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

# Lubricants and their classification

Engineering Chemistry · Chapter 8: Lubricants and Paints

- Asked in: 2072 Magh, 3 marks
- Video: [Classification of Lubricants - Lubricants - Engineering Chemistry 1](https://www.youtube.com/watch?v=gqp4foT7How) — Ekeeda
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/lubricants-classification
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-8

## Question

What are lubricants? How are they classified?

## Short answer

A lubricant is a substance introduced between moving surfaces to reduce friction and wear. They are classified as liquid (animal/vegetable, mineral, synthetic oils), semi‑solid (calcium, sodium, lithium soap greases) and solid (graphite, MoS₂, talc).

## Answer

A **lubricant** is a substance introduced between two moving or sliding surfaces to reduce friction and wear between them. It reduces friction, wear and heat.

#### Classification
```text
Lubricants
 ├── Liquid   (oils)
 │     ├── Animal/vegetable oils: castor, olive, tallow
 │     ├── Mineral (petroleum) oils
 │     ├── Synthetic oils: silicones, polyglycols
 │     └── Blended oils (oil + additives)
 ├── Semi-solid  (greases): Ca, Na, Li soap greases
 └── Solid: graphite, MoS₂, talc, soapstone
```
- **Liquid lubricants** form a fluid film; mineral oils are the most common as they are cheap and stable.
- **Greases** are oils thickened with metallic soap.
- **Solid lubricants** have layered structures and are used at high temperatures and loads.

## Check yourself

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

1. What is the main purpose of a lubricant?
   A. To increase friction for grip
   B. To fuse two metal surfaces
   C. To reduce friction and wear between moving surfaces
   D. To only paint the surface
   Answer: C. A lubricant is introduced between moving surfaces to reduce friction, wear and heat. (Key point: Definition)

2. How are lubricants classified by physical state?
   A. Liquid (oils), semi-solid (greases) and solid
   B. Natural, artificial and mixed
   C. Acidic, basic and neutral
   D. Hot, warm and cold
   Answer: A. Lubricants are liquid, semi-solid (greases) or solid. (Key point: Classification)

3. What is a grease?
   A. A mixture of graphite and water
   B. A pure synthetic oil
   C. An oil thickened with metallic soap
   D. A solid mineral like talc
   Answer: C. Greases are oils thickened with metal soaps such as Ca, Na or Li. (Key point: Grease)

4. Which are the types of liquid lubricants?
   A. Animal/vegetable, mineral, synthetic and blended oils
   B. Graphite, MoS2, talc and soapstone
   C. Calcium, sodium and lithium greases
   D. Oil paint, enamel and varnish
   Answer: A. Liquid lubricants include animal/vegetable, mineral, synthetic and blended oils. (Key point: Liquid types)

## Related in Engineering Chemistry

- [Liquid lubricants and their classification](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Aliquid-lubricants-classification) — Engineering Chemistry, chapter 8
- [Solid lubricants and their use](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Asolid-lubricants) — Engineering Chemistry, chapter 8, asked 2×
- [Classification of air pollutants; sources, effects](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Aair-pollutants-classified) — Engineering Chemistry, chapter 3

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