Metals and Non-metals — NCERT Solutions, AI Tutor & Practice

Understand: Metals share a set of typical physical properties, but a few metals behave differently.

Four properties with exceptions:

1. Lustrous (shiny): Most metals have a shine when freshly cut. Exception: Sodium and potassium are dull and can be cut easily.
2. Hard: Most metals are hard. Exception: Sodium and potassium are so soft they can be cut with a knife.
3. High melting point: Most metals melt at high temperatures. Exception: Gallium and caesium have such low melting points that they melt in the palm of the hand.
4. Sonorous (produce ringing sound): Metals like iron and copper make a ringing sound when struck. Exception (density/state): Mercury is the only metal that is liquid at room temperature.

Conclude: Physical properties are general trends; sodium, potassium, mercury, gallium and caesium are common exceptions.

Understand: Metals combine with oxygen to form metal oxides, which are usually basic in nature.

Example — Magnesium burning in air:

$2Mg + O_2 \rightarrow 2MgO$

Magnesium burns with a dazzling white flame to form white magnesium oxide.

Nature of the oxide: MgO is a basic oxide. When dissolved in water it forms an alkali:

$MgO + H_2O \rightarrow Mg(OH)_2$

Special case: Some metal oxides such as aluminium oxide ($Al_2O_3$) and zinc oxide ($ZnO$) are amphoteric — they react with both acids and bases.

Conclude: Metals + oxygen → metal oxides (mostly basic; some amphoteric).

Understand: The reactivity series ranks metals by how vigorously they react.

Decreasing order of reactivity:

$Na > Zn > Fe > Cu$

Reaction with cold water:

1. Sodium (Na): Reacts very vigorously, even catching fire: $2Na + 2H_2O \rightarrow 2NaOH + H_2\uparrow$
2. Zinc (Zn): Does not react with cold water; reacts only with steam.
3. Iron (Fe): Does not react with cold water; reacts slowly with steam.
4. Copper (Cu): Does not react with water at all (it is below hydrogen in the series).

Conclude: Only sodium reacts with cold water among these; the others need steam or do not react.

Understand: Sodium is a very reactive metal.

Analyze: In open air, sodium reacts rapidly with oxygen and moisture. It reacts so violently with water/moisture that it can catch fire:

$4Na + O_2 \rightarrow 2Na_2O$ $2Na + 2H_2O \rightarrow 2NaOH + H_2\uparrow + \text{heat}$

The heat released can ignite the hydrogen gas produced.

Conclude: To prevent this dangerous, spontaneous reaction with air and moisture, sodium (and potassium) is stored under kerosene oil, which keeps oxygen and water away from the metal surface.

Understand: Ionic bonds form by transfer of electrons between a metal and a non-metal.

Formation of NaCl:

• Sodium (2,8,1) loses 1 electron to become $Na^+$ (2,8).
• Chlorine (2,8,7) gains 1 electron to become $Cl^-$ (2,8,8).

$Na \rightarrow Na^+ + e^-$ $Cl + e^- \rightarrow Cl^-$

The oppositely charged ions attract each other by strong electrostatic force to form $NaCl$.

Two properties of ionic compounds:

1. They have high melting and boiling points due to strong ionic attractions.
2. They conduct electricity in molten state or in aqueous solution (because ions are free to move), but not in the solid state.

Conclude: NaCl is formed by complete transfer of one electron from Na to Cl.

Understand: An alloy is a homogeneous mixture of two or more metals, or of a metal with a non-metal.

Why alloys are preferred:

• They are usually harder and stronger than pure metals.
• They have greater resistance to corrosion.
• Their electrical conductivity and melting point can be lower, which is useful for specific applications (e.g. solder).

Composition:

• Brass: Copper + Zinc.
• Bronze: Copper + Tin.

Conclude: Alloying improves the useful properties (hardness, strength, corrosion resistance) of metals for practical use.

Understand: In a displacement reaction, a more reactive metal displaces a less reactive metal from its salt solution.

Analyze:

1. M displaces copper from $CuSO_4$: $M + CuSO_4 \rightarrow MSO_4 + Cu$ So M is more reactive than copper.

2. M cannot displace zinc from $ZnSO_4$: So M is less reactive than zinc.

Conclude: In the reactivity series:

$Zn > M > Cu$

Metal M lies below zinc but above copper.

Understand: Roasting and calcination are methods to convert a concentrated ore into its metal oxide before reduction.

Roasting: Heating a sulphide ore strongly in the presence of excess air.

$2ZnS + 3O_2 \xrightarrow{\Delta} 2ZnO + 2SO_2$

Calcination: Heating a carbonate ore strongly in the absence (or limited supply) of air.

$ZnCO_3 \xrightarrow{\Delta} ZnO + CO_2$

Difference: Roasting is for sulphide ores in the presence of air; calcination is for carbonate ores in limited/no air.

Conclude: Both give metal oxide, which is then reduced to obtain the metal.

Understand: Metals high in the reactivity series (Na, K, Ca, Mg, Al) have great affinity for oxygen, so their oxides cannot be reduced by carbon.

Method of extraction — Electrolytic reduction:

These metals are extracted by electrolysis of their molten chlorides or oxides. The metal is deposited at the cathode and the non-metal is liberated at the anode.

Example — extraction of sodium by electrolysis of molten NaCl:

• At cathode (reduction): $Na^+ + e^- \rightarrow Na$
• At anode (oxidation): $2Cl^- \rightarrow Cl_2 + 2e^-$

Similarly, aluminium is obtained by electrolysis of molten aluminium oxide (Hall–Héroult process).

Conclude: Highly reactive metals are obtained by electrolytic reduction of their molten compounds.

Understand: Corrosion is the gradual eating away of a metal surface due to reaction with substances in air and moisture. Rusting of iron is a common example.

Conditions necessary for rusting: Iron rusts only when both

1. Oxygen (air), and
2. Moisture (water)

are present together. The reddish-brown rust formed is hydrated iron(III) oxide, $Fe_2O_3\cdot xH_2O$.

Two methods of prevention:

1. Galvanisation: Coating iron with a layer of zinc.
2. Painting / oiling / greasing: Applying a protective coat that keeps air and moisture away. (Other methods: electroplating, alloying to stainless steel.)

Conclude: Preventing contact of iron with oxygen and moisture stops rusting.

Understand: Melting point depends on the strength of forces between particles.

Ionic compounds — high melting point: They consist of ions held together by strong electrostatic forces of attraction. A large amount of energy is needed to break this rigid lattice, so their melting and boiling points are high.

Covalent compounds — low melting point: Many covalent (molecular) compounds are held together by weak intermolecular forces between molecules. Only little energy is needed to separate the molecules, so melting/boiling points are low.

Conduction in metals: Metals conduct electricity because they have free (delocalised) electrons that move through the metal when a voltage is applied. Here conduction is due to mobile electrons, not ions. (Ionic compounds conduct via ions only when molten or dissolved.)

Conclude: Strong ionic bonds → high m.p.; weak intermolecular forces → low m.p.; metallic conduction is due to free electrons.

Understand: Gold is very unreactive (a noble metal) and does not react with air, water, or single ordinary acids.

Aqua regia: It is a freshly prepared mixture of concentrated hydrochloric acid and concentrated nitric acid in the ratio 3 : 1.

$\text{Aqua regia} = 3\ HCl : 1\ HNO_3$

It is a highly corrosive, fuming liquid and is one of the few reagents that can dissolve gold and platinum.

Why gold is noble: Gold lies very low in the reactivity series, so it has very little tendency to lose electrons and react. It resists corrosion and reaction with most acids, which is why it stays shiny and is used in jewellery.

Conclude: Aqua regia (3:1 HCl:HNO₃) can dissolve even the noble metal gold, which otherwise does not react with individual acids.