---
title: "Lubricants and their functions"
subject: "Engineering Chemistry"
chapter: 8
key: "semester_2/engineering_chemistry:lubricants-functions"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/lubricants-functions
site: BCT Engineering
---

# Lubricants and their functions

Engineering Chemistry · Chapter 8: Lubricants and Paints

- Asked in: 2075 Baisakh, 2 marks; 2074 Bhadra, 2.5 marks; 2072 Asoj, 2 marks; 2068 Bhadra, 2 marks (asked 4 times)
- Video: [Lubricants || Lubricant Important Functions And Classification ||Lubrication ||Engineering Chemistry](https://www.youtube.com/watch?v=Kuj2X6ncPBc) — Chemistry by Dr. Anjali Ssaxena
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/lubricants-functions
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-8

## Question

What are lubricating oils? Write the chief functions of lubricants.

## Short answer

Lubricating oils are liquid lubricants, mainly mineral oils, that form a film to reduce friction. Their chief functions are reducing friction and wear, cooling, sealing against gas leakage, preventing corrosion, cleaning away debris, and saving energy.

## Answer

A **lubricant** is a substance introduced between two moving or sliding surfaces to reduce friction and wear between them. Lubricating oils are liquid lubricants, mostly mineral oils from petroleum, which form a thin film between the surfaces and thereby replace solid–solid friction by fluid friction inside the oil.

#### Chief functions
1. **Reduces friction** between moving surfaces by forming a thin film that separates them.
2. **Reduces wear and tear**, which increases the life of machine parts.
3. **Acts as a coolant:** carries away heat produced by friction.
4. **Acts as a seal:** prevents leakage of gases, e.g. between piston rings and cylinder.
5. **Prevents corrosion** by coating metal surfaces from air and moisture.
6. **Cleans** by washing away dirt, metal particles and carbon deposits.
7. **Saves energy:** smoother motion means less power loss and higher efficiency; also reduces noise and vibration.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What does a lubricant do?
   A. Increases friction
   B. Reduces friction and wear between moving surfaces by forming a thin film
   C. Joins surfaces
   D. Removes metal
   Answer: B. It replaces solid-solid friction with fluid friction. (Key point: Lubricant definition)

2. How does a lubricant act as coolant?
   A. It freezes the surface
   B. It absorbs light
   C. It produces cold gas
   D. It carries away heat produced by friction
   Answer: D. Oil circulation removes frictional heat. (Key point: Cooling)

3. How does a lubricant act as a seal?
   A. It prevents leakage of gases between piston rings and cylinder
   B. It glues parts
   C. It evaporates
   D. It dissolves metal
   Answer: A. The oil film fills gaps. (Key point: Sealing)

4. How does a lubricant prevent corrosion?
   A. By reacting with metal
   B. By heating metal
   C. By coating metal surfaces from air and moisture
   D. By removing oxygen from air entirely
   Answer: C. The film excludes air and moisture. (Key point: Corrosion)

## Related in Engineering Chemistry

- [Lubricating greases and their functions](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Agreases-functions) — Engineering Chemistry, chapter 8, asked 3×
- [Lubricant functions; solid lubricants](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Alubricant-functions-solid) — Engineering Chemistry, chapter 8
- [Lubricant functions and paint [partly illegible]](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Alubricant-paint-partial) — Engineering Chemistry, chapter 8
- [Lubricant functions; grease and solid lubricant use](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Alubricant-functions-grease-solid) — Engineering Chemistry, chapter 8

## Related in other subjects

- [Lubricating oil functions and lubricating system](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Adiesel-lubricating-system) — Power Plant Equipment, chapter 5
- [Diesel components, lubricating and cooling functions](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Adiesel-components-lube-cooling) — Power Plant Equipment, chapter 5
- [Diesel fuel supply and lubrication functions](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Adiesel-fuel-supply-lubrication) — Power Plant Equipment, chapter 5
- [Lubricant properties, additives and SAE grading](https://www.bctengineer.com/question.md?key=semester_6%2Finternal_combustion_engines%3Alubricant-properties-additives-sae-grading) — Internal Combustion Engines, chapter 6