii) Sodium carbonate + $H_2SO_4$:

$$Na_2CO_3(s) + H_2SO_4(aq) \rightarrow Na_2SO_4(aq) + H_2O(l) + CO_2(g)\uparrow$$

Type of reaction:

• Double displacement (referring to Chapter 1) — ion exchange.
• Gas-evolution reaction.

Daily life:

• In fire extinguishers.
• The reason food rises (baking powder).
• In treatment of acidity.

Example 6: NCERT — Baking Soda on Ant Stings

Why do we apply baking soda on an ant sting? Write the chemical reaction.

Solution:

Cause: An ant's sting contains formic acid ($HCOOH$) — which causes burning on the skin.

Action of baking soda:

• $NaHCO_3$ is basic.
• It neutralises the formic acid.
• The acid's effect is reduced — pain and burning are relieved.

Reaction:

$$NaHCO_3(s) + HCOOH(aq) \rightarrow HCOONa(aq) + H_2O(l) + CO_2(g)\uparrow$$

(sodium formate + water + $CO_2$)

Daily life:

• A bee sting is also acidic — baking soda is effective.
• A wasp sting is alkaline — treated with vinegar/lemon (acid).

[Repeatedly asked in board]

Example 7: Fire Extinguisher

A type of fire extinguisher uses baking soda and dilute acid. How does it work? Write the reaction.

Solution:

Principle:

• $CO_2$ gas is effective in extinguishing fires ($CO_2$ is heavier than air, prevents oxygen from reaching the fire).
• When the button is pressed, $NaHCO_3$ and dilute $H_2SO_4$ mix and release $CO_2$.

Reaction:

$$2NaHCO_3 + H_2SO_4 \rightarrow Na_2SO_4 + 2H_2O + 2CO_2\uparrow$$

(or in single equation form:)

$$NaHCO_3 + HCl \rightarrow NaCl + H_2O + CO_2\uparrow$$

How does it extinguish fire?

1. $CO_2$ is released — heavier than air, forms a 'blanket' over the fire.
2. Oxygen is cut off — the fire is put out.
3. Water is also produced — provides cooling.

Modern extinguisher: $CO_2$ cylinder direct — pre-stored.

[Board Important — daily life]

Example 8: Numerical — Gas Volume

How much $H_2$ gas is liberated at STP from the complete reaction of 3.25 g of Zn with dilute $H_2SO_4$? (Zn=65, molar volume=22.4 L)

Solution:

1. Reaction: $$Zn + H_2SO_4 \rightarrow ZnSO_4 + H_2$$
2. Moles of Zn: $\frac{3.25}{65} = 0.05$ mol
3. Moles of $H_2$: 0.05 mol (1:1 ratio)
4. Volume at STP:

$$V_{H_2} = 0.05 \times 22.4 = 1.12 \text{ L}$$

5. Answer: 1.12 L of hydrogen gas.

Lesson: Stoichiometry — first find the mole ratio, then convert to volume.

Example 9: Dilute vs. Concentrated Acid

Both dilute and concentrated $HCl$ react with Zn. What is the difference in their reaction?

Solution:

The basic reaction is the same in both:

$$Zn + 2HCl \rightarrow ZnCl_2 + H_2$$

Differences:

Aspect	Dilute $HCl$	Concentrated $HCl$
$H^+$ concentration	Lower	Higher
Rate of reaction	Slow / moderate	Fast
Bubbles	Slow	Quick and brisk
Heat released	Some	More
Final products	$ZnCl_2$ + $H_2$	Same

Special note:

• Concentrated $H_2SO_4$ behaves differently — it acts as an oxidising agent, sometimes giving $SO_2$ instead of $H_2$.
• Concentrated $HNO_3$ also — does not give $H_2$; instead $NO_2$ or $NO$.
• This is a Class 11 level concept.

Caution: When working with concentrated acids, goggles and gloves are essential.

Example 10: Which Metal Will Not React with Acid?

Which of the following metals will not react with dilute $HCl$? (a) Zn (b) Fe (c) Cu (d) Mg (e) Ag (f) Au

Solution:

Activity series (relevant part): $Mg > Zn > Fe > Pb > H > Cu > Hg > Ag > Au$

Rule: Metals above H can displace $H_2$ from a dilute acid; those below cannot.

Metal	Position vs H	Reacts with acid?
Zn	Above	Yes ✓
Fe	Above	Yes ✓
Cu	Below	No ✗
Mg	Above	Yes ✓
Ag	Below	No ✗
Au	Below	No ✗

Answer: Cu, Ag, and Au will not react with dilute $HCl$.

Daily life: This is why silver and gold ornaments don't deteriorate easily — they are not affected by most acids.

[Common question in boards]

Example 11: Test for Hydrogen Gas

When Zn was added to dilute $HCl$, hydrogen gas was produced. How would you identify this gas? Describe the test.

Solution:

The famous 'pop' test for hydrogen:

1. Procedure:

• Collect the evolved gas in an inverted test tube (by water displacement).
• Bring a burning matchstick close to the mouth of the tube.

1. Observation:

• The gas burns with a 'pop' (small explosive) sound.
• Sometimes a faint blue flame is also seen.

1. Chemical reaction (during the pop):

$$2H_2 + O_2 \rightarrow 2H_2O + \text{Energy}$$

4. Special test — soap bubbles:

• Pass the gas through a soap solution.
• Soap bubbles form and rise in the air ($H_2$ is lighter than air).
• Touch them with a burning matchstick — the pop sound results.

Caution: $H_2$ + $O_2$ mixtures can be explosive. Perform with care.

[Frequently asked in boards]

Example 12: Test for $CO_2$

An unknown gas was passed through lime water — the lime water turned milky. Which gas is it? Give the chemical reaction.

Solution:

1. Gas: Carbon dioxide ($CO_2$).
2. Reaction:

$$Ca(OH)_2(aq) + CO_2(g) \rightarrow CaCO_3(s)\downarrow + H_2O(l)$$

3. Cause of the milky appearance: $CaCO_3$ is insoluble — it disperses as fine particles, making the solution milky.
4. On bubbling more $CO_2$:

$$CaCO_3(s) + H_2O(l) + CO_2(g) \rightarrow Ca(HCO_3)_2(aq)$$

The white precipitate dissolves (calcium bicarbonate is soluble).

Where does $CO_2$ come from in daily life?

• Released in respiration.
• Released in combustion.
• Carbonated drinks.
• In fire extinguishers.

This is the most famous test for $CO_2$.

Example 13: Three Reactions — A Mini Numerical

Balance and state the type of each: (a) $Mg + HCl \rightarrow MgCl_2 + H_2$ (b) $Na_2CO_3 + HCl \rightarrow NaCl + H_2O + CO_2$ (c) $NaHCO_3 + HCl \rightarrow NaCl + H_2O + CO_2$

Solution:

(a): $Mg + 2HCl \rightarrow MgCl_2 + H_2\uparrow$

• Type: Acid + active metal → salt + $H_2$ (displacement).

(b): $Na_2CO_3 + 2HCl \rightarrow 2NaCl + H_2O + CO_2\uparrow$

• Type: Acid + carbonate → salt + water + $CO_2$ (double displacement + gas evolution).

(c): $NaHCO_3 + HCl \rightarrow NaCl + H_2O + CO_2\uparrow$

• Type: Acid + bicarbonate → salt + water + $CO_2$ (double displacement + gas evolution).
• (already balanced)

Example 14: NCERT — Strong vs. Weak Acid

We know $HCl$ is a strong acid and $CH_3COOH$ (acetic) is a weak acid. What happens if Zn is added to solutions of equal concentration of each?

Solution:

$HCl$ + Zn (strong acid):

$$Zn + 2HCl \rightarrow ZnCl_2 + H_2\uparrow$$

• Brisk bubbles appear.
• High concentration of $H^+$, so the reaction is fast.

$CH_3COOH$ + Zn (weak acid):

$$Zn + 2CH_3COOH \rightarrow Zn(CH_3COO)_2 + H_2\uparrow$$

• Slow bubbles appear.
• Low concentration of $H^+$ (only partial ionisation).

Comparison:

Aspect	$HCl$	$CH_3COOH$
Ionisation	Complete	Partial
$H^+$ concentration	High	Low
Reaction rate with Zn	Fast	Slow
pH (1M solution)	~ 0	~ 2.4

Lesson: The practical difference between a strong and a weak acid — the vigour of chemical reactions.

[Board Important]

Example 15: Reactions That Will Not Occur

Which of the following reactions will not occur? Give reasons: (a) $Zn + 2HCl \rightarrow$ (b) $Cu + 2HCl \rightarrow$ (c) $Mg + H_2SO_4 \rightarrow$ (d) $Ag + HCl \rightarrow$

Solution:

Activity series: $Mg > Zn > Fe > Pb > H > Cu > Ag$

Analysis:

Reaction	Metal vs H	Will it occur?
(a) Zn + HCl	Zn > H	Yes ✓
(b) Cu + HCl	Cu < H	No ✗
(c) Mg + $H_2SO_4$	Mg > H	Yes ✓
(d) Ag + HCl	Ag < H	No ✗

Answer: Reactions (b) and (d) will not occur.

Reason: Cu and Ag are below H in the activity series — they cannot displace $H_2$ from a dilute acid.

Exception: Concentrated $HNO_3$ or concentrated $H_2SO_4$ can react with Cu — but those are oxidation-reduction, not simple displacement.

[Common question in boards]

Example 16: A Concise Multi-Topic Question

(a) Write the reaction of dilute $HCl$ + $CaCO_3$. (b) Which gas is evolved? (c) What is the test for this gas? (d) What happens if more $CO_2$ is passed through lime water?

Solution:

(a) Reaction:

$$CaCO_3(s) + 2HCl(aq) \rightarrow CaCl_2(aq) + H_2O(l) + CO_2(g)\uparrow$$

(This is the same reaction used to prepare $CO_2$ in the laboratory.)

(b) Gas evolved: Carbon dioxide ($CO_2$).

(c) Test for the gas:

• Lime water test: pass the gas through $Ca(OH)_2$ solution.
• Observation: lime water turns milky.
• Reaction: $$Ca(OH)_2 + CO_2 \rightarrow CaCO_3\downarrow + H_2O$$

(d) On passing more $CO_2$:

• The white $CaCO_3$ precipitate begins to dissolve.
• Because calcium bicarbonate $Ca(HCO_3)_2$ is formed — which is soluble.

$$CaCO_3 + CO_2 + H_2O \rightarrow Ca(HCO_3)_2$$

Lesson: Lime water remains milky only for a while — it returns to clear (with excess $CO_2$).

[Board 5-mark question]