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

# Liquid lubricants and their classification

Engineering Chemistry · Chapter 8: Lubricants and Paints

- Asked in: 2068 Magh, 3 marks
- Video: [1.LIQUID LUBRICANT OILS @sadhanadhananjaya CHEMISTRY WINS #chemistry](https://www.youtube.com/watch?v=02Nfi3cgK8U) — CHEMISTRY WINS BY DHANANJAYA
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/liquid-lubricants-classification
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-8

## Question

Describe liquid lubricants with emphasis on their classification.

## Short answer

Liquid lubricants include animal/vegetable oils (good oiliness, unstable), mineral oils (cheap, stable), synthetic oils (extreme temperature use) and blended oils (mineral with additives).

## Answer

Liquid lubricants (lubricating oils) are liquids placed between moving surfaces to form a fluid film that reduces friction. They are the most widely used lubricants because they flow easily, carry away heat and are cheap.

#### Classification
```text
Liquid lubricants
 ├── Animal and vegetable oils
 ├── Mineral (petroleum) oils
 ├── Synthetic oils
 └── Blended oils
```
1. **Animal and vegetable oils:** castor oil, olive oil, palm oil, tallow oil, whale oil. They have good oiliness (stick well to metal) but oxidise, become gummy and decompose at high temperature. Used in light machinery and as additives.
2. **Mineral oils:** obtained by fractional distillation of petroleum. They are cheap, stable and available in many grades, but have poor oiliness. Used in engines, turbines, gears.
3. **Synthetic oils:** silicones, polyglycols, organic esters, chlorinated hydrocarbons. They keep their viscosity over a wide temperature range and are used in jet engines and extreme temperatures.
4. **Blended oils:** mineral oil with additives (oiliness agents, antioxidants, pour-point depressors, viscosity-index improvers) to give special properties.

## Check yourself

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

1. What is the main weakness of animal and vegetable oils as lubricants?
   A. They have poor oiliness
   B. They are too thin at all temperatures
   C. They are derived from petroleum
   D. They oxidise, turn gummy and decompose at high temperature
   Answer: D. They have good oiliness but oxidise and decompose. (Key point: Animal/vegetable)

2. Why are mineral oils the most common?
   A. They have the best oiliness
   B. They are cheap, stable and available in many grades
   C. They are the only liquid lubricants
   D. They are made by blending vegetable oils
   Answer: B. Mineral oils from fractional distillation of petroleum are cheap and stable, but have poor oiliness. (Key point: Mineral oils)

3. When are synthetic oils preferred?
   A. For cheap light machinery
   B. Only as additives to grease
   C. For watches only
   D. In jet engines and at extreme temperatures
   Answer: D. They keep their viscosity over a wide temperature range. (Key point: Synthetic oils)

4. What are blended oils?
   A. Animal fats mixed with water
   B. Mineral oil with additives such as antioxidants and viscosity-index improvers
   C. Greases with soap
   D. Pure synthetic oils
   Answer: B. Additives give special properties. (Key point: Blended oils)

## Related in Engineering Chemistry

- [Lubricants and their classification](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Alubricants-classification) — Engineering Chemistry, chapter 8

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