Engineering Chemistry (BT-101) Unit 3 Notes

How unit 3 is examined

Lubrication mechanisms (thick film, thin film, polarizability) and viscosity index carry the marks; flash/fire, aniline, cloud/pour points and the acid-number and VI numericals are short but repeatedly asked.

Introduction

<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">Not asked since 2022</span>

Definition. <mark>A lubricant is a substance introduced between two moving or sliding surfaces to reduce friction, wear and heat generation between them.</mark> Lubrication is the process of applying it.

Key points.

Lubricants reduce friction and wear, so machine parts last longer and less energy is wasted.
They carry away frictional heat, acting as a coolant.
They act as a seal, keep out dust and moisture, and reduce corrosion of the metal surfaces.
An efficient lubricant needs suitable viscosity and viscosity index, a high flash and fire point, a low pour point, a low acid value and a low emulsification tendency.

Mechanism of lubrication

Definition. <mark>Lubrication mechanism is the way in which a lubricant keeps two moving surfaces apart or protects them, either by a thick fluid film (hydrodynamic), a thin adsorbed film (boundary) or an extreme pressure film.</mark>

Polarizability. Polarizability is the ease with which the electron cloud of an atom or molecule is distorted by an electric field, forming an induced dipole. Larger, more loosely held electron clouds are more polarizable. Polar, polarizable molecules (fatty acids, alcohols) stick strongly to metal surfaces, which is why they make good boundary lubricants.

Diagram. <figure class="ds-fig" style="margin:1.4rem 0;overflow-x:auto"><svg xmlns="http://www.w3.org/2000/svg" id="dsfig-u3-01" viewBox="0 0 510 80" width="510" height="80" role="img" aria-label="Journal bearing. Sh = rotating shaft, Oil = thick oil wedge film, Brg = fixed bearing. The film separates Sh from Brg completely."><style>#dsfig-u3-01 .e{stroke:#454C5A;stroke-width:1.4;fill:none}#dsfig-u3-01 .e.hi{stroke:#2340B8;stroke-width:2.6}#dsfig-u3-01 .n{fill:#FFFFFF;stroke:#16181D;stroke-width:1.4}#dsfig-u3-01 .n.hi{fill:#E3E9FC;stroke:#2340B8;stroke-width:2.2}#dsfig-u3-01 .n.rb-b{fill:#16181D;stroke:#16181D}#dsfig-u3-01 .n.rb-r{fill:#BD3227;stroke:#BD3227}#dsfig-u3-01 text{font-family:"JetBrains Mono",ui-monospace,Menlo,Consolas,monospace;font-size:13px}#dsfig-u3-01 .t{fill:#16181D;font-weight:500}#dsfig-u3-01 .t.inv{fill:#FFFFFF;font-weight:700}#dsfig-u3-01 .kd{stroke:#16181D;stroke-width:1.2}#dsfig-u3-01 .dot{fill:#16181D}#dsfig-u3-01 .ann{fill:#2340B8;font-size:11px;font-weight:700}#dsfig-u3-01 .lbl{fill:#6F7787;font-family:system-ui,-apple-system,sans-serif;font-size:12px;font-weight:700}#dsfig-u3-01 .ptr{fill:#2340B8;font-size:12px;font-weight:700}#dsfig-u3-01 .ah{fill:#454C5A}#dsfig-u3-01 .ah.hi{fill:#2340B8}#dsfig-u3-01 .wl rect{fill:#FFFFFF;stroke:#DCE0E7}#dsfig-u3-01 .wl .t{font-size:12px;font-weight:700}#dsfig-u3-01 .wl.hi rect{fill:#2340B8;stroke:#2340B8}#dsfig-u3-01 .wl.hi .t{fill:#FFFFFF}html.dark #dsfig-u3-01 .e{stroke:#B1B7C3}html.dark #dsfig-u3-01 .e.hi{stroke:#8FA3FF}html.dark #dsfig-u3-01 .n{fill:#161920;stroke:#E6E8ED}html.dark #dsfig-u3-01 .n.hi{fill:#1E2748;stroke:#8FA3FF}html.dark #dsfig-u3-01 .n.rb-b{fill:#E6E8ED;stroke:#E6E8ED}html.dark #dsfig-u3-01 .n.rb-r{fill:#FF7E71;stroke:#FF7E71}html.dark #dsfig-u3-01 .t{fill:#E6E8ED}html.dark #dsfig-u3-01 .t.inv{fill:#0F1115}html.dark #dsfig-u3-01 .kd{stroke:#E6E8ED}html.dark #dsfig-u3-01 .dot{fill:#E6E8ED}html.dark #dsfig-u3-01 .ann{fill:#8FA3FF}html.dark #dsfig-u3-01 .lbl{fill:#858D9C}html.dark #dsfig-u3-01 .ptr{fill:#8FA3FF}html.dark #dsfig-u3-01 .ah{fill:#B1B7C3}html.dark #dsfig-u3-01 .ah.hi{fill:#8FA3FF}html.dark #dsfig-u3-01 .wl rect{fill:#161920;stroke:#2A2E37}html.dark #dsfig-u3-01 .wl.hi rect{fill:#8FA3FF;stroke:#8FA3FF}html.dark #dsfig-u3-01 .wl.hi .t{fill:#0F1115}</style><defs><marker id="ah4" 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="ahh4" 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="M59,40 L234,40" marker-end="url(#ah4)"/><path class="e" d="M274,40 L449,40" marker-end="url(#ah4)"/><circle class="n" cx="40" cy="40" r="18"/><text class="t" x="40" y="40" dy=".35em" text-anchor="middle">Sh</text><circle class="n" cx="255" cy="40" r="18"/><text class="t" x="255" y="40" dy=".35em" text-anchor="middle">Oil</text><circle class="n" cx="470" cy="40" r="18"/><text class="t" x="470" y="40" dy=".35em" text-anchor="middle">Brg</text></svg><figcaption style="font-size:.82em;opacity:.72;margin-top:.45rem">Journal bearing. Sh = rotating shaft, Oil = thick oil wedge film, Brg = fixed bearing. The film separates Sh from Brg completely.</figcaption></figure>

Key points.

Thick film (hydrodynamic) lubrication separates the two moving surfaces completely by a fluid layer at least about 1000 Å thick, so there is no metal-to-metal contact.
Friction arises only from internal resistance (shear) of the fluid, so the lubricant's viscosity decides the friction.
In a journal bearing the rotating shaft drags oil into a converging wedge, which builds pressure and lifts the shaft on the oil film.
It suits light and heavy journal bearings and high speeds with moderate loads; a liquid of suitable viscosity (mineral oil) is used.
Thin film (boundary) lubrication acts when speed is low or load is high, so a thick film cannot form and the surfaces come close.
Polar molecules of fatty acids or oils are adsorbed on the metal surface as a monomolecular or polymolecular layer, which prevents direct contact; this is the mechanism for delicate instruments (watches, balances, sewing machines) using vegetable oils, blended mineral oils, silicones.
Extreme pressure lubrication is used under very high load and temperature, where additives such as chlorinated esters or sulphur and phosphorus compounds react with the metal to form a durable low-shear film of chloride, sulphide or phosphate.

Basis	Thick film	Thin film
Film	Fluid layer, above 1000 Å	Adsorbed layer of molecules
Contact	None	Occasional
Governing property	Viscosity	Oiliness, polarity
Load and speed	Moderate load, high speed	Low speed, high load
Examples	Journal bearings	Delicate instruments, gears

Answer frame. Open with the definition of the mechanism asked; draw the journal bearing; then develop points 1-4 for thick film or 5-6 for delicate instruments; add the table if both types are named; close by stating the example and the property (viscosity or polarity) that controls it. For the Nov 2022 note, first give the classification (see next topics), then points 1, 5 and 7 with uses.

Asked: [14 marks] (Jun 2022) Write short notes on (i) Polarizability (ii) Thin film lubricants and fluid film lubricants Asked: [14 marks] (Nov 2022) Write a note on lubricants with special reference to their (i) classification (ii) mode of action with examples and uses Asked: [7 marks] (Jun 2023) Describe the mechanism of lubrication that is applied to delicate instruments Asked: [7 marks] (Dec 2024) Give short note on mechanism thick film of lubrication process

Classification of lubricants

Definition. <mark>Lubricants are classified by physical state into liquid, semi-solid and solid lubricants.</mark>

<figure class="ds-fig" style="margin:1.4rem 0;overflow-x:auto"><svg xmlns="http://www.w3.org/2000/svg" id="dsfig-u3-02" viewBox="0 0 308 130" width="308" height="130" role="img" aria-label="Lubricants by physical state"><style>#dsfig-u3-02 .e{stroke:#454C5A;stroke-width:1.4;fill:none}#dsfig-u3-02 .e.hi{stroke:#2340B8;stroke-width:2.6}#dsfig-u3-02 .n{fill:#FFFFFF;stroke:#16181D;stroke-width:1.4}#dsfig-u3-02 .n.hi{fill:#E3E9FC;stroke:#2340B8;stroke-width:2.2}#dsfig-u3-02 .n.rb-b{fill:#16181D;stroke:#16181D}#dsfig-u3-02 .n.rb-r{fill:#BD3227;stroke:#BD3227}#dsfig-u3-02 text{font-family:"JetBrains Mono",ui-monospace,Menlo,Consolas,monospace;font-size:13px}#dsfig-u3-02 .t{fill:#16181D;font-weight:500}#dsfig-u3-02 .t.inv{fill:#FFFFFF;font-weight:700}#dsfig-u3-02 .kd{stroke:#16181D;stroke-width:1.2}#dsfig-u3-02 .dot{fill:#16181D}#dsfig-u3-02 .ann{fill:#2340B8;font-size:11px;font-weight:700}#dsfig-u3-02 .lbl{fill:#6F7787;font-family:system-ui,-apple-system,sans-serif;font-size:12px;font-weight:700}#dsfig-u3-02 .ptr{fill:#2340B8;font-size:12px;font-weight:700}#dsfig-u3-02 .ah{fill:#454C5A}#dsfig-u3-02 .ah.hi{fill:#2340B8}#dsfig-u3-02 .wl rect{fill:#FFFFFF;stroke:#DCE0E7}#dsfig-u3-02 .wl .t{font-size:12px;font-weight:700}#dsfig-u3-02 .wl.hi rect{fill:#2340B8;stroke:#2340B8}#dsfig-u3-02 .wl.hi .t{fill:#FFFFFF}html.dark #dsfig-u3-02 .e{stroke:#B1B7C3}html.dark #dsfig-u3-02 .e.hi{stroke:#8FA3FF}html.dark #dsfig-u3-02 .n{fill:#161920;stroke:#E6E8ED}html.dark #dsfig-u3-02 .n.hi{fill:#1E2748;stroke:#8FA3FF}html.dark #dsfig-u3-02 .n.rb-b{fill:#E6E8ED;stroke:#E6E8ED}html.dark #dsfig-u3-02 .n.rb-r{fill:#FF7E71;stroke:#FF7E71}html.dark #dsfig-u3-02 .t{fill:#E6E8ED}html.dark #dsfig-u3-02 .t.inv{fill:#0F1115}html.dark #dsfig-u3-02 .kd{stroke:#E6E8ED}html.dark #dsfig-u3-02 .dot{fill:#E6E8ED}html.dark #dsfig-u3-02 .ann{fill:#8FA3FF}html.dark #dsfig-u3-02 .lbl{fill:#858D9C}html.dark #dsfig-u3-02 .ptr{fill:#8FA3FF}html.dark #dsfig-u3-02 .ah{fill:#B1B7C3}html.dark #dsfig-u3-02 .ah.hi{fill:#8FA3FF}html.dark #dsfig-u3-02 .wl rect{fill:#161920;stroke:#2A2E37}html.dark #dsfig-u3-02 .wl.hi rect{fill:#8FA3FF;stroke:#8FA3FF}html.dark #dsfig-u3-02 .wl.hi .t{fill:#0F1115}</style><defs><marker id="ah5" 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="ahh5" 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><line class="e" x1="144" y1="37" x2="47.5" y2="101"/><line class="e" x1="144" y1="37" x2="146" y2="101"/><line class="e" x1="144" y1="37" x2="240.5" y2="101"/><rect class="n" x="95" y="22" width="98" height="30" rx="8"/><text class="t" x="144" y="37" dy=".35em" text-anchor="middle">Lubricants</text><rect class="n" x="14" y="86" width="67" height="30" rx="8"/><text class="t" x="47.5" y="101" dy=".35em" text-anchor="middle">Liquid</text><rect class="n" x="97" y="86" width="98" height="30" rx="8"/><text class="t" x="146" y="101" dy=".35em" text-anchor="middle">Semi-solid</text><rect class="n" x="211" y="86" width="59" height="30" rx="8"/><text class="t" x="240.5" y="101" dy=".35em" text-anchor="middle">Solid</text></svg><figcaption style="font-size:.82em;opacity:.72;margin-top:.45rem">Lubricants by physical state</figcaption></figure>

Key points.

Liquid lubricants (lubricating oils) are mineral oils, animal and vegetable oils, and synthetic oils; they suit high speeds and light loads.
Semi-solid lubricants (greases) are oil thickened with a soap of calcium, sodium or lithium; they stay in place and suit slow, heavily loaded bearings.
Solid lubricants such as graphite and molybdenum disulphide have layered structures that slip easily, and are used at very high temperatures or loads where oil would burn or squeeze out.
Additives and blends of these are used to improve specific properties.

significance & determination of Viscosity and Viscosity Index

Definition. <mark>Viscosity is the property of a fluid by which it resists flow, i.e. the internal friction between its layers. Viscosity Index (VI) is an arbitrary number showing how little the viscosity of an oil changes with temperature.</mark>

Key points.

Viscosity is the most important property of a lubricant, because it decides the film thickness and the friction in fluid film lubrication.
Too low a viscosity lets the film break under load; too high a viscosity wastes energy as fluid friction and heat.
The viscosity of every oil falls as temperature rises, so a good oil should fall as little as possible.
Viscosity is determined with a Redwood, Saybolt or Engler viscometer, in which the time for a fixed volume (50 ml) of oil to flow through a standard orifice at a set temperature is measured.
Dynamic viscosity is measured in poise or Pa s; kinematic viscosity (dynamic viscosity divided by density) is in stokes.
VI is found by comparing the test oil with two reference oils that have the same viscosity at 210 °F (99 °C): Pennsylvanian oil (high VI, VI = 100, small change with temperature) and Gulf oil (low VI, VI = 0, large change).
A high VI means a flat viscosity-temperature curve, which is desirable for oils in engines working over wide temperature ranges.

Formula. L = viscosity at 100 °F of the Gulf oil, H = of the Pennsylvanian oil, U = of the test oil.

$$\text{VI} = \frac{L - U}{L - H} \times 100$$

Oil	Viscosity-temperature behaviour
Pennsylvanian	Flattest curve, VI = 100
Test oil	Between the two, VI = value calculated
Gulf	Steepest curve, VI = 0

Answer frame. Open with the definitions of viscosity and VI; draw the viscosity (y-axis) versus temperature (x-axis) graph with three curves labelled Pennsylvanian, test and Gulf, all meeting the same viscosity at 210 °F; give points 6-7 and the formula with symbols; close with significance (a high VI oil is preferred).

Asked: [14 marks] (Jun 2022, Dec 2023) Write a short note on (i) Viscosity index (ii) Flash point; also: What is lubricant? With neat diagrams explain (i) Viscosity index (ii) Flash and fire point

Flash & Fire Points

Definition. <mark>Flash point is the lowest temperature at which an oil gives off enough vapour to ignite momentarily with a flash when a flame is brought near; fire point is the lowest temperature at which the vapour burns continuously for at least 5 seconds.</mark>

Key points.

Flash point is determined in the Pensky-Martens closed cup (higher-flash oils), the Abel closed cup (low-flash oils) or the Cleveland open cup, where the oil is heated slowly with stirring and a test flame is passed over the cup at intervals.
The fire point is found in the same apparatus by heating further after the flash until burning is sustained; it is usually 5 to 40 °C above the flash point.
Significance: a lubricant's flash point must be well above its working temperature, so it serves as a safety guide in storage, transport and handling.
A low flash point also indicates contamination with volatile fuel.

Asked: [7 marks] (Dec 2024) Explain about flash and fire point and their significance Asked: [7 marks] (Jun 2025) Explain the following properties (any two) i) Aniline Point ii) Cloud and Pour Point iii) Flash and Fire Point

Cloud & Pour Points

Definition. <mark>Cloud point is the temperature at which wax crystals first begin to separate, making the oil cloudy; pour point is the lowest temperature at which the oil still flows under standard conditions.</mark>

Key points.

Both are found by cooling the oil in a test jar and observing it at every 3 °C fall: cloudiness marks the cloud point, and the temperature 3 °C above the level at which the oil no longer flows is the pour point.
Pour point is always lower than cloud point, and both indicate the wax content of the oil.
A lubricant used in cold conditions must have a pour point below the lowest working temperature, otherwise it will not flow and will block oil lines.

Aniline Point

Definition. <mark>Aniline point is the lowest temperature at which equal volumes of the oil and aniline are completely miscible.</mark>

Key points.

It is determined by heating equal volumes of oil and aniline in a tube until they form one clear phase, then cooling slowly and noting the temperature at which turbidity appears.
Aromatic hydrocarbons dissolve in aniline more easily than paraffins, so a low aniline point means a high aromatic content.
A higher aniline point means lower aromatic content and better lubricating quality, because aromatics attack rubber seals and gaskets.

Asked: [7 marks] (Jun 2023) Explain about aniline point and saponification Number

Acid Number

Definition. <mark>Acid number (neutralization value) is the number of milligrams of KOH required to neutralize the free acids in 1 g of oil.</mark>

Key points.

It is found by titrating a weighed oil sample dissolved in alcohol against standard alcoholic KOH, with phenolphthalein indicator.
A high acid number means oxidation of the oil, which causes corrosion of the machine parts and gumming, so a good lubricant has a low value.

Formula. $$\text{NV} = \frac{V \times N \times 56}{w}$$

Example. Given w = 1.5 g, V = 24 ml, N = 0.0125 N, limit 8.0.

$$\text{NV} = \frac{24 \times 0.0125 \times 56}{1.5} = \frac{16.8}{1.5}$$

NV = 11.2 mg KOH/g. Since 11.2 > 8.0, the oil is NOT safe to use in the machinery.

Properties of an efficient lubricant. Suitable viscosity and high viscosity index, high flash and fire point, low pour point, low acid value, low emulsification tendency, thermal and oxidation stability, and good oiliness.

Answer frame. List the properties first; then write the formula, the substitution and the answer; close with the comparison to 8.0 and the verdict.

Asked: [7 marks] (Jun 2025) What are the important properties that an efficient lubricant should possess? 1.5 g of oil dissolved in alcohol needs 24 ml of 0.0125 N alcoholic KOH. Find the neutralization value; should not exceed 8.0, is it safe?

Saponification Number

Definition. <mark>Saponification number is the number of milligrams of KOH required to saponify 1 g of oil or fat.</mark>

Key points.

It is found by refluxing a known weight of oil with excess standard alcoholic KOH and titrating the unused KOH against standard HCl, along with a blank.
Animal and vegetable oils are esters of fatty acids and are saponified, so they have a high saponification number, while mineral oils are hydrocarbons and have almost none.
It therefore shows whether the oil is animal, vegetable or mineral, and detects adulteration of mineral oil with fatty oils.

Steam Emulsification Number

Definition. <mark>Steam emulsification number (SEN) is the time in seconds taken by 20 ml of oil and 20 ml of water, in a tube heated by steam, to separate into two layers.</mark>

Key points.

Steam is passed through the oil at 90 °C and the time for the oil and water to separate is noted.
A good oil separates quickly (SEN below 20 s), meaning it does not form an emulsion, while a high SEN means a stable emulsion that washes away the film.
It matters for oils used in steam engines and turbines, where the lubricant meets water or steam.

Last-minute revision

Lubricant: reduces friction and wear between moving surfaces and acts as coolant and seal.
Thick film needs more than 1000 Å of fluid and depends on viscosity; thin film depends on oiliness and adsorption.
Extreme pressure lubrication uses additives that form chloride, sulphide or phosphate films.
VI = (L - U)/(L - H) x 100; Pennsylvanian oil = 100, Gulf oil = 0.
Flash point: momentary flash; fire point: burns for at least 5 s; Pensky-Martens, Abel, Cleveland.
Pour point is below cloud point; cloud point is where wax separates.
Aniline point: high value means low aromatics, and low value means aromatic oil that attacks rubber.
NV = V x N x 56 / w mg KOH/g; the Jun 2025 answer is 11.2, above 8.0, so unsafe.
Saponification number: mg KOH per g of fat; high for fatty oils, near zero for mineral oils.
SEN: seconds for oil and water to separate; low is good.
Jun 2025 VI = 338/697 x 100 = 48.5.

Memory hooks

"Thick = viscosity, Thin = polarity": film type decides which property matters.
PG: Pennsylvanian is Perfect (100), Gulf is Gone (0).
Flash flickers, fire fights on for 5 seconds.
Cloud comes before pour as the oil cools.
Aniline: high point, low aromatics, safe rubber.

Coverage checklist

Introduction: covered (properties of an efficient lubricant).
Mechanism of lubrication: Jun 2022, Nov 2022, Jun 2023, Dec 2024.
Classification of lubricants: covered for the Nov 2022 note.
significance & determination of Viscosity and Viscosity Index: Jun 2022, Dec 2023.
Flash & Fire Points: Dec 2024, Jun 2025 (any two).
Cloud & Pour Points: Jun 2025 (any two).
Aniline Point: Jun 2023, Jun 2025 (any two).
Acid Number: Jun 2025.
Saponification Number: Jun 2023.
Steam Emulsification Number: covered.
related numerical problems: Jun 2025.

Account

Engineering Chemistry (BT-101) - Unit 3 Short Notes

How unit 3 is examined

Introduction

Mechanism of lubrication

Classification of lubricants

significance & determination of Viscosity and Viscosity Index

Flash & Fire Points

Cloud & Pour Points

Aniline Point

Acid Number

Saponification Number

Steam Emulsification Number

related numerical problems

Last-minute revision

Memory hooks

Coverage checklist