How unit 5 is examined
Covers the water phase diagram, the Cu-Ag eutectic diagram and corrosion; all three carry high marks, with corrosion asked in almost every paper.
Phase diagram of single component system (Water)
<span style="display:inline-block;padding:.16em .6em;border:1.5px solid currentColor;border-radius:999px;font-size:.68em;font-weight:700;letter-spacing:.06em;text-transform:uppercase;opacity:.75">High weight</span>
Definition. <mark>The triple point is the point where solid, liquid and vapour coexist in equilibrium; for water it is at 0.0098 °C (0.01 °C) and 4.58 mm Hg.</mark>
Phase rule terms. A phase (P) is a physically distinct, homogeneous, mechanically separable part of a system. A component (C) is the least number of independent chemical species needed to express the composition of every phase. Degrees of freedom (F) is the number of variables (T, P, concentration) that can be changed independently without disturbing the number of phases.
$$F = C - P + 2$$
For water C = 1, so $F = 3 - P$.
Diagram.
<figure class="ds-fig" style="margin:1.4rem 0;overflow-x:auto"><svg xmlns="http://www.w3.org/2000/svg" id="dsfig-u5-01" viewBox="0 0 553 372.4" width="553" height="372.4" role="img" aria-label="Water P-T diagram. O is the triple point; OA vaporization curve, OB sublimation curve, OC fusion curve, OA' (drawn as OD) metastable curve. Areas: ice left of OC/OB, water between OC and OA, steam below OA/OB."><style>#dsfig-u5-01 .e{stroke:#454C5A;stroke-width:1.4;fill:none}#dsfig-u5-01 .e.hi{stroke:#2340B8;stroke-width:2.6}#dsfig-u5-01 .n{fill:#FFFFFF;stroke:#16181D;stroke-width:1.4}#dsfig-u5-01 .n.hi{fill:#E3E9FC;stroke:#2340B8;stroke-width:2.2}#dsfig-u5-01 .n.rb-b{fill:#16181D;stroke:#16181D}#dsfig-u5-01 .n.rb-r{fill:#BD3227;stroke:#BD3227}#dsfig-u5-01 text{font-family:"JetBrains Mono",ui-monospace,Menlo,Consolas,monospace;font-size:13px}#dsfig-u5-01 .t{fill:#16181D;font-weight:500}#dsfig-u5-01 .t.inv{fill:#FFFFFF;font-weight:700}#dsfig-u5-01 .kd{stroke:#16181D;stroke-width:1.2}#dsfig-u5-01 .dot{fill:#16181D}#dsfig-u5-01 .ann{fill:#2340B8;font-size:11px;font-weight:700}#dsfig-u5-01 .lbl{fill:#6F7787;font-family:system-ui,-apple-system,sans-serif;font-size:12px;font-weight:700}#dsfig-u5-01 .ptr{fill:#2340B8;font-size:12px;font-weight:700}#dsfig-u5-01 .ah{fill:#454C5A}#dsfig-u5-01 .ah.hi{fill:#2340B8}#dsfig-u5-01 .wl rect{fill:#FFFFFF;stroke:#DCE0E7}#dsfig-u5-01 .wl .t{font-size:12px;font-weight:700}#dsfig-u5-01 .wl.hi rect{fill:#2340B8;stroke:#2340B8}#dsfig-u5-01 .wl.hi .t{fill:#FFFFFF}html.dark #dsfig-u5-01 .e{stroke:#B1B7C3}html.dark #dsfig-u5-01 .e.hi{stroke:#8FA3FF}html.dark #dsfig-u5-01 .n{fill:#161920;stroke:#E6E8ED}html.dark #dsfig-u5-01 .n.hi{fill:#1E2748;stroke:#8FA3FF}html.dark #dsfig-u5-01 .n.rb-b{fill:#E6E8ED;stroke:#E6E8ED}html.dark #dsfig-u5-01 .n.rb-r{fill:#FF7E71;stroke:#FF7E71}html.dark #dsfig-u5-01 .t{fill:#E6E8ED}html.dark #dsfig-u5-01 .t.inv{fill:#0F1115}html.dark #dsfig-u5-01 .kd{stroke:#E6E8ED}html.dark #dsfig-u5-01 .dot{fill:#E6E8ED}html.dark #dsfig-u5-01 .ann{fill:#8FA3FF}html.dark #dsfig-u5-01 .lbl{fill:#858D9C}html.dark #dsfig-u5-01 .ptr{fill:#8FA3FF}html.dark #dsfig-u5-01 .ah{fill:#B1B7C3}html.dark #dsfig-u5-01 .ah.hi{fill:#8FA3FF}html.dark #dsfig-u5-01 .wl rect{fill:#161920;stroke:#2A2E37}html.dark #dsfig-u5-01 .wl.hi rect{fill:#8FA3FF;stroke:#8FA3FF}html.dark #dsfig-u5-01 .wl.hi .t{fill:#0F1115}</style><defs><marker id="ah7" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="7" markerHeight="7" orient="auto-start-reverse"><path class="ah" d="M0,1 L9,5 L0,9 z"/></marker><marker id="ahh7" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="7" markerHeight="7" orient="auto-start-reverse"><path class="ah hi" d="M0,1 L9,5 L0,9 z"/></marker></defs><path class="e" d="M228.7,202.9 L496.3,57.7"/><path class="e" d="M196.4,222.9 L55.6,321.5"/><path class="e" d="M204.9,194.4 L150.3,57.6"/><path class="e hi" d="M229,220.5 L332.6,272.3"/><circle class="n" cx="212" cy="212" r="18"/><text class="t" x="212" y="212" dy=".35em" text-anchor="middle">O</text><circle class="n" cx="513" cy="48.6" r="18"/><text class="t" x="513" y="48.6" dy=".35em" text-anchor="middle">A</text><circle class="n" cx="40" cy="332.4" r="18"/><text class="t" x="40" y="332.4" dy=".35em" text-anchor="middle">B</text><circle class="n" cx="143.2" cy="40" r="18"/><text class="t" x="143.2" y="40" dy=".35em" text-anchor="middle">C</text><circle class="n" cx="349.6" cy="280.8" r="18"/><text class="t" x="349.6" y="280.8" dy=".35em" text-anchor="middle">D</text></svg><figcaption style="font-size:.82em;opacity:.72;margin-top:.45rem">Water P-T diagram. O is the triple point; OA vaporization curve, OB sublimation curve, OC fusion curve, OA' (drawn as OD) metastable curve. Areas: ice left of OC/OB, water between OC and OA, steam below OA/OB.</figcaption></figure>
Key points.
- The diagram is a pressure-temperature plot with three areas: solid (ice), liquid (water) and vapour (steam).
- In each area only one phase exists, so $F = 1 - 1 + 2 = 2$ (bivariant); both T and P must be stated to define the system.
- Curve OA (vaporization curve) shows water in equilibrium with steam; it ends at the critical point (374 °C, 218 atm) and $F = 1$ (univariant).
- Curve OB (sublimation curve) shows ice in equilibrium with vapour, and $F = 1$.
- Curve OC (fusion or melting curve) shows ice in equilibrium with water; it slopes to the left because ice melts at lower temperature when pressure rises, since ice is less dense than water; $F = 1$.
- At the triple point O three phases coexist, so $F = 1 - 3 + 2 = 0$ (invariant); neither T nor P can be changed without losing a phase.
- Curve OA' is the metastable curve, the extension of AO below the triple point, showing supercooled water in equilibrium with vapour; it is unstable and on disturbance water freezes to ice.
- Significance of the triple point: it is a fixed, reproducible reference point (0.0098 °C, 4.58 mm Hg) used to define the kelvin scale.
| Region | Phases | F |
|---|---|---|
| Area | 1 | 2 |
| Curve (OA, OB, OC, OA') | 2 | 1 |
| Point O | 3 | 0 |
Answer frame. Open with the definition of triple point and the phase rule $F = C - P + 2$ with the terms P, C, F defined; draw the P-T diagram with OA, OB, OC, OA' and the areas labelled; then develop the areas (F = 2), the three curves (F = 1), metastable curve, and triple point (F = 0) in that order; close with the significance of the triple point.
Asked: [7 marks] (Jun 2022, Jun 2025) What is triple point? Explain water system with the help of neat phase diagram; define and explain the terms of the phase rule, draw the labelled diagram, and give the significance of the triple point and metastable curve. Asked: [14 marks] (Nov 2022) Write brief note on (any two): phase diagram of single component systems (other options are from Units 6 and 7). Pitfall: Do not write the fusion curve OC with a positive slope; for water it slopes backward (to the left).
Phase diagram of binary Eutectic System (Cu-Ag.)
<span style="display:inline-block;padding:.16em .6em;border:1.5px solid currentColor;border-radius:999px;font-size:.68em;font-weight:700;letter-spacing:.06em;text-transform:uppercase;opacity:.75">High weight</span>
Definition. A phase diagram is a graph of the conditions (temperature, pressure, composition) at which different phases of a system are stable at equilibrium. <mark>A eutectic is the mixture of two solids that has the lowest melting (freezing) point of all compositions and melts and freezes sharply at that one temperature like a pure substance.</mark>
Formula. For a condensed system pressure is fixed, so the reduced phase rule is used:
$$F' = C - P + 1$$
Diagram.
<figure class="ds-fig" style="margin:1.4rem 0;overflow-x:auto"><svg xmlns="http://www.w3.org/2000/svg" id="dsfig-u5-02" viewBox="0 0 510 226.2" width="510" height="226.2" role="img" aria-label="Cu-Ag temperature vs composition (Cu left, Ag right). Cu melts at 1085 C, Ag at 961 C. E is the eutectic point at 779 C and 71.9% Ag; CuE and AgE are liquidus curves, LR is the solidus (eutectic) line."><style>#dsfig-u5-02 .e{stroke:#454C5A;stroke-width:1.4;fill:none}#dsfig-u5-02 .e.hi{stroke:#2340B8;stroke-width:2.6}#dsfig-u5-02 .n{fill:#FFFFFF;stroke:#16181D;stroke-width:1.4}#dsfig-u5-02 .n.hi{fill:#E3E9FC;stroke:#2340B8;stroke-width:2.2}#dsfig-u5-02 .n.rb-b{fill:#16181D;stroke:#16181D}#dsfig-u5-02 .n.rb-r{fill:#BD3227;stroke:#BD3227}#dsfig-u5-02 text{font-family:"JetBrains Mono",ui-monospace,Menlo,Consolas,monospace;font-size:13px}#dsfig-u5-02 .t{fill:#16181D;font-weight:500}#dsfig-u5-02 .t.inv{fill:#FFFFFF;font-weight:700}#dsfig-u5-02 .kd{stroke:#16181D;stroke-width:1.2}#dsfig-u5-02 .dot{fill:#16181D}#dsfig-u5-02 .ann{fill:#2340B8;font-size:11px;font-weight:700}#dsfig-u5-02 .lbl{fill:#6F7787;font-family:system-ui,-apple-system,sans-serif;font-size:12px;font-weight:700}#dsfig-u5-02 .ptr{fill:#2340B8;font-size:12px;font-weight:700}#dsfig-u5-02 .ah{fill:#454C5A}#dsfig-u5-02 .ah.hi{fill:#2340B8}#dsfig-u5-02 .wl rect{fill:#FFFFFF;stroke:#DCE0E7}#dsfig-u5-02 .wl .t{font-size:12px;font-weight:700}#dsfig-u5-02 .wl.hi rect{fill:#2340B8;stroke:#2340B8}#dsfig-u5-02 .wl.hi .t{fill:#FFFFFF}html.dark #dsfig-u5-02 .e{stroke:#B1B7C3}html.dark #dsfig-u5-02 .e.hi{stroke:#8FA3FF}html.dark #dsfig-u5-02 .n{fill:#161920;stroke:#E6E8ED}html.dark #dsfig-u5-02 .n.hi{fill:#1E2748;stroke:#8FA3FF}html.dark #dsfig-u5-02 .n.rb-b{fill:#E6E8ED;stroke:#E6E8ED}html.dark #dsfig-u5-02 .n.rb-r{fill:#FF7E71;stroke:#FF7E71}html.dark #dsfig-u5-02 .t{fill:#E6E8ED}html.dark #dsfig-u5-02 .t.inv{fill:#0F1115}html.dark #dsfig-u5-02 .kd{stroke:#E6E8ED}html.dark #dsfig-u5-02 .dot{fill:#E6E8ED}html.dark #dsfig-u5-02 .ann{fill:#8FA3FF}html.dark #dsfig-u5-02 .lbl{fill:#858D9C}html.dark #dsfig-u5-02 .ptr{fill:#8FA3FF}html.dark #dsfig-u5-02 .ah{fill:#B1B7C3}html.dark #dsfig-u5-02 .ah.hi{fill:#8FA3FF}html.dark #dsfig-u5-02 .wl rect{fill:#161920;stroke:#2A2E37}html.dark #dsfig-u5-02 .wl.hi rect{fill:#8FA3FF;stroke:#8FA3FF}html.dark #dsfig-u5-02 .wl.hi .t{fill:#0F1115}</style><defs><marker id="ah8" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="7" markerHeight="7" orient="auto-start-reverse"><path class="ah" d="M0,1 L9,5 L0,9 z"/></marker><marker id="ahh8" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="7" markerHeight="7" orient="auto-start-reverse"><path class="ah hi" d="M0,1 L9,5 L0,9 z"/></marker></defs><path class="e" d="M57.2,48.1 L332.4,178.1"/><path class="e" d="M455.1,103.3 L364.5,174.5"/><path class="e hi" d="M102,186.2 L425.2,186.2"/><path class="e" d="M45.4,58.2 L77.6,168"/><path class="e" d="M465,109.9 L449.2,167.9"/><circle class="n" cx="40" cy="40" r="18"/><text class="t" x="40" y="40" dy=".35em" text-anchor="middle">Cu</text><circle class="n" cx="470" cy="91.6" r="18"/><text class="t" x="470" y="91.6" dy=".35em" text-anchor="middle">Ag</text><circle class="n" cx="349.6" cy="186.2" r="18"/><text class="t" x="349.6" y="186.2" dy=".35em" text-anchor="middle">E</text><circle class="n" cx="83" cy="186.2" r="18"/><text class="t" x="83" y="186.2" dy=".35em" text-anchor="middle">L</text><circle class="n" cx="444.2" cy="186.2" r="18"/><text class="t" x="444.2" y="186.2" dy=".35em" text-anchor="middle">R</text></svg><figcaption style="font-size:.82em;opacity:.72;margin-top:.45rem">Cu-Ag temperature vs composition (Cu left, Ag right). Cu melts at 1085 C, Ag at 961 C. E is the eutectic point at 779 C and 71.9% Ag; CuE and AgE are liquidus curves, LR is the solidus (eutectic) line.</figcaption></figure>
Key points.
- Cu-Ag is a simple two-component system in which the metals are completely miscible as liquid but almost immiscible as solids; pressure is fixed, so the diagram is temperature against composition.
- The upper curves from the melting points of Cu (1085 °C) and Ag (961 °C) are liquidus curves; on them liquid is in equilibrium with solid Cu or solid Ag, and the melting point of each metal falls as the other is added.
- The two liquidus curves meet at the eutectic point E, at 779 °C with 71.9% Ag and 28.1% Cu, the lowest temperature at which any liquid can exist.
- The horizontal line through E is the solidus (eutectic line); below it everything is solid.
- Above the liquidus there is a single liquid phase, so $F' = 2 - 1 + 1 = 2$ (bivariant).
- On a liquidus curve two phases coexist (liquid and one solid), so $F' = 2 - 2 + 1 = 1$ (univariant).
- At E three phases coexist (liquid, solid Cu, solid Ag), so $F' = 2 - 3 + 1 = 0$ (invariant); temperature and composition are both fixed.
- On cooling a liquid of eutectic composition, it solidifies entirely at 779 °C to a fine mixture of Cu and Ag crystals; any other composition first deposits the excess metal, and the remaining liquid moves along the curve to E.
| Region | Phases | F' |
|---|---|---|
| Liquid | 1 | 2 |
| Liquid + solid Cu or Ag (under liquidus) | 2 | 1 |
| Eutectic point E | 3 | 0 |
Answer frame. Open with the definition of a phase diagram and the reduced phase rule $F' = C - P + 1$; draw the Cu-Ag diagram with 1085 °C, 961 °C, E at 779 °C and 71.9% Ag, and label liquid, liquid + solid, and solid; then develop liquidus, solidus, eutectic point and F' for each region; close with the cooling of eutectic composition. For "eutectic point" as a short note, give the definition, 779 °C / 71.9% Ag and F' = 0.
Asked: [7 marks] (Dec 2023) What is meant by a phase diagram? Explain two-component system by taking Cu-Ag system as an example. Asked: [14 marks] (Jun 2023) Write brief note on (any two): eutectic point; caustic embrittlement (given under Corrosion); the other options are from Units 3 and 7. Pitfall: Eutectic composition is 71.9% Ag, not 50%; F' at E is 0, not 1.
Corrosion: Types, Mechanisms & prevention
<span style="display:inline-block;padding:.16em .6em;border:1.5px solid currentColor;border-radius:999px;font-size:.68em;font-weight:700;letter-spacing:.06em;text-transform:uppercase;opacity:.75">High weight</span>
Definition. <mark>Corrosion is the destruction or deterioration of a metal by chemical or electrochemical reaction with its environment.</mark> Rusting of iron is the common example.
Types. Dry (chemical) corrosion is direct attack by gases; wet (electrochemical) corrosion occurs in the presence of a conducting liquid.
Key points.
- Oxidation corrosion (dry): direct attack of oxygen on a metal with no moisture, most rapid at high temperature; the metal is oxidised, $M \rightarrow M^{n+} + ne^-$, and oxygen is reduced, $\tfrac{n}{2}O_2 + ne^- \rightarrow nO^{2-}$, to form the oxide $M_2O_n$.
- Mechanism: a thin oxide film forms first on the surface; further growth needs metal ions to diffuse outward (or oxide ions inward) through the film, and the film thickens.
- Nature of the film decides the rate: a stable, non-porous film protects (Al, Cr, Cu); an unstable film decomposes (Au, Pt oxides); a volatile film evaporates (MoO$_3$); a porous film cracks and lets attack continue (Fe, Mg, alkali metals).
- Pilling-Bedworth rule: an oxide is protective if the volume of oxide formed is not less than the volume of metal consumed (ratio at least 1); if the ratio is below 1 the film is porous and non-protective (Na, K, Mg).
- Electrochemical (wet) corrosion has a separate anode and cathode. At the anode the metal is oxidised, $Fe \rightarrow Fe^{2+} + 2e^-$. At the cathode, in acidic medium hydrogen is evolved, $2H^+ + 2e^- \rightarrow H_2$; in neutral or alkaline medium with oxygen, $\tfrac12 O_2 + H_2O + 2e^- \rightarrow 2OH^-$.
- Rust forms as $Fe^{2+} + 2OH^- \rightarrow Fe(OH)_2$, which oxidises to yellow rust $Fe_2O_3 \cdot 3H_2O$.
- Galvanic corrosion: two dissimilar metals in contact in a conducting medium; the metal higher in the electrochemical series is the anode and corrodes (Zn-Cu couple, steel screw in brass). Prevent by choosing metals close in the series or insulating them.
- Waterline corrosion is differential aeration corrosion: metal just below the water level in a tank or ship hull is less aerated so it is the anode and corrodes, while the well-aerated part above is the cathode. Prevent with paints or cathodic protection.
- Pitting is localised attack where the protective film breaks or is defective (dirt, scratch, chloride); the small pit is the anode and the large surrounding area is the cathode, so pits deepen fast and perforate the metal.
- Stress corrosion cracking (SCC) is cracking from the combined action of tensile stress and a specific corrosive environment; examples are caustic embrittlement of boiler steel (NaOH in rivets and hairline cracks, prevented by sodium phosphate or tannin) and season cracking of brass (ammonia).
- Prevention: proper design and pure metals or alloys; protective coatings (paint, galvanising with Zn, tinning, electroplating); cathodic protection (sacrificial anode of Mg or Zn, or impressed current); and inhibitors (chromates, phosphates, amines).
Example (Jun 2025). Mass of rust from 100 kg Fe.
| Step | Working |
|---|---|
| Given | Fe = 100 kg, atomic weight 55.85, rust $Fe_2O_3 \cdot 3H_2O$ = 214 |
| Reaction | 2Fe gives 1 rust; $2 \times 55.85 = 111.7$ g gives 214 g |
| Mass | $100 \times 214 / 111.7$ |
Rust formed = 191.58 kg (about 191.6 kg).
Answer frame. Open with the definition of corrosion and the two types; draw the electrochemical cell (anode, cathode, moisture) where the answer needs it; then develop the mechanism, the types asked and prevention in that order; close with the anodic and cathodic reactions or a real example. For "oxidation corrosion", cover film formation, the film types and the Pilling-Bedworth rule. For "chemical versus electrochemical", give a 5-row comparison and end with the water phase diagram from the first topic.
Asked: [14 marks] (Jun 2023) Describe with examples: (i) chemical corrosion and electrochemical corrosion; (ii) phase diagram of one-component water system (see first topic). Asked: [7 marks] (Jun 2022, Dec 2024) What is oxidation corrosion and how does it take place? Describe the mechanism of oxidation corrosion. Asked: [7 marks] (Jun 2022) Explain pitting corrosion and stress corrosion. Asked: [7 marks] (Dec 2024) Explain water line corrosion and galvanic corrosion. Asked: [7 marks] (Jun 2025) Define corrosion. Write types and prevention of corrosion. Explain theory of mechanism of corrosion. How much rust ($Fe_2O_3 \cdot 3H_2O$) forms when 100 kg of iron has rusted away? (Mol. wt. of rust = 214). Pitfall: Anode is where the metal is oxidised and dissolves; do not swap anode and cathode in the galvanic and waterline answers.
Last-minute revision
- Triple point of water: 0.0098 °C and 4.58 mm Hg, F = 0.
- Phase rule: $F = C - P + 2$; reduced rule for condensed systems $F' = C - P + 1$.
- Water: areas F = 2, curves OA, OB, OC F = 1, point O F = 0; OA' is the metastable supercooled-water curve.
- Curve OC slopes backward because ice is less dense than water.
- Cu-Ag: Cu 1085 °C, Ag 961 °C, eutectic 779 °C at 71.9% Ag, 28.1% Cu.
- Eutectic point: F' = 0, lowest melting mixture, three phases.
- Corrosion is deterioration of a metal by chemical or electrochemical reaction with its environment.
- Pilling-Bedworth ratio of oxide to metal volume below 1 means a porous, non-protective film.
- Anode: $Fe \rightarrow Fe^{2+} + 2e^-$; cathode: $H_2$ evolution (acid) or $OH^-$ formation (oxygen).
- Waterline corrosion: less aerated area is the anode.
- SCC examples: caustic embrittlement and season cracking of brass.
- 100 kg Fe gives 191.58 kg rust ($214 / 111.7$).
Memory hooks
- OA-OB-OC: A for Atmosphere (vapour curve), B for Below zero (sublimation), C for Cold-melt (fusion).
- Triple point = zero freedom: three phases, F = 0.
- Eutectic = "easy to melt": lowest melting mixture.
- Anode Always Attacked; Cathode Collects electrons.
- Less oxygen means more damage: the poorly aerated spot is the anode.
Coverage checklist
- Phase diagram of single component system (Water): Nov 2022 (short note), Jun 2022 / Jun 2025 (triple point, water system).
- Phase diagram of binary Eutectic System (Cu-Ag.): Dec 2023 (phase diagram, Cu-Ag), Jun 2023 (eutectic point note).
- Corrosion: Types, Mechanisms & prevention: Jun 2023 (chemical and electrochemical), Jun 2022 / Dec 2024 (oxidation corrosion), Jun 2022 (pitting, stress), Dec 2024 (waterline, galvanic), Jun 2025 (define, types, prevention, rust numerical).