Reactions of Metals with Oxygen and Water

Reaction of Metals with Oxygen

When metals react with oxygen, they form metal oxides.

General Form

$$\text{Metal} + \text{Oxygen} \rightarrow \text{Metal Oxide}$$

Reactions According to Reactivity

Highly Reactive Metals (Na, K):

• Burn in air even at room temperature.
• That is why they are stored in kerosene.

$$4Na + O_2 \rightarrow 2Na_2O$$

$$4K + O_2 \rightarrow 2K_2O$$

Moderately Reactive Metals (Mg, Al, Zn, Fe):

• React when heated in a flame.
• Magnesium burns with a bright dazzling white flame.

$$2Mg + O_2 \xrightarrow{\Delta} 2MgO$$

$$4Al + 3O_2 \xrightarrow{\Delta} 2Al_2O_3$$

$$2Zn + O_2 \xrightarrow{\Delta} 2ZnO$$

$$3Fe + 2O_2 \xrightarrow{\Delta} Fe_3O_4$$

Least Reactive Metals (Cu, Ag, Au):

• Copper forms a green coating on the surface (in moist air).
• Silver forms a black coating.
• Gold remains unaffected — called a 'noble metal'.

$$2Cu + O_2 \xrightarrow{\Delta} 2CuO$$

(Black colour — refer to Section 1)

Nature of Metal Oxides

Metal oxides are generally basic in nature — they react with acids and give $OH^-$ ions in water.

Basic Oxides

Most metal oxides are basic.

Examples:

• $Na_2O + H_2O \rightarrow 2NaOH$ (forms a base)
• $CaO + H_2O \rightarrow Ca(OH)_2$
• $K_2O + H_2O \rightarrow 2KOH$

That is, when dissolved in water they produce alkalis.

Reaction with Acids

Metal Oxide + Acid → Salt + Water

Examples: $$CuO + 2HCl \rightarrow CuCl_2 + H_2O$$ (Black CuO turns bluish-green)

$$MgO + 2HCl \rightarrow MgCl_2 + H_2O$$

Amphoteric Oxides

Some metal oxides react with both acids and bases.

Main Examples: $Al_2O_3$ and $ZnO$

Behaviour of Al₂O₃:

With acid (acts as a base): $$Al_2O_3 + 6HCl \rightarrow 2AlCl_3 + 3H_2O$$

With base (acts as an acid): $$Al_2O_3 + 2NaOH \rightarrow 2NaAlO_2 + H_2O$$

(Sodium aluminate)

ZnO behaves similarly: $$ZnO + 2HCl \rightarrow ZnCl_2 + H_2O$$ $$ZnO + 2NaOH \rightarrow Na_2ZnO_2 + H_2O$$

An Interesting Point

Not all metal oxides dissolve in water.

• $Na_2O, K_2O$ — soluble in water → form alkalis.
• $CuO, ZnO, Fe_2O_3$ — insoluble in water.
• $Al_2O_3$ — insoluble; but reacts with acids/bases.

Reaction of Metals with Water

The reaction of metals with water depends on their reactivity.

General Reaction

$$\text{Metal} + \text{Water} \rightarrow \text{Metal Hydroxide} + H_2$$

or

$$\text{Metal} + \text{Steam} \rightarrow \text{Metal Oxide} + H_2$$

Highly Reactive Metals (Na, K, Ca)

Na and K — react vigorously with cold water:

$$2Na + 2H_2O \rightarrow 2NaOH + H_2\uparrow + \text{heat}$$

The hydrogen evolved can immediately catch fire due to the heat released.

$$2K + 2H_2O \rightarrow 2KOH + H_2\uparrow + \text{more heat}$$

Ca — reacts with cold water but less vigorously:

$$Ca + 2H_2O \rightarrow Ca(OH)_2 + H_2\uparrow$$

Ca pieces sink and float repeatedly in water — due to hydrogen bubbles sticking to them.

Moderately Reactive Metals

Mg — reacts slowly with hot water:

$$Mg + 2H_2O \xrightarrow{\text{hot}} Mg(OH)_2 + H_2\uparrow$$

Mg/Al/Zn/Fe — react with steam (when heated):

$$3Fe + 4H_2O(\text{steam}) \rightarrow Fe_3O_4 + 4H_2\uparrow$$

(Refer Chapter 1)

$$Zn + H_2O(\text{steam}) \rightarrow ZnO + H_2\uparrow$$

$$2Al + 3H_2O(\text{steam}) \rightarrow Al_2O_3 + 3H_2\uparrow$$

Least Reactive Metals

Cu, Ag, Au — do not react with water at all (at any temperature).

This is why utensils/jewellery made from them last for very long.

Reactivity Table — Reaction with Water

Metal	Cold Water	Hot Water	Steam
K	Violent (explosive)	—	—
Na	Vigorous	—	—
Ca	Reacts	—	—
Mg	Slow	Reacts	—
Al	No	No	Reacts
Zn	No	No	Reacts
Fe	No	No	Reacts (slow)
Pb, Cu	No	No	No
Hg, Ag, Au	No	No	No

Special Cases and Exceptions

Combustion of Magnesium

When Mg is burnt in air, it burns with a bright dazzling white flame.

$$2Mg + O_2 \rightarrow 2MgO + \text{heat} + \text{light}$$

Features:

• The flame is very intense — harmful to the eyes.
• Goggles must be worn while burning.
• Used in flash-bulbs and fireworks.

(Already studied in Section 1 of Chapter 1.)

Protective Layer on Aluminium

When exposed to air, aluminium immediately forms a thin layer of $Al_2O_3$ — which protects the inner Al from further corrosion.

This is why aluminium utensils last for years — even though Al is a reactive metal.

Anodising

The process of forming a thicker layer of $Al_2O_3$ on aluminium.

Method:

• Aluminium is made the anode in an electrolytic cell.
• Dilute $H_2SO_4$ is used as electrolyte.
• Oxygen liberated at the anode combines with Al to thicken the $Al_2O_3$ layer.

Uses:

• Window frames
• Kitchen utensils
• Decorative items

The Danger with Sodium

Na + H₂O — a hazardous demonstration:

Even when a small piece is dropped in water:

• Vigorous bubbling (H₂)
• Flames (yellow, due to sodium)
• Sometimes explosion

This is why:

• Na is shown carefully in classrooms.
• Never to be held with bare hands.
• A tiny piece is enough for demonstration.

The Principle of Reactivity

Why are some metals more reactive?

Principle: Metals that lose electrons easily are more reactive.

Highly Reactive (Na, K, Ca):

• 1-2 outer electrons.
• Lose electrons easily.
• Highly reactive.

Moderately Reactive (Mg, Al, Zn, Fe):

• 2-3 outer electrons.
• Lose electrons with moderate ease.

Least Reactive (Cu, Ag, Au):

• More stable outer shell.
• Difficult to lose electrons.
• Less reactive.

Practical Use of Reactivity

1. Storage:

• Na, K → in kerosene.
• Other metals → can be kept in the open.

2. Uses:

• Highly reactive Na, K — not to be exposed in open.
• Less reactive Au, Ag — used in jewellery.
• Moderate Fe, Al — used in structures.

3. Method of Extraction:

• Highly reactive — by electrolysis.
• Moderate — by reduction with carbon (coke).
• Less reactive — found directly.

(Discussed in detail in Section 7.)

Summary Table

Metal	Reaction with $O_2$	Reaction with Water	Reaction with Acid
K, Na	Vigorous, at room temp	Vigorous (cold water)	Explosive
Ca	On burning	With cold water	Vigorous
Mg	On burning	With hot water	Reacts
Al	On burning	With steam	Reacts
Zn	On burning	With steam	Reacts
Fe	On burning	With steam (slow)	Reacts
Cu	On strong heating	No	No (with dilute)
Ag	No	No	No
Au	No	No	No

[Board Important] This table is frequently asked in 5-mark questions.

🧠 Memory Capsule

A quick glance just before the board exam.

1. Metal + Oxygen

$$\text{Metal} + O_2 \rightarrow \text{Metal Oxide}$$

Essential Reactions:

• $4Na + O_2 \rightarrow 2Na_2O$
• $2Mg + O_2 \rightarrow 2MgO$ (white flame)
• $4Al + 3O_2 \rightarrow 2Al_2O_3$
• $3Fe + 2O_2 \rightarrow Fe_3O_4$
• $2Cu + O_2 \rightarrow 2CuO$ (black)

2. Nature of Metal Oxides

• Most: Basic
• Exceptions ($Al_2O_3, ZnO$): Amphoteric (react with both acids and bases)

3. Metal + Water

$$\text{Metal} + H_2O \rightarrow \text{Hydroxide/Oxide} + H_2$$

With cold water (Na, K, Ca):

• $2Na + 2H_2O \rightarrow 2NaOH + H_2\uparrow$
• $Ca + 2H_2O \rightarrow Ca(OH)_2 + H_2\uparrow$

With hot water/steam (Mg, Al, Zn, Fe):

• $3Fe + 4H_2O(\text{steam}) \rightarrow Fe_3O_4 + 4H_2$
• $Zn + H_2O(\text{steam}) \rightarrow ZnO + H_2$

No reaction (Cu, Ag, Au): Do not react with water.

4. Anodising

Process of forming a thicker $Al_2O_3$ layer on Al.

• Method: Al is made the anode in dilute $H_2SO_4$.
• Uses: window frames, utensils.

5. Reactivity Table — with Water

Metal	Cold Water	Steam
K, Na, Ca	Yes (vigorous)	—
Mg, Al, Zn, Fe	No/Slow	Yes
Cu, Ag, Au	No	No

6. Board's 'Golden' Questions

1. Difference between basic and amphoteric oxides.
2. Write the reaction of Na with water — why is it dangerous?
3. What is anodising?
4. What is the protective layer on Al? How does it protect?
5. Reaction of iron with steam.

Final Formula: More reactive metal = loses electrons easily = reacts faster with oxygen/water/acid.

Solved Examples

Example 1: NCERT — Combustion of Magnesium

What happens when magnesium is burnt in air? Write the chemical reaction.

Solution:

Observations:

• Magnesium burns with a bright dazzling white flame.
• A white powder ($MgO$) is formed.
• Intense heat is released.

Reaction:

$$2Mg(s) + O_2(g) \xrightarrow{\Delta} 2MgO(s) + \text{heat} + \text{light}$$

This reaction is:

• Combination
• Exothermic
• Oxidation (Mg gains O)

Nature of MgO: Basic. Slightly soluble in water giving $Mg(OH)_2$.

Precaution: Wear safety goggles to protect the eyes.

[NCERT — refer to Chapter 1]

Example 2: NCERT — Sodium and Water

How does sodium react with water? Why is this dangerous?

Solution:

Reaction:

$$2Na(s) + 2H_2O(l) \rightarrow 2NaOH(aq) + H_2(g)\uparrow + \text{heat}$$

Observations:

1. The small piece of Na floats on water (low density).
2. Vigorous bubbling (hydrogen gas).
3. Yellow flame — sometimes explosion.
4. The solution becomes basic.

Why is it dangerous?

1. Highly exothermic — releases a lot of heat.
2. Hydrogen gas catches fire immediately — risk of explosion.
3. Pieces may jump out — danger to skin/eyes.
4. NaOH is also corrosive — burns the skin.

Precautions:

• Use a very small piece in classroom.
• Wear safety goggles.
• Never touch with bare hands.
• Watch from a safe distance.

This is why Na/K are stored in kerosene.

[NCERT — repeatedly asked]

Example 3: NCERT — Protection of Aluminium

Why do aluminium articles remain shiny for a long time — even though Al is a reactive metal?

Solution:

Principle: A protective layer is formed on Al.

Mechanism:

1. On exposure to air, Al immediately reacts with O₂: $$4Al + 3O_2 \rightarrow 2Al_2O_3$$

2. This $Al_2O_3$ layer is thin and dense, formed on the surface of Al.

3. This layer is:

• Very thin (a few micrometers)
• Dense and pore-free
• Transparent — Al's shine remains visible
• Protects Al from further corrosion

1. The Al beneath this layer is protected.

Why isn't this like rust?

The $Fe_2O_3$ layer formed on iron is loose and porous — so the iron beneath also rusts.

The layer on Al is dense — does not allow further oxygen to penetrate.

Applications:

• Window frames.
• Kitchen utensils.
• Food packaging.
• Aircraft parts.

Anodising: The process of deliberately thickening this layer.

[3-mark question in board]

Example 4: NCERT — Anodising

What is anodising? Describe its method and uses.

Solution:

Definition: The electrolytic process of forming a thicker layer of $Al_2O_3$ on aluminium.

Method:

1. The aluminium article is made the anode (+).
2. Immerse it in dilute $H_2SO_4$ solution.
3. Pass an electric current.
4. Oxygen is liberated at the anode — reacts with Al to form a thicker $Al_2O_3$ layer.

Reaction:

$$4Al + 3O_2 \rightarrow 2Al_2O_3$$

Different from the natural layer:

• Natural layer: a few micrometers.
• Anodised layer: tens of micrometers (much thicker).

Advantages:

1. Better corrosion-resistance.
2. Colours can be added — dyes get trapped in the layer.
3. Durable.
4. Scratch-resistant.

Uses:

• Window frames (in various colours)
• Mobile phone cases
• Kitchen utensils
• Decorative items
• Spacecraft parts

[NCERT textbook question]

Example 5: NCERT — Amphoteric Oxides

What are amphoteric oxides? Give two examples and their reactions.

Solution:

Definition: Metal oxides that react with both acids and bases to give salt and water are called amphoteric oxides.

Main Examples: $Al_2O_3$, $ZnO$

Aluminium Oxide ($Al_2O_3$):

Reaction with acid (acts as a base):

$$Al_2O_3 + 6HCl \rightarrow 2AlCl_3 + 3H_2O$$

Reaction with base (acts as an acid):

$$Al_2O_3 + 2NaOH \rightarrow 2NaAlO_2 + H_2O$$

(Sodium aluminate)

Zinc Oxide (ZnO):

Reaction with acid:

$$ZnO + 2HCl \rightarrow ZnCl_2 + H_2O$$

Reaction with base:

$$ZnO + 2NaOH \rightarrow Na_2ZnO_2 + H_2O$$

(Sodium zincate)

Comparison with Other Oxides

Type of Oxide	With Acid	With Base
Basic (Na₂O)	Yes	No
Amphoteric (Al₂O₃)	Yes	Yes
Acidic (SO₃)	No	Yes
Neutral (CO)	No	No

Key Insight: Amphoteric oxides act like a 'bridge' — reacting with both kinds.

[Board Important — 5-mark question]

Example 6: NCERT — Behaviour of Cu and Au

Why do copper and gold not react with water and dilute acids?

Solution:

Principle: Cu and Au lie below H in the reactivity series.

Reactivity Series: $...Pb > H > Cu > Hg > Ag > Au$

Rule: Only metals above H react with water/acid to release $H_2$.

Behaviour of Cu:

With water: No reaction (at any temperature).

With dilute $HCl$/$H_2SO_4$: No reaction.

Exception — Concentrated $H_2SO_4$ or $HNO_3$: $$Cu + 2H_2SO_4(\text{conc.}) \rightarrow CuSO_4 + 2H_2O + SO_2$$ (This is not displacement — it is redox)

Behaviour of Au:

No reaction with any acid.

Exception — Aqua Regia: Conc. $HNO_3$ + Conc. $HCl$ (1:3) — can dissolve gold.

('Aqua Regia' means 'Royal Water' — because it can dissolve the 'king of metals' (gold).)

Practical Consequences

1. Cu utensils: Acidic foods can be stored — no direct reaction.

2. Au jewellery: Lasts for thousands of years — corrosion-resistant.

3. Ag jewellery: Also less reactive — but turns black with $H_2S$.

Key Insight: The reactivity series gives a deep understanding of metal behaviour.

[Board Important]

Example 7: NCERT — Calcium and Water

When pieces of calcium are dropped in water, they sink and float repeatedly. Why?

Solution:

Reaction:

$$Ca + 2H_2O \rightarrow Ca(OH)_2 + H_2\uparrow$$

Observations:

1. The Ca piece sinks in water (Ca's density is greater than water).
2. Hydrogen gas bubbles are formed.
3. These bubbles stick to the Ca piece.
4. The bubbles lift Ca up.
5. On reaching the surface, the bubbles burst.
6. Ca sinks back down.
7. The cycle continues — sinking, floating, sinking.

Why is this different from Na?

• Na reacts with water vigorously — can catch fire.
• Ca is slightly less reactive — not explosive, just rapid bubbling.
• Ca's density is greater — it sinks (Na floats).

Other Observations:

• The solution slowly becomes milky — due to low solubility of $Ca(OH)_2$.
• Yet the solution is basic — increases pH.

Effect of Temperature:

• Ca + cold water — slow but continuous reaction.
• Ca + hot water — fast reaction.

[NCERT textbook question]

Example 8: NCERT — Iron and Steam

How does iron react with steam? Write the chemical reaction.

Solution:

Reaction:

$$3Fe(s) + 4H_2O(g) \xrightarrow{\Delta} Fe_3O_4(s) + 4H_2(g)\uparrow$$

(Iron + Steam → Ferrosoferric Oxide + Hydrogen)

Special Note:

• $H_2O$ with (g) — meaning steam.
• Fe does not react with cold or hot water.
• Only with steam (very hot water vapour).

What kind of reaction?

• Displacement — Fe displaced H.
• Redox — Fe oxidised ($Fe → Fe^{3+}$), H reduced.

Compound formed:

• $Fe_3O_4$ = Iron (II, III) oxide
• Black colour
• Mineral 'magnetite' is also made of this

Other Metal+Steam Reactions

Zinc: $$Zn + H_2O(\text{steam}) \rightarrow ZnO + H_2$$

Aluminium: $$2Al + 3H_2O(\text{steam}) \rightarrow Al_2O_3 + 3H_2$$

Magnesium: $$Mg + H_2O(\text{steam}) \rightarrow MgO + H_2$$

(Mg with hot water gives $Mg(OH)_2$, but with steam gives $MgO$.)

Fe + Water — A Special Reason

Why doesn't Fe react with water at room temperature?

• In the reactivity series, Fe is in the middle.
• Not as active as Mg, Ca.
• Only at the high temperature/pressure of steam is enough energy provided.

[Board Important — 3-mark]

Example 9: A Comparison — Na vs Ca

Both sodium and calcium react with water. Compare their behaviours.

Solution:

Reactions:

$$2Na + 2H_2O \rightarrow 2NaOH + H_2\uparrow + \text{heat}$$

$$Ca + 2H_2O \rightarrow Ca(OH)_2 + H_2\uparrow + \text{heat}$$

Comparative Table:

Basis	Na	Ca
Reactivity	Very high	High (but less than Na)
Density	0.97 g/cm³ (floats)	1.55 g/cm³ (sinks)
Intensity of Reaction	Explosive	Rapid bubbling
Fire	Can catch fire	Usually not
Product	NaOH (soluble)	Ca(OH)₂ (less soluble)
Colour of Solution	Colourless	Milky (due to low solubility)
Heat	Very high	Less

Visual Demonstration:

• Na: Vigorous, flame, jumping.
• Ca: Sinking-floating (due to bubbles).

Key Insight: Na is above Ca in the reactivity series — hence more vigorous.

Caution: Goggles, gloves are essential while working with both.

[Board: 3-5 marks]

Example 10: Numerical — Mg + O₂

When 12 g of magnesium is completely burnt, how much $MgO$ will be formed? (Mg=24, O=16)

Solution:

Reaction:

$$2Mg + O_2 \rightarrow 2MgO$$

Molecular Mass:

• $2Mg = 48$ g
• $2MgO = 80$ g (40 each)

Ratio: 48 g Mg → 80 g MgO

From 12 g Mg:

$$\text{MgO} = \frac{80}{48} \times 12 = 20 \text{ g}$$

Answer: 20 g of $MgO$ will be formed.

Verification (Conservation of Mass):

• Mg + O₂ → MgO
• 12 g + ? g → 20 g
• Oxygen = $20 - 12 = 8$ g (= 0.25 mol O = 0.5 mol… wait)
• Oxygen used = 8 g = 0.25 mol O₂ (since O₂ = 32 g/mol)
• 12 g Mg = 0.5 mol; 0.25 mol O₂ — ratio 2:1 ✓

[Board: 2-3 marks]

Example 11: NCERT — Reactivity of Five Metals

Arrange the following metals in increasing order of reactivity: Cu, Mg, Na, Fe, Au

Solution:

Reactivity Series (Relevant Part):

$$\text{Au} < \text{Cu} < \text{Fe} < \text{Mg} < \text{Na}$$

In Increasing Order of Reactivity:

1. Au (Gold) — Least reactive (noble metal)
2. Cu (Copper)
3. Fe (Iron)
4. Mg (Magnesium)
5. Na (Sodium) — Most reactive

Comparison Showing Reactions:

Metal	With Water	With Acid	With $O_2$
Na	Vigorous with cold water	Explosive	At room temperature
Mg	Slow with hot water	Reacts	On burning
Fe	Slow with steam	Reacts	On burning (rust)
Cu	No	Only with conc. acid	On strong heating
Au	No	Only aqua regia	No

Practical Uses:

• Na - stored in kerosene.
• Mg - flash-bulbs, fireworks.
• Fe - construction, machinery.
• Cu - wires, utensils.
• Au - jewellery, electronics.

[Board Important]

Example 12: An Interesting Question — Food and Utensils

Why should acidic food items (lemon, tomato) not be stored in aluminium utensils?

Solution:

Principle: Aluminium is a reactive metal. It can react with acids.

Surface Protective Layer:

• Normally Al has a thin protective layer of $Al_2O_3$.
• This protects Al from base.

Effect of Acid:

• Acid dissolves this $Al_2O_3$ layer.
• Then the Al underneath is exposed.
• Al reacts with the acid.

Reaction (Al + acid):

$$2Al + 6HCl \rightarrow 2AlCl_3 + 3H_2\uparrow$$

Or with lemon (citric acid) — similar reaction.

Result:

1. Al gets mixed in food (not toxic, but undesirable).
2. Taste of food may change.
3. Surface of utensil is damaged.

Right Alternatives:

• Stainless steel utensils (inert).
• Glass utensils.
• Earthen traditional utensils.
• Copper/Brass utensils (for some foods).

However: Common acidic foods (curd, buttermilk, tomato) cause very mild reaction with Al — not very concerning.

Best Practice: Stainless steel is best for modern kitchen.

[Board + Daily Life]

Example 13: NCERT — A Mixed Question

Balance the following reactions: (a) $Mg + O_2 \rightarrow MgO$ (b) $Fe + H_2O(\text{steam}) \rightarrow Fe_3O_4 + H_2$ (c) $Al + O_2 \rightarrow Al_2O_3$ (d) $Ca + H_2O \rightarrow Ca(OH)_2 + H_2$

Solution:

(a) Mg + O₂: $$2Mg + O_2 \rightarrow 2MgO$$

Check: Mg=2=2, O=2=2 ✓

(b) Fe + H₂O (steam): $$3Fe + 4H_2O \rightarrow Fe_3O_4 + 4H_2$$

Check: Fe=3=3, H=8=8, O=4=4 ✓

(c) Al + O₂: $$4Al + 3O_2 \rightarrow 2Al_2O_3$$

Check: Al=4=4, O=6=6 ✓

(d) Ca + H₂O: $$Ca + 2H_2O \rightarrow Ca(OH)_2 + H_2$$

Check: Ca=1=1, H=4=4, O=2=2 ✓

Key Tips for Balancing:

• Start from the most complex compound.
• Change coefficients, not formulae.
• Balance H and O at the end.

[Board: 3-mark]

Example 14: NCERT — Behaviour of Sodium and Potassium

Sodium and potassium release intense heat on reacting with water. What is the result of this heat?

Solution:

Reactions:

$$2Na + 2H_2O \rightarrow 2NaOH + H_2 + \text{heat}$$

$$2K + 2H_2O \rightarrow 2KOH + H_2 + \text{more heat}$$

Result of Heat:

1. Hydrogen gas's temperature rises sharply.

2. The temperature is so high that H₂ + O₂ (from air) burn immediately:

$$2H_2 + O_2 \rightarrow 2H_2O + \text{more heat}$$

3. A flame is observed:

• With Na — yellow
• With K — violet (more intense)

4. Sometimes explosion — especially with K.

Na vs K — Difference in Intensity

Basis	Na	K
Reactivity	High	Very high (more than Na)
Heat	More	Even more
Flame	Yellow	Violet
Explosion	Sometimes	More likely

Precautions:

• Use very small pieces in classroom.
• Never hold Na/K with bare hands.
• Wear safety goggles.
• Watch from a distance.
• The teacher demonstrating must be a professional.

That is why: Na and K are stored in kerosene — to prevent contact with air.

[NCERT textbook — daily life safety]

Example 15: An Interesting Comparison — Which Oxide is What?

Classify the following oxides as basic, acidic, or amphoteric — $Na_2O, SO_2, Al_2O_3, CO_2, ZnO, CaO, NO_2$

Solution:

Rules:

• Metallic oxides are usually basic.
• Non-metallic oxides are usually acidic.
• Some metallic oxides (Al₂O₃, ZnO) are amphoteric.

Analysis:

Oxide	Metal/Non-metal?	Type
$Na_2O$	Metal	Basic
$SO_2$	Non-metal	Acidic
$Al_2O_3$	Metal (but)	Amphoteric
$CO_2$	Non-metal	Acidic
$ZnO$	Metal (but)	Amphoteric
$CaO$	Metal	Basic
$NO_2$	Non-metal	Acidic

Verification (Reaction with Water):

Basic:

• $Na_2O + H_2O \rightarrow 2NaOH$
• $CaO + H_2O \rightarrow Ca(OH)_2$

Acidic:

• $SO_2 + H_2O \rightarrow H_2SO_3$
• $CO_2 + H_2O \rightarrow H_2CO_3$
• $3NO_2 + H_2O \rightarrow 2HNO_3 + NO$

Amphoteric:

• $Al_2O_3 + 6HCl \rightarrow 2AlCl_3 + 3H_2O$ (with acid)
• $Al_2O_3 + 2NaOH \rightarrow 2NaAlO_2 + H_2O$ (with base)

[Board: 5-mark question]

Example 16: A Concluding Question

(a) Difference between basic and amphoteric oxides. (b) Reactions of Mg and Fe with O₂ and H₂O. (c) Why is Na stored in kerosene? (d) Purpose of anodising.

Solution:

(a) Basic vs Amphoteric:

Property	Basic	Amphoteric
What does it react with?	Only acid	Both acid and base
Examples	Na₂O, CaO	Al₂O₃, ZnO
With water	Form bases	Mostly insoluble

(b) Reactions of Mg and Fe:

Mg + O₂: $$2Mg + O_2 \xrightarrow{\Delta} 2MgO$$

Mg + H₂O (hot): $$Mg + 2H_2O \rightarrow Mg(OH)_2 + H_2$$

Fe + O₂ (hot): $$3Fe + 2O_2 \rightarrow Fe_3O_4$$

Fe + H₂O (steam): $$3Fe + 4H_2O \rightarrow Fe_3O_4 + 4H_2$$

(c) Na stored in kerosene:

• Na is highly reactive.
• Reacts immediately with O₂ and moisture in air.
• Risk of fire.
• Kerosene keeps Na away from $O_2$ and $H_2O$.

(d) Purpose of anodising:

• To form a thicker layer of $Al_2O_3$ on Al.
• Corrosion-resistant.
• Helps in adding colour.
• Used in window frames, utensils.

[Board: 5-mark mixed question]