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Step-by-step answers for all textbook questions in this chapter
Physical Properties of Metals
Most metals share these physical properties:
| Property | Description | Exceptions |
|---|---|---|
| Lustre | Shiny surface | — |
| Malleability | Can be beaten into thin sheets | — |
| Ductility | Can be drawn into thin wires | — |
| Conductivity | Good conductors of heat and electricity | — |
| Sonorous | Produce a ringing sound when struck | — |
| Hardness | Generally hard | Na and K are soft; cut with a knife |
| Melting point | Generally high | Mercury is liquid at room temperature; gallium melts in hand |
| Density | Generally high | Na, K, Li are less dense than water |
Most ductile metal: Gold (1 g of gold can be drawn into 2 km of wire) Most malleable metal: Gold and silver Best conductor of heat: Silver, then copper Hardest natural substance: Diamond (a non-metal — allotrope of carbon) Poorest conductor among metals: Lead
Physical Properties of Non-metals
| Property | Description | Exceptions |
|---|---|---|
| Not lustrous | Dull appearance | Iodine is lustrous; diamond is brilliant |
| Brittle | Break when hammered | — |
| Poor conductors | Don't conduct heat/electricity | Graphite (carbon) conducts electricity |
| Not sonorous | Don't ring when struck | — |
| Low melting point | Generally low | Diamond (carbon) has very high melting point |
Chemical Properties of Metals
1. Reaction with Oxygen
Most metals react with oxygen to form metal oxides (basic in nature):
4Na + O₂ → 2Na₂O 2Mg + O₂ → 2MgO 4Al + 3O₂ → 2Al₂O₃
Aluminium oxide (Al₂O₃) is amphoteric — it reacts with both acids and bases:
- With acid:
Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O - With base:
Al₂O₃ + 2NaOH → 2NaAlO₂ + H₂O
Important reactivity notes:
- Na and K react violently with air and are stored under kerosene
- Ag, Au, Pt do not react with oxygen at all
- Iron does not burn in air but burns in pure oxygen
2. Reaction with Water
| Metal | Reaction with water | Products |
|---|---|---|
| K, Na | Very vigorous (even with cold water) | Metal hydroxide + H₂ |
| Ca | Less vigorous with cold water | Metal hydroxide + H₂ |
| Mg | Very slowly with cold water; fast with steam | MgO + H₂ |
| Al, Fe, Zn | Only with steam | Metal oxide + H₂ |
| Cu, Ag, Au | No reaction with water | — |
2Na + 2H₂O → 2NaOH + H₂↑ + Heat Mg + H₂O(steam) → MgO + H₂ 3Fe + 4H₂O(steam) → Fe₃O₄ + 4H₂
3. Reaction with Dilute Acids
Metal + Dilute acid → Salt + Hydrogen gas
Zn + H₂SO₄ → ZnSO₄ + H₂↑ Fe + 2HCl → FeCl₂ + H₂↑
Metals below hydrogen in reactivity series (Cu, Hg, Ag, Au) do NOT react with dilute acids.
4. Reaction with Salt Solutions (Displacement)
A more reactive metal displaces a less reactive metal from its salt solution.
Fe + CuSO₄ → FeSO₄ + Cu Cu + 2AgNO₃ → Cu(NO₃)₂ + 2Ag
The Reactivity Series
From most reactive to least reactive:
Mnemonic: Please Stop Calling Me A Zebra, I Never Show Pity — Can He Say Aye Please?
Uses of reactivity series: 1. Predict whether a metal will displace another from a salt solution 2. Determine if a metal will react with a dilute acid 3. Guide metal extraction methods
How Metals are Extracted (Extraction of Metals)
The method of extraction depends on the reactivity of the metal:
| Reactivity | Examples | Method of extraction |
|---|---|---|
| Very high (K, Na, Ca, Mg, Al) | Aluminium, Sodium | Electrolytic reduction |
| Medium (Zn, Fe, Pb, Sn) | Iron, Zinc | Reduction with carbon (coke) |
| Low (Cu, Hg, Ag, Au) | Gold, Silver | Very easily obtained; some just by heating ores |
Key Terms in Metallurgy
- Mineral: A naturally occurring element or compound in the earth's crust
- Ore: A mineral from which a metal can be extracted profitably
- Gangue: Earthly/rocky impurities mixed with ore
- Concentration/dressing of ore: Removing gangue from ore before extraction
- Calcination: Heating ore strongly below its melting point in limited air
- Roasting: Heating ore strongly in excess air (converts sulphide → oxide)
- Smelting: Reduction of metal oxide using coke at high temperature
- Flux: A substance (like limestone) added to remove gangue as slag
- Slag: The fusible product formed when flux reacts with gangue
Extraction of Iron — Blast Furnace
Fe₂O₃ + 3CO → 2Fe + 3CO₂
(Coke burns to form CO, which reduces iron oxide to molten iron. Limestone removes SiO₂ as slag CaSiO₃.)
Extraction of Aluminium — Electrolytic Process
- Ore: Bauxite (Al₂O₃·2H₂O)
- Purified to alumina (Al₂O₃)
- Electrolysis of molten alumina: Al³⁺ → Al at cathode; O²⁻ → O₂ at anode
Ionic Compounds and How They Form
An ionic bond is formed by the complete transfer of electrons from a metal to a non-metal.
Formation of NaCl
- Na has 1 electron in outer shell. It gives 1 electron → becomes Na⁺ (2,8)
- Cl has 7 electrons in outer shell. It takes 1 electron → becomes Cl⁻ (2,8,8)
- Electrostatic attraction between Na⁺ and Cl⁻ forms the ionic bond
Formation of MgCl₂
- Mg has 2 electrons in outer shell. It gives 2 electrons (one to each Cl) → becomes Mg²⁺
- Each Cl takes 1 electron → becomes Cl⁻
- So: 1 Mg²⁺ ion + 2 Cl⁻ ions → MgCl₂
Properties of Ionic Compounds
- High melting and boiling points (strong electrostatic forces)
- Solid at room temperature (usually)
- Conduct electricity only when molten or dissolved in water (ions are then free to move)
- Generally soluble in water (polar solvent)
Corrosion of Metals
Corrosion = gradual destruction of a metal by chemical reaction with its environment.
Types of Corrosion
| Metal | Corrosion Product | Colour |
|---|---|---|
| Iron | Fe₂O₃·xH₂O | Orange-brown (rust) |
| Copper | CuCO₃·Cu(OH)₂ | Green (patina) |
| Silver | Ag₂S | Black tarnish |
| Aluminium | Al₂O₃ | White film (protective!) |
How Aluminium is Protected by Corrosion
Aluminium rapidly forms a thin, hard oxide layer (Al₂O₃) that prevents further corrosion. This is why aluminium cookware is widely used despite Al being a reactive metal. The process of deliberately making this oxide layer thicker is called anodising.
Prevention of Corrosion
1. Painting — blocks contact with air and moisture 2. Oiling/Greasing — used for machine parts 3. Galvanising — coating iron with zinc; even if zinc layer scratches, zinc (being more reactive) corrodes preferentially, protecting iron 4. Tinning — coating iron with tin (tin is less reactive; protects only as long as coating is intact) 5. Electroplating — coating with chromium, nickel, etc. 6. Alloying — mixing metals to reduce reactivity (stainless steel = Fe + Cr + Ni)
Alloys
An alloy is a homogeneous mixture of two or more metals (or a metal + non-metal), having properties superior to the individual metals.
| Alloy | Composition | Properties/Uses |
|---|---|---|
| Brass | Cu + Zn | Harder than Cu; used in utensils, taps |
| Bronze | Cu + Sn | Tough; used in sculptures, bells, medals |
| Solder | Pb + Sn | Low melting point; used for joining metals |
| Stainless steel | Fe + Cr + Ni | Rust-proof; used in utensils, cutlery |
| Duralumin | Al + Cu + Mg + Mn | Light and strong; used in aircraft |
| Amalgam | Mercury + another metal | Dental fillings |
Pure gold is 24 carat. 22-carat gold = 22 parts gold + 2 parts silver/copper. Alloying makes gold harder and more durable for jewellery.
Board Exam Tips
- 5–6 marks typically from this chapter
- Reactivity series is must-memorise — used in multiple question types
- Know the 3 types of extraction (electrolysis, coke reduction, simple heating)
- Ionic bond questions often ask you to show the electron transfer diagram for NaCl or MgCl₂
- Thermite reaction (
Fe₂O₃ + 2Al → Al₂O₃ + 2Fe) — remember it's used to join railway tracks - "Why does aluminium not corrode despite being reactive?" — standard 2-mark question
- Galvanising vs Tinning: galvanising provides protection even after scratching (Zn is more reactive than Fe); tinning does not (Sn is less reactive)
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Intext Questions
Page 40, Q1: Give an example of a metal which is a liquid at room temperature.
Page 40, Q2: Give an example of a metal which is the best conductor of heat.
Page 40, Q3: Give an example of a non-metal which is a good conductor of electricity.
Page 46, Q1: What are amphoteric oxides? Give an example.
Zinc oxide (ZnO) is another amphoteric oxide.
- With acid:
Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O - With base:
Al₂O₃ + 2NaOH → 2NaAlO₂ + H₂O
Page 49, Q2: Why should the metals placed low in the activity series not be extracted by reduction with carbon or aluminium?
Chapter-End Exercise
Q1. Which of the following pairs will give displacement reactions? (a) NaCl + Cu (b) MgCl₂ + Fe (c) FeSO₄ + Ag (d) AgNO₃ + Cu
Q2. What are the various steps in the extraction of metals from low, medium, and high reactivity?
| Reactivity level | Metals | Extraction method |
|---|---|---|
| Low (below H in series) | Cu, Ag, Au, Pt | Found free in nature; sometimes heating oxide in air |
| Medium (Zn, Fe, Pb, Sn) | Zinc, iron, lead, tin | Reduction of oxide with carbon (coke) |
| High (Na, K, Ca, Mg, Al) | Sodium, potassium, calcium | Electrolytic reduction (can't use carbon) |
Highly reactive metals cannot be extracted by carbon reduction because they are more reactive than carbon and would not be displaced.
Q3. What is meant by refining of metals? Describe electrolytic refining.
Electrolytic refining:
- The impure metal is made the anode (positive electrode)
- Pure metal is made the cathode (negative electrode)
- A solution of the metal's salt is used as the electrolyte
- On passing current: anode (impure metal) dissolves → metal ions move to cathode → pure metal deposits on cathode
- Impurities from the anode collect below it as anode mud
- Metals like copper, zinc, tin, nickel, silver, and gold are refined this way
Q4. What is an alloy? Give two examples with their compositions.
Examples: 1. Brass — Copper (Cu) + Zinc (Zn) — harder than copper, used for utensils, musical instruments 2. Steel — Iron (Fe) + Carbon (C, 0.02–2%) — harder than iron, used for construction 3. Bronze — Copper (Cu) + Tin (Sn) — used for statues, medals 4. Solder — Lead (Pb) + Tin (Sn) — low melting point, used for electrical joints
Alloys are stronger, harder, and more resistant to corrosion than pure metals.
Q5. Explain why: (a) Platinum, gold, and silver are used to make jewellery. (b) Sodium is kept under kerosene.
- They are at the bottom of the activity series — very low reactivity
- They do not tarnish or corrode in air or water
- They maintain their lustre over long periods
- They are malleable and ductile — can be shaped into intricate designs
(b) Sodium is stored under kerosene because:
- Sodium is highly reactive — it reacts vigorously with oxygen and water in air
4Na + O₂ → 2Na₂O(immediate oxidation)2Na + 2H₂O → 2NaOH + H₂↑(with moisture — releases flammable H₂)- Kerosene is a non-reactive liquid that prevents contact with air and moisture
Q6. Give reasons why copper is used to make hot water tanks but not steel.
- Copper does not react with water even at high temperatures, so it does not corrode inside the tank
- Copper is a good conductor of heat — it transfers heat quickly and efficiently
- Steel (iron alloy) reacts with hot water and oxygen to form rust (Fe₂O₃), which weakens the tank over time and contaminates the water
- Therefore, copper is preferred for hot water storage despite being more expensive